COMMUNICATIONS TO THE EDITOR
526 Me
Me
I1 IV 1% mg.) was irradiated6 (best results in dilute pentane solution under nitrogen) with ultraviolet light (Hanovia medium-pressure quartz mercury-vapor lamp, 200 w., Pyrex filter) for 10 min. a t 20'. The solution was concentrated to 5 ml. and cooled to -70' for several hours. The precipitated solid (ca. 60 mg.) was7 largely methyl p-isopropenylbenzoate (IV), m.p. 51-52', Further concentration of the mother liquors gave crystals of 11, which, however, were more or less contaminated with IV. A more satisfactory separation was achieved by rapid chromatography a t -5' of the mother liquors over basic alumina (pretreated with diethylamine) and elution with 8 : 1 pentane-methylene chloride. Crystallization then gave pure 11, m.p. 32-32.5'. Anal. Calcd. for CUH120?: C, 74.99; H , 6.82; mol. wt., 176. Found: C, 75.08; H , 6.92; mol. wt. (Rast, in dicyclopentadiene), 163. The ultraviolet absorption spectrum showed A,",.,"" 238, 279, and 286 mk (sh); log 6 3.95, 3.41, 3.41, and 3.39, respectively. The infrared spectrum (liquid film) showed strong bands a t 1725, 1296, 1216, 1097, and 758 cm.-'. The n.m.r. spectrum (60 Mc./sec.) of I1 in carbon tetrachloride solution showed the following features given in 7-values (assignments shown in parentheses) : 8.38, 6 protons (gem-dimethyl); 6.1& 3 protons (ester methyl); 2.84, quartet, 1 proton (H6, J o y t h o = 6.6 and J p a v a= 1.9 c.P.s.); 2.3, quartet, 1 proton (Hz, J p a r a= 1.9 and JmeLa= 0.7 c.P.s.); 2.14, doublet,8 1 proton (H4, J o r t h o = 6.6 c.P.s.). The assignments of the aromatic bands are based on the knownga low-field shift of aromatic protons ortho to a carbomethoxy group and on the magnitude of J o y t h o . It is interesting that J p a r ais greater than J m e i a , the reverse of the usual order, and that J o y r h o is a little smaller than Such variations are not unexpected, as there could be considerable distortion of the benzene ring in 11. The aromatic character (e.g., the ring current effect) of the benzene ring is not greatly disturbed in 11, as shown by the normal chemical shifts of protons Hz, H1, and H6, as well as by the similarity of the ultraviolet absorption spectrum of I1 to t h a t of methyl benzoate. The equivalence of the gem-dimethyl protons in the n.m.r. spectrum of 11, even a t - 150' (CC12F2 solution), is consistent with a planar or a rapidly inverting, and therefore presumably nearly planar, structure for the benzocyclopropene skeleton.I0 ( 6 ) T h e preparation of cyclopropenes hg irradiation of pyrazolenines has been reported recently: G I. Closs and W. Boll, A n g e w C h e m I n t e r n E d . E q i , 9 , 399 (1!463), G Ege, Tetrahedron Letters, 1667 (1963). ( 7 ) T h e structure was deduced from t h e n m . r . spectrum and was confirmed by comparison of the photolysis product with a sample prepared from methyl p~isoi'rup)'lbenzoate by bromination with N-bromosuccinimide followed by dehydrobrumination Hydrolysis of I V gave p-isopropenylhenzuic acid. rn p. 1603, G B Bachmann, C . 1,. Carson, a n d M Robinson, J A m i hem Soc , 73, 1964 (19.51), report the same melting point for this acid (8) Both components u f the doublet were rather broad and t h u s obscured the small coupling due t o Jm,t, which should also he present. This could be dii? t a a very small long-range coupling o f , e g , 0.3 c p.s. with the gemdimethyl g r r n p :Y) I, bl Jackman, "Applications of Nuclear Magnetic Resonance Spectroscopy to Organic Chemistry," Pergamon Press, New York, N . Y , 1969 (a) p 6 3 ; ( h ) p 8.5 ( I O ) T h e possibility of a nonplanar structure for benzocyclopropene has been discussed b y Ullman and Buncel
Vol. 86
Although I1 is stable for several hours a t room temperature. i t is completely destroyed in refluxing benzene after 15 min. The compound is very sensitive tu acids, but not to bases. The transformation products and the rate of decomposition of I1 are being investigated; preliminary n.m.r. evidence shows that a t least one of the products contains an isopropenyl group. OF CHEMISTRY DEPARTMENT ~JNIVERSITYOF OTTAWA OTTAWA, ONTARIO, CAKADA RECEIVED I~ECEMBRR 14, 1963
