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Catalytic Reactions with Hydrosilane and Carbon Monoxide [New synthetic methods (30)].

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Catalytic Reactions with Hydrosilane and Carbon Monoxide
By Shinji Murai and Noboru Sonodat*]
New synthetic
methods (30)
The reagent hydrosilane/carbon monoxide opens up new possibilities for organic synthesis.
Four cases will be discussed: 1. The reaction of olefins with hydrosilane (trialkylsilane) and
carbon monoxide in the presence of Co, Ru, and Rh complexes leads to enol silyl ethers having
one more carbon atom than the olefins. 2. Cyclic ethers undergo carbonylative ring opening to
w-siloxyaldehydes when reacted with hydrosilane and carbon monoxide in the presence of
C O ~ ( C Oas) ~
catalyst. 3. Aldehydes are catalytically converted into the next higher a-siloxyaldehydes or 1,2-bis(siloxy)alkenesdepending on the reaction conditions used. 4. The reaction of
alkyl acetates proceeds in various ways depending on the nature of the alkyl group; enol silyl
ethers or alkenes are obtained.-Mechanisms for these C O ~ ( C Ocatalyzed
reactions using hydrosilane and carbon monoxide are discussed in which HCo(CO), or R3SiCo(CO),L function
as catalytically active agents. With these species there are four types of catalytic cycles.-The
synthetic possibilities of these catalytic reactions have still not been fully explored.
1. Introduction
Not only olefins but also various oxygenated compounds
such as cyclic ethers and carbonyl compounds have been
The hydroformylation of olefins with molecular hydrogen
found to react with hydrosilane and carbon monoxide in the
and carbon monoxide to produce aldehydes has been extenpresence of group VIII transition metal complexes. In most
sively studied for almost forty years; the interest attached to
cases C O ~ ( C Ohas
) ~ been used as catalyst. Both olefins and
homogeneous catalysis and the growing industrial imporoxygenated compounds incorporate carbon monoxide and
tance of such reactions are continually stimulating new inthe silyl moiety to give stable 0-silylated derivatives of
usually unstable or otherwise inaccessible hydroxy comA naive idea occurred to us about four years ago: what
pounds such as enols, hydroxy aldehydes, or ene-diols.
happens if a hydrosilane (HSiR3) is employed in place of
In this article we present a survey of these new catalytic
molecular hydrogen in the hydroformylation? The idea was
reactions, almost all of which were discovered in our laborabased on the known similarity in reactivity between molecutory. Emphasis will be laid on the basic features of the reaclar hydrogen and hydrosilanes toward transition metal comtions and hence on the possible role of the catalytic agents.
plexes (cf. Section 2.1). A few months later, clean but rather
We shall also focus attention on the unsolved problems in
puzzling reactions were found. For example, the C O ~ ( C O ) ~ this area, for these reactions are still in an early stage of decatalyzed reaction of cyclohexene with a hydrosilane and
carbon monoxide gave a silyl ether of cyclohexanecarbaldehyde-enol (vide infra). This finding led to the discovery of a
series of new catalyzed reactions using hydrosilane and car2. Catalytic Reactions of Olefins with Hydrosilane
bon monoxide, opening up a new field embracing organosiliand Carbon Monoxide
con chemistry, homogeneous catalysis, and synthetic organic
2.1. Similarity in Reactivity of Hydrosilanes and Molecular
Hydrogen toward Transition Metal Complexes
[*] Prof. Dr. S. Murai, Prof Dr. N. Sonoda
Department of Petroleum Chemistry, Faculty of Engineering
Osaka University, Suita, Osaka 565 (Japan)
Angew. Chem. Inf. Ed. Engl. 18, 837-846 (1979)
It is interesting to note that reactions of hydrosilanes
(HSiR3) with transition metal complexes parallel those of
0 Verlag Chemie, GmbH, 6940 Weinheim, 1979
$ 02.50/0
molecular hydrogen. Both hydrosilanes[ll and molecular hydrogenI2]undergo oxidative addition[31to group VIII metal
R3Si(Hz)Co(P P h 3 ) ,
Scheme 1. Similarity of hydrosilane and molecular hydrogen. R = OEt
complexes. The
shown in Scheme 1 may serve as
illustrative examples of the similar behavior of hydrosilanes
and molecular hydrogen. Catalytic hydr~silylation~'~~~,
i. e.
the addition of hydrosilanes to unsaturated compounds with
H SiR3
the aid of transition metal complexes [eq. (a)], is formally
analogous to catalytic hydrogenation [eq. (b)]['I. Mechanistically, these two reactions seem to be very closely related[''21.
Various attempts have been made at the insertion of carbon monoxide into the Si-H bondI6I or into silicon-transition metal bonds['], but all have proven unsuccessful. This
parallels the fact that no definite example of carbonyl insertion into a hydrogen-transition metal bond has been recordedis1,although this reaction to give formylmetal complexes is suggested to be important as the initial step in the
Fischer-Tropsch synthesis[*"]or in the Union Carbide ethylene glycol synthesis"'. In this context, the copper(I1) catalyzed insertion of cyclohexyl isocyanide, which is isoelectronic to carbon monoxide, into the Si-H bond of triethylsilanei101is very intriguing. Carbon monoxide, however, does
not react with the silane under the same conditions["].
transition metal c o m p l e ~ e s ~ ' ~The
- ' ~ ~important
works of
Colleuille~i'l,as well as those o f c h a l k and Harrod16."I, will
be referred to at the end of this section.
As shown in eq. (d), the reaction of diethyl(methy1)silane
(1) and carbon monoxide in the presence of a catalytic
amount of C O ~ ( C O )gives
[diethyl(methyl)siloxymethylene]cyclohexane (2), an enol silyl ether, as sole product and
no trace of the acylsilane (3) or the p-silyl aldehyde (4), as
would be expected as products by direct formal analogy with
the Roelen reaction['i,14,151.Similarly, the (siloxymethy1ene)cycloalkanes of type (2) were obtained from cyclopentene (48%), cycloheptene (74%), and cyclooctene (69%)[14."I.
The reaction conditions were: catalyst 0.2-0.4 mmol, olefin
30-200 mmol, HSiEt,Me (1) 10 mmol, initial pressure of
CO 50-80 bar at 25 "C, reaction temperature 140"C, 20 h.
Since two reaction sites are available in a terminal olefin,
four isomers of siloxymethylene derivatives (regio- and stereoisomers) are expected in this case. All these isomers have
been obtained (80%)and characterized in the C O ~ ( C Ocata)~
lyzed reaction of l-hexene[1i*16'
[eq. (e)].
