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Organosilicon Chemistry.

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C O N F E R E N C E REPORTS
Organosilicon Chemistry
The Second International Symposium on Organosilicon
Chemistry at Bordeaux (July 9-12, 1968) - like the previous
Symposium at Prague in 1965 - aroused world-wide interest;
it was organized by J. Vulude. 13 plenary lectures and almost
100 discussion lectures, presented in three concurrent subconferences, confronted the participants, numbering about
400, with such a broad spectrum of the present state of organosilicon chemistry that, apart from a few notable exceptions, the reasoning and scientific depth of individual papers
became somewhat lost in the sheer volume of subject material.
isomerization of the cis-@ compound. On the basis of the
analogy between CH and CSi bonds, C. Eaborn (Brighton)
presented a review of the cleavage of S i c bonds and compared this bond with GeC, SnC, and PbC bonds. Triorganosilyl-substituted aryl carboxylic acid derivatives, whose reactivity is determined by the interplay of inductive and resonance effects, represent suitable model substances. EthynylSi compounds behave similarly to their aryl analogs; the high
reactivity of the =CSi bond has permitted the development
of a new synthesis of polyacetylene compounds.
Organosilicon technology (direct synthesis as well as manufacture and characterization of polymers) seems to make few
bounds forward; developments appear to be overshadowed
by the more academically oriented investigation of the
“monomeric” compounds. Before very long there will probably be only few chemists working with organosilicon compounds that will remember their benefactors who provided,
and still generously provide, the “minerals” that form the
very basis of this field.
E. Frainnet (Bordeaux) gave an account of the addition of triorganosilanes to carbonyl compounds such as ketones,
aldehydes, esters, carboxylic anhydrides, and carboxylic acid
halides, and of the reaction of alkoxysilanes with aromatic
compounds, esters, and lactones for an extremely wide range
of reactants. Anionic rearrangements of organosilicon compounds were discussed by R. West (Madison, Wisconsin).
They are responsible for the isomerization of silyl hydrazines
in the presence of catalytic amounts of alkyllithium, proceed
via an intramolecular mechanism, and occur frequently in
organosilicon chemistry because silicon can expand its
valence shell and thus stabilize the intermediates required by
the rearrangement. Thermodynamic preference of the 1,ldisilylhydrazines is probably due to the increased stability
of the SiNSi group. On the other hand, disilylethylenediamines undergo 1,4 rearrangement; electronic effects such as
phenyl-substitution can reverse the direction of rearrangement. A report on the preparation, structure, and reactions
of silyl-substituted alkali-metal amides was presented by
U. Wunnagat (Braunschweig). It appears from recent
publications that the great versatility and reactivity of the
alkali-metal bis(triorganosily1)amide is exceeded by LiN(SiCl3)z. Vibration-spectroscopic and other physicochemical
measurements on LiN[Si(CH3)3]2 indicate the presence of
dimeric molecules in the fused compound and in solution,
whereas a n X-ray structure analysis has established that the
crystalline compound exists as the trimer in the form of a sixmembered (LiN)3 ring having 4 SiNSi = 118 ’ and d SiN =
1.72 A[ll.
Plenary Lectures
Except for the contributions by V. BaZunt (Prague): “Studies
o n the mechanism of direct synthesis”, K. A . Andrianov
(Moscow): “Polymerization of ring-shaped organosilicon
compounds”, and H . Kriegsmunn (Berlin): “Intramolecular
and intermolecular interactions in some organosilicon compounds”, which were concerned with vibration spectroscopic,
mass’spectrometric. and IH-NMR spectroscopic studies o n
polysiloxane chains, and particularly with the range of electronic effects in a siloxane chain, all the plenary lectures were
about the chemistry of monomeric organosilicon compounds.
