2514
COMMUNICATIONS TO THE EDITOR
descend slowly into a cylindrical furnace a t the temperature of about 100' and having its lower part cooled to create a temperature gradient of a t least i 0 / c m . in the melting zone. Such a crystal was cut to 14 mm. length and placed in a thin-walled cylindrical container made of Plexiglas. The measurements were carried out as already described in ref. 3 and 4. Two series of measurements were made a t about 23', a t the frequencies of 223.5 and 267.9 Mc.p.s. for E1Br and 59Br, respectively. The asymmetry parameter was 7 0.045 0.002 for both the isotopes, in substantial agreement with Shimomura's value. The results concerning the crystal structure confirmed the data of Croatto and Bezzi. In fact, we found two nonequivalent directions of the Z-axis of the electric field gradient, which we identified with the Br-Br directions of the two molecules contained in the unit cell. The angle between these two directions, as well as the dihedral angle between the planes of the two molecules, calculated assuming that the X-axis of the electric field gradient is perpendicular to the plane of the molecule, is reported in Table I , together with the values calculated from the data of Croatto and Bezzi (=
*
V O l . 86
intramolecular hydride ion transfer to produce the new and more stable cation 11, which is rapidly cleaved to aldehyde and amide. Confirmation of the presence of such an intermediate, particularly in view of the previous exclusion3 of a radical-chain path, would provide strong evidence for the proposed2a hydride ion transfer process.6 Furthermore. since the species I1 is representative of a new class of unstable cationic intermediates, its properties would be of considerable general interest.
I
I1
I11
TABLE I
We now wish to report the identification of the intermediate I1 in this reaction, its independent preparation, T h i s work and some of its chemical properties. Decomposition of the hexafluorophosphate salt of I a t 60" in dry ethylene Angle between the two chloride solution produced a tan solution, the infrared Br-Br directions 71" 30' i 2 ' 15' f 10' spectrum of which exhibited absorption a t 1745 cm. - l . This band, which is present neither in the diazonium Dihedral angle between the ion nor in the products obtained by adding water to the t w o molecular planes 120° 1 1 8 O i2 O reaction mixture (see below), is assigned to the carbonyl group of II.8 The n.m.r. spectrum of the solution exAcknowledgment.-We thank Prof. Eo10 Scrocco hibited a weak singlet a t r 0.53 p.p.m. (half-width = for advice and encouragement during the work. 4 c.P.s.)which is attributed to the aldehydic type proton of the -CH===N< system, and another singlet a t T ISTITCTODI CHIMICAFISICA P BCCCI 4.45 p,p.m. (half-width = 3 c.P.s.) which is attributed UNIVERSITADI PISA P CECCHI PISA, ITALY A. COLLICIANI to the benzylic methylene group.g The characteristic spectral properties of I1 disappeared upon addition of RECEIVED APRIL24, 1964 chloride ion in the form of tribenzylamine hydrochloride or of water. In the latter case, benzaldehyde absorption immediately appeared a t 0.10 p.p.m. As expected, the hydrogen transfer product N-benzylbenzA New Type of Cationic Intermediate from amide (111, 18% yield) was identified as one of the coman Intramolecular Hydrogen Transfer Reaction' pounds obtained from the hydrolyzed reaction mixture. Two other products were benzoic acid and benzylidineSir. benzylamine (117), obtained in equivalent quantities I t has recently been r e p ~ r t e dthat ~ , ~ thermal decom(15% yield). position of the diazonium ion I derived from o-aminoThe first step of the independent preparation of I1 inK,N-dibenzylbenzamide