Origin of the ".beta.-Oxygen Effect" in the Barton Deoxygenation Reaction

Abstract

Photolysis of O-neopentyl 5-tributylstannyl dithiocarbonate with hexaphenyl distannane, and 4-methoxyacetophenone as sensitizer, results in crossover of the stannyl groups. The reaction of O-octyl 0'-(2-butoxyethy!) thiocarbonate with tributyltin deuteride or tris(trimetbylsilyl)siiane and a radical initiator shows no significant preference for the cleavage of either C-O bond. Intermolecular competitions between O-octyl O'-phenyl thiocarbonate and 0-(2-butoxyethyl) O'-phenyl thiocarbonate for a deficiency of stannane or silane also indicated no significant preference for reaction of the /3-oxygen-substituted substrate, leading to the conclusion that in conformationally unrestricted systems there is no significant /3-oxygen effect in the Barton deoxygenation reaction. Competition experiments between the cisand frans-0-(4-phenylcyclohexyl) 5-methyl dithiocarbonates and the cisand ;raws-0-(2-phenyli,3-dioxan-5-yl) 5-methyl dithiocarbonates for reaction with tributylstannane reveal that in every case the heterocyclic system is more reactive. The di-isomers of 4-phenylcydohexyl 5-methyl dithiocarbonate and 0-(2-phenyl-1,3dioxan-5-yl) 5-methyl dithiocarbonate, with their axial xanthates, are more reactive than the corresponding transisomers. Molecular mechanics calculations suggest that the greater reactivity of the di-series with respect to the trans is due to the greater relief of strain on fragmentation,

Chemistry of β-(Acyloxy)alkyl and β-(Phosphatoxy)alkyl Radicals and Related Species: Radical and Radical Ionic Migrations and Fragmentations of Carbon−Oxygen Bonds

“…It is of some interest to note the high regioselectivity in the addition of HPO 4 2-to 263, 290, and 291 and to contrast it with the shifts of acyloxy groups away from the anomeric center in nucleosides in benzene solution as described by Tanaka and Chatgilialoglu (Table 1, entries [29][30][31][32][33][34][36][37][38][39]. Clearly, the regioisomeric equilibrium of such radicals is very finely balanced and significantly altered by relatively small structural differences and solvent effects.…”

“…It is well appreciated [35][36][37] that in the case of thiocarbonyl derivatives, sulfur is readily and reversibly 38,39 attacked by stannyl, thiyl, and certain silyl 40 radicals. Indeed, this process forms the basis for the Barton deoxygenation and decarboxylation reactions.…”

Erratum

“…A similar oxygen-carbon P-bond effect has already been noticed. [34] To summarize, we described a novel route for the preparation of 1-substituted vinylphenylphosphinates and demonstrated their usefulness in the synthesis of a variety of 1 -functionalised-2-aminoethylphenylphosphinic acids. …”

Utilization of α‐Oxygenated Radicals in Synthesis

“…Moreover, 2-tetrahydropyranyl radicals prefer to be axial in order to maximize overlap with the lone pair of the ring ether oxygen; ab initio calculations predict the axial radical to be >2 kcal/mol more stable than the equatorial radical [2]. In terms of molecular orbital theory, the stabilization between the axial oxygen lone pair and the singly occupied molecular p-orbital (SOMO) of an axial carbon-336 4.3 Utilization of a-Oxygenated Radicals in Synthesis have suggested this effect to be significant [ 5 ] , although firm quantitative evidence for the existence of a 'P-oxygen effect ' has not yet been forthcoming [6].…”