+ CO
C6H6.140 "C, 80 %
( Z ) - ( 5 ) , 16%
( E )- (5), 1 6 %
The regioisomers (5) and (6) may correspond to those [(7)
and (S)] obtained by the Roelen reaction'121[eq. (01. Methyl
acrylate, a terminal olefin having an electron withdrawing
group, also gave a mixture of four isomeric enol silyl eth-
2.2. Reaction of Olefins with Hydrosilanes and Carbon Monoxide Catalyzed by Co,(CO),
The well known Roelen reaction (so-called hydroformylation or 0x0 reaction)[l2]involves addition of hydrogen and
carbon monoxide to olefins [eq. (c)]. The similarity between
hydrosilanes and molecular hydrogen in transition metal
reactions urged us to study the reaction of olefins with hydrosilanes and carbon monoxide in the presence of various
O + H 2 + C 0
ers["f. When the reaction was carried out at higher temperatures or with higher concentrations of catalyst, migration of
the double bond in the product took place. The reaction of
cyclohexene at 200 "C gave I-(siloxymethy1)cyclohexene (9)
in addition to (2) in a ratio 1:5[151.
0siE t z Me
C6H6, 140°C
When migration of the double bond gives the more stable
isomer, complete isomerization is observed as in eq. (g)["l.
So far, we have examined only a few types of olefins. Besides olefins, there still remain dienes and acetylenes to be
studied. The effective range of transition metal complexes as
catalysts has been studied and will be discussed in Section
Angew. Chem. Inr. Ed. Engl. 18, 837-846 (1979)
workers reported that the reaction of I-hexene was too complex, which is in contrast to our findings.
2.3. Catalysts
4 9%
2.3. While the reactivity of the H- -H bond in H2 obviously
cannot be changed, that of the Si-H bond can be varied by
changing the substituents on the silicon atom. This is one of
the interesting aspects of the reactions using hydrosilanes.
The reactivity of the hydrosilanes (HSiR'R2R3 where
R" = H, alkyl, aryl, alkoxy, halogen etc.) in hydrosilylation
depends upon the s u b s t i t u e n t ~ [A~ ~
~ ~ ,examination of the
reactivity of hydrosilanes has been made for the C O ~ ( C O ) ~
catalyzed reaction of cyclohexene [eq. (d)]["I. In this particular case the trialkylsilanes (alkyl = Me and/or Et) appear to
be the reaction partners of choice. Although hydrosilanes
having alkoxy or chloro groups proved to be very reactive,
the reactions were complex and gave mixtures of many products whose structures have not yet been clarified. Diethyl(methy1)silane ( I ) was used exclusively in the present
study because it is easy to handle (b.p. 76 "C, very stable towards water and oxygen). Moreover, the NMR spectrum of a
compound containing an Et,MeSi group(s) shows nice sharp
singlet(s) due to the methyl group(s). This is of great experimental utility, because it enables the purity of a product or
the number of the products formed to be ascertained immediately.
Pioneering work on the C O ~ ( C Ocatalyzed
reactions of
olefins with hydrosilanes and carbon monoxide has also been
carried out independently by Colleuille and his results have
appeared in the patent literature["]; we first became aware of
this only very recently. Although details for the characterization of products are not given, as is usual in patents, it has
been clearly demonstrated that the corresponding enol triethylsilyl ethers were obtained from ethylene (44%), propene
(70%), 1-butene (27%), isobutene (44%), cyclohexene (87%),
and norbornene (65%) on using HSiEt3 and CO. With excep-
C H z = C H 2 + HSiEts
CH3CH=CH2 + HSiEt3
0 s iE t 3
A search for good catalysts for a certain reaction will never
be complete since there are numerous possible combinations
of metals, ligands, additives, reactants, and reaction conditions. Even for the Roelen reaction[121,which has forty years
of history, the research in this field is still in progress. So far,
we have examined the catalytic activity of various transition
metal complexes only for the reaction of 1-hexene according
to eq. (e).
For this particular reaction the following complexes show
little or no catalytic activity["]: cuCl2, Cu2C12, W(CO),,
IrC1(CO)(PPh3)2, Ni(C0)4,
Ni(CO)2(PPh3)2, Ni(acac),, PdC12(PPh3)2,Pd(PPh3)4, Pdblack, H2PtC16,Pt-black. The effective complexes found are
listed in Table 1[11,161.
Table 1 . Reaction of 1-hexene with HSiEt2Me and CO to give the linear isomer
(6) and the branched isomer (5) [a].
Catalyst (mmol)
(5) + (61 [%I
C O ~ ( C O(0.4)
(0.4) [c]
C02(C0)8 (0.4) + Ph3P (1.0)
C O ~ ( C O(0.4)
) ~ + n-Bu3P (1.0)
RhCI(PPh,), (0.4)
RhCI(PPh,)] (0.4) + Et,N (1.0)
tion of the reaction with propene, however, in all the cases
mentioned considerable amounts of unidentified by-products (1 to 0.15:1)were obtained which could not be separated from the enol silyl ethers by fractional distillation.
These by-products are likely to be double bond-isomers similar to (9) and/or hexaethyldisiloxane. No comment is made
about the presence of stereoisomers, as in the reaction of 1hexene [eq. (e)]. Chalk and Harrod have also reported the
C O ~ ( C Ocatalyzed
reaction of ethylene with HSiEt, and
CO to give 1-(triethylsiloxy)-1-propene[
Both groups of
Angew. Chem. Int. Ed. Engl. 18, 837-846 (1979)
[a] Reaction conditions: 1-hexene (30 mmol), HSiEt2Me (10 mmol), CO (50 bar,
initial pressure at 25 "C), catalyst (as given), solvent (benzene, 20 ml), 140 "C, 20
h. [b] For comparison: Relative yield of linear isomer in the Roelen reaction [I91
[eq. (01in (%), [(8)/(7) + (SJ] x 100. [c] 200 mmol 1-hexene.
These effective complexes for the reaction of 1-hexene
were also tested in the reaction of cyclohexene [eq. (e)]; the
results are shown in Table 2. Interestingly, whereas
C O ~ ( C O was
) ~ more effective for cyclohexene, R U ~ ( C O ) ~ ~
and RhCl(PPh3)3 were more effective in the case of l-hexene. Further investigations in this area should open up the
possibility of selective reaction of different double bonds in
one molecule.
Table 2. Reaction of cyclohexene with HSiEt2Me and CO [a].
Catalyst (mmol)
R u ~ C O ) I(0.2)
Co2(CO)n (0.2)
C02(CO)s (0.4) PI
RhCI(PPh1), (0.2)
RhCI(PPh,), (0.2)
Yield of (2) [%I
+ Et,N (1.0)
[a] Reaction conditions: cyclohexene (30 mmol), HSiEt2Me (10 mmol), CO (80
bar, initial pressure at 25"C), catalyst (as given), solvent (benzene, 20 ml),
140"C, 20 h. [b] Cyclohexene (200 mmol), CO (50 bar, initial pressure at
25 "C).
Since the present reaction requires the activation of both
hydrosilane and carbon monoxide, it seems of interest to
compare these results with those of hydr~silylation[',~~
the Roelen reactionfl21.The effective catalysts found for the
reaction of I-hexene with HSiEt,Me and CO, i. e. R U ~ ( C O ) ~ Z ,
and RhCl(PPh3)3, are known to be active in the
Roelen reaction. The latter two complexes are also effective
in hydrosilylation['l. However, among the complexes which
show little or no activity in the present reaction there are
some which are effective catalysts for the Roelen reaction
[for example, Rh203 and IrCl(CO)(PPh3)3][121
and for hydrosilylation [PdC12(PPh3)2,H2PtC1,, etc.]['l.A tentative conclusion that could be drawn here is that an effective catalyst in
the present reaction may also be effective as a catalyst in
both the Roelen reaction and in hydrosilylation[201.However,
more extensive studies are obviously necessary in this field.
It is noteworthy that the change in regioselectivity of the
reaction of I-hexene [eq. (e), Table I], is similar to that observed in the Roelen reaction [eq. (01when using Co2(CO),/
PR3 as catalyst['61.This implies that the interaction of l-hexene with HCO(CO)~is also important as initial step in the
reaction using hydrosilane and carbon monoxide (cf Section
the mechanism given in Scheme 2 for the C O ~ ( C Ocata)~
lyzed reaction of cyclohexene with DSiEt,Me and CO.
While it is obvious that the carbon and oxygen atoms of
carbon monoxide are incorporated into the oxymethylene
group of the product, the enol silyl ethers, it is much more
difficult to establish which hydrogen atom in the product
stems from the hydrosilane used in the reaction. To clarify
The effects of a change in reaction conditions have been
examined for the Co,(CO), catalyzed reaction of I-hexene
with HSiEt2Me and CO [eq. (e)]. The reaction conditions described in footnote [a] of Table 1 have been regarded as the
standard conditions. Under these standard conditions (5)
and (6) were obtained in a combined yield of 57%. Some of
the more important results" 'I will be summarized below.
The combined yield of (5) and (6) markedly increases on
using a higher molar ratio of I-hexene to hydrosilane: The
yields were 7,57, and 80% at olefin/hydrosilane ratios of 1:1,
3: 1 and 20: 1, respectively. Good yields were obtained with
Co2(C0),:hydrosilane in the molar ratio 1 :25. The use of
smaller amounts of catalyst gave lower yields, while the use
of a higher catalyst: hydrosilane ratio (1 : 10) resulted in the
formation of a hydrosilylation product (n-C6H13SiEt2Me)at
the expense of the enol silyl ethers (5) and (6). The effect of
CO pressure was examined in the range between 3 and 150
bar (initial pressure at 25 "C). The yield increased from 14%
at 3 bar to 68% at 80 bar and remained almost unchanged at
higher pressures. The best results for the reaction of I-hexene
were obtained at a reaction temperature of 140°C. The
yields of the enol silyl ethers decreased with decreasing temperature, and no reaction took place below 80 "C. At 200 "C,
migration of the double bond in the products took place to
some extent. The distribution of the four isomers [(qand
(I?)+) (branched), (3-and (E)-(6) (linear)], and hence the
proportion of linear isomers, did not change significantly under the various reaction conditions described above. Nor did
changes in the reaction time have any influence on the distribution. The reactions were almost complete within 7 h under
the standard conditions.
2.5. Mechanistic Aspects
The mechanism of the catalyzed reaction of olefins with
hydrosilane and carbon monoxide is one of the most intnguing problems. An understanding of the mechanism is, however, essential for further development of new reactions. On
the basis of the arguments outlined below, we propose here
this problem we carried out the reaction of cyclohexene with
DSiEt,Me and CO using C O ~ ( C Oas
) ~a catalyst["]: the deuterium was found predominantly (91%) in the vinylic position of the product (2a).
1niti at i on :
2.4. Reaction Conditions
+ CO
C o z ( C O ) , + DSiEtzMe
DCo(CO), + MeEtzSiCo(CO),
The first catalytic cycle:
Succeeding catalytic cycles:
Scheme 2. C O ~ ( C Ocatalyzed
reaction of cyclohexene with DSiEt,Me and CO.
(The step (7) is not involved in the catalytic cycle; see text.) Abbreviations: SiCo
erc. = MeEt,SiCo(CO). efc., n = 3 or 4.
The course of the deuterium incorporation may be best explained in terms of the catalytic cycle shown in Scheme 2.
For simplicity, the alkyl groups on the silicon and the CO ligands on the cobalt have been omitted. The most important
reactions of hydrosilanes with C O ~ ( C Ohave
) ~ been studied
by Chalk and Harrodf6I and later by Baay and MacDiarThe reaction of HSiR3 with CO,(CO)~takes place
smoothly even at room temperature to give HCo(CO), and
R3SiCo(CO), [eq. (h)]. In the presence of excess amounts of
the hydrosilane, the former reacts with the silane with evolution of molecular hydrogen [eq. (i)]. The catalytic cycle may
+ C O ~ ( C O-+
) ~HCo(C0)4 + R3SiCo(C0)4
HSiR3 + HCO(CO)~
--t R3SiCo(C0)4+ Hz
Angew. Chem. Int. Ed. Engl. 18, 837-846 (1979)
start by the insertion of olefin into the H---Co bond (Step 1).
