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Ionic Hydrogenations and Dehydrogenations.

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ANGEWANDTE CHEMIE
VOLUME 1 - N U M B E R 1 2
DECEMBER 1 9 6 2
PAGES 6 1 3 - 6 7 2
Ionic Hydrogenations and Dehydrogenations
BY PROF. DR. H. MEERWEIN, DR. KL. WUNDERLICH, AND DR. K. FR.ZENNER
CHEMTSCHES INSTITUT DER UNIVERSITAT MARBURG/LAHN (GERMANY)
Dedicated in friendship to Prof. Dr. Otto Bayer on the occasion of his 60th birthday
In acetonitrile, 4-chlorobenzenediazonium fluoroborates readily add onto trimethylamine
to give 4-trimethylammoniumbenzenediazonium chloride fluoroborates. These addition
products have apronounced tendency to accept two hydrogen atoms in the form of a hydride
ion and a proton from donors such as I,4-dihydrobenzene, 1,3-dioxolxans, I ,4-dioxan etc. The
diazonium group is thereby reduced, and the trimethylammonium group is split of as trimethylammonium fluoroborate and replaced by the chloride ion. In the presence of a trace of
copper, the reaction mechanism is changed from an ionic to a free-radical one. In this case,
just as in the absence of copper, the diazonium group is reduced, but the trimethylammonium
group is retained. Alcohols behave in the same way as the donors of ionic hydrogen cited.
1. Addition of 4-Chlorobenzenediazonium
Fluoroborates onto Trimethylamine
In acetonitrile, 4-chlorobenzenediazonium fluorborate
readily adds onto trimethylamine to give Ctrimethylammoniumbenzenediazonium chloride fluoroborate ( I ) .
CI
I
I
the number of chlorine atoms increases. The yields
obtained range from 88 to 96 %. The rate and course of
addition can be readily followed by determining the
amount of chloride ion formed.
An excess of trimethylamine should be avoided to
prevent reduction of the diazonium group [l]. For
example, if a solution in acetonitrile of 2,4,6-trichlorobenzenediazonium fluoroborate is added at -35 "C to a
solution in acetonitrile of an excess of trimethylamine,
trimethyl-3,5-dichlorophenylammoniumchloride (4) is
obtained in 50 % yield:
NG N]@
(1)
Similarly, 2,4-dichloro- and 2,4,6-trichlorobenzenediazonium fluoroborates yield the addition products (2)
and (3).
(4)
The salts form slightly yellowish crystals, which can be
stored in a refrigerator for a long time without undergoing alteration. Their stability decreases somewhat as
Angew. Chem. internat. Edit. / Vol. 1 (1962) / No. 12
Recrystallization of the chloride from an aqueous
fluoroboric acid solution gives particularly well-formed
crystals of fluoroborate, m.p. 220-221.5 "C.
[l] H. Meerwein et al., Angew. Chem. 70, 211 (1958).
613
2. Ionic Hydrogenation of the Addition Products
The trimethylammoniumbenzenediazonium chloride
fluoroborates (I), ( 2 j , and (3j exhibit a pronounced
tendency to accept two hydrogen atoms in the form of
a hydride ion and a proton from donors. In this “ionic
hydrogenation” [2], the diazonium group is reduced,
the trimethylammonium group ‘being split off as trimethylammoniumfluoroborate and replaced by chloride
ion:
He
CI
He
Thus, the chloride ion formed when trimethylamine is
added to 4-chlorobenzenediazonium fluoroborates disappears in the course of ionic hydrogenation and reoccupies its original position on the benzene ring.
Most of the experiments described below were carried
out with the diazonium salt (3), obtained from 2,4,6trichlorobenzenediazonium fluoroborate and trimethylamine, since the 1,3,5-trichlorobenzene,which is formed
as a result of the ionic hydrogenation, can be isolated
very easily and quantitatively.
The following compounds were used as donors of ionic
hydrogen : 1,4-dihydrobenzene, 1,4-dihydronaphthaIene,
various 1,3-dioxolans and 1,Cdioxan. Their ability to
donate hydride ions is known [3]. In our case, however,
the loss of a hydride ion is followed or accompanied
simultaneously by a loss of a proton, resulting in
stabilization of the cation initially formed,
The course of reaction is readily understood when 1,4dihydrobenzene or 1,4-dihydronaphthalene is used. In
Table 1, a list is given of the yields of 1,3,5-trichlorobenzene, trimethylammonium fluoroborate and benzene
or naphthalene obtained with addition product (3).
Benzene was identified and determined quantitatively as
2,4-dinitrobenzene,and naphthalene as its picrate.