Attention was focused primarily on papers dealing with
formation and cleavage of S i c linkages. H . Gitman (Ames,
Iowa) reported on the silylation of halogenohydrocarbons
after metalation with Li or Mg. Thus propyne. hexahalogenobenzenes, and hexachloropropene react with Li and
The impressive advances made in the chemistry of silicon(CH3)3SiCl in situ to give [(CH3)3Si]zC=C=C[Si(CH3)3]2;
metal compounds in recent years became evident in two
under these conditions perchloroalkanes and ethylene yield
lectures: G. A . Razuvuev (Gorki) dealt mainly with the silyl
(CH3)3SiC=CSi(CH3)3; and trichlorobenzene and pentaand germyl derivatives of main-group elements such as Hgrzl,
chloropyridine afford tris(trimethylsi1yl)trichlorobenzene and
and A . G. MacDiarmid (Philadelphia, Pennsylvania) con4-trimethy1silyltetrachloropyridine,respectively. Novel reaccentrated o n carbonyl complexes containing SiMn, SiFe, and
tion routes have also become available via copper compounds
SiCo bonds. The thermal stability of such bonds is strongly
such as R3SiCu and R3GeCu, which can be obtained from
dependent on the nature of the substituents and decreases in
the corresponding lithium compounds and C6H5Cu or
the series SiFe-SiMn-SiCo; these bonds show a remarkable
c6Cl&u. D . R . Weyenberg (Midland, Michigan) has found
stability
toward, e.g., PF3, PF5, and SO;?.The energy of the
the pyrolysis of alkoxypolymethylsilanes at 225 OC to be a
SiCo bond (105-124 kcal/mole, as determined by mass
suitable reaction for the production of dimethylsilylene
spectrometry) is exceptionally high and suggests SiCo-d,dx
(CH&Si:, which can be detected by mass spectrometry and
bonds. In Cl3SiCo(CO)4, the Si atom occupies a n axial
intercepted, e.g., by diphenylacetylene to form the 1,Cdisilaposition of a trigonal bipyramid; the CO groups cis to the Si
2.5-cyclohexadiene system. Pyrolytic formation of the silylene
are displaced by 4.8 toward the latter. (CH3)3SiMn(CO)s
occurs only when the (CH3)2Si group is bonded to a n atom
has an octahedral structure; d MnSi is 2.50 i 0.02 A, a value
bearing a lone pair of electrons; the same applies to the
lower than that expected for a single bond.
silylenes (CH3)CH30Si: and (CH30)zSi:. In the absence of a
M . G . Voronkov (Riga) reported on the biological activity of
silylene interceptor such as an alcohol or a diene the resulting
compounds containing silicon. Thus compounds containing
RZSi: can also be used for the lengthening of the alkoxypolyS i c bonds occur in certain bacteria, and silicon compounds
silane chain. Investigations on the tributylamine-catalyzed
are undoubtedly partly responsible for silicosis. The silatranes,
addition of trichlorosilane to phenylacetylene were the subpolycyclic alkoxysilanes of the general formula RSi(OCH2ject of a paper delivered by R . A . Benkeser (Lafayette, InCH2)3N, are unexpectedly poisonous; with a lethal dosage of
diana). An essential step in this reaction is the formation of
0.2 mg/kg they are considerably more toxic than strychnine.
the ion pair R3NH+SiCl;. The primary products, a-and cis-@Their toxicity is strongly dependent on the nature of the subtrichlorosilylstyrene, react further with HSiCI3 to give .$stituent R: the compound with R = p-tolyl is more than l o 4
bis(trichlorosily1)ethylbenzene. trans-@-Trichlorosilylstyrene
times more poisonous than that with R = methyl.
is also found as a product but its formation is attributed to
526
Angew. Chem. internat. Edit.
Vol. 8 (1969)/ No. 7
Discussion Lectures
Preparation and properties of monomeric organosilicon
compounds were also dealt with in most of the discussion
lectures. In a number of cases the part of the molecule investigated was so far removed from an organosilicon group
that the effects observed could hardly be attributed with
certainty to the silicon. This report will now consider an
arbitrary selection of the topics that were discussed.
Whereas the papers presented by M . Dvogak (Prague), N . P .
Lobusevich (Moscow), and H . Ya. Zueva were devoted to the
catalyst systems used in direct syntheses and the effects of
foreign atoms, that by A . I. Gorbunav (Moscow) dealt with
kinetic aspects of direct synthesis. M. Barfholin (Villeurbanne) has succeeded in incorporating Al and Cr into phenylpolysiloxanes in aqueous media at p H values between 2.5
and 5, and J . A . C. Wntt (Stevenston) has found the perphenylpoly-p-silphenylene,/asbestos system to be a stable
combination up to 400OC. I . M . White (Farnborough) has
introduced P(0)R and isophthaloyl groups into neopentylsubstituted polysiloxanes by co-condensation; however, the
properties of the products are no more favorable than those
of the usual silicones. The stereochemistry of the polymerization of a trimethyltriphenylcyclotrisiloxanein the presence of
various catalysts has been studied by E. E. Bostick and J. B.
Bush Jr. (Schenectady, New York).
