Photo-arbuzov rearrangements of dimethyl benzyl and dimethyl p-acetylbenzyl phosphite

The direct ultraviolet irradiation of dimethyl benzyl phosphite (1) and dim ethyl p-acetylbenzyl phosphite (8) was investigated in acetonitrile, cycloh exane, and benzene. Phosphite 1 gives predominantly the photo-Arbuzov produ ct, dimethyl benzylphosphonate (2), in 67-81% accountability yields, based of phosphite consumed, along with minor amounts of bibenzyl (20) and dimeth yl phosphite (10). The quantum yield for formation of 2 in cyclohexane, phi (p), is 0.43. By contrast, irradiation of phosphite 8 yields only 7-13% of photo-Arbuzov phosphonate (9) but relatively large amounts of radical diffu sion products: dimethyl phosphite (10) the p-acetylbenzyl radical dimer (11 ); and p-acetyltoluene (12). Evidently 8, closely related to acetophenone, reacts predominantly via the triplet excited estate to generate long-lived, triplet, free-radical pairs (6 and 7a). In benzene, further products (15, 16, 17a and 17b) are identified that result from addition of the phosphinoy l radical (6) to benzene to give cyclohexadienyl radical 14, followed by co mbination and disproportionation reactions with radical 7a. (Total product quantum yields in benzene (Sigma phi(i)) = 0.47.) In benzene, accountabilit ies of radical 6 from photolysis of 8 as high as 56% are encountered along: with up to 92% accountabilities of p-acetylbenzyl (7a) radicals. Addition of radical;scavengers PhSH, PhCH2Br, and TEMPO in the three solvents establ ishes the cage yield of 9 as 3-5%. The products of radical trapping provide further proof of the radical-pair nature of the photolysis of phosphite 8, including a 95% accountability of 6 with PhCH2Br in benzene. It is propose d that the CH2-O scission of triplet 8 must occur concertedly with partial phosphoryl (P=O) bond formation. The trapping of radicals 6 and 7b from irr adiation of phosphite 1 as the benzene adducts 22 and 23, analogous structu rally to those (16 and 17) from phosphite 8, supports the postulation that photoisomerization of 1 to 2 proceeds via short-lived, presumably singlet, free-radical pairs.