produces, among other prodvolved the treatment of benzylidinebenzylamine (IV) ucts, benzaldehyde and X-benzylbenzamide (111), with 1 equiv. of benzoyl chloride to produce the a-chloSimilar dealkylations have been observed in analogous roamide V. lo The latter in ethylene chloride excases in which the benzyl groups are replaced by methyl2 and by cyclohexy14 groups and in related sys(6) An a l t e r n a t i v e h y d r o g e n alom t r a n s f e r h a s been suggested a s o n e possibility for t h i s t y p e of r e a c t i o n . ' terns5 ( 7 ) 11. N B r o w n , I) H H e y , a n d C. W. R e e s , J . Chem. SOC., 3873 (1961). I t has been postulated2 that the benzene carbonium (8) As e x p e c t e d , i t is s h i f t e d a b o u t 100 c m . - ' t o higher f r e q u e n c y t h a n ion generated by loss of nitrogen from I suffers an t h e c a r b o n y l a b s o r p t i o n of t h e uncharged ~ - , S ~ d i b e n z ~ , l b e n z a m i d e . Croatto a n d Bezzi
(1) S u p p o r t e d b y t h e N a t i o n a l Science F o u n d a t i o n t h r o u g h G r a n t S S F G-2Y70.i. ( 2 ) T C o h e n , R h1 h f n r a n , a n d G. S o w i n s k i , J . O i g , Chem , 2 6 , 1 (1961). 13) T C o h e n , A . H Ilinwoodie. a n d I, I). M c K e e v e r . % b i d , 27, 3385 (1462) :4) J. I.ipowitz, unpublished results. ( 5 ) F o r p e r t i n e n t references, see t h e first p a p e r in t h i s series
(9) T h e s e a s s i g n m e n t s a r e based o n t h e s p e c t r u m of t h e i n d e p e n d e n t l y h c o m p a r i s o n of t h e i n t e n s i t i e s of t h e p r e p a r e d s p e c i m e n ; s e e below t w o p e a k s was n o t possihle b e c a u s e t h e high-field p e a k o v e r l a p p e d with benzylic p e a k s of o t h e r p r o d u c t s of t h e reaction a (10) T . C J a m e s a n d C u' J u d d . J Chrm SOL.. 100, 1427 (1914) (11) T h e infrared s p e c t r u m of t h e a d d u c t V a l w a y s exhibited a v e r y weak T h e adduct b a n d f o r t h e c a r b o n y l group of u n r e a c t e d benzoyl chloride f o r m a t i o n is t h u s p r e s u m a b l y reversible
June 20, 1964
COMMUNICATIONS TO THE EDITOR
hibited carbonyl absorption a t 1658 cm.-l and benzylic methylene absorption a t T 5.45 p.p.m. (half-width = 2 c.P.s.). The benzylic proton on the chlorine-bearing carbon atom absorbed in the aromatic region. Integration of the n.m.r. spectrum indicated t h a t there are 16 protons in the aromatic region and 2 in the aliphatic region.
2515
The Question of a Benzene Cation Insertion Reaction. A Novel Intramolecular Electrophilic Substitution'
Sir: Three of the products obtained upon thermal decomposition of the diazonium ion I in acetic acid containing sulfuric acid have been shown to be benzaldehyde, Nbenzylbenzamide, and 1-phenyl-2-benzylphthalimidine (III).3 Similar products were obtained from the N,Ndimethyl analog of I.3 Evidence has been presented that the first two of these products arise from solvolytic cleavage of the cation I1 presumably formed by intramolecular abstraction of a hydride ion by the benzene cation which is produced by loss of nitrogen from I . 4
I Addition of antimony pentachloride to an ethylene chloride solution of V converted most of V into the cation 11, having absorption a t 1748 cm.-' ( s ) (c=O) 8+
Ci" I
6+
and 1585 cm.-l (vs) (-CH-=N) in its infrared spectrum and a t T 0.48 (half-width = 7 c.P.s.) and 4.30 p.p.m. (half-width = 3 c.P.s.) in its n.m.r. spectrum12 (area ratio 1:2). The addition of chloride ion, in the form of trimethylamine hydrochloride, to the solution of I1 caused the regeneration of V. Addition of water caused the cleavage of 11, furnishing benzaldehyde and N-benzylbenzamide (111). The hydrolysis also produced small quantities of benzoic acid and benzylidine benzylamine (IV). The yield of the latter two products increased substantially when the hydrolysis was performed on a solution of I1 which had been heated a t 70' for 1.5 hr.13 T h e production of the same f o u r hydrolysis products