This is in agreement with the observed similarity of the product linearities with those in the Roelen reaction (Section 2.3)
and with the fact that better yields of the product have been
obtained at higher concentrations of olefin (Section 2.4), suggesting competition between insertion (Step 1) and hydrogen
evolution [eq. (i)]. Although the reaction with DSiEt2Me
proceeds primarily via DCO(CO)~,this catalytically active
species may be converted into HCO(CO)~
in the first catalytic
cycle (Scheme 2). In the second and all succeeding catalytic
cycles the reaction of the olefin is catalyzed by HCO(CO)~
(Step 1)which is repeatedly regenerated by p-elimination in
the product-forming step (Step 6). Thus, the deuterium taken
up in the oxidative addition (Step 3) moves to the acyl position of the intermediate aldehyde in the reductive elimination step (Step 4), and finally appears at the vinylic position
of the enol silyl ether ( 2 4 (Step 6). The possible intermediacy of an aldehyde will be discussed later.
This deuterium labeling experiment clearly rules out the following two alternative mechanisms. The first one involves
intervention of molecular hydrogen and is represented by
two successive reactions [eq. (c) and (i)]. At first glance, it
might be surmised that the present reaction involves the well
In order to decide which mechanism is operating, one
could attempt to isolate the aldehyde. Examination of Scheme 2 suggests that introduction of an efficient by-path (Step
7) to convert the silyl(carbony1)cobalt compound (SiCo in
the Scheme) into HCo(CO), (HCo in the Scheme) would
leave the aldehyde intact without breaking the catalytic cycle. It occurred to us that tert-butyl acetate would be suitable
for this purpose. The conversion could be expected to proceed via (10) or (11) (see Section 3.4).
+ Hz
known Roelen reaction[''] [eq. (c)] utilizing molecular hydrogen which could be generated in situ by dehydrogenative
[eq. (i)]of the aldehyde thus formed. The second
alternative mechanism involves initial formation of an acylsilane such as (3) as an intermediate, followed by Brook rearrangement[22]to give (2). This mechanism has been proposed
by Chalk and Harrod to account for their observations['x1.
Do,5 OSiEtZMe
Indeed, cyclohexanecarboxaldehyde was obtained in 52%
yield when the C O ~ ( C Ocatalyzed
reaction of cyclohexene
(90 mmol) with HSiEt'Me (10 mmol) and CO (50 bar) was
carried out in the presence of tert-butyl acetate (30 mmol) in
benzene at 140 "C for 20 h["l. The enol silyl ether (2) (20%),
diethyl(methy1)silyl acetate, and isobutene were also formed
[eq. (l)]. For this reaction we suggest the catalytic cycle
+ (2) + MeEtzSiOAc
+ CHz=C(CHs)z
C6H6.140 "C
5 2%
shown in Scheme 3, and that an aldehyde is formed as intermediate in the reaction (d); but further verification of Scheme 3 is required, since the possible intervention of the carbene complex [eq. (k)] is still not out of the question.
According to the first or the second alternative mechanism
the reaction using DSiEt'Me should afford (2b) or (24, re~pectively['~].However, the formation of ( 2 4 (vide supra) is
In the proposed mechanism (Scheme 2), the C-Co bond
is first cleaved in the reductive elimination step (Step 4)giving an aldehyde-and is then reformed again in the subsequent step (Step 5). Whether an aldehyde is first formed as
an intermediate or not may be an important
a more direct path via a carbene-metal complex, in which the
C --Co bond remains unbroken-with no aldehyde being
produced [eq. (k)]rz51-is also conceivable.
Angew. Chem. Int. Ed. Engl. 18, 837-846 (1979)
Scheme 3. Formation of an aldehyde from cyclohexene. (The numbering of the
steps and abbreviations correspond to those in Scheme 2.)
It should be noted that the formation of an aldehyde according to eq. (1) is a new type of hydroformylation of an olefin and differs from the Roelen[''] or Reppe reactions["]. It is
hoped that still better trapping agents than terf-butyl acetate
will be found.
There are still many unsettled questions regarding the
mechanism. Above all, a detailed knowledge of the numerous steps involved and of the structures of the catalytically
effective intermediates is still lacking. A further interesting
question is whether the mechanism suggested above is also
valid or not for reactions with other catalysts, such as Rh and
Ru complexes.
3. C O ~ ( C O )Catalyzed
Reactions of Oxygenated
Compounds with Hydrosilane and Carbon monoxide
and is regenerated in the subsequent reaction (i) with evolution of molecular hydrogen. Recently, Sakurai, Miyoshi, and
Nakadaira have found that the dehydrogenative silylation of
ketones with hydrosilanes is effectively catalyzed by a
(additive: amines, phosphanes e t ~ . ) [ ~ ~ ] .
Again, silyl(carbony1)cobalt must play an important role in
3.1. Reactivity of Silyl(carbony1)cobalt Compounds
Reaction of a hydrosilane with C O ~ ( C Ogives
) ~ HCo(CO),
and R,S~CO(CO)~,
[eq. (h)]. As we have already seen,
is important in the catalyzed reaction of olefins. It
has now been found that there are other reactions of hydrosilane and carbon monoxide in which R3SiCo(C0)4 seems to
play the key role as catalyst. In these reactions, the catalytic
cycles evidently begin with interaction between the substrates and R3SiCo(C0), and terminate with the regeneration of this compound. The substrates may be oxygenated organic compounds such as cyclic ethers, aldehydes, and esters.
These compounds incorporate carbon monoxide and form siloxyaldehydes or enol silyl ethers having one or two more
carbon atom@).So far, we have studied these reactions onIy
with CO,(CO)~as catalyst (or catalyst precursor). Before discussing such reactions, it may be appropriate to take a brief
look at the characteristic reactivity of R3SiC~(C0)4[271.
Trialkylsilyl(tetracarbonyl)cobalt, R3SiCo(C0)4, may be
obtained by the reaction of hydrosilane with C O ~ ( C Oas
shown in eqs (h) and (i) and can be isolated by distillation or
by crystallization when desired[6.2'.28.2yl.
The most important
property of these silyl(carbony1)cobalt compounds is perhaps
that they are very susceptible toward nucleophilic attack at
the silicon atom even by weak bases such as alcohols [eq.
Since the stereochemistry of this reaction involves
+ R3SiCo(C0)4 + R'OSiR, + HCO(CO)~
this useful reaction. Chalk has reported that C O ~ ( C Oin) ~the
presence of a twofold or greater excess of HSiEt, catalyzes
the cationic polymerization of tetrahydrofuran, an oxetane,
and propylene oxide[301.
A silyloxonium ion of type (13) has
been p o ~ t u l a t e d as
~ ~intermediate.
These examples imply that the central silicon atom in
R3SiCo(C0), is strongly acidic and strongly electrophilic.
