Four-centre mechanism

Co (Con) exists in equilibrium with the hydrido form, (HCo(Con) + (pXa x 1.0), which gradually decomposes to Co (Con) and H2. The hydrido complexes are more reactive nucleophiles than are Co1 (Con), and their addition to unsaturated bonds gives cis products probably via an unsym-metric four-centre mechanism (Scheme 100, iii, iv). The Co—C bond is also formed between Con(Con) + and alkyl radicals (Scheme 100, v), and Com(Con) 2+ and carbanions (Scheme 100, vi). 

Jensen and Rickborn60 have severely criticised the work of Dessy on the acidolysis of dialkylmercurys by hydrogen chloride. They state that, in their view, no firm mechanistic conclusions can be drawn from the published results , although a four-centred mechanism at present seems to be reasonable. 

The same alcohol could be made by the Baeyer-Villiger rearrangement but the stereochemistry would have to be set up before the Baeyer-Villiger step. Hydroboration has the advantage that stereochemistry is created in the hydroboration step. We have discussed the details of this step. In drawing the mechanism it is usually best to draw it as a simple concerted four-centre mechanism providing you remember that the regioselectivity is controlled by the initial interaction between the nucleophilic end of the alkene and the empty p orbital on boron. 

According to the rules for orbital symmetry conservation Ihe four-centre mechanism in Scheme I would appear to be forbidden, but very few attempts have been made to describe the orbital transformations in the reactions of Grignard reagents. Much evidence has been presented for the operation of cyclic 6-ccntrc concerted reaction mechanisms (Scheme 1.9), although it has been questioned whether they are in accordance with orbital symmetry rules 5. ... 

Tetrahedral phosphorus in a five-membered ring is highly strained. To relieve this strain, such compounds tend to expand the coordination of the phosphorus to five. It was reasoned that if indeed the thermal fragmentation should proceed by the four-centred mechanism outlined above, it would proceed much more readily in a five-membered cyclic a-hydroxyiminophosphonate. To test this, a five-membered cyclic benzoylphosphonate was reacted with hydroxylamine . This reaction yielded only the (E )-oxime along with a considerable amount of benzonitrile (equation 123). This result is consistent with the... 

Since no evidence for an initial cfj-isomer could be obtained, a transoid free-radical addition was preferred to a four-centre mechanism. No fluoro-trimethylsilane was formed in this reaction, which precludes the occurrence of the addition-elimination sequence proposed for reactions of the mercurial with fluoro-olefins. Perfluoropropyne behaves differently and its reaction with the mercurial yields mainly the substitution product 3,3,3-trifluoro-l-trimethylsilylpropyne (86%), an almost equal amount of fluorotrimethyl-sitane, and only 5% of a disilylpropene, CFa C SiMe3) CF SiMcs. The latter product was also obtained by treating hexafiuoropropene with the mercurial, but its geometry could not be firmly established and further developments are awaited to resolve the interesting mechanistic possibilities. 

Cycloalkenes, such as cyclooctene, afford linear polymers on metathesis. This is in accord with the carbene mechanism, but not with the four-centre mechanism which predicts the formation of large rings (Fig. 12.11). 

Fig. 12.11 Metathesis of cycloocetene to give a linear polymer (alkylidene mechanism) rather than large rings (four-centre mechanism).

It has been claimed that the previously proposed four-centre mechanism plays no part in the reduction of ketones with borohydride whether this mechanism is involved in the photochemically accelerated borohydride reduction of ketones has yet to be clarified. ... 

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