The Oxiranyl Radical

As was the case with ll g, the rates of inversion of the oxiranyl radicals, 12J, and 12J, could be measured by EPR line broadening over a wide range of temperatures 

However, the pre-exponential factor in this equation is roughly 2 orders of magnitude smaller than would be expected for a classical (over the barrier) inversion (compare with Eq. (28.11)). It is therefore highly probable that QMT is also important in the D-atom inversion over the temperature range covered. 

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Rearrangement of the oxiranyl radical (274) might lead to CH2CHO which would explain the formation of propionaldehyde (268). 

Barton ester system for the generation of specific radicals. In one example, the oxiranyl radicals (116) have been formed by the uranyl-glass-filtered irradiation of the substrate (117) and cyclize to yield (118) as the final product. ... 

The stereochemical outcome for addition of r-l,3-dioxolan-4-yl and oxiranyl radicals to phenyl vinyl sulfone has been probed. The results indicated that the symanti selectivity could be altered by changing the group next to the radical in the diox-olanyl case but not in the oxiranyl case (bulky groups had a large xyn-directing effect) (Scheme 39). Several alkenyl-lactones and -lactams have been subjected to hydrosilylation conditions using carbohydrate-derived thiols as homochiral polarity reversal catalysts (yields 25-96% ee 5-95%) " ... 

Trisubstituted carbon-centred radicals chemically appear planar as depicted in the TT-type structure 1. However, spectroscopic studies have shown that planarity holds only for methyl, which has a very shallow well for inversion with a planar energy minimum, and for delocalized radical centres like allyl or benzyl. Ethyl, isopropyl, tert-butyl and all the like have double minima for inversion but the barrier is only about 300-500 cal, so that inversion is very fast even at low temperatures. Moreover, carbon-centred radicals with electronegative substituents like alkoxyl or fluorine reinforce the non-planarity, the effect being accumulative for multi-substitutions. This is ascribed to no bonds between n electrons on the heteroatom and the bond to another substituent. The degree of bending is also increased by ring strain like in cyclopropyl and oxiranyl radicals, whereas the disubstituted carbon-centred species like vinyl or acyl are bent a radicals [21]. 

The resulting oxiranyl radicals isomerize exothermically (AH 120 kJ mol ) to a variety of products. 

Alkenyl epoxides have been analogously used in the synthesis of bicyclic compounds [69]. Addition of a thiyl radical to a suitably protected epoxyketone enolate generated the oxiranyl carbinyl radical which initiated a cascade of uni-molecular steps (i.e. ring-opening, 1,5-radical translocation, 5-exo cyclization, and ejection of the PhS radical) to produce the final enol product in good yield and high diastereoselectivity (equation (31)). The PhS radical-induced fragmentation of alkenyl epoxy silanes has been found to afford a-trimethylsilyl aldehydes [70]. 

About 10 years ago, Hasegawa discovered that oxiranyl methyl radicals such as 54 that possess an electron-donating tributylstannyloxy substituent at the radical center undergo selective C 0 bond cleavage to give the oxy radicals 55 in the free-radical reaction of epoxy ketones 52 with tributyltinhydride, which finally produce (3-hydroxy ketones 53 in high yields (Scheme 14). ... 

Brown and Suzuki have shown that treatment of trialkylboranes with ethenyl-(Scheme 42, Eq. 42a) and ethynyloxiranes (Scheme 42, Eq. 42b) in the presence of a catalytic amount of oxygen, affords the corresponding allylic or allenic alcohols. The mechanism may involve the addition of alkyl radicals to the unsaturated system leading to l-(oxiranyl)alkyl and l-(oxiranyl)alkenyl radicals followed by rapid fragmentation to give alkoxyl radicals that finally complete the chain process by reacting with the trialkylborane [104-106]. 

In another early example, C radicals, which were generated from the reaction of trialkylboranes with dioxygen, O2, were added to a-epoxy alkynes 6 to give allenes 7 (Scheme 2.2). In this reaction, the initially formed vinyl radicals (or a-oxiranyl... 

Fused indoles have been synthesized using various radical strategies. Some of the more renowned works include Ziegler s Photochemically-induced cyclization of aUcyl, vinyl, oxiranyl, and aziridinyl radicals [87-89], Chuang s benzenesulfonyl radical induced oxidative cyclization of l-(4-allylsulfonylbutyl)indole [90], and Moody s tin-mediated cyclization of l-co-iodoalkylindoles [91]. 

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