3.2. Catalytic Reactions of Cyclic Ethers with Hydrosilane
and Carbon Monoxide
The reaction of cyclic ethers (tetrahydrofuran, oxetane,
and 1,2-epoxycyclohexane) with HSiEt2Me and CO using
C O ~ ( C Oas
) ~catalyst has been found to give w-[diethyl(men
(IS), 40%
inversion at the silicon
the reaction most likely proceeds via the intermediate (12)[301.Aprotic bases such as ter-
tiary amines or phosphanes react similarly[271.
For example,
the reaction of trimethylsilyl(tetracarbony1)cobalt with trimethylphosphane gives a silylphosphonium salt [eq. (n)] and
not a phosphanecobalt complex obtainable by ligand exchanger3". Strong bases such as MeLi and MeMgBr substiMe3P + Me3SiCo(C0)4 --* [Me3P-SiMe3]
tute triphenylsilyl(tetracarbonyl)cobalt[32~at the silicon atom,
while PhLi attacks at the carbonyl ligand[331.
Co2(C0)8 is known to catalyze the silylation of alcohols,
carboxylic acids, and amide~[~'I.
In these silylations, the acEtOH
cat. co2(CO)8
EtOSiEt, + Hz
tive catalyst must be Et3SiCo(CO),, which is generated by
the reaction (h); it silylates the alcohol according to eq. (m),
( e x c e s s)
(16), 51%
The reaction conditions were: cyclic
ether (3-5 equiv.), HSiEt,Me (1 equiv.), CO with initial
pressure of 60 bar at 25 "C, C O ~ ( C O(0.02
) ~ equiv.), in benzene at 70-140°C for 20 h. For the synthesis of the w-siloxyaldehydes (14)to (16)excess amounts of cyclic ether are
essential (see Section 3.3). The reaction appears to be limited
to three-, four-, and five-membered cyclic ethers. Neither tetrahydropyran nor diethyl ether reacted under these conditions. Interestingly, diethyl ether could even be used as a solvent in the reaction of tetrahydrofuran. The enhanced reactivity of tetrahydrofuran toward R3SiCo(C0)4 has been
known for some time[30.361.
Angew. Chem. inl. Ed. Engl. 18, 837-846 (1979)
which interaction of the carbonyl group of an aldehyde with
R3SiCo(C0)4 seems to be the key step.
Reaction of 3 equivalents of aldehyde with 1 equiv. of
HSiEt,Me and CO (50 bar) in the presence of 0.04 equiv. of
Co2(C0)8PPh3(1 :1) in benzene at 100 "C for 20 h, afforded
the a-siloxyaldehydes (18) having one more carbon atom
[eq. (s)][~'].The yields of (18) were 49% (R=n-C3H7), 50%
(R = n-C,HI3), and 54% (R = cyclohexyl). Although it is not
always necessary to use PPh, as a co-catalyst, it was found
effective in suppressing the simple hydrosilylation of aldehydes [eq. (t)].
Scheme 4. C O ~ ( C Ocatalyzed
reaction of T H F with HSiEt2Me and CO
Although a detailed reaction mechanism has not as yet
been formulated, the catalytic cycle shown in Scheme 4
would appear to account for the present carbonylative ring
opening. Two important aspects of this mechanism are: firstly, R3SiCo(C0)4,and not HCO(CO)~,
undergoes initial interaction with the cyclic ether, and is later regenerated; secondly, the well known high affinity of the organosilyl group for
the oxygen atom[371could be regarded as the important driving force for formation of the intermediate having a C--Co
bond like (17). In other words, Scheme 4 illustrates a new
method for the formation of a carbon-transition metal bond;
the carbonyl insertion is made possible by using the strong
affinity of silicon for oxygen.
The ring-opening of tetrahydrofuran to the alkylcobalt
complex (17) could proceed via the oxonium ion (13). In this
context, the configuration of the product (16) from 1,2epoxycyclohexane is very interesting. The trans configuration of (16) could be understood by assuming an SN2-type
ring-opening of the oxonium ion intermediate.
In contrast to reaction (o), the Roelen reaction["] of tetrahydrofuran with Co2(CO), as a catalyst gives several products [eq. (r)]f3'1.
We suggest the catalytic cycle shown in Scheme 5 for the
reaction (s). Here also, the active catalyst is a silyl(carb0ny1)cobalt compound, M ~ E ~ , S ~ C O ( C O(L) ~
CO or PPh3)
(19), and this reacts with the aldehyde to give the a-siloxyalkylcobalt (20). The strong affinity between silicon and oxy-
R K 0"
Scheme 5. Coz(CO),/PPh, catalyzed conversion of a aldehyde to the next higher
a-siloxyaldehyde using MeE1,SiH and CO. (The substituents on the silicon are
not shown.) L = CO or PPh,.
gen forces bond formation between the carbonyl carbon and
the cobalt atom. In contrast, under the conditions of the
Roelen reaction[121
(H2 and CO) aldehydes are known to give
formates. This implies that the interaction of aldehydes with
HCo(CO), (n = 3 or 4) takes place in the opposite direction to
give (2,?)i4OI.
2 R+H
- HCdC0)s
3.3. Catalytic Reactions of Aldehydes with Hydrosilane and
Carbon Monoxide
We have tested whether the above described new entry to
intermediates having a C-Co bond is also applicable to aldehydes, and have revealed unique catalytic reactions in
Angew. Chem. Int. Ed. Engl. 18, 837-846 (1979)
si ---- +.o+ ____ co
Insertion of carbon monoxide into the C-Co bond of (20)
takes place to give the acylcobalt (21). Very recently, inser843
tion of carbon monoxide into an a-siloxyalkylmanganese
complex having similar structure to (20) has been reported["'I. Such reactions involving a-hydroxyalkylmetal
compounds are very important with regard to the mechanism
of the Fischer-Tropsch reaction['"] and the Union Carbide
ethylene glycol synthesi~''~in which a-hydroxyalkylmetal
compounds [(23) and (24), respectively] are thought to play a
key role.
nation of the catalytic cycles given in Schemes 5 and 6. The
use of PPh3 was essential in these cases to suppress reaction
Instead of the mechanisms shown in Schemes 5 and 6
some alternative ones could be suggested in which ionic species such as (27) or (28) could play a role. These possibilities
remain open.
