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of a Si-0 bond when we allowed ~ B u , S ~ = N - S ~ ~ B Uto
react with benzophenone in diethyl ether. A potential precursor of 2 having constitution 3b, according to the X-ray
structure analysis (see below), was thereby isolated.
Adduct 3 b forms dark red, hydrolytically labile crystals
from pentane at -78°C;"41 it is readily soluble in diethyl
ether and moderately soluble in pentane and benzene, but
dissolves in tetrahydrofuran (THF) only with replacement
of Ph,C=O by THF. There are obvious steric reasons why
3b is stable toward transformation to the [2+2] cycloadduct at room temperature, whereas other addition compounds such as Me2Si=C(SiMe3),. P h 2 C = 0 and 3a"" undergo spontaneous transformation to the corresponding
[2 21 cycloadducts even at very low temperatures.
Gentle warming (60-80°C) of a dark red benzene solution of 3b results in dissociation of 3b to form a lighter
solution of the educts rBu2Si=N-SitBu, (pale yellow) and
Ph,C=O. Thus, 3b does not rearrange to the [2+2] cycloadduct 2b. Cooling of the solution results once again in
the formation of 3b. When benzophenone is used as solvent, the thermal dissociation is suppressed so that such
solutions remain initially dark red even at 90°C. Since, under these conditions, 3b is transformed slowly into uncharacterized products, however, solutions of 3b in benzophenone also turn lighter within a few hours. In this case,
cooling does not result in reappearance of the dark red
Fig. I . Molecular structure of 3b (ORTEP, displacement ellipsoids SO%,
without H atoms). The silyl group Si2 is slightly displaced backwards out of
the plane of the figure (Sil-N-Si2 169.3(2)'). Important distances [A1 and angles ['']: Sil-N 1.601(2), Si2-N 1.678(2), S i l - 0 1.927(2), SiI-Cl 1.930(3), SilC2 1.923(3), Si2-C3 1.950(3), Si2-C4 1.961(3), Si2-C5 1.961(3), 0 - C 6 1.254(3);
0-SiI-N 106.9(1), 0-Sil-CI 91.3(1), 0-Sil-C2 102.5(1), N-Sil-CI 120.4(1),
N-SiI-CZ 117.5(1), CI-Sil-C2 1l2.4(1), Sil-O-C6 153.7(2).
The X-ray structure analysis['61 reveals that 3b may be
regarded as a loose benzophenone adduct at the unsaturated Si atom of the silanimine tBu2Si=N-SitBu, (Fig. 1).
This is shown especially by the unusually long S i - 0 bond
(1.927(2)A, compared with a normal value of 1.521.75
the comparably short C - 0 distance (1.254(3) A,
compared with 1.231 in Ph,C=O"*'), and the incompletely attained tetrahedral geometry of the unsaturated Si
atom Sil (trigonal-planar geometry in free silanimine["]).
The addition is associated with a slight leagthening of the
N=SiI bond (from 1.568(3) A to 1.601(2) A); o n the other
hand, the N-Si2 bond becomes somewhat shorter and the
0 VCH Verlagsgesellschaft mbH. 0-6940 Weinheim. 1988
valence angle at the N atom decreases from 177.812)" to
169.3(2) .
Received: December 18, 1987;
revised: February 12, 1988 [ Z 2546 IE]
Publication delayed at authors' request
German version: Angew. Chem. 100 (1988) 979
[I] N. Wiberg, K. Schurz, Chem. Be?. 121 (1988) 581.
[2] Reviews: L. E. Gusel'nikov, N. S. Nametkin, V. M. Vdovin, Arc. Chem.
Res. 8 (1975) 18; L. E. Gusel'nikov, N. S. Nametkin, Chem. Reu. 79
(1979) 529; G . Raabe, J. Michl, ibid. 85 (1985) 419.
131 Silaethenes
0x0 compounds: D. N. Roark, L. H. Sommer, J. Organomet. Chem. 66 (1974) 29; R. D. Bush, C. M. Golino, G. D. Homer, L.
