Effect of amino substituents on the stereochemical outcome of the photo-Arbuzov rearrangements of 1-arylethyl phosphorodiamidites.

The essentially stereochemically pure 1-arylethyl phosphorodiamidites 8 and 9 were irradiated by UV light in acetonitrile, benzene, and cyclohexane (Tables 1-4). Reaction via singlet free-radical pairs, formed by carbon-oxygen bond scission (Scheme 1), which are somewhat longer lived than those from the analogous phosphites 5 and 6, is proposed. Tetramethyl 1-phenylethylphosphorodiamidite (8) gives the photo-Arbuzov rearrangement product 10 in 59% +/- 2% GC yield, based on percent 8 consumed (Tables 1 and 4), along with the free radical dimerization product 2,3-diphenylbutane, 12a, in in amounts corresponding to ca. 19% of the potentially formed 1-phenylethyl radicals. Similarly, from 9, the photorearrangement product 11 is generated in 64 +/- 4% yield (Tables 2 and 4) along with a 18 +/- 2% accountability of the 1-naphthylethyl radicals as 12b. The photorearrangement of stereochemically enriched 8 (R/S = 99:1) gives 10 in which an apparent 67 +/- 2% (100y, eq 3, Table 4) of the initial radical pairs [3,14] recombine with retention of configuration at the stereogenic carbon (34 +/- 3% net retention, eq 5). With TEMPO present, 70% (100y, eq 3) of the initial 1-phenylethyl radicals, 14, from 8 combine with radicals 3 in the solvent cage with retained configuration at carbon (40% percent net retention, eq 5). The yield of product 10 is reduced to 54%, and 12a is absent. Similarly, the five-membered ring naphthylethyl analogue, phosphorodiamidite 9 (R/S = 98:2), affords largely (R)-11 with apparent 34 +/- 3% net retention. The degree of stereorandomization observed in these systems is higher than was reported previously for phosphites 5 and 6. The neglect of reconversion of pro-S 14 to pro-R 14 on the results of these studies is addressed. Estimated maximum values (eq 4) of kcomb/krot (2.3) for the proximate radical pairs [3,14] from 8 with TEMPO present appear to be at least 6-fold smaller than those of the analogous phosphite (R)-5 (average kcomb/krot = 13 with TEMPO present). Possible origins for this effect are proposed.

A facile route to vinyl- and arylphosphonates by vinyl and aryl radical trapping with (MeO)3P.

The generation of vinyl or aryl radicals under classical, thermal AIBN/n-Bu(3)SnH conditions at 80 degrees C in the presence of an excess of (MeO)(3)P gives rise to the corresponding vinyl- or arylphosphonates in good yields. This approach complements the photochemical reactions of the same systems previously used. Reactions with the individual stereoisomers of MeCH=CHMeBr (thermal AIBN/n-Bu(3)SnH conditions) afford a radical-equilibrated 96/4 E/Z ratio of vinylphosphonates. Substitution of (TMS)(3)SiH for n-Bu(3)SnH yields an approximately 1/1 ratio of separable E and Z vinylphosphonate diastereomers.

Trimethyl phosphite as a trap for alkoxy radicals formed from the ring opening of oxiranylcarbinyl radicals. Conversion to alkenes. Mechanistic applications to the study of C-C versus C-O ring cleavage.

Trimethyl phosphite, (MeO)(3)P, is introduced as an efficient and selective trap in oxiranylcarbinyl radical (2) systems, formed from haloepoxides 8-13 under thermal AIBN/n-Bu(3)SnH conditions at about 80 degrees C. Initially, the transformations of 8-13, in the absence of phosphite, to allyl alcohol 7 and/or vinyl ether 5 were measured quantitatively (Table 1). Structural variations in the intermediate oxiranylcarbinyl (2), allyloxy (3), and vinyloxycarbinyl (4) radicals involve influences of the thermodynamics and kinetics of the C-O (2 --> 3, k(1)) and C-C (2 --> 4, k(2)) radical scission processes and readily account for the changes in the amounts of product vinyl ether (5) and allyl alcohol (7) formed. Added (MeO)(3)P is inert to vinyloxycarbinyl radical 4 and selectively and rapidly traps allyloxy radical 3, diverting it to trimethyl phosphate and allyl radical 6. Allyl radicals (6) dimerize or are trapped by n-Bu(3)SnH to give alkenes, formed from haloepoxides 8, 9, and 13 in 69-95% yields. Intermediate vinyloxycarbinyl radicals (4), in the presence or absence of (MeO)(3)P, are trapped by n-Bu(3)SnH to give vinyl ethers (5). The concentrations of (MeO)(3)P and n-Bu(3)SnH were varied independently, and the amounts of phosphate, vinyl ether (5), and/or alkene from haloepoxides 10, 11, and 13 were carefully monitored. The results reflect readily understood influences of changes in the structures of radicals 2-4, particularly as they influence the C-O (k(1)) and C-C (k(2)) cleavages of intermediate oxiranylcarbinyl radical 2 and their reverse (k(-1), k(-2)). Diversion by (MeO)(3)P of allyloxy radicals (3) from haloepoxides 11 and 12 fulfills a prior prediction that under conditions closer to kinetic control, products of C-O scission, not just those of C-C scission, may result. Thus, for oxiranylcarbinyl radicals from haloepoxides 11, 12, and 13, C-O scission (k(1), 2 --> 3) competes readily with C-C cleavage (k(2), 2 --> 4), even though C-C scission is favored thermodynamically.