(27) [ R T H ] C O ( C O ) , ~
' MH
Two significant aspects of the reaction (s) may be worth
mentioning. Firstly, the reaction (s) represents the first definite example of a catalytic reaction in which carbonylation
with carbon monoxide takes place directly at the carbon of a
carbonyl group. To our knowledge no reaction of this type,
except for a few equivocal ones[42],has so far been reported,
probably because no appropriate entry to the required a-hydroxyalkylmetal precursor has been available. Secondly, so
far as we know, reaction (s) (Scheme 5 ) constitutes the first
example of the interception of a plausible intermediate of
carbonyl hydrosilylation [e. g. (t)], viz. of the compound
To obtain the a-siloxyaldehydes (18) by reaction (s), it is
essential to use the starting aldehydes in excess over the hydrosilane; otherwise, the yields of (18) decrease. This implies
that the catalyst species R3SiCo(CO)3L (19) can react not
only with the starting aldehydes but also with the a-siloxyaldehydes (18) that are formed. Thus, the Co2(CO)dPPh3catalyzed reaction of an aldehyde with an excess of HSiEt2Me
(1 :3) and CO gave a different product, the 1,2-bis(siloxy)al-
+ HSiEtzMe
C6H6, 140°C
kene (25) was obtained instead of (18) [eq. (u)]["'I. The yields
were 67% (R = n-C3H7),66% (R = n-C6H13), and 37% (R = cyclohexyl). Apparently, (25) may have been obtained as a result of dehydrogenative silylation of the initially formed (18).
Scheme 6 shows the simplified catalytic cycle for the conversion of (18) into (25); the active catalyst (19) is regenerated,
similarly as in reaction (i), with evolution of molecular hydrogen. The overall reaction (u) may be regarded as a combi-
HS i
Since the product from tetrahydrofuran is the aldehyde
(14) [see eq. (o)],it can be expected that the use of excess
amounts of HSiEt2Me will convert it into the corresponding
1,2,6-tris(siloxy)hexene (29). This is indeed the case, as is
shown in eq. (v)'"']. In this reaction, it was unnecessary to use
bS iE t zM e
( 2 9 ) , 89%
PPh3 as a co-catalyst since the starting material was not an
aldehyde [cf. reactions (s), (t), and (u)]. The course of the
reaction (v) may be explained by a sequence of three catalytic cycles corresponding to Schemes 4, 5, and 6. The route
from tetrahydrofuran to the product (29) is possibly very
long, most likely involving more than a dozen steps. By using
excess amounts of HSiEt,Me, oxetane gave a product similar
to (29), but 1,2-epoxycyclohexaneunexpectedly afforded (2)
as the major product["].
A fascinating possibility could be envisaged on considering this type of C-C bond forming reaction with carbon
monoxide. If one could carbonylate the a-siloxyaldehyde
(18) in the same manner as in eq. (s), one would obtain a
higher a$-bis(si1oxy)aldehyde. By repeating this process,
one would in principle be able to "copolymerize" hydrosilane and carbon monoxide to persilylated polyhydroxymethylenes (30) [eq. (w)][""]. However, various other problems
still have to be solved before seriously attempting this "copolymerization".
0 si
HC o A S i O M H
Scheme 6. Catalytic conversion of a-siloxyaldehydes ( I S ) into 1,2-bis(siloxy)alkenes. (The substiluents on the silicon are not shown. For simplicity, the ligands
on the cobalt whose number may possibly change during the cycle are also not
shown.) Abbreviations: SiCo etc. = MeEt2SiCo(CO),L efc.; n = 3 or 4, L = CO or
3.4. Catalyzed Reactions of Alkyl Acetates with Hydrosilane
and Carbon Monoxide
There are many promising candidates available as partners for a catalyzed reaction with hydrosilane and carbon
Angew. Chem. Inf. Ed. Engl. 18, 837-846 (1979)
monoxide. The reactions of oxygenated compounds such as
esters, acid anhydrides, ketones, enones, acetals, ketals, and
orthoesters as well as those of organic halides would be of
great interest. So far, only a few substrates have been examined; some preliminary results of experiments with alkyl acetates are described below[“’.
Alkyl acetates were allowed to react with HSiEt2Me (3 :1)
and CO (50 bar) in the presence of C O ~ ( C Oin
) ~benzene at
140 “C for 20 h [eq. (x)-(z)].
We wish to express our sincere gratitude to our able former
students: Dr. Y. Seki (now at Kagawa University) for his conceptual and experimental contributions, Messrs. A Hidaka, I.
Yamamoto, Y. Agari, S. Makino, and M . Fukutani for their
skill and insight, and also to our present students (T. Kato, Y.
Hatayama, and N . Chatani). We thank Proj K. Kawamoto
(Kagawa University) for valuable discussions.
O _ O A c _
6 2%
2 8‘10
Received: April 3, 1979 [A 295 IE]
German version: Angew. Chem. 91. 896 (1979)
Although not shown in eqs. (x)-(z), a silyl acetate
MeEtzSiOAc was obtained in all cases. From the primary alkyl acetate [eq. (x)], only a trace amount of the branched
isomers (5) was obtained, thus indicating little isomerization
of the intermediary n-hexyl(carbony1)cobalt compound under these reaction conditions. Interestingly, up to three molecules of carbon monoxide have been incorporated [eq. (x)].
The reaction of the secondary alkyl acetate [eq. (y)] seems to
be promising as a synthetic reaction, In the plausible catalytic cycle of reaction (z), H(C0)4Co is very likely to arise in the
product forming step. This process has already been formulated for step 7 in Schemes 2 and 3. In the present reaction
(z), HCO(CO)~
can be converted into a silyl(carbony1)cobalt
compound [eq. (z)].
4. Outlook
There are four types of catalytic cycles or overall catalytic
transformations for the reaction with hydrosilane and carbon
monoxide. One cycle begins with HCO(CO)~(“HCo”), another with a silyl(carbony1)cobalt compound (“SiCo”); both
terminate with regeneration of the same initial catalyst species in the product forming step [Scheme 2 for “HCo”; and
Schemes 4 and 5 for “SiCo”]. In the other two catalytic cycles or overall catalytic transformations, regeneration of the
initial catalyst species occurs after the product is formed
[Scheme 3 for “HCo”; and the transformation (u) outlined
by Schemes 5 and 6 for “SiCo”]. Availability of the two catalyst species, “HCo” and “SiCo”, and hence of the four types
of catalytic cycles or catalytic transformations provides a
unique opportunity for designing a reaction. Moreover, in
planning a new reaction, it will also be of great help to recall
the close analogy between the reaction patterns of “SiCo”
Angew. Chem. Int. Ed. Engl. 18, 837-846 (1979)
described in this paper and those of the well studied Me3SiX,
where X is a soft base such as CN, N3, SR, I e t ~ . [ ~ ’ , ~ ’ I .