H. Sommer, ibid. 80 (1974) 37.
0x0 compounds: C. M. Golino, R. D. Bush, L. H. Som[4] Silanimines
mer, J . Am. Chem. Sac. 96 (1974) 614; D. R. Parker, L. H. Sommer, J .
Organornet. Chem. 110 (1976) C I .
[S] Review: T. J Barton, f i r e Appl. Chem. 52 (1980) 615.
[6] T. J. Barton, G. P. Hussmann, Organometallics 2 (1983) 692.
[7] A compound synthesized later than l a [8] and falsely reported as the
first oxasilacyclobutane 191 has in the meantime been placed in doubt by
other researchers 161. For other known oxasilacyclobutanes, cf. [lo].
[8] Preliminary communication: N. Wiberg, G . Preiner, 0. Schieda, Chem.
Ber. 114 (1981) 3518.
[9] W. Ando, A. Sekiguchi, T. Sato, J. Am. Chem. Soc. 104 (1982) 6830.
[lo] G. Markl, M. Horn, Tetrahedron Lett. 24 (1983) 1477; A. G. Brook, W. J.
Chatterton, J. F. Sawyer, D. W. Hughes, K. Vorspohl, Organometallics 6
(1987) 1246.
[ I l l N. Wiberg, K. Schurz. G. Reber. G. Miiller. J . Chem. SOC.Chem. Commun. 1986. 591.
2a: M.p.= 104°C; dec. > I S O T ; 'H-NMR (C6D6): b= 1.10 (s, SitBu,),
1.3611.41 (s/s, SitBu,), 6.58 (s, CH), 7.1W7.77 (m, Ph); "CC('H)-NMR
(C6D6): 6=23.9 (3 m e ) , 26.1/28.4 (2 m e , ) , 29.1/30.8 (2 CMe,), 31.8
(3 CMe,), 95.0 (CH), 127.7/129.1/129.8/145.3 (Ph); 29Si-NMR (C,D,):
S = 2.40 (SitBu,), 26.2 (SitBu,).
Remark on the mechanism of decomposition of 1 and 2 : Increasing
bulkiness of the substituents of 1 and 2 increases the stability of the
compounds. This is evidence against the intermediary formation of silanones in the lransformation of 1 and 2 to polysiloxanes: N. Wiberg, K.
Schurz, unpublished results.
3b: 'H-NMR (C6D6): 6=1.14 (s, SitBu,), 1.36 (s, SitBu,), 7.0W7.69
(m/m, 2 Ph); "C('H}-NMR (C6D6): 6=23.5 ( 5 m e 3 ) , 29.6 (2 CMe,),
31.5 (3 CMe,), 128.3/130.2/132.1/138.3 (2 Ph), 196.0 (CO); 29Si-NMR
(C6D6): 6 = -9.6 (SitBu3), 54.2 (SitBu,).
Evidence for the formation of an intermediate 3a during the reaction of
rBu,Si=N-SitBu, with PhHC=O is provided by the deep blue color,
which appears immediately after the addition of the first drop of
PhHC=O to a pentane solution of the silanimine at - 125°C and disappears shortly after addition of the last drop.
Crystal structure data: C33HssNOSi2,monoclinic, space group P2,fi,
a = 13.247(2), b = 15.689(2), C = 17.073(2) &fl= 110.53(1)", V=3323.0 A,,
1.075 g cm-',p(MoKa)= 1.3 cm-', Z = 4 . 5179 unique reflections
up to (sin8//2)=0.571, 3789*with Fo>Z.Oo(Fo) (Enraf-Nonius CAD4,
MoKnradiation, A=0.71069 A, T= -45'C). Solution by direct methods
(SHELXS-86). R ( R , ) = 0 . 0 5 0 (0.039), w = I/02(Fo) for 379 refined parameters (anisotropic, H constant, CH, as rigid groups, SHELX-76).