SIC Compounds
Silylboranes. which possess considerable potential as polymer
building units, were presented by S. Papetti (New Haven,
Connecticut), silicon-containing four-, five-, and ten-membered rings having C-C, C = C , and C = C groups by G . Fritz
(Karlsruhe), and a sila-steroid by S. Barcza (Hanover, New
Jersey). According to Sheng-lieh Liu (Taipeh), four (CH&
C6H5Si groups may be bonded to a central carbon atom.
H . Schmidbaur (Wiirzburg) has observed stabilization of the
P = C bond by (CH&Si groups in (CH3)3P=CHSi(CH3)3;
the silyl shift involved in the conversion of (CH3)3SiCHzP(CH3)2=CHSi(CH3)3 into (CH&P=C[Si(CH3)& is remarkable. According to M. J . Newlands (Manchester), 1,3-disilacyclobutanes can undergo either electrophilic or nucleophilic
cleavage. On cleavage with bromine, the reactive dislacyclo( B ~be) $intercepted
HZ
butane C H ~ ( C ~ H ~ ) ? ~ C H ~ S ~ C H ~can
and the Br replaced by H on treatment with LiAIH4. The
rate of catalytic chlorination of arylsilanes and the CJ constants of various organosilyl groups, determined from halfwave potentials and the rate of bromination of phenylsilanes,
respectively, were shown by V. Chvalovskj (Prague) and
F. Mares (Prague), respectively, to evidence pronounced
(p +d)r interactions in the Si-phenyl bond.
within a few days. K. Riihlinoim (Berlin) discussed the merits
of bis(trimethylsi1ylimino) compounds for the synthesis of
new heterocycles. Olefins add to silyl azides to give N-silyltriazolines, which yield silylaziridines on thermolysis. Benzimidazolines bearing a metal substitutent (Li, Na, Fe) attached to the nitrogen atom constituted the subject of the paper
by D . Kummer (Karlsruhe). These thermally unstable compounds can be converted into the N,N-bis(trimethylsily1)
derivatives; in their chemical behavior they resemble alkalimetal disilylamides. I. Schlrintrnn-Ruidisch (Wiirzburg) reported on the synthesis of mono- and bis(trimethylsily1)tert-butylamine; one of the Si atoms can also be replaced by
Ge, Sn, o r Pb. The long-sought-after pale blue compound,
bis(trimethy1silyl)diimine (azotrimethylsilane) [31, which decomposes to tetrakis(trimethylsily1)hydrazine and N2 at
-35 “C has been prepared by N . Wiberg (Miinchen). The diimine ignites in the presence of 0 2 ; controlled oxidation affords bis(trimethylsi1yl) peroxide. R. P. Bush (Barry) has
studied the rearrangement of N-metalated six- and eightmembered siloxane rings; ring contraction to cyclodisilazanes and six-membered rings, respectively, always proceeds
t i n excision of a OSiR2 unit. As shown by V . L . Sheludjakov
(Moscow), rj-eliminations in SiNCX systems (+ S i x i
-NC-) determine the courses of many reactions in SiN
chemistry: decomposition of N-silylurethanes, N-silylcarbamoyl chlorides, N-silylimino carboxylic acid derivatives.
0. J. Scherer (Wurzburg) has shown scission of SIN bonds
by thionyl halides to be a universally applicable reaction for
the preparation of sulfinylamine derivatives such as (CH&SiN=S=O, NC-N-S-0,
and RS-N=S=O. Alkali-metal
silylamides [MNR’(SiR3)] react with transition-metal carbG.
onyls M’-C=O to give MOSiR3 and M’-C=NR‘.
Schirawski (Braunschweig) has elucidated the mechanism of
this reaction and exploited it for the preparation of new isocyanide complexes.
Other Silicon Compounds
SiO and SiN Compounds
The synthesis of (C6F5)3SiSi(CbF5)3 has been accomplished
by E. Hengge and G. Starz (Graz); the intermediate (CsF43SiH is characterized by an unusually high vSiH vibration at
2295 cm-1. E. A. Chernyshev (Moscow) has studied interception reactions of dichlorosilylene :Sic12 with naphthylsilanes
and obtained a large number of derivatives of 1,2-disiIacyclopentane and -hexane. Preparation and vibration spectra of
methylated isotetrasilanes and neopentasilane were treated
in the lecture by U. Goetze (Braunschweig). The compound
[(CH&Si]3SiH, with vSiH = 2051 cm-1 and J(JH29Si) =
155 :k 2 Hz, provides a particularly clear illustration of the
extreme electron-releasing effect of the (CH3)3Si group. Like
other phosphines, [(CH3)3Si]3P can also act as ligand in
transition-metal complexes; according to H. Schumann
(Wiirzburg), it forms air-stable compounds, admittedly of
low thermal stability, with the groups Ni(C0)3, Fe(C0)4,
Cr(C0)5, and Co(C0)2NO.
