Photo-Arbuzov Rearrangements of 1-Arylethyl Phosphites:  Stereochemical Studies and the Question of Radical-Pair Intermediates

Abstract

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.

New Insights into the Chemistry of gem‐Bis(phosphonates): Unexpected Rearrangement of Michael‐Type Acceptors

“…The stable radical TEMPO is often used to scavenge free radical pairs that escape the initial solvent cage and appeared applicable to the study of this reaction. [38,39] When this reaction was accordingly conducted in the presence of TEMPO, a significant impairment of formation of 8 was observed, CG-MS analysis showing the presence of TEMPO, TEMPO-H, and a small amount of compound 8. In addition, no trapping products formed from radical species and TEMPO were detected, but some other random free radical coupling products -such as tetraethyl ethane-1,1-diylbis(phosphonate), tetraethyl 2-hydroxyethane-1,1-diylbis(phosphonate), [(EtO) 2 P(O)] 2 , etc.…”

“…A lack of ability of TEMPO for radical trapping, in spite of producing an inhibitory effect on the course of the reaction, has already been described. [38][39][40] These findings may be due either to steric hindrance of the TEMPO radical [38] or to thermal instability [39,40] of the coupling products. The addition of TEMPO or quinhydrone retards the formation of compound 8, which suggests that a radical mechanism was involved in this reaction.…”

Photochemical and Photophysical Studies of and in Bulk Polymers

“…Although the rates of the two processes can be mediated by polarity and viscosity of the solvent 151,152 or by changing the spin multiplicity of the radical pairs, 153 stereochemistry is not retained in the photoproducts in many coupling reactions of prochiral radical pairs in fl uid media. 153 However, cavities in constrained polymeric media have been shown to provide substantial enantioselectivity in some radical pair reactions. Signifi cant enantioselectivity was found during the recombination of decarbonylated radical pairs from the irradiation of 1 -naphthyl ( R ) -2 -phenylpropanoate (( R) -1NCc ) in PE fi lms (Scheme 11.19 ).…”

Radical Reactions with Double Memory of Chirality (2MOC) for the Enantiospecific Synthesis of Adjacent Stereogenic Quaternary Centers in Solution: Cleavage and Bonding Faster than Radical Rotation

“…Knowing that the time constants for rotation of medium size molecules in fluid solvents (e.g., benzene) fall in the 5–20 ps range, 14 one may expect that competent 2 MOC photochemical reactions will occur from the singlet excited state with reactants that have essentially no barrier for each of the two bond-cleavage steps, and which also have a spin-allowed bond forming reaction. 15 While it would appear that finding reactants capable of satisfying these conditions should be very unlikely, there are a few notable examples of radical pair reactions that occur with a significant MOC at one chiral center in solution, such as the photo Arbuzov reaction of optically enriched arylmethyl phosphites 16 and arylmethyl phosphordiamidites, 17 and the photodecarboxylation of optically enriched esters from (+)- or (−)-2-methylbutyric acid and 2,4,6-trimethylphenol. 18 In this paper, we are pleased to report our results on bis -( N -methyl-phenylpyrrolidinonyl)-ketones ( R,S )- 1b , ( RR )- 1b , and ( S,S )- 1b (Scheme 2), which undergo a photodecarbonylation reaction with double memory of chirality through radical intermediates formed at the two α-carbons.…”