From the synthetic point of view, the new catalyzed reactions have not as yet been fully explored. It should be noted,
however, that the products obtained are 0-silyl ethers of hydroxy compounds which are otherwise unstable or hardly accessible, i. e. enols, enediols, and a-and w-hydroxyaldehydes.
The synthetic utility of enol silyl ethers was first demonstrated by our research
and has now been well established[37.45.471
(11 J. F. Harrod, A. J. Chalk in I . Wender, P. Pino: Organic Synthesis via Metal
Carbonyls. Vol. 2. Wiley, New York 1977, p. 673ff.
f2] R. E. Harmon, S. K. Gupta, D. J. Brown, Chem. Rev. 73, 21 (1973).
[3] J. P. Collman, W. R. Poper, Adv. Organomet. Chem. 7, 53 (1968).
141 N. J. Archer, R. N . Haszeldine. P. Y. Parish, Chem. Commun. 1971, 524.
(51 E. Lukevics, Z. V. Belyakova, M. G. Pomerantseua, M. G. Voronkou, J . Organomet. Chem. Libr. 5, 1 (1977).
[6] A . J. Chalk, J. F. Harrod, J . Am. Chem. SOC.89, 1640 (1967).
[7] B. J. Aylett. J. M. Campbell, J. Chem. SOC.A 1969. 1910, 1916; Inorg. Nucl.
Chem. Lett. 3, 137 (1976); F. de Charenrenay, J. A . Osborn, G. Wilkinson, J .
Chem. SOC.A 1968, 187; A. P. Hargen, L. McAmis, M. A . Stewart, J . Organomet. Chem. 66, 127 (1974).
[S] a) G. Henrici-Olive, S. Olive, Angew. Chem. 88, 144 (1976); Angew. Chem.
Int. Ed. Engl. IS, 136 (1976): b) F. Calderarzo, ibid. 89, 305 (1977) and 16.
299 (1977), respectively.
[9] R. L. Pruett, Ann. N. Y . Acad. Sci. 295, 239 (1977).
[I01 T. Saegusa, Y [to, S. Kobayushi, K. Hirota, J . Am. Chem. SOC.89, 2240
11 11 S. Murai, N . Sonoda, unpublished results.
[12] a) 0.Roelen, DRP 849548 (1938); b) For recent reviews, F. Piuncenti, M.
Bianchi, P. Pino in [I], Chapters 1-3; c) In the present paper, we use the
term “Roelen reaction” for the catalyzed reactions of organic compounds
with H2 and CO. The term ‘‘0x0 reaction” is not recommended because
many ‘‘0x0 reactions” do not give 0x0 (i.e. carbonyl) compounds [12bj. The
popular term “hydroformylation”. introduced by H. Adkins and G. Krsek [J.
Am. Chem. SOC.71,3051 (1949)], may also be confusing since it has, unfortunately, often been used not only to show the type of transformation (the
addition of a hydrogen atom and a formyl group) but, improperly, to indicate the reactants (H, and CO). In the present paper, the term “hydroformylation” will be used only in the context of its original meaning, i. e. to show
the type of transformation. Besides hydroformylation by the Roelen reaction (H2 and CO), hydroformylation has also been observed in the Reppe
reaction ( H 2 0 and CO) (13) and in the reaction dealt with here (HSiR3 and
CO), see Section 2.5.
[I31 W. Reppe, H. Vetter, Justus Liebigs Ann. Chem. 582, 133 (1953); N. uon Kutepow, H. Kindler, Angew. Chem. 72, 802 (1960); H. Kang, C. H. Mauldin,
T. Cole, W. Skger, K. Cann, R. Pettit, J . Am. Chem. SOC.99, 8323 (1977); R.
M . Laine, ibid. 100, 6451 (1978).
1141 Y. Seki. A . Hidaka, S. Murai, N. Sonoda, Angew. Chem. 89,196 (1977); Angew. Chem. Int. Ed. Engl. 16, 174 (1977).
[IS] Y Seki, A. Hidaka, S. Makino, S. Murai, N . Sonoda, J . Organomet. Chem.
140, 361 (1977).
1161 Y. Seki. S. Murai, A . Hidaka, N. Sonoda, Angew. Chem. 89,919 (1977); Angew. Chem. Int. Ed. Engl. 16, 881 (1977).
[I71 Y. Col1euiIle. Holl. Pat.-Appl. 6513584 (1966); Chem. Abstr. 6.5, 8959
(1966); DBP 1248050 (1967); US-Pat. 3450737 (1969).
[IS] A . J. Chalk, J. F. Harrod, Adv. Organomet. Chem. 6, 119 (1968).
1191 E. R. Tucci, Ind. Eng. Chem. Prod. Res. Dev. 9, 516 (1970).
[ZO] The hydrosilylation of olefins using Ru,(CO),~is in progress.
[211 Y L. Baay, A . G. MacDiurmid, Inorg. Chem. 8, 986 (1969).
(221 a) A . R. Bassindale, A . G. Brook, J. Harris, J. Organomet. Chem. 90, C 6
(1975); b) A . G. Brook, Account Chem. Res. 7, 77 (1974).
[23] Details will be published later.
[24] A control experiment, in which n-heptanal did not give the enol silyl ethers
(6) but afforded n-C,HliOSiEt2Me under the standard reaction conditions,
led us to suggest that an aldehyde might not be an intermediate [14]. Later,
we found that this conversion into the alkoxysilane was complete before the
temperature of the reaction vessel reached at 140°C. When n-heptanal was
injected under pressure at 140 “C, the enol silyl ethers (6) were found among
the products [1 11.
1251 The silyl group migration to give the metal-carbene complex finds its topo-
logical analog in the known rearrangement of a-silyl ketones to enol silyl
ethers (22bj. Hydrogen migration to a carbene ligand similar to thar in eq.
(k) has been suggested in some reactions [26].
[26] a) K. J. Ivin, J. J. Rooney, C. D. Stewarf, M. L. H. Green, R. Mahfab, J.
Chem. SOC.Chem. Commun. 1978, 604; b) R. B. Culvert, J. R. Shapley, J.
Am. Chern. SOC.99, 5225 (1977).