Table 1. Ionic hydrogenation of addition compound (3) with
dihydroaromaticcompounds
Product
1,3,5-Trichlorobenzene
Trimethylammonium
fluoroborate
Benzene or naphthalene
Yields [ %]
with 1.4dihydrobenzene
with 1,4dihydronaphthalene
49.5
69.8
65.6
13.3
52.2
Table 2. Ionic hydrogenations of addition product (3)
with 1,3-dioxolans
Expt.
No.
1.3-Dioxolan
74.6
Yield of 1,3,5trichlorobenzene [%I
Yield of trimethylammonium
fluoroborate
82
73.3
70.4
62.5
84.5
71.4
79.7
66.7
[%I
1
2
3
4
I
NENl@
chloride ions disappear almost completely, while,
simultaneously, nitrogen is evolved vigorously. The
yields of isolated 1,3,5-trichlorobenzeneand trimethylammonium fluoroborate are given in Table 2.
1.3-Dioxolan
2-Methyl-1,3-dioxolan
Z,Z-Dimethyl-l.3-dioxolan
2-Phenyl-l.3-dioxolan
So far we have been unable to isolate the 1,3-dioxoles
one would expect to obtain according to the following
equation;
(CHd,NI@
I
N=NI@
CI
This is due to the great tendency of these compounds to
polymerize [4]. In order to trap the 1,3-dioxoIe in its
nascent state, we carried out an ionic hydrogenation of
addition product (3) with 2,2-dimethyl-l,3-dioxolan
in
glacial acetic acid. The yield of 1,3,5-trichlorobenzene
was 51.9 % and that of trimethylammoniumfluoroborate
89.3% [5], 4-Acetoxy-2,2-dimethyl-l,
3-dioxolan, formed
from 2,2-dimethyl-l,3-dioxolebut not isolated, was
hydrolyzed with cold dilute hydrochloric acid. The
resulting glycol aldehyde was identified by converting
it into its p-nitrophenylosazone of glyoxal (m.p. 307
to 312 O C ; yield: 43.8 %). This proves that the ionic
hydrogenation and dehydrogenation of addition products (I), (2), and (3) with 1,3-dioxolans occur as
indicated above.
Some time ago, we were able to show [3] that, in acetonitrile, diazonium fluoroborates are reduced by 1,3dioxolans, e.g. 2-phenyl-l,3-dioxolan, to give ArH compounds and 1,3-dioxolenium fluoroborates:
[
Ar-NEN
]
BF4
+
\ /H
+
I~III’c\GH~
ArH+
[
“’7-0;
H2C-0
[2] Cf. H. Meerwein et al., Liebigs Ann. Chem. 635, 3 (1960),
footnote 4.
[3] H . Meerwein et al., Liebigs Ann. Chem. 635, 1 (1960).
6 14
C-CsHs
1
BF4
+ Nz
[4] N . D . Field, J. Amer. chem. SOC.83, 3504 (1961).
[5] The disproportionately high yield of trimethylammonium
fluoroborate compared to that of 1,3,5-trichlorobenzene is due
to the fact that a part of addition product (3) reacts with glacial
acetic acid with elimination of the trimethylammonium group,
which is activated by the diazonium group. Quaternary ammonium salts of 2,Z-dinitroaniline are known to undergo a
similar reaction [Brit. Pat. 805761 (1955); National Research
Development Corp.; Inventor: J. H . Wilkinson; Chem. Abstr.
53, 8074 (195911. In agreement with this, 40% of the ionogenic
chlorine remain.
Angew. Chem. internat. Edit. 1 Vol. I (1962) / No. 12
Hence, we initially assumed that the first step in the
reaction of 4 - trimethylammoniumbenenediazonium
chloride fluoroborateswith 1,3-dioxolansoccurs similarly, and that the resulting 1,3-dioxolenium fluoroborates
are then stabilized by loss of a proton and formation of
1,3-dioxoles:
able to isolate p-nitrophenylosazone of glyoxal when
the reduction was carried out without acetic acid. However, the yield was low (1.8 %) and the reduction took
place only with addition product (I), which is not very
reactive and which contains no chlorine on the ring.
3. Free-radical Reduction of the Addition Products
It was possible to prove that, in the reduction of
addition products (I), (21, and (3) with 1,3-dioxoIans,
hydrogen is actually transferred in the form of a hydride
ion and a proton and not as hydrogen atoms.
