The direct ultraviolet irradiation of dimethyl benzyl phosphite (1) and dimethyl p-acetylbenzyl
phosphite (8) was investigated in acetonitrile, cyclohexane, and benzene. Phosphite 1 gives predominantly the
photo-Arbuzov product, dimethyl benzylphosphonate (2), in 67−81% accountability yields, based of phosphite
consumed, along with minor
amounts of bibenzyl (20) and dimethyl phosphite (10). The quantum yield for
formation of 2 in cyclohexane, φP, is 0.43. By contrast, irradiation of phosphite 8 yields only 7−13% of
photo-Arbuzov phosphonate (9) but relatively large
amounts of radical diffusion 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 phosphinoyl
radical (6) to benzene to give cyclohexadienyl radical 14, followed by combination and disproportionation
reactions with radical 7a. (Total product quantum yields in benzene (Σφi) = 0.47.) In benzene, accountabilities
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
establishes 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 proposed that the CH2−O scission of triplet 8 must occur concertedly with partial phosphoryl (PO)
bond formation. The trapping of radicals 6 and 7b from irradiation of phosphite 1 as the benzene adducts 22
and 23, analogous structurally 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.