[271 H. G. Ang, P. T. Lau, Organomet. Chem. Rev. A 8,235 (1972); C. S. Cundy,
R. M. Kingston, M . F. Lappert, Adv. Organomet. Chem. 11, 253 (1973).
[28] D.
L Morrison, A . P. Hagen, Inorg. Synth. 13, 65 (1972).
1291 L. H. Sommer, J. E. Lyons, H. Fujimofo, J . Am. Chem. SOC. 91, 7051
[30] A. J. Chalk, Chem. Commun. 1970, 847.
1311 B. J. Alvlett. J. M. Camobe~l.1. Chem. SOC. A 1969. 1910: J. M. Burlitsh. J.
Am. Chem. Soc. 91,4562 (1969); J. F. Bald, Jr., A . G. MacDiarmid, J. Organomet. Chem. 22, C 2 2 (1970); H. Schaifr, A . G. MacDiarmid, Inorg. Chem.
15, 848 (1976).
E. Colomer, R. J. P. Corriu, J. Chem. Soc. Chem. Commun. 1976, 176.
E. Colomer, R. J. P. Corriu, J. C. Young, J. Chem. SOC.Chem. Commun.
1977, 73.
H. Sakurai, K. Miyoshi, Y. Nakadaira, Tetrahedron Lett. 1977, 2671.
Y. Seki, S. Murai, 1. Yamamoto, N. Sonoda, Angew. Chem. 89, 81 8 (1977);
Angew. Chem. Int. Ed. Engl. 16, 789 (1977).
1361 W. M. h g l e , G. Prefi, A. G. MacDiarmid, J. Chem. SOC.Chem. Commun.
1973, 497; B. K Nicholson. B. H. Robinson, J. Simpson, J. Organomet.
Chem. 66, C 3 (1974); B. K. Nicholson, J. Simpson, ibid. 155, 237 (1978).
(371 E. W. Coluin, J. Chem. Soc. Rev. 1977. 15.
[38] W. Reppe, H. Kroper, H. J. Pisfor, 0. Weissbarth, Justus Liebigs Ann.
Chem. 587, 87 (1953).
1391 S.Mural, 7: Kafo, N. Sonoda, Y. Seki, K Kawamoto, Angew. Chem. 91,421
(1979); Angew. Chem. Int. Ed. Engl. 18, 393 (1979).
[401 L. Marko, Proc. Chem. Soc. 1962,67; L. Marko, P. Srabo, Chem. Technol.
(Berlin) 13, 482 (1961); Chem. Abstr. 56, 7102 (1962); M. Polieuka, E. J.
Misfrik, Chem. Zvesti 26, 149 (1972); Chem. Abstr. 77, 113388 (1972). See
[12b], pp. 12 and 84, and also (48aj.
[411 J. A. Gladyz, J. C. Selouer, C. E. Sfrouse. J. Am. Chem. SOC. 100, 6766
( 1 978); see also [48 b].
142) T. Yukawa, H. Wakamatsu, Brit. Pat. 1408857 ( 1 974); S. K. Bhattacharyya,
S. K . Palit, A . D. Das, Ind. Eng. Chem. Prod. Res. Dev. 9, 92 (1970).
(431 Y. Seki, S. Murai, N. Sonoda, Angew. Chem. 90,139 (1978); Angew. Chem.
Int. Ed. Engl. 17, 119 (1978).
The possibility of “co-oligomerization” of HSiR,
and CO as an entry to persilylated sugar derivatives was suggested by Prof. A . Nakamura at the Summer Organometallic Chemistry Seminor, Yunoyama, Japan, June 1978.
(451 For reviews see: F Hudrlik, J. Organomet. Chem. Library 1, 127 (1976); S.
S. Washburne, J. Organomet. Chem. 83, 155 (1974); c) E. Cooper, Chem.
Ind. (London) 1978, 794.
I461 S. Mural, Y. Kuroki, T. Aya. N. Sonoda, S. Tsutsumi. J. Chem. SOC.Chem.
Commun. 1972, 741; S. Murai, Y. Kuroki, K . Hasegawa, S. Tsufsumi, ibid.
1972, 946. For the latest paper of this series: I. Ryu, S.Murai, Y. Hatayama,
N. Sonoda, Tetrahedron Lett. 1978. 3455.
[471 J. K. Rasmussen, Synthesis 1977, 91.
[481 Note added in proof: a) The reaction of HCHO with HCo(CO), is reported
to give HOCHZCHO;J. A. Rofh, M. Orchm, J. Organement. Chem. 172, C
27 (1979). b) An a-siloxyalkylmanganese complex was obtained by reaction
of C,H,CHO with (CH,),SiMn(C0)5; D. L. Johnson, J. A. Gladyre, submitted for publication.
Neurophysins: Molecular and Cellular Aspects[**]
By Roger Acher“]
Neurophysins are linear cystine-rich proteins containing 93-95 amino acid residues which
like the neurohypophysial hormones oxytocin and vasopressin are formed in the hypothalamus
and travel from there to the hypophysial posterior lobe. A species usually contains two (or
three) neurophysins which differ only slightly in chain length and/or sequence. Many observations suggest that both oxytocin and one of the neurophysins as well as vasopressin and the
other neurophysin have a common precursor whose long chain is split into neurophysin and
hormone. It can be shown on rats having considerable diabetes insipidus that a single gene
controls the biosynthesis of the vasopressin and one of the neurophysins.
1. Neurosecretion and Neurohormones: First Investigations
The history of the neurophysins cannot be dissociated
from that of neuroendocrinology. In fact, the early efforts of
biochemists to purify the “posterior lobe hormones” of the
Prof. Dr. R. Achei
Laboratoire de Chimie Biologique
Universite de Paris VI
96 Boulevard RasDail. F-75006 Paris (France)
[**I This review IS dedicated lo the memory of Ernst Scharrer, who some fifty
years ago discovered neurosecretion
0 Verlag Chemie, CmbH, 6940 Weinheim, 1979
pituitary, the activities of which have been known since the
beginning of this century, were paralleled by cytologists’
studies aimed at identifying the cells synthesizing these substances and at elucidating the secretory mechanism involved.
It was Ernst Scharrer[’]who in 1928, on the basis of his observations in certain hypothalamic neurons of a teleost fish,
Phoxznus Iaeuis (Fig. l), for the first time clearly proposed the
concept of neurosecretion. At the time, this idea was indeed
revolutionary, nerve cells being considered as highly specialized for the conduction of nervous impulses and totally different from regular gland cells. For a long time thereafter,
the posterior pituitary was interpreted as an autonomic endo-
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