Sigma bonds metal insertion

Metal-Halogen Counpounds. One of the few examples of an olefin insertion into a metal-halogen compound has been reported by Tsuji. The reaction, which also supports the idea that sigma-bonded metal-carbon compounds are intermediates in the palladium chloride-olefin oxidation reaction, was the addition of carbon monoxide to the ethylene palladium chloride 7r-complex in nonaqueous solvents to produce a moderate yield of 3-chloropropionyl chloride (96). 

Equations 11, 12, and 13 constitute a process for the formation of the metal-carbon sigma bond. Carbonyl insertion may now occur as shown in Equation 14. 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

From this example one can judge that insertions of metal atoms into sigma bonds may be precluded by unfavorable spin or steric/promofion factors, despite the formal possibility of attractive interactions such as depicted in Fig. 4.62. Long-range s2 steric/promofion barriers may be expected to block direct insertion of metal atoms... 

In contrast to the severe difficulty of cracking a sigma bond, insertion of a transition metal into a pi bond can proceed in facile fashion. This can be illustrated by the attack of Ti on the pi bond of ethylene, which leads to metallacycle formation in the reaction... 

These reactions resemble those described in section 2.2.1 because both types involve insertion of the transition metal into the Si-H or Ge-H bond. However, here the molecule eliminated is not a neutral ligand but is formed by deinsertion of two ligands which are sigma bonded to the transition metal after the addition of R3MH. 

Attempt to prepare Jt-complexes of triafulvenes and related methylene cyclopro-parenes285,427 428 directly by ligand exchange reaction