Silicon- Metal Compounds
Siloxycarbenes have been obtained by A . G . Brook (Toronto)
on irradiation of a-silyl ketones. They are nucleophilic in
character and may be intercepted with ketones and fumaric
esters. J. CaussP (Bordeaux) reported that triorganosilanes
add to ketenes to form enoxysilanes; both possible stereoisom a s are formed with ketenes of the type RR’C=C=O. Careful examination of the reaction of (R0)zPONa with triorganochlorosilanes led N . L . Orlov (Leningrad) to the conclusion that the reaction product is not (R0)2P(O)SiR3 but a
mixture of (RO)zPOSiR3 and R(RO)P(O)OSiR3, the latter
arising by isomerization of the former. The major product on
irradiation of acetone/triorganosilane mixtures has been
found by A . Ritter (Mulheim) to be isopropoxysilane; with
benzophenone, o n the other hand, O-triorganosilylbenzopinacol could be isolated as main product. Optically active
2-sila-5-oxazolidones have been obtained by J. F. Klebe
(Schenectady, New York) on reaction of optically active Nsubstituted amino acids with diaminosilanes at room temperature. The diastereomers differ in their proton magnetic
resonance spectrum; equilibrium is established in solution
In addition to the Si-alkali-metal compounds [E. Amberger
(Munchen)] and tris(trimethylsilyl)thallium, which was reported upon by A . G . Lee (Cambridge), Si-transition-metal
compounds received considerable attention. Homolysis o f
(R3Si)zHg produces R3Si radicals, which add readily to unsaturated systems [ W. P. Neumann (Giessen)]. Compounds
having Si-Ti linkages have also been obtained for the first
time: according to H . Zimmermann (Graz), reaction of Tic14
and (C&)3SiK affords [(C6Hs)3Si]4Ti[41; the synthesis of
silyl cyclopentadienyltitanium derivatives has also been accomplished by M. F. Lappert (Brighton). 57Fe-MCissbauer
and I R spectra show the compounds (SiH3),FeHz_,(C0)4
( n = 1,2) to have a cis-octahedral structure; other SiH3
compounds such as H3SiCo(C0)4 and HsSiMn(C0)s have
also been prepared and studied by E . J. Aylett (London).
F. Glockling (Durham) has utilized the reaction between PtH
and HSi groups, which is exothermic to the extent of 20
kcal/mole, for the synthesis of new SiPt compounds. H2.
HCI, CH31, and H 2 0 cleave the SiPt bond under mild conditions.
Angew. Chem. internat. Edit. / Val. 8 (1969) / No. 7
52 7
Physicochemical Measurements
In conclusion, a broad selection of physicochemical studies
were reported, ranging from structure determinations o n new
cage-type disilylmethylenes by nuclear magnetic resonance
and mass spectrometry [W. J. Owen (Barry)] to quantumchemical calculations o n benzyl, aryl-, and alkenylsilanes
[J. Nugy (Budapest)]. H. Bock (Miinchen) considered why
silyl ketones are yellow, as opposed to the colorless aryl
ketones. On the basis of ionization energies, half-wave reduction potentials, and charge-transfer and electron-excitation
energies, he has drawn up an experimental MO scheme which
shows that the reduced excitation energy of the long-wavelength n+x* transition is to be attributed to promotion of
the oxygen n-electron pair by the strong inductive effect of
the R3Si group and a simultaneous lowering of the x*
energy level by back-donation of electrons in the sense
(O=C), +Si. Dipole-moment measurements and pnd, contributions have been correlated by S. Ferenczi-Gresz (Budapest) for organo-oxysilanes and by V. Vuisarovd (Prague) for
phenylhalogenosilanes. Interactions between proton donors
of the type -CH. -NH. or -OH and SiO or SIN compounds
have been detected by infrared spectroscopy [M.Jukoubkovd
(Prague)], and the assignment of the infrared spectra of siloxanes and alkoxysiloxanes was reported on by A . Marchand
(Bordeaux). A simple universal set of force constants for
representation of vibration spectra of phenyl-Si compounds
was presented by F. Hoj¶er (Munster). The paper read by
H. Burger (Braunschweig) was concerned with normal coordinate analysis of all trimethylsilyl compounds of Group
IV-VII elements (El) that contain a full complement of
(CH&Si groups. The relevant calculations afford not only
the SiEl stretching force constants and the limits of the
SiElSi bond angle but also correlations between C3Si deformation vibrations and the mass of the fourth substituent. With
the aid of nuclear magnetic resonance studies, E. G. Rochow
(Cambridge, Massachusetts) has shown that methylpolysilazane chains are flexible and that their flexiblity is increased
by branching; free rotation of the CH3 group is retained in
such chains. J . Schraml (Prague) has analyzed the IH-NMR
spectra of a large number of silyl-2,2-dichlorocyclopropanes
and reached the conclusion that no x bond can be detected,
a t least by N M R spectroscopy, between the cyclopropane
ring and Si.