As we previously observed during the reduction of
diazonium fluoroborates which do not contain a triThat this course is not followed is evident from the fact
methylammonium
group with 1,3-dioxolanes, the
that ionic hydrogenation occurs readily also with 2,2relatively
slow
reaction
is remarkably accelerated by
dimethyl-l,3-dioxolan, which cannot give rise to a 1,3of copper bronze. The
addition
of
a
small
amount
dioxolenium salt (see experiment 3, Table 2). We were
i.e.
the
ArH
compound and the 1,3reduction
products,
also able to show that, in the reaction of trimethyl-2,5dioxolenium
fluoroborate,
remain
unaltered [6].
dichlorophenylammonium chloride (4) with 2-methylCopper exerts the same effect on the reduction of
1,3-dioxolenium fluoroborate, the triethylammonium
addition products (I), (2), and (3) with 1,3-dioxolans.
group is not split off, but that a simple exchange of
However, in this case, the result is different. The dianions takes place :
azonium group is reduced normally, just as in the
(CH33NlC1
absence of copper, but the trimethylammonium group
is not split off. When 2-methyl-1,fdioxolan is allowed
/C-CHs
BF4 4
c1to react with 4-trimethylammonium-2,6-dichlorobenHz -0
zenediazonium chloride fluoroborate (3) in the presence
(4)
of
a trace of copper, practically no 1,3,5-trichloro(CH33NlBF4
I
benzene or trimethylammonium fluoroborate is formed.
Instead, a 70.1 % yield of trimethyl-3,5-dichlorophenyl+
c1ammonium fluoroborate ( 4 4 and a 75.5 % yield of pchloroethyl acetate are obtained. The latter is formed
(44
by rearrangement of the 2-methyl-1,3-dioxolenium
0
chloride initially formed. The reaction mechanism
,C!-CH3
I
changes from an ionic to a free-radical one owing to the
HzC-0
uptake of an electron released by the copper according
Ionic hydrogenation of 4-trimethylammonium-2,6-di- to the equation:
chlorobenzenediazonium chloride fluoroborate (3) with
1,4-dioxan occurs as readily as that with 1,3-dioxolans.
The yield of 1,3,5-trichlorobenzene is 68 %, that of
trimethylammonium fluoroborate 63 %. Here, too, it
I
has not yet been possible to isolate the dehydro-1,4N=N]@
dioxan, which should be formed according to the
(CHhNIBF4
equation :
b-clI"%"
+
]
.1
c'H2y
.
+
c1-
clHzi!
0
,8--CH3
H2C-0
+
Hzi-o>&CH3
H2C-0
[
+ Cle + Nz
-1
H2C-0
1
>C-CH3]
CI+e
H2C-0
Ct
In this case also, as with 2,2-dimethyl-l,3-dioxolan,
we
carried out a reduction in glacial acetic acid, and
isolated the glyoxal-p-nitrophenylosazone,m. p. 307 to
309 'C, described above in 23.6 % yield. We were also
Angew. Chem. internat. Edit. ,/ Vol. I (1962) 1 No. 12
The electron initially taken up by the diazonium ion is
again reIeased on conversion of the 2-methyl-1,3dioxolan radical into the 2-methyl-1,3-dioxoleniumion.
In this case, the transfer involves a hydrogen atom and
not a hydride ion; subsequent or simultaneous transfer
of a proton, which takes place in ionic hydrogenation,
does not occur.
[6] See 111, p. 214.
615
The result of this experiment shows that free-radical
reduction takes place much faster than the ionic
hydrogenation, so that the latter practically does not
take place.
In view of the different opinions which exist with regard
to the mechanism of the reduction of diazonium salts
with alcohols, it is of interest that it is possible, in this
case, to make the reaction occur either by an ionic or a
free-radical mechanism.
4. Reduction of the Additionproducts with Alcohols
5. Experimental Procedures
Alcohols, the classical reducing agents for diazonium
salts, are also donors of ionic hydrogen. The reaction
of 4 - trimethylammonium- 2,6 - dichlorobenzenediazonium chloride fluoroborate (3) with ethanol is similar
to that with other donors of ionic hydrogen. It is also
markedly accelerated by the addition of acetonitrile and
takes place exothermically at 30-40°C. We were able
to isolate 93 % of the 1,3,5-trichlorobenzene, 41 % of
the trimethylammonium fluoroborate, and 95 % of the
acetaldehyde (as its p-nitrophenylhydrazone) formed
according to the equation :
I
N=N]@
c1
I
The chloride ion disappears almost completely.
It has already been demonstrated that the hydride ion
undergoing transfer in the reduction of the diazonium
salts with alcohols is an a-hydrogen atom by both Melander [7] and Miklukhin and Rekasheva [8] who used
tritiated and deuterated ethanol, respectively. The
proton undergoing transfer in ionic hydrogenation is
the hydrogen atom of the hydroxyl group. This is
indicated by the fact that the reaction with methanol is
as smooth as that with ethanol.
