Triplet-Sensitized Photorearrangements of 2-Phenylallyl Phosphites

The triplet-sensitized photorearrangements of
2-phenylallyl phosphites 1, 3, and 4
to the corresponding
2-phenylallylphosphonates 1a, 3a, and
4a are shown to proceed with complete regioselectivity
(5 → 6 and Scheme
). A mechanism is proposed in which the 1,2-biradical-like styryl
triplet, 9, adds oxidatively to
three-coordinate
phosphorus to generate a cyclic, triplet, phosphoranyl
1,3-biradical, 10, that undergoes rapid
subsequent β scission
to generate product 2−phenylallylphosphonate, 11.
Phosphonate 1a is formed near-quantitatively from
dimethyl
2-phenylallyl phosphite, 1. Phosphites
2−4, with phosphorus contained in a five-,
six-, or seven-membered ring,
respectively, photorearrange much more slowly, although yields of
phosphonates 3a and 4a of 50−70% are
generated.
Quantum yields for the formation of phosphonates 1a,
3a, and 4a, on sensitization by triplet
triphenylene, were
determined to be 0.25, 0.003, and 0.005, respectively, in benzene.
Similar values were found in acetonitrile and
benzene using benzophenone as triplet sensitizer. Rate constants,
kq, for efficient quenching of the triplets
of
benzophenone (phosphorescence quenching) and triphenylene (photothermal
lense measurements) were all in the
range 4.0−5.4 × 109 M-1
s-1. The low quantum yields for
phosphonate formation from 3 and 4 (and
presumably
2), therefore, result from the relative inefficiencies of
reactions of their 2-phenylallyl triplets toward
phosphorus.
This is ascribed to the large reduction in the rate of
isomerization of the kinetically formed initial 1,3-biradical,
23,
to the thermodynamically more stable species, 24, an effect
found in spiro phosphoranyl monoradicals. A reduction
in the rate of β scission of biradical 23, caused by its
spiro structure, also may play a role. The failure of
the
3-phenylallyl phosphite 7 to undergo the same cyclic
photorearrangement as 1 is ascribed to the inability of
the
benzyl radical-like terminus of triplet 7 to react with
phosphorus. Phosphite homolog 8 also is inert toward
cyclic
photorearrangement via a six-membered ring 1,3-biradical
(28) analogous to 10. The reactivity
patterns of phosphites
1-4, 7, and 8 can be
rationalized in terms of the 1,2-biradical nature of the triplet styryl
moiety and factors known
to govern the formation and permutational properties of phosphoranyl
monoradicals.