f r o m both the synthetic cation 11, and the diazonium i o n I after its thermal decomposition, provides powerful chemical evidence, in addition to the spectroscopic evidence already cited, for the existence of' the cation 11 a s an intermediate in the diazonium i o n decomposition reaction. Another interesting property of I1 is its conversion to N,N-dibenzylbenzamide by hydride ion abstraction from tribenzylamine. 1 4 , l5 This reduction serves as further proof for the structure of 11. An unusual intramolecular electrophilic substitution reaction of the cation I1 and its mechanistic implications are discussed in the accompanying communication. l6 (12) A fairly good model for I1 is the hydrochloride of I V , the nonaromatic protons of which absorb at 7 0.52 and 4.94 p.p.m. (13) Upon heating, the hexachloroantimonate and hexa0uorophosphate salts of I1 apparently decompose partially to benzoyl halide and the imine (complexed with the Lewis acid). (14) H. Meerwein, V. Hederich. H. Morschel, and K. Wunderlich, A n n . . 656, 1 (1960). (15) The ion I1 failed to abstract a hydride ion from triphenylmethane under the same conditions. (16) T. Cohen and J. Lipowitz, J . A m . Chem. Soc., 8 6 , 2515 (1964).
DEPARTMENT OF CHEMISTRY UNIVERSITY OF PITTSBURGH PITTSBURGH, PENNSYLVANIA 15213
I1
THEODORE COHEN LIPOWITZ
JONATHAN
RECEIVED APRIL 6, 1964
I11 One possible mode of formation of I11 involves an intramolecular electrophilic substitution by 11. However, it is questionable whether a relatively stable cation of this type could execute an electrophilic substitution a t a rate comparable to that of its solvolytic c l e a ~ a g e . ~ , ~ Furthermore, one might expect the positive carbon atom of I1 to attack the benzyl ring more rapidly than it attacks the deactivated benzoyl ring. The preference for attack on the deactivated ring can, of course, be explained by 'invoking the argument2 t h a t the ortho position of the benzoyl ring is constrained by a rwonanceimposed planarity of the system to a position closer to the positive carbon atom than that occupied by the ortho position of the benzyl ring. Nevertheless, i t is entirely possible that the ring closure to a phthalimidine only occurs because i t very closely follows or is concomitant with the transfer of the hydrogen. Precedents for the formation of rings by the attack of cationic carbon on saturated carbon atoms are An intriguing possibility is t h a t this type of carbon-carbon bond formation is particularly favorable in the present system because of the (1) Supported by the National Science Foundation through Grant N S F G-23705. (2) T . Cohen, A. H. Dinwoodie, and L. D. McKeever, J . Org. Chem., 27, 3385 (1962). (3) 1'.Cohen, R . M. Moran. and G. Sowinski, i b i d . , 26, I (1961); A. H. Lewin, unpublished results. (4) T. Cohen and J. Lipowitz, J . A m . Chem. Soc., 8 6 , 2514 (1964). (5) The N,N-dimethyl analog of I gives substantial quantities of 2methylphthalimidine even in aqueous solution.3 (6) A somewhat similar electrophilic substitution is involved in the Pictet-Spengler reaction.' In this case strong activating groups are usually required on the benzene ring being substituted. (7) W. M. Whaley and T. R . Govindachari, Org. Reocfions, 6 , 151 (1951). (8) L. de Vries and S. Winstein, J . A m . Chem. Soc., 81, 5363 (1960); P. S.Skell and I. Starer, ibid., 84,3962 (1962).