H. Burger [VB 202 IE]
German version: Angew. Chem. B I , 500 (1969)
111 D . Moorz, A. Zinnius, and B. Bottcher, Angew. Chem. 81,
398 (1969); Angew. Chem. internat. Edit. 8, 378 (1969).
121 N . S . Vyazankin, G. A. Razuvuev, and 0 . A . Kruglaya, Organometallic Chem. Rev. A 3 , 323 (1968).
[3] N . Wiberg, W.-Ch. Joo, and W. Uhlenbrock, Angew. Chem.
80, 661 (1968); Angew. Chem. internat. Edit. 7, 640 (1968).
[4] E. Hengge and H. Zimmermann, Angew. Chem. 80, 153
(1968); Angew. Chem. internat. Edit. 7, 142 (1968).
weight, but they are also responsible for the chemical and
molecular regularity of the polyoxymethylene.
Owing to their predominantly regular molecular structure
polyoxymethylenes crystallize extremely readily, two crystal
modifications being encountered: an orthorhombic modification and an hexagonal one. The former rearranges irreversibly
to the latter at temperatures above 80 “C. Many of the physical properties arise from this highly crystalline state, as for
example the insolubility of polyoxymethylenes in organic
solvents at room temperature, the high modulus of elasticity,
and the favorable combination of hardness and rigidity.
Lecture at Freiburg on April 25, 1969 [VR 203 IE]
German version: Angew. Chem. 81, 502 (1969)
[‘I Dr. H. Cherdron
Farbwerke Hoechst AG
623 Frankfurt (Main), Postfach 8003 20 (Germany)
Intramolecular Motions in Enamines and
Hydrazones. Separation of Rotational Isomers
By A . Munnschreck and U.Koller*I
Rotation about the =C-N bond of enamines ( I ) occurs more
slowly than the corresponding process in saturated amines,
whereas cis-trans isomerization at the C = C bond in ( I ) is
faster than in alkenes containing no hetero atom[l,zl. Some
H5C6
FSH5
h-w
( I ) , X = CH
(3), X = C H
(2). X = N
(4). X = N
indications of similar phenomena in hydrazones (2) have
been obtained [I]. However, the free enthalpies of activation
AG* for rotation about the =N-N bond in (2) are lower by
about 4 kcal/mole than the values for the corresponding
motion at =C-N in ( I ) . This was concluded, for example,
from the coalescence of the 1H-NMR methyl signals of (31,
AG: = 21.5 kcal/mole at 160°C. and of (4). AG? = 17.6
kcal/mole at 73 OC, in diphenyl ether. These findings are in
agreement with the fact, deduced from 1H/IH coupling constants in fulvenes (31,that dipolar resonance structures contribute more to the ground state of enamines than to that of
hydrazones.
The high barrier to rotation about the =C-N bond in (3) led
us to attempt the separation of isomers in suitable cases. For
instance, ( 5 b ) . m.p. 185-186OC, was obtained in the pure
Polyoxymethylenes
By H. Cherdronc*J
Polyoxymethylenes can be prepared in a number of ways:
1) Cationic or anionic polymerization of formaldehyde,
2) Polycondensation of formaldehyde in aqueous solution
(via the unstable methylene glycol),
3) Cationic polymerization of cyclic trimers and tetramers of
formaldehyde.
The homopolymerization and copolymerization of trioxane,
which exhibit a variety of particular features, have been
thoroughly studied. These ring-opening polymerizations are
characterized by various side reactions of the growing polyoxymethylene cations such as chain transfer, transacetalization, and hydride shifts. Not only d o the side reactions
determine the nature of the end group and the molecular
528
H“
Angew. Chem. internat. Edit.
Vol. 8 (1969) / N o . 7
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