Cyclobutanol, formation mechanism

Still another reaction which is readily susceptible to our n-7r model is the Norrish Type II with concomitant cyclobutanol formation (i.e. the Yang reaction 33)). This mechanism was described in detail by the author 1,3,12), again in those early papers. Here the two dimensional circle-dot-y notation suffices and is convenient. Note Equation 8. 

Due to the very limited amount of experimental data available, a choice between the two mechanisms is even more difficult for reaction III than for reaction II. Orban et investigated the photochemical rearrangement, in pentane, of an aliphatic, optically active ketone with a single asymmetric carbon atom in the y position. Their results demonstrate a partial retention of configuration during photochemical cyclobutanol formation, which can be explained neither by a biradical mechanism alone, if this involves a long-lived radical, nor by a concerted mechanism alone. The results are reconcilable with the competitive participation of both mechanisms, but they are just as compatible with the assumption of the production of a short-lived biradical whose rates of racemization and of cyclization are comparable. 

Scheme 9.—Proposed Mechanism for Formation of Cyclobutanol from the Photolysis of 1,3,4,5,6-Penta-O-acetyl-fceto-L-sorbose (18).

Observation of Scheme 54 shows a close analogy between these results and the ones obtained in arynic condensations. Thus we observe the formation of normal ketones 138, transposed ketones 139 or 140, and methylene cyclobutanols 141. However, we met large difficulties with the mechanism. Indeed, starting from the hypothesis of cyclohexyne being the intermediary (what is consistent with our works on amine condensations) and by analogy with arynic condensations, the reaction should be written as in Scheme 55. 

Larock and Reddy obtained the 2-alkylidenecyclopentanone 72 by the reaction of l-(l-alkynyl)cyclobutanol 71 with iodobenzene. The bicyclononanone 74 was obtained from 73. Selective formation of 74 demonstrates that the more substituted bond a in the eyelobutanol 73 undergoes exclusive cleavage (or migration) [8,13]. Larock proposed the mechanism of the reaction of 75 involving ring expansion of 76 to form palladacycle 77 and reductive elimination to give 78. 

Upon irradiation in solution aliphatic ketones containing 7-hydrogens from cyclobutanols as well as undergo photoelimination Cc g-) Eq. (46) Two mechanisms have been considered to account for the photochemical formation of cyclobutanols. The first is a stepwise mechanism, Eq. (59), and the second is a concerted process, Eq. (CO). 

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