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Further evidence for a singlet as opposed to a triplet mechanism is provided by the striking observation that the only substituted stilbenes which have been found not to undergo photochemical conversion to phenanthrenes are those with nitro I1 probably is the result of a ring closure correspond- or acetyl substituents; it is well known" that these ing to that which would give I as opposed to 11. substituents enhance intersystem crossing rates Finally, if the intermediate dihydrophenanthrene and thereby decrease excited singlet state lifehad the cis configuration 11, it might be expected times by several orders of magnitude. \Ye have developed this reaction into a valuable to give phenanthrene in the absence of an oxidant by ejection of molecular hydrogen since it is esti- synthetic method for obtaining 1-, 3- or 9-submated that this process would be exothermic by stituted phenanthrenes in 60-90y0 yields starting about 49 kcal./niole. The lack of such a reaction from the readily accessible 0-,p- or a-substituted in solution is in accord with the tram configura- stilbenes, respectively, with substituents such as tion I since concerted elimination of a hydrogen CH3, CHaO, F, C1, Br, CO1H, CF3 and CsHb. inolecule from I would involve a drastic molecular m-Substituted stilbenes give mixtures of 2 - and distortion and a large activation energy; also, 4-substituted phenanthrenes. These reactions are it is estimated that the stepwise loss of two hy- carried out by irradiation under an air atmosphere drogen atoms from I would be endothermic by of solutions of 2-3 g. of the stilbene and 3-5 mole-Y0 about 46 kcal.,'mole for the first hydrogen and of iodine in 500 ml. of cyclohexane in a quartz endothermic by about 9 kcal.;'mole for the second vessel. For preparative work iodine is far superior to oxygen as the oxidant since it gives faster and hydrogen. cleaner reactions with higher yields. The recent observation that the photolysis of (11) (a) M. Kasha, Disc. F a r a d a y SOC.,9, 14 (1950); (b) W. X. stilbene in the vapor phase gives phenanthrene and Moore, G . S. Hammond and R. P. Foss, J . A m . Chem. Soc., 83, 2789 hydrogen8 does not necessarily mean that the in- (1961). termediate must be I1 rather than I. The diFRANKR. MALLORY OF CHEMISTRY CLELIAS. WOOD hydrophenanthrene is formed from excited cis- DEPARTMENT MAWRCOLLEGE JANICET. GORDON stilbene with an estimated 50-73 kcal .;'mole excess BRVN BRVNMAWR,PEXNSYLVASIA LOISC. LINDQUIST vibrational energy which would be transferred MAXINEL. SAVITZ to solvent molecules very rapidly in solution but RECEIVED OCTOBER 10, 1962 which would be retained by the dihydrophenanthrene for much longer periods in the vapor phase. ORGANOBORON COMPOUNDS. XIV. RAPID Under the latter conditions this excess energy ALKOXY EXCHANGE BETWEEN DIALKYLALKOXYBORANES AND could conceivably allow either the concerted or the ALKYLDIALKOXY BORANES',' stepwise loss of two hydrogens from I as described Sir : above. Distillation studies in This Laboratory3 have The fact that the reaction is not quenched by oxygen in solution suggests that the excited state shown that facile exchange of alkoxy groups occurs undergoing ring closure is not a triplet. Convincing among alkylalkoxyboranes and trialkoxyboranes evidence that triplet states of stilbene are not in- on fractional distillation somewhat above room volved in the ring closure reaction has been ob- temperature. More recently Mikhailov and tained recently from a studySQin which i t was Vasil'ev4 reported a similar result and, without found that the initial rate of phenanthrene forma- citing any experimental evidence, added that the tion is finite starting from solutions of pure cis- exchange is slow a t room temperature and rapid stilbene but zero starting from solutions of pure a t higher temperatures. We have investigated trans-stilbene; phenanthrene is formed from ir- the speed of this exchange reaction a t room temradiation of tvans-stilbene only after some cis- perature, using proton magnetic resonance specstilbene has been produced by photoisornerization. tra,?for analysis of equilibrium mixtures, and have Thus, the precursor to the dihydrophenanthrene is found the exchange to be extremely rapid. The p.1n.r. spectrum of diisobutylmethoxyan excited state which can only be produced by absorption of a photon by cis-stilbene. It is borane in 33y0 by volume of benzene has a singlet generally acceptedg that the triplet state which is a t low field (3.69 p.p.m. above benzene) due to the reached by intersystem crossing from excited rnethoxy hydrogens. The spectrum of isobutyl!vans-stilbene singlet is either identical with or in dimethoxyborane in the same solvent has a similar rapid equilibrium a t room temperature with the singlet, due to the dimethoxy hydrogens, a t sometriplet state which is similarly reached from excited what higher field (3.77 p.p.m. above benzene). ( I ) Previous paper, P. A. hIcCusker, J . V. Marra and 0 . F. Henc-is-stilbene singlet. On the other hand, i t has J. A m . Chem. Soc., 83, 1924 (19til). been shown that the excited siiiglet states of cis- nion, ) Contribution from the Radiation Laboratory operated by stilbene and trans-stilbene do not interconvert.sas10 the( 2University of Notre Dame under contract with the Atomic Energy f X ) Unpublished work of R. Srinivasan and J. C. Powers, I B M Research Center: the private communication of these results is gratefully acknowledged. (9) (a) R. H. Dyck and D. S. McClure, J . Chem. Phy.s., 36, 2326 (1962): (b) S. Slalkin and E. Fischer, Symposium on Reversible Photochemical Processes, preprints of papers, p. 281; (c) H. Stepemeyer, private communication. (10) G . S.Lewis. T. 1'. Magel and n. L i p k i n . J An?. C h c m . SOC., 62, Xl7.3 ~ 1 : l I ~ l ~ .
Commission. (3) Abstracts of Papers, 140th Meeting of the American Chemical Society, 2 2 - 5 , Chicago, September, 1961. ( 4 ) B. ivl. hlikhailov and L. S. Vasil'ev, Btlll. A c a d . Sci. U . S . S . K . (Div. of Chem. Sci.) Eng. Trans., 11, 1962 (1961). ( 5 ) Proton magnetic resonance spectra were obtained using a Varian Associates Model H-60, high resolution, 110 mc., N M R spectrometer. Chemical shifts were obtained using t h e side-band technique w i t h henzene as i n t e r n a l stand:ird
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