Ap,,,= f0.27 e k 3Further
details of the crystal structure investigation
may be obtained from the Fachinformationszentrum Energie, Physik,
Mathematik GmbH, D-75 14 Eggenstein-Leopoldshafen 2 (FRG), on
quoting the depository number CSD-52884, the names of the authors,
and the journal citation.
Organomet. Chem. 271
1171 Cf. N. Wiberg, G. Wagner, G. Muller, J. Riede, .
(1984) 381, and references cited therein.
[I81 E. B. Fleischer, N. Sung, S. Hawkinson, J. Phys. Chem. 72 (1968) 4311.
By n o m a s van der Does and Friedrich Bickelhaupt*
The cyano radical C N @could combine with other radicals R@by bond formation at C or N, resulting in either
nitriles RCN or isonitriles RNC.['.'I It dimerizes only
[*I Prof. Dr. F. Bickelhaupt, Drs. T. van der Does
Scheikundig Laboratorium, Vrije Universiteit
De Boelelaan 1083, NL-1081 HV Amsterdam (The Netherlands)
[**I We thank R . F. Schmirz, Amsterdam, for carrying out the difficult recording of the mass spectrum.
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Angew. Chem. Int. Zd. Engl. 27 (1988) No. 7
through carbon, however, to give cyanogen 1, which has
been known since 1815;131to the best of our knowledge, the
isomeric cyanoisocyanide NCNC 2 and diisocyanogen
CNNC 3 have been prepared neither by CN' dimerization nor via other routes. Theoretical calculations141may
provide an explanation for these observations; 2 and 3 are
about 17 and 65 kcal mol-', respectively, less stable than
1. Our research on phosphaalkenes,lSa1in particular 7-norbornadienylidenephosphane,'sblprompted us to investigate
the corresponding nitrogen analogues. This work led to the
first synthesis of 3, the least stable by far of the three isomers, namely by cheletropic elimination from the azine 6.
the I4N coupling usually observed for is~cyanides."~]
the I4N-NMR
3 (6= -239, external standard
MeNO, (6= 0)) also differs in a charact&istic fashion from
1 (6= - 118);1'51here, too, the signal of 3 vanishes above
-30°C. The identity of 3 was corroborated by mass spectroscopy (see Experimental Procedure). The molecular ion
C2N;@ is the base peak and the intensities of the fragment
ions differ significantly from those observed for 1
Owing to its high volatility, its ready decomposition in
solution, and its spectral data, the new compound can be
unambiguously identified as diisocyanogen 3. The reactivity of this new species is currently being investigated.
Noteworthy, however, is the high thermal stability of 3 in
the gas phase. Normally, isonitriles undergo rearrangement at temperatures above 200°C to the corresponding
nit rile^;^'^^ in contrast, 3 affords no 1, even at 500°C. This
surprising stability of the N-N bond of 3 can be explained
by its theoretically predicted double-bond character.I4].
Experimental Procedure
Cheletropic eliminations proceed extremely smoothly
for norbornadienes if, in addition to benzene, a second stable fragment is formed.16' In the case of norbornadienones,
the second species is the extremely stable carbon monoxide and the fragmentation occurs even below -70°C. In
contrast, aryl isocyanides are eliminated from N-arylnorbornadien-7-ylidenamines at 20-80"C[Sb36d1
and from benzonorbornadiene derivatives at ca. 120"C.r71This approach
can be used to synthesize 3 from 6 as starting material. We
obtained 6 in 65% yield by catalytic isomerization of
4 ;[6d.81
the catalyst employed, sublimate, was reported by
Rood and Klumpp to be particularly well-suited for the
rearrangement of quadricyclanes to norbornadienes.19JAs
expected, the isomerization of 4 to 6 proceeds stepwise
via 5 , which could be detected in the reaction mixture by
'H-NMR spectroscopy.
In solution ([D8]toluene) 6 decomposes above 80°C with
polymerization; neither benzene nor other soluble products are formed. Therefore, 6 was subjected to flash vacuum pyrolysis and the products were collected in a trap
cooled with liquid nitrogen. At 300-450°C a nearly quantitative mixture of 6, benzene, and 3 was obtained. The
monofragmentation product N-isocyanonorbornadien-7ylidenamine was not found. Above 500°C the reaction of 6
was complete. Since 3 decomposes above -30°C with
formation of a brown
the volatile products
were condensed at low temperature along with [D,]toluene
in an NMR tube cooled with liquid nitrogen. At -65"C,
only the resonance of benzene (6=7.07) was observed in
the 'H-NMR spectrum; in the I3C-NMR spectrum the resonance of benzene (6= 128.5) was observed along with a
resonance at 6 = 172, which vanished at - 30°C. We assign
this signal to the C atoms of 3, since it lies in the region
characteristic of i ~ o c y a n i d e s . The
~ ' ~ ~13C-NMR resonance
of 1, which, to the best of our knowledge, has not been
previously measured, was found at 6=93. The I3C-NMR
signal of 3 is sharp ( A V , , ~ =1.2 Hz) and does not exhibit
Angew. Chem Int. Ed. Engl. 27(1988) No. 7
6 : A solution of 4 (340 mg, 1.63 mmol) [6d, 81 and HgC12 (200 mg, 0.74
mmol) in 25 m L of chloroform was stirred for 24 h. After removal of the solvent under vacuum, the residue was extracted with toluene and the filtrate
was evaporated. The residue was sublimed (50-60°C.
mbar) and afforded 222 rng (1.07 mmol, 65%) of 6 as colorless crystals (m.p. = 140- 142°C
(dec)).-MS (70 ev): rn/z 208 (5%; high resolution, found 208.1005, calcd
208.1000), 130 (8%). 104 (30%), 78 (loo%), 77 (32%).-'H-NMR (CDCI,, 250
MHz): 6=6.88 (m. 4H), 6.79 (m. 4H), 4.47 (m, 2H), 4.01 (m, 2H).-"CNMR (CDCI,, 62.89 MHz): 6 = 180.1 ( s ) , 139.7 (d, 'J(CH)= 178.5 Hz), 138.5
(d, 'J(CH)= 178.5 Hz), 51.6 (d, 'J(CH)= 153 Hz), 47.6 (d, 'J(CH)= 156 Hz).
'H-"C double resonance experiments confirmed the expected couplings.Correct C,H,N analyses.-If the reaction mixture is evaporated after about
3 h, and the residue dissolved in CDC13, then, in addition to the signals of 4
and 6, those of 5 are observed in the 'H-NMR spectrum: 5 : 6=6.95
(m, 2H), 6.89 (rn, 2H), 4.76 (m, IH), 4.09 (m, IH), 2.05 (br. s, 5H), 1.57
(m, 1 H).
3: 6 (100 mg, 0.48 mmol) was heated at 50-60°C and
mbar in a flash
vacuum pyrolysis apparatus [I81 and underwent sublimation over ca. 30 h
through a hot tube (aluminum oxide, 25-cm length, 1.5-cm diameter, 500°C).
The products were collected in a cold trap (liquid nitrogen). The cold trap
was then removed from the cooling bath and the volatile products (3, benzene) were distilled into a second trap cooled with liquid nitrogen. n-Pentane
was distilled into the trap and the mixture was slowly heated. At -30°C
brown flakes began to separate out; this process was complete after several
hours at 0°C. After removal under vacuum of pentane and benzene, 14 mg of
a brown powder (assumed lo be (CN), [ iOj. 60%yield) was obfdined. In order
to record the NMR spectra, the volatile products were collected (as described
above) in an NMR tube sealed to the apparatus, followed by distillation of
[Da]toluene into the tube, which was then sealed.-In order to carry out the
mass spectral studies, the pyrolysate was distilled into a tube (see above)
cooled with liquid nitrogen and connected via a capillary with the direct inlet
of a Varian MAT CH-5-DF mass spectrometer, followed by warming of the
pyrolysate using a cryostat. Above - 140°C the signals of 3 (without benzene
signals) were recorded; above - 100°C the signals of benzene were also observed. MS (70 e y : m/z 52 (100%; high-resolution: found 52.0059, calcd
52.0061), 53 (2.7%), 38 (2.7%), 26 (14%), 24 (23%).