Here too, the addition of a trace of copper changes the
ionic reaction mechanism into a free-radical one. The
reaction is so vigorous that cooling is necessary. 1,3,5Trichlorobenzeneand trimethylammonium fluoroborate
are formed only in small amounts. Instead, we isolated
trimethyl - 3,s - dichlorophenylammonium fluoroborate
(4a) and acetaldehyde in 92.3 % and 95 %
, yields,
respectively. One mole of hydrogen chloride was
liberated. Hence, the reaction takes place as follows:
+
.. +
CH3-CH-OH
CIS
+ Nz
I
CH3-CH0
+ HCl
4
t [CH3-CH=OHlCI
+e
[7] L. Melander, Chem. Abstr. 46, 7534 (1952).
[ 8 ] G. P. Miklukhin and A . F. Rekasheva, Chem. Abstr. 47,4855
(1953).
616
Addition of 2,4,6-trichlorobenzenediazoniumfluoroborate onto trimethylamine to give 4-trimethylammonium-2,5-dichlorobenzenediazoniumchloride ffuoroborate (3)
In a 500-ml flask, fitted with a dropping funnel, stirrer, and
a suction tube containing a sealed-in sintered glass filter,
74.5 g (0.252 mole) of 2,4,6-trichlorobenzenediazonium
fluoroborate are suspended in 200ml of acetonitrile and 200ml
of a 1.26 N trimethylamine solution in acetonitrile added
dropwise at -18°C over a period of 4 hours with good
agitation. The trimethylamine odor disappears instantly.
The solution turns yellow as soon as a few ml have been
added, and stays yellow until the end of the addition. Too
rapid addition is indicated by the appearance of a brown
coloration Following completion of the addition, the mixture is stirred for 0.5 hr at -18OC. The yellow product is
sucked off with exclusion of moisture, washed with agitation
in succession with 200 mI of ice-cold acetonitrile, 200 ml of
ice-cold carbon tetrachloride, and two or three times with
200-ml portions of ice-cold methylene chloride, and then
sucked dry. Maximum yield: 86.5 g or 96.5 %; slightly
yellowish crystals, which decompose at 94-96 "C. The salt is
insoluble in all hydroxyl-free organic solvents. It dissolves
readily in water and dilute acids, but cannot be recrystallized.
The reactions of 4-chloro- and 2,4-dichlorobenzenediazonium
fluoroborates with trimethylamine take place similarly to
give addition product (I) (yield: 90 %; decomp. 104-108 "C)
and (Z), respectively (yield: 88 %; decomp. 87-89 "C).
Reaction of addition product (3) with 1,3-dioxolan:
In a 300-ml Erlenmeyer flask protected with a calcium
chloride tube, 19.2 g of addition product (3) are treated with
100 ml of ice-cold l,3-dioxolan and then stirred at 0 "C with
a magnetic stirrer. Nitrogen is evolved sfowly; it takes two
days for all the diazonium salt to react. The precipitate of
trimethylammonium fluoroborate is filtered by suction and
the filter cake washed with ether. Yield of nearly pure salt:
6 g. The filtrate give an additional 0.7 g of impurer salt; thus,
the total yield is 6.7 g, or 84.5 %.
After removal of the ether from the filtrate, steam-distillation
of the residue gave 8.0 g (82 %) of 1,3,5-trichlorobenzene,
m.p. 62-63 "C.
Reduction of addition product (3) with 2-methyl-1,3dioxolan in the presence of copper:
Ten grams of addition product (3) (28.2 miltimoles) are added
portionwise, at -40°C to 35 ml of 2-methyl-l,3-dioxolan,
freshly distilled from sodium and containing some copper
bronze. When the initially vigorous evolution of nitrogen has
subsided, the cooling bath is removed, and more copper
added. The addition product does not dissolve but forms
a beautifully crystalline reaction product. After 30 minutes,
all of the diazonium salt has reacted. The mixture is treated
with 100 ml of ether, filtered off by suction, and the filter
cake washed with ether. Thus, 6.9 g (83.5 %) of a mixture
of 84 % trimethyl 3 3 - dichlorophenylammonium fluoroborate (4a; m.p. 220-221 "C) and 16 % trimethyl 3,4,5 trichlorophenylammonium fluoroborate (m.p. 294-296 "C)
are formed as a result of a Sandmeyer reaction. Separation
of the mixture by crystallization from water is difficult.
Distillation of ether from the filtrate yields 2.60 g (75.5 %)
of P-chloroethylacetate of 95 % purity, as shown by gas
chromatography.
