Group transfer reactions reduction

In volume 7 reactions of metallic salts, complexes and organometallic compounds are covered. Isomerisation and group transfer reactions of inert metal complexes and certain organometallics (not involving a change in oxidation state) are considered first, followed by oxidation-reduction processes (a) between different valency states of the same metallic element (b) between salts of different... 

A new radical chain group transfer reaction which does not involve tin reagents has been reported. The reaction proceeds by a photosensitized electron transfer reductive activation of PhSeSiR.3 using 1,5-dimethoxynaphthalene as the sensitizer [95ACIE2669]. In contrast to the tellurium transfer described above, the selenium transfer reaction gave higher diastereoselectivity (4 1 vs 2 1). 

PET (photosensitized electron transfer) reduction of PhSeSiRs and the subsequent radical chain group transfer reaction have been reported (Scheme 18). The radical anion [PhSeSiRa] generated by electron transfer from... 

A well-known example of group transfer reaction with p = 0, q = 2 is represented by the concerted reduction of alkenes and alkynes with the reactive intermediate diimide stereoselectively in the cis fashion (Scheme 6.2). The driving force of the reaction is the formation of the stable nitrogen molecule. 

The transfer of one or more groups from one molecule to another in a concerted process is known as group transfer reaction. In most of the cases hydrogen is transferred. Only a few reactions of this class are known. Among them, the most common are ene reactions and diimide reduction. 

Diimide reduction of alkenes and alk3mes are also group transfer reactions. Delivery of two hydrogen atoms to an alkyne or alkene takes place in a concerted process involving suprafacial delivery of two hydrogens in a TS. These reactions are per-icyclic in nature. 

Among other group transfer reactions, the reduction of 1,2-dimethylcyclohexene with 9,10-dihydronaphthalene is used frequently [4]. 

The Clemmensen reduction can be formulated to proceed by a sequence of one-electron and proton transfer reactions. It is a heterogenous reaction, taking place at the zinc surface. Initially an electron is transferred from zinc to the carbonyl group of ketone 1, leading to a radical species 3, which is presumed to react further to a zinc-carbenoid species 4 ... 

From a study of the decompositions of several rhodium(II) carboxylates, Kitchen and Bear [1111] conclude that in alkanoates (e.g. acetates) the a-carbon—H bond is weakest and that, on reaction, this proton is transferred to an oxygen atom of another carboxylate group. Reduction of the metal ion is followed by decomposition of the a-lactone to CO and an aldehyde which, in turn, can further reduce metal ions and also protonate two carboxyl groups. Thus reaction yields the metal and an acid as products. In aromatic carboxylates (e.g. benzoates), the bond between the carboxyl group and the aromatic ring is the weakest. The phenyl radical formed on rupture of this linkage is capable of proton abstraction from water so that no acid product is given and the solid product is an oxide. 

Our group has also reported that the alkylation products of 4-cyano-l,3-diox-anes can serve as substrates for radical atom transfer reactions [41]. One such example is shown below (Eq. 17). Slow addition of tributyltin hydride/AIBN to a refluxing solution of cyanohydrin 115 generated the radical nitrile transfer product 116. This method, though somewhat limited in scope, can provide access to syn-l,3-diols which maybe unstable to the vigorous Li/NHg reduction conditions. 

In the case of the reduction of the complex Co(NH3)5l by Ru(NH3)6 the kinetic concentration of Ru(NH3)5l is higher than the estimated equilibrium concentration, and thus the reaction would appear to occur by direct group transfer. 

For polymerizations of butadiene in toluene at 50°C with the Ba-Li catalyst, we have observed a reduction in molecular weight and the incorporation of benzyl groups in chains of polybutadiene. We conclude from this result that proton abstraction from toluene occurs to give benzyl carbanions which are capable of forming new polymer molecules in a chain transfer reaction. 

A) denotes the main type of reaction (1) oxidation-reduction (2) functional group transfers (3) hydrolysis (4) lysis (5) isomerisation and (6) ligation (see Table 1) ... 

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