Received: February 8. 1988;
supplemented: March 21, 1988 [Z 2610 IEj
German version: Angew. Chem. 100 (1988) 998
CAS Registry numbers:
3, 114861-40-4; 4, 53414-44-1; 5, 114861-38-0; 6, 114861-39-1; benzene,
71 -43-2.
[I1 Z . Rappoport (Ed.): llre Chemistty of the Cyuno Group, Wiley, London
(21 G. Tennant, Compr. Org. Chem. 1979, No. 2, p. 385.
[3] L. J. Gay-Lussac, Ann. Chim. (Paris) 95 (1815) 175.
[4] M. Sana, G. Leroy, J. Mol. Stnrct. 76 (1981) 259.
[51 a) T. van der Does, F. Bickelhaupt, Phosphonrs Sulfur 30 (1987) 515; b)
T. van der Does, unpublished results.
[6] a) R. B. Woodward, R. Hoffmann, Angew. Chem. 81 (1969) 797: Angew.
Chem. I n f . Ed. Engl. 8 (1969) 781, and references cited therein; b) R. W.
Hoffmann, Angew. Chern. 83 (1971) 595; Angew. Chem. Inr. Ed Engl. I0
(1971) 529; C. W. L. Mock in A. P. Marchand, R. E. Lehr (Eds.): Pericyclic Reactions, Vol. B. Academic Press, New York 1977, p. 141 ; d) A.
Riemann, R. W. Hoffmann, Chern. Ber. 118 (1985) 2544; e) R. W. Hoffmann, W. Barth, R. Schiittler, B. Mayer, ibid. 119 (1986) 3297.
0 VCH Verlugsgesellschafi rnbH, 0-6940 Weinheim, 1988
OS70-0833/88/0707-0937 $ 02.50/0
171 R. S. Atkinson, M. J. P. Harger, J . Chem. SOC.Perkin Trans. 1 1974.
2619, and references cited therein.
[8] H. Sauter, H.-G. Horster, H. Prinzbach, Angew. Chem. 85 (1973) 1106;
Angew. Chem. Int. Ed Engl. 12 (1973) 991.
191 I. D. C. Rood, G. W. Klumpp, R e d . J. R. Nerh. Chem. SOC.103 (1984)
[lo] The elemental analysis (found C 50.16, H 2.12, N 41.37; (CN), calcd
C 46.14, N 53.86) merely reveals that the ratio C : N (1.4 : 1) roughly corresponds to the value expected; possibly, other substances are present.
The brown paracyanogen (CN), [ I l l formed from 1 at higher temperatures is also difficult to analyze [ I Ib]. It should be noted here that diisocyanomethane CH2(NC)>,like 3, polymerizes at -30°C [12].
[ I I] a) T. K. Brotherton, J. W. Lynn, Chem. Reu. 59 (1959) 841; b) L. L. Bircumshaw, F. M. Tayler, D. H. Whiffen, J. Chem. SOC.1954, 931.
1121 R. Neidlein, Angew. Chem. 76 (1964) 440: Angew. Chem. Int. Ed. Engl. 3
(1964) 382.
[I31 R. W. Stephany, M. J . A. de Bie, W. Drenth, Org. Magn. Res. 6 (1974)
[I41 8. E. Mann in R. K. Harris, B. E. Mann (Eds.): NMR and the Periodic
Table. Academic Press, London 1983, S. 96, and references cited therein.
[ 151. D. Herbison-Evans, R. E. Richards, Mol. Phys 8 (1964) 19.
[I61 NBS Library Compilorion. disk library number 55, Finnigan Corporation, San Jose, CA, USA 1984.