RAGIXIANET F. A L .INET
The Synthesis of Methylenecyclopropanes via an a-Elimination Reaction Sir : Methylene carbenes (RzC=C :) have been suggested as intermediates in the formation of butatrienes from exocyclic vinyl halides and sodium amide' or phenyllithium. * With simpler 2,2-diaryl vinyl bromides rearrangement occurs to give diary1 acetylenes. Stereochemical evidence has been p r e ~ e n t e dto ~ . prove ~ that this reaction does not produce a methylene carbene. The formation of substituted methylenecyclopropanes in low yields by the reaction of vinyl chlorides, olefins, and potassium t-butoxide has recently been d i s c l ~ s e d . ~ This reaction is assumed to involve intermediate methylene carbenes. We have found t h a t aliphatic vinylidene bromides give a-elimination reactions with methyllithium. While 1,1-dibromopropene, methyllithium, and olefins gave methylacetylene ; 1,l-dibromo-2-methylpropene (1), methyllithium, and olefins gave moderate yields (1270%) of isopropylidenecyclopropanes (2). (CHB)?C=CBr2 1
+ CH&i + RIRPC-CFIR1 --+
RiKzC - CHK3 \ / ~
c
II ~(CHB~Z 2
The reaction 1 + 2 has been run successfully a t -80 to +SO". A solution of methyllithium in ether was added slowly to an equimolar amount of 1 dissolved in a three- to tenfold excess of olefin. The products were isolated by standard separation and distillation procedures or by preparative scale vapor phase chromatography. The highest yields of 2 were obtained with nucleophilic olefins such as vinyl ethers. The reaction with ethyl isobutenyl ether gave a 59% yield of 2 (KI = Rz = CH3, R3 = OCHZCH3), b.p. 85" (115 mm.). Anal. Calcd. for ClaHlsO: C , 77.9; H , 11.8. Found : C, 77.9; H , 12.0. Isopropylidenecyclopropanes were also obtained from ethyl vinyl ether (70?&), methyl vinyl ether (600j0), cyclohexene (41%)- isobutylene (35%), norbornene (22%), 2-methyl-2-butene (12%), and trans-2-butene (23%).6 All of the products 2 showed infrared absorption near 5.60 p which was attributed to the strained double bond exocyclic to the cyclopropane ring. Two common features of the proton magnetic resonance spectra of 2 also assisted in the identification of the products: high-field resonances of the hydrogen atoms on the cyclopropane ring and long-range coupling (-2 c.P.s.) between the ring hydrogens and the isopropylidene (1) C . R Hauser a n d D . Lednicer, J Org C h e m . . 94, 1248 (1457) (2) D . Y Curtin and W H . Richardson, J . A m C h e m Sor . 81, 'ti'l!' (1959). (3) A A. Bothner-By, ;bid., 77, 3294 f1SC56) ( 4 ) D Y Curtin, E W , Flynn, and R F. S y s t r o m . ibid , 8 0 , 4599 !958) (6) M . Tanabe and R. 4 Walsh, ibid.,86, 3522 (1963) (6) Satisfactory elemental analyses and spectral d a t a were obtained f o r ai( these compounds. I
Feb. 5 , 1964
COMMUNICATIONS TO THE EDITOR
hydrogens. Thus, the spectrum of the product obtained from isobutylene (2, R1 = Rz = CH3, R3 = H ) showed the resonances of the ring methyls (singlet) at T 8.85, the methylene (seven peaks) centered a t T 9.21, and the olefinic methyls (four peaks) centered a t r
527
Careful analysis of the hydrocarbon fraction of the product by V.P.C. revealed only quadricyclene (Ib)5,6 and a trace ( < l % ) of an unknown material. Even /X NaBH,
8.24.
2-Bromo-3-methyl-2-butene (3) was formed in all the reactions of 1, methyllithium, and olefins. The yields of 3 were in inverse relation to those of the olefin adducts 2. With weakly nucleophilic olefins, such as ethylene or vinyl chloride, only traces of methylenecyclopropanes were formed, and 3 was the major product. These results suggest that an intermediate is formed which can react with olefin or methyl bromide. The variation in yield of 2 with olefin structure is that expected of an electrophilic intermediate. It is suggested that lithium-bromine exchange occurs and is followed by, a t least, partial loss of lithium b r ~ m i d e . ~
(a)50% aq. CH3CH,0H
(c) d r y diglyme
Ib
Ia, c Ia, X b,X c, X
= = =
C1 H OBs
d, X e, X
= =
OH OCH2CH3
more remarkably, reaction of the p-bromobenzenesulfonate (IC)with sodium borohydride in dry diglyme' a t 40" produced only quadricyclene (Ib) along with a trace (
