Reduction free-radical

Tin-free radical reduction by an organophosphite [17] and phosphinic acid can also be initiated by Et3B/02. Radical cyclizations using phosphinic acid neutralized with sodium carbonate and Et3B/02 as a radical initiator... 

Free radical allylation free radical reduction (homogeneous)... 

The lithiophosphonate adds to carboxylic esters to afford ketophosphonates, which were trapped with Grignard reagents in situ [76]. Free radical reduction of the tertiary alcohol adducts afforded the products of formal secondary alkylation (Eq. 20) [76]. 

Guengerich, F.P. (1994) Metabolism and genotoxicity of dihaloalkanes. Adv. Pharmacol., 27,211-236 Guha, S.N., Schoneich, C. Asmus, K.D. (1993) Free radical reductive degradation of vic-dibromoalkanes and reaction of bromine atoms with polyunsaturated fatty acids possible involvement of Br(.) in the 1,2-dibromoethane-induced lipid peroxidation. Arch. Biochem. 

The demercuration of (3-alkoxymercurials is usually best effected using alkaline sodium borohydride. Few rearrangements during this free radical reduction process have been observed 429 When the mercury moiety is positioned a to a carbonyl group, alkaline H2S430,431 or 1,3-propanedithiol316 provide alternatives that afford complementary stereochemical results (equation 262). The use of sodium-mercury amalgam is also useful for stereospecific reduction.432 433... 

A high stereoselectivity was found for the free-radical reduction of glycopy-ranosyl halides43,44 and related 1-C-nitro-sugars45 with tri-n-butyltin hydride. 

Asymmetric Free-Radical Reductions Mediated by Chiral Stannanes, Germanes, and Silanes... 

As a result of the limited configurational stability of optically active organotin compounds, in which the chirality is on the tin atom, most advances in enantioselective free-radical reductions involve organostannanes where the elements of chirality are contained in the organic substituents. Selected... 

FIGURE 27.3 Examples of menthyltin hydrides evaluated by Chirogen for enantioselective free-radical reductions. 

Free-radical reductions mediated by chiral stannanes, germanes, and silanes may occur with enantioselectivities in excess of 99% ee. Owing to the involvement of radical intermediates and the mild reaction conditions, this process is applicable for a large variety of simple or even complex target molecules that are incompatible to asymmetric reductions that require ionic reaction conditions. 

Dumartin and associates described the preparation of in situ polymer-supported organ-otin hydrides for use as clean reducing agents (equation 15)47, while Deleuze and coworkers reported the preparation of a novel, macroporous polymer-supported organotin hydride (37), for use in catalytic free-radical reductions (equation 16)48,49. 

During the time frame covered by this chapter, stannanes have been reported to have been involved in radical chemistry not involving the more traditional functionalities. For example, Marzi and coworkers reported that the reduction of cyclic thionocarbonates (e.g. 82) with BusSnH under standard radical conditions affords cyclic acetals that can then be further transformed into 1,2-diols (equation 57)219. This transformation represents a new approach to the protection of these diols. Zehl and Cech described the use of Bu3SnH in the reduction of azide (83) to the corresponding amine (equation 58)305, while Hanessian and his associates reported the Ph3SnH-mediated free-radical reduction of the tertiary oxalate (84) (equation 59)309. This transformation represents a departure from the more typical reduction of a pyridinethioneoxycarbonyl (PTOC) oxalate ester321. 

Lastly, Gualtieri reported the use of binaphthyl-substituted germanes (8, 9) in enan-tioselective radical chemistry26. For example, an enantioselectivity of 59% was reported for the reaction of 126 with 8 at —60° (equation 127). To the best of our knowledge, this represents the first account of the use of a chiral germanium hydride in free-radical reduction chemistry. 

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