[ 171 a) See [I], p. 859; b) W. Reichen, C. Wentrup, Helm Chim.Actn 59 (1976)
2618; c) C . Wentrup, U. Stutz, H. J. Wollweber, Angew. Chem. 90 (1978)
731; Angew. Chem. Int. Ed. Engl. 17 (1978) 688, and references cited
1181 P. A. Kraakman, E. T. J. Nibbering, W. H. de Wolf, F. Bickelhaupt,
Tetrahedron 43 (1987) 5109, and references cited therein.
Upon acidification the fulminates furnish exclusively formonitrile oxide 1, whereas the cyanates give only isocyanic acid 3 (Raman@"]and IR[6b1spectrum). In agreement with these observations the structural data indicate
that resonance structure l e is the most important one in
the case of silver fulminate and that the canonical form 3
is the main contributor in the case of silver cyanate.
Only with the help of the matrix isolation technique has
it been possible to demonstrate the existence of cyanic acid
4 directly."' Herein, we report that the last and only missing isomer, namely the real carboxime 2 (isofulminic
acid)[*]-altogether there are four "chemically reasonable"
linear arrangements (cf. formulas 1-4) for a compound of
the formula CHNO-is detectable by the same technique.
Carboxime (Isofulminic Acid)**
By Giinther Maier, * Joaquim Henrique Teles,
B. Andes Hess, Jr., and Lawrence J. Schaad
When, after some controversy (1823-1 826),['] it became
clear that silver fulminate (Liebig) and silver cyanate
( Wohler) have the same composition but different chemical
properties, the theoretical interpretation followed inevitably: the phenomenon of isomerism had been observed
(Berzelius, 1830"l). The order of the C and N atoms in the
two salts is interchanged.[*]
The discussion concerning the free acids 1-4 related to
the fulminates (1'++2') and cyanates (3'++4') has continued until today: For fulminic acid only dimeric structures were considered initially.[31 NdT1 in 1894, was the
first to recognize the monomeric constitution and ascribed
to it the carboxime formula 2. Seventy more years were to
pass until fulminic acid was unequivocally characterized,
on the basis of its spectroscopic data (IR[5a.b1and microwave spectrumf5']), as formonitrile oxide 1 (instead of 2).
0 @
+ H-N=C=O
Irradiation of dibromoformoxime 519] in argon at 12 K
with a low-pressure mercury lamp leads to disappearance
of all IR bands of 5 within 20 minutes, and one obtains the
IR spectrum shown in Figure 1, which we assign to the
carboxime 2. Our arguments in support of this assignment
are: 1. The new species shows a spectrum different from
that of fulminic acid, and even more different from that of
isocyanic acid or cyanic acid. 2. The asymmetric stretching
vibration of the O N C unit in 2 appears at 2190 cm-'. This
CO. 2
- 3icm-']
Fig. I . IR spectrum of isofulminic acid in an Ar matrix at 12 K.
[*] Prof. Dr. G. Maier, DipLChem. J. H. Teles
Institut fur Organische Chemie der Universitat
Heinrich-Buff-Ring 58, D-6300 Giessen (FRG)
Prof. Dr. B. A. Hess, Jr., Prof. Dr. L. J. Schaad
Department of Chemistry, Vanderbilt University
Nashville, T N 37235 (USA)
This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen 1ndustrie.-B. A . H . and L. J. S. thank the
National Science Foundation (Grant CHE 8605951) for financial support.
0 VCH Verlagsgesellschafl mbH. 0-6940 Weinheim. 1988
position almost coincides with the position of the corresponding band of fulminic acid 1 (2193 cm-'), but is located at much lower wavelength than the corresponding
absorption of isocyanic acid 3 (2260 cm -') o r cyanic acid
4 (2286 cm-'). Calculations also predict that carboxime 2
should show the smallest wave number for this vibration.
3. The difference of 96 cm-' between the wavelengths at
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Angew. Chem. Int. Ed. Engl. 27 (1988) No. 7
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