Nucleophilic attack 5- coordinate intermediate structure

In contrast, spectroscopic and crystal structure analysis indicates that nucleophilic attack of hydride on 72 occurs on the face of the ligand which is coordinated to the metal (Scheme 17). No intermediate species could be detected for this latter reaction. Monitoring of the reduction of the rhenium analog 74 with sodium borohydride indicated the intermediacy of a rhenium formyl complex 75, presumably formed by attack on a coordinated carbon monoxide. Signals for 75 eventually disappear and are replaced by those of the (diene)rhenium product 76 (Scheme 18)95. 

The Michaelis-Arbuzov reaction is the most used and well-known method for the synthesis of phosphonates and their derivatives and may also be used to synthesize phosphinates and tertiary phosphine oxides. The simplest form of the Michaelis-Arbuzov reaction is the reaction of a trialkyl phosphite, 3, with an alkyl halide, 4, to yield a dialkyl alkylphosphonate, 6, and new alkyl halide, 7 (Scheme 2). During this transformation the phosphorus atom of a ter-valent phosphorus(III) species (3) acts as a nucleophile resulting in the formation of an intermediate alkoxy phosphonium salt 5, containing a new [P—C] bond. The precise structure of the intermediates 5 is a subject of debate—as reflected by common reference to them as pseudophosphonium salts —with a penta-coordinate species (containing a [P—X] bond) being proposed and detected in some cases.18 Decomposition (usually rapid under the reaction conditions) of the intermediate 5 by nucleophilic attack of X- on one of the alkyl groups R1, with concomitant formation of a [1 =0] bond yields the product pentavalent phosphorus(V) compound (6) and the new alkyl halide, 7. 

The situation is completely different in the presence of 150 mol % LiNTf2- Indeed, whether isomer 110 or 111 is used, the formation of a anomer 110 predominates (a /i = >99 <1). It has been proposed that the intermediate oxocarbenium ion 1-3 is involved in the anomerization mechanism. In the presence of the trityl cation, the equilibrium between anomers 110 and 111 might shift to the more thermodynamically stable anomer 111. The significant role of perchlorate and bis(sulfonimide) anions has been consistently claimed. Thus, intermediate 1-3 would be efficiently stabilized by these anions located at the sterically less-encumbered / side of anomeric centers, achieving nucleophilic attack from the a side. It is also suggested that the Li cation is likely to effect the selective stabilization of product a anomer 110 by forming coordination complexes as exemplified by putative chelated structures C-1 and C-2 (Fig. 10). 

The addition-elimination reaction proceeds via addition of a nucleophile at phosphorus to generate a five-coordinate intermediate or activated complex. TTie phosphorus atom is readily able to use its d orbitals to expand its coordination number and it is argued that an attacking nucleophile such as OH" adds to the four-coordinate phosphorus to form a pentacoordinate phosphorane (143). It appears that generally the pentacoordinate intermediate will have a trigonal bipyramidal structure, although spirocyclic phosphoranes seem to favor square pyramidal structures (80). 

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