Organometallic compounds reaction pathways

The synthesis of compounds containing boron-group IIB bonds also occurs by two pathways. The most convenient method is using anionic borane or carboranc species, as in the group-IB case, to react with Zn, Cd, or Hg halides. Moreover, reactions between organometallic compounds (Zn, Cd) with unchanged boranes arc also realized, so Zn and Cd decaboranes, e.g., MB H 2 n solvent (M = Zn, Cd), result from reactions of decaborane(14) with MRj . 

Both overt carbanions and organometallic compounds, such as Grignard reagents, are powerful nucleophiles as we have seen in their addition reactions with C=0 (p. 221 et seq.) they tend therefore to promote an SN2 pathway in their displacement reactions. Particularly useful carbanions, in preparative terms, are those derived from CH2(C02Et)2, (3-ketoesters, l,3-( 3-)diketones, e.g. (55), a-cyanoesters, nitroalkanes, etc.—the so-called reactive methylenes ... 

The strong similarities between HDS and DHN results suggest that the type of catalytic sites involved in these two reactions are similar. This would mean that the routes for DHN of THT and HDS of thiophene include some identical reactional step(s), at least for the kinetically limitative one(s). Angelici and coworkers, in a detailed work [10] concerning the HDS of thiophene, THT, 2,3- and 2,5-dihydrothiophene showed that the dihydrothiophenes are much more reactive than the other molecules. On this basis and an additional study [11] on organometallic compounds, these authors have suggested that dihydrothiophenes are reaction intermediates and proposed a mechanistic pathway including this step. 

An aryl halide such as chlorobenzene is relatively unreactive towards nucleophilic substitution. The S l and Sj. 2 pathways involve mechanisms that are not open to aryl halides. The greater s character of the sp bond makes it more difficult to cleave the bond to generate a carbo-cation. However, these restrictions do not apply to radical or carbanion chemistry. Hence, aryl halides undergo radical coupling reactions and metal insertion reactions, leading to organometallic compounds. 

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