-
-
Angew. Chem. infernat. Edit. / VoI. 1 (1962) / No. 12
Proof of the formation of 2,2-dimethyl-l,3-dioxolein
the ionic hydrogenation of addition product (3) with
2,2-dimethyl-1,fdioxolan :
A mixture of 12.5 g of 2,2-dimethyl-l,3-dioxolan
and 25 ml
of glacial acetic acid is added to 10 grams of addition product
(3). The mixture is then stirred and heated to 60°C. The
diazonium salt dissolves, and after 10 minutes, the reaction is
completed. After working up as described above, 89.3 % of
trimethylammonium fluoroborate and 51.9 % of 1,3,5-trichlorobenzene are obtained.
To demonstrate the presence of the 4-acetoxy-2,2-dimethyl1,3-dioxolan formed, another reaction mixture on the same
scale was treated with 2 N hydrochloric acid and 50 ml of
water; the mixture was agitated for 3 hours, and the acetone
formed removed by vacuum distillation. After filtering off the
precipitate of 1,3,5-trichlorobenzene, the filtrate was treated
with a solution of 10 g ofp-nitrophenylhydrazine in 150 ml
of 2 N hydrochloric acid. The resulting mixture was allowed
of crude
to stand for 2 days. This yielded 4.06 g (43.8
glyoxal-p-nitrophenylosazone,which, after recrystallization
from pyridine, melted at 307-312°C.
We are indebted to the Fonds der Chemie, Farbenfabriken
Bayer, and Chemische Fabrik Schering A.G. for generous
support for our work.
Received, July 31 st, 1962
[A 232152 IE]
Isocyanates of Esters of Some Acids of Phosphorus and Silicon
BY DR. H. HOLTSCHMIDT AND DR. G. OERTEL
WISSENSCHAFTLICHES HAUPTLABORATORIUM DER FARBENFABRIKEN BAYER AG.
LEVERKUSEN (GERMANY)
Dedicated to Prof. Dr. Otto Bayer on the occasion of his 60th birthday
Several aminoaryl esters of phosphoric and thiophosphoric acids were prepared by the
reaction of phosphorus pentahalides with nitrophenols, followed by catalytic hydrogenation,
or by treating aminophenols with phosphorus trihalides and oxidation to pentavalent phosphorus. These amino esters were then converted into isocyanato esters by the action of
phosgene. Isocyanates of phosphonates have been synthetized on the same principle, as well
as via the Arbusov reaction of halogen-substituted isocyanates with trialkyl phosphites. The
reaction of silicon halides or alkylhalogenosilanes with aminophenols yielded aminoat-yI
esters of silicic acid or its derivatives, which could also be treated with phosgene to convert
them into isocynnato esters.
1. Introduction
The preparation of isocyanates bearing an ester group
in the molecule was described in 1906 by Morel El], who
used phosgenation of the hydrochloride of ethyl aminoethanoate to form ethyl isocyanatoacetate ( 1 ) as an
example. Siefken and Petersen 121 later prepared isocyanates from esters of other aliphatic and aromatic
amino acids, such as ethyl E-isocyanatohexanoate and
v
H~C~+C-CHZ-NCO
(I)
p-ethoxycarbonylphenyl isocyanate, while Miiller et
al. [3] synthetized higher functional isocyanate esters,
e.g. the bis-m-isocyanatobenzoateof diethylene glycol
(2). Isocyanato esters with polymenzable groups, e.g.
(3-isocyanatoethyl acrylate and methacrylate (3), have
also become known as the result of recent investigations [4].
We have been occupied for some time with the preparation of isocyanates of esters of inorganic acids; this work
will be discussed in the following.
A. Isocyanates of Esters of Phosphorus Acids
I. Isocyanates of Phosphates and Thiophosphates
OCN'
NCO
[I] A. Morel, C. r. hebd. Seances Acad. Sci. 143, 119 (1906).
[2] A. Siefken and S . Petersen, Liebigs Ann. Chem. 562, 75
(1949).
Angew. Chem. internat. Edir. / Vol. I (1962) / N o . 12
Nitration of triphenyl phosphate with concentrated nitric
acid yields tris-(p-nitrophenyl) phosphate (4), which
can easily be hydrogenated with Raney nickel to give
[3] German Pats. 1078 115 (Dec. 13th, 1957), 1081667 (April 4th,
1959), and 1085869 (Dec. 16th, 1959); inventors: E. Miiller,
0. Bayer, H. Wilms, F. v. Spulak, and M. Theis.
[4] German Pat. 1018050 (April 26th, 1954); inventor: H. HaltSchmidt.
617
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