Radical stereoselectivity oxidative coupling

It is important to select stoichiometric co-reductants or co-oxidants for the reversible cycle of a catalyst. A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid. Taking these interactions into account, the requisite catalytic system can be attained through multi-component interactions. Stereoselectivity should also be controlled, from synthetic points of view. The stereoselective and/or stereospecific transformations depend on the intermediary structure. The potential interaction and structural control permit efficient and selective methods in synthetic radical reactions. This chapter describes the construction of the catalytic system for one-electron reduction reactions represented by the pinacol coupling reaction. 

A wide series of oxidants, spanning from TiCLj to iodine, has been used in the oxidative homocoupling of chiral 3-arylpropionic acid derivatives aimed at the preparation of lignans. The /f,/f-selectivity in the reactivity of 34 has been explained by a radical coupling mechanism (equation 20). The initially formed lithium (Z)-enolate may transform into the titanium enolate 35, which undergoes oxidation to the radical intermediate 36 via a single electron transfer process. The iyw-Z-type radicals 36 couple each other at the less hindered S-side si face) to give the R,/f-isomers 37 stereoselectively. 

Prochiral carbon radicals have enantiotopic faces reaction with chiral nitroxides can result in two possible diastereomeric products (Scheme 9). Our laboratory has been investigating the ability of chiral nitroxides to differentiate between the two enantiotopic faces of a transient prochiral carbon radical. In many of the examples, the prochiral radical is generated by the lead dioxide oxidation of a secondary benzylic hydrazine. Early work utilized a camphor-derived nitroxide 27, which was coupled to a secondary benzylic prochiral carbon radical with low but reproducible stereoselectivity (Scheme 10) [26]. The stereoselectivity jumped dramatically upon moving to a conformationally rigid nitroxide in the form of the steroid doxyl radical... 

Vinyl (or cyclopropyl) silyl ethers have also been used to generate carbon-centered radicals by treatment with CAN. 3,6-Dihydroxyphthalate esters are produced by dimerization when bisenolsilylated 1,3-diketones are treated with CAN. An elegant example is the three-component condensation of cyclopropyl silyl ether, cyclopentenone, and methyl vinyl ether (eq IS). Oxidation of cyclopropyl silyl ether gives the 8-ester radical, which undergoes tandem radical addition processes apparently controlled by electronic effects. Subsequent oxidation and trapping affords the 2,3-disubstituted cyclopen-tanone in an excellent stereoselectivity. Other substrates include tertiary aminocyclopropanes, iV-(silylmethyl)amides, and VUV-dialkylanilines. For example, CAN-mediated oxidation of IVUV-dialkylanilines in water affords the coupling products -tetraalkylbenzidines. ... 

The reaction between W =C(OMe)CH=CHMe (CO)5 and cyclohexene oxide proceeds by a radical coupling process in the presence of Cp2TiCl2 to form complex 108 with a high level of stereoselectivity at the new stereogenic centres. Other cyclic epoxides give similarly high levels of... 

Dianions derived from chiral imidazolines have been reported to undergo selective one-electron oxidation reactions followed by stereoselective radical coupling. ... 

Experimental data have provided evidence for the involvment of quantum mechanical tunnelling in the coupled cobalt-carbon bond homolysis-substrate radical generation reaction catalysed by methylmalonyl-CoA mutase. ortho-Metallated dirhodium(II) complexes have been used as catalysts in the dia-stereoselective cyclopropanation of menthyl-a-diazo-P-keto ester and C-H insertion of ot-diazo ester. The iridium complexes [Ir(Me)2(L)(r -C5Me5)] (L = PMes, PPhs) have been found to catalyse the dehydrogenative coupling of dimethylphenylsilane in the presence of one-electron oxidants to yield Me2PhSiSiPhMe2. ... 

For the synthesis of (-)-magellanine and related Lycopodium alkaloids, Mukai et al. used a Ueno-Stork cycliza-tion coupled with a Keck allylation to generate an advanced intermediate. Thus, bromoacetal derivative 97 was exposed to conventional radical conditions in the presence of allyltributyltin to give the tricyclic compound 98 (after an oxidation step) with the quaternary carbon center formed and an allyl group installed stereoselectively (Scheme 25.46). 

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