Concerted reductive elimination

A comprehensive experimental and theoretical study was undertaken on the reaction, which was shown to be a concerted reductive elimination process kinetic studies were consistent with reductive elimination, and DFT calculations on complex 3 (Fig. 13.1) supported an associative reductive elimination pathway with a small... 

Anti stereospecificity is associated with a concerted reductive elimination, whereas single-electron transfer fragmentation leads to loss of stereospecificity and formation of the more stable A-stereoisomer. 

Some years ago, Dr. Cross and I put forward a description of concerted reductive elimination (and, by implication, concerted oxidative addition) processes at transition metal centres, assuming the conservation of orbital symmetry, within a single dominant configuration, for the most obvious reaction path This picture had unexpected implications which some recent work has rendered quite explicit, and which are discussed in Part II of this article. 

Symmetry requirements for concerted reductive elimination of dialkyls have been considered (314), and for trialkylgold(III) species reductive elimination from a trigonal, three-coordinate intermediate was found to be symmetry forbidden. Solvent participation or the involvement of T-shaped species, however, was suggested as possible. Charge transfer to the high-oxidation state gold(III) center and reductive elimination from such a charge transfer state was proposed as an alternative reaction pathway. 

Disadvantage Deactivation of catalyst due to concertive reductive elimination of the NHC ligand and the catalytically active alkyl or aryl group. 

The theoretical studies confirm intermediacy of tantalacy-cloheptane in the trimerization process. The conversion of tantalacycloheptane to the Ta-(l-hexene) complex follows. The dimerization is disfavored by the nonexistence of such concerted reductive elimination process for the tantalacy-clopentane. The bimolecular process of tetramerization and possible higher oligomerization cannot compete with the facile reductive elimination (see Reductive Elimination) ... 

For aromatic ether formation, electron-withdrawing groups on the arene accelerate the rate of reductive elimination. Fm-ther, the more basic alkoxide groups form ethers faster than phenoxides. These facts have led to the proposal that, in addition to the usual concerted reductive elimination mechanism, some substrates can undergo reductive elimination via a Meisenheimer type intermediate (equation 31). 

To explain this degradation, two different reaction pathways have been proposed (Scheme 29). The reaction may proceed by concerted reductive elimination or alternatively by migratory insertion, giving the intermediate (175) and subsequent heterolytic cleavage. Kinetic studies and DFT calculations supported the second proposed pathway. An... 

Intramolecular coupling reactions of group 4 imino-acyl complexes yielding the corresponding free imines are rather rare and mainly involve titanium complexes [35 — 38]. The mechanism of these reactions still remains unclear although a concerted reductive elimination step, that initially leads to an -irnine intermediate complex of the type II, has already been postulated [35] (Fig. 4). 

Loss of an L ligand (usually a phosphine) to give a 14-e complex (T-or Y-shaped), followed by concerted reductive elimination to form R-R (see preceding paragraph) ... 

By one pathway, reductive elimination of R and the electrophile gives R-E with retention of stereochemistry at the metal-bound carbon (recall from Chapter 8 that concerted reductive elimination occurs with retention of configuration at this site). For example, the f/ireo-3,3-dimethylbutyl-l,2-(ij iron complex shown in Equation 12.16 reacts with mercuric chloride to form f/zreo-3,3-dimethylbutyl-l, 2-mercuric chloride. Similar results have been obtained for the reactions of t/u eo-PhCHDCHDFe(CO)2Cp and tra)is-(tlzreo-PhCHDCHD)... 

Figure 5 - Change in total energy and overlap population for concerted reductive elimination of ethane and propene.

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