Abstract:The stereochemistry of the photo-Arbuzov rearrangement of the benzylic phosphite trans-(R,R‘)-10 to the corresponding phosphonate, 11, has been determined by 31P NMR spectroscopy and X-ray
crystallography. The reaction is shown to occur with predominant retention of configuration at
the stereogenic migratory carbon center of configuration R‘ in starting trans-(R,R‘)-10 and the
predominant product cis-(R,R‘)-11. Thus, reaction of optically active phosphoramidite 13 (R/S
ratio 98/2, 96% ee) with 1-phenylethanol … Show more
“…Further evidence for the formation of radical pairs in the direct photolysis of 1 and related phosphites comes from research from this laboratory on 3 and the analogue of phosphite 3 with a methyl substituent at the ring carbon next to oxygen . The photo-Arbuzov rearrangement of essentially a single enantiomer of the diastereomer with ring methyl and 1-phenylethoxy ring substituents oriented in cis fashion was carried out.…”
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.
“…Further evidence for the formation of radical pairs in the direct photolysis of 1 and related phosphites comes from research from this laboratory on 3 and the analogue of phosphite 3 with a methyl substituent at the ring carbon next to oxygen . The photo-Arbuzov rearrangement of essentially a single enantiomer of the diastereomer with ring methyl and 1-phenylethoxy ring substituents oriented in cis fashion was carried out.…”
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.
“…A classic example is the radical pair generated by the thermolysis of Ph 2 CCN−CHPhMe that combines to form Ph 2 C(CN)CHPhMe . Singlet proximate pairs are expected to undergo a high degree of recombination within the solvent cage as shown for 1 (Ar = Ph, , 1-naphthyl , ) with predominant retention of configuration if a stereogenic migratory carbon center is involved ( 5 → 6 , direct irradiation, eq 1) …”
The direct UV irradiation of the 1-arylethyl phosphites 7, 8, and 9 was carried out in acetonitrile, benzene, and cyclohexane, as was the triphenylene-sensitized reaction of 9. Dimethyl 1-phenylethyl phosphite, 7, gives the photo-Arbuzov rearrangement product, dimethyl 1-phenylethylphosphonate (10), in 67% average yield and minor amounts (2%) of 2,3-diphenylbutane (11a) in quantum yields of 0.32 and 0.02, respectively. The photorearrangement of optically active, predominantly (R)-1-phenylethyl phosphite 7 (R/S = 97/3; 94% ee), at 35-40 degrees C proceeds with a high degree of stereospecificity at the stereogenic migratory carbon to give predominantly (R)-10 (R/S = 86/14, 72 +/- 2% ee). Use of the nitroxide radical trap TEMPO affords phosphonate 10, presumably all cage product, from predominantly (R)-7 (R/S = 97/3; 94% ee) in 64% yield (80% ee, R/S = 90/10). By contrast, the 1-(4-acetylphenyl)-ethyl phosphite, predominantly (S)-8 (S/R = 98/2, 96% ee), on direct irradiation gives the corresponding phosphonate (12) in only 20% yield along with dimer 11b in 40% accountability yield. Phosphonate 12 is nearly racemic (R/S = 52/48). Direct irradiation of predominantly (R)-9 (R/S = 98/2, 96% ee), a 1-(1-naphthyl)ethyl phosphite, results in a product distribution similar to that from predominantly (R)-7, but with a somewhat higher degree of retention of configuration in the product phosphonate 13 (R/S = 93/7, 86 +/- 3 ee). By contrast, the triplet triphenylene-sensitized photorearrangement of largely (R)-9 (R/S = 98/2, 96% ee) leads to product distributions similar to those from direct irradiation of predominantly (S)-8 and is accompanied by almost total loss of stereochemistry in its product phosphonate, 13 (R/S = 51/49). The partial loss of stereochemistry on direct irradiation of 7 and 9 provides evidence for radical pair formation. Furthermore, these stereochemical results are diagnostic of the multiplicity of the initial radical pair formed. Values for kcomb/krot for the proximate free radical pairs from 7 and 9, derived experimentally, are severalfold larger than those for the proximate singlet pair from Ph2C=C=N-CHPhMe, corrected to 35 degrees C. The possibility that kcomb is increased for the pairs from 7 and 9 is proposed.
“…For phosphonate 9 , the tert -butyl and phosphoryl oxygen (PO) are the priority groups. This convention for phosphonates is opposite to what we have used in other studies that involved closely related cyclic phosphonates, , but it follows the usual priority rules for the substituents on rings: O > C on phosphorus and C > H on carbon; i.e., the cis diastereomer of 9 has the tert -butyl and PO cis to one another.…”
The stereochemistry at phosphorus of the SET-induced photorearrangement of diastereomeric 4-tert-butyl-2-phenylallyl-1,3,2-dioxaphosphorinanes (8) to the corresponding 2-phenylallylphosphonates (9), which involves exicted singlet 1,4-dicyanonaphthalene (1DCN*) as one-electron oxidant, was investigated. The rearrangement occurs with close to complete retention of configuration at phosphorus. The previously postulated mechanism for this photorearrangement is shown to be consistent with the stereochemical finding. Thus, one-electron reduction by DCN.- of the presumably stereospecifically formed distonic cyclic 1,3-cation radical intermediate 15, generated from cis-8 (Scheme 2), yields the thermodynamically stable diradical 16. beta scission of 16 forms phosphonate cis-9. An alternative mechanism involving beta scission of 15 to a styryl cation radical, prior to one-electron reduction to 15, is discounted on the basis of unpublished trapping studies using MeOH. The direct, kinetically controlled formation of diradical 16 rather than the thermodynamically less stable 21 with CH2 bonded apically to phosphorus is argued to be consistent with the essentially equal values of the quantum yield for phosphonate formation (phi P) on SET-induced rearrangement of the acyclic 2-phenylallyl phosphite 1 and phosphite 7 with phosphorus incorporated in a six-membered (1,3,2-dioxaphosphorinane) ring. This mechanism is contrasted to that for the previously reported triplet-sensitized photorearrangements of phosphites 1 and 7, which have greatly different phi P values. For these reactions, kinetic formation of the triplet analogue of 21, but without the tert-butyl substituent, requires a permutation of substituents for conversion to diradical 16 prior to intersystem crossing and beta scission to form the phosphonate corresponding to 7. The preparative-scale SET-induced photorearrangement of the thymidine-based 2-phenylallyl 3',5'-phosphite 10 gave both diastereomers of phosphonate 11 that were separated by HPLC. The 2-phenylallyl functionality provides an opportunity for further functionalization. As reported elsewhere, 11 was not formed in useful amounts via triplet-sensitized reaction of 10.
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