May 5, 1961
COMMUNICATIONS TO THE EDITOR
2195
tetraphenylfuran.8 Aqueous sodium nitrite oxida- involved in the formation of 111, is given in a tion of tetramethylcyclobutadiene nickel chloride separate communication. complex to cis-dihydroxytetramethylcyclobutene EASTERN -SEARCH LABORATORY H. H. FREEDMAN COMPANY was utilized by Criegees to establish the identity of THEDow CHEMICAL MASSACHUSETTS the organic portion of his cyclobutadiene metal FRAMINGHAM, RECEIVED JANUARY 31, 1961 complex. An analogous diol has not been isolated in our work to date, but its ready dehydration to tetraphenylfuran is perhaps not unexpected. TETRAPHENYLCYCLOBUTADIENE DERIVATIVES. In general, 111 is quite soluble in solvents which 11.1 CHEMICAL EVIDENCE FOR THE TRIPLET STATE can further complex with the available d orbitals of Sir: the nickel, yielding deep blue or violet solutions Theoretical considerations suggest that cyclowhich are readily decomposed by the application of butadiene will have a triplet ground state.* The heat or the addition of water. I n contrast, attempts to remove the nickel by treatment of a di- method employed for the preparation of tetramethylformamide solution of I11 with aqueous phenylcyclobutadiene nickel bromide complex desodium acetate or sodium hydroxide led only to ex- scribed in the accompanying report,’ presents a change of the bromide with the production of the unique opportunity for experimental evidence red, ether-soluble, oxygen-sensitive nickel acetate bearing on these theoretical predictions, since here or nickel hydroxide complex of tetraphenylcyclo- the complex is formed in solution from an intermediate of finite lifetime. butadiene. This intermediate, formed by the loss of diCatalytic hydrogenation of the complex 111 in methyltin dibromicie from the stannole dibromide tetrachloroethane proceeded only a t high pressures I’ a t approximately 1503, exhibits properties con(PtOz, 800 Ib. H P , 75’) and afforded an 80% yield sistent with either structure IIa, the predicted? of white needles, m.p. 122-123’. The mass triplet ground state of tctraphenylcyclobutadiene, spectral pattern of this product unequivocally or, less likely, its open chain equivalent, IIb. assigns to i t a molecular formula of C%HZ2 and this C,,Hj CtjlIis supported by analysis (Found: C, 93.6; H, 6.4). The choice between the two most likely -15 . CCHj-’ \-C,Hor 1,2,possibilities, 1,2,3,4-tetraphenylcyclobutene 3,4-tetraphenylbutadiene,was resolved in favor of Sn-Br Br the former on the basis of the spectral properties. / ‘Ale Me Thus, the ultraviolet spectrum in heptane (A max. 1 293 mp, E 26,000)resembles t h a t of stilbene and not Me2SnBrz that of phenyl-substituted l,&dienes. T h e int frared spectrum in carbon tetrachloride solution shows absorption a t 2915 ern.-’, characteristic of alkane hydrogen, effectively eliminating the possibility that the hydrogenation product is a 1,2,3,4tetraphenylbutadiene. The n.m.r spectrum in carbon tetrachloride shows no bands in the region IIa IIb characteristic of the olefinic protons, but absorption does occur in the region characteristic for Evidence t h a t the intermediate in question is protons on a cyclobutane ring.g However, the indeed a diradical species is inferred from the transn m.r. spectrum shows, besides phenyl protons, ient green color observed when I is decomposed in two singlet bands a t -4.90 and -4.03 p.p.m. solution and its avid reactivity with characteristic (relative to tetramethylsilane as an internal stand- radical reagents. The ready reaction of the interard) in the relative ratio of approximately 1:2. mediate with oxygen (itself in a triplet state) is Inasmuch as the molecular formula does not allow particularly significant and affords an 80y0 yield the presence of more than two non-phenyl equiva- of cis-dibenzoyl~tilbene~(IV) v i a the indicated lent protons, we are forced to conclude t h a t the electron redistribution of the postulated interhydrogenation product is in fact a mixture of the mediate peroxide, I I I a or b. cis and trans-tetraphenylcyclobutenes.lo Rigorous proof of this conclusion is in progress. Attempts t o prepare complexes analogous t o I11 with other metal bromides, as well as some preliminary findings in regard t o the intermediate OLO IIIb
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(8) We are grateful to Prof. R. R.Summerbell for a sample of tetraphenylfuran. IIIa (9) The n.m.r. spectrum of a tetraphenylcyclobutane has been reAddition of I1 to dienophiles takes place stereoported by J. A. Pople, W. G. Schneider and H. J. Bernstein, “High Resolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., specifically and in high yields. Dimethyl maleate New York, N.Y., 1959, p. 291. (10) It is conceivable that rearrangement has occurred during the (1) H. H. Freedman, Part I, J. A m . Chcm. Soc., 83,2194 (1961). hydrogenation with the production of 1,2,3-triphenyl-1,4-dihydro(2) (a) J. D. Roberts, A. Streitweiser, Jr., and C. M. Regan, {bid., naphthalene, a structure which would fit all the data reported above 74, 4579 (19521, and references cited therein; (b) W. D. Hobey and and, in particular, the n.m.r. spectrum. However, this compound has A. D. McLachlan, J . Chcm. Phrs., 83, 1695 (1960). been prepared previously (E. Bergmann and 0. Zwecker. Ann., 487, (3) We are indebted to Prof. Peter Yates for the sample of cis155 (1931)), and is reported to melt at 165’. dibenzoylstilbene used for comparison purposes.
gives the known methyl cis-dihydrophthalic ester cluding tetraphenylfuran and 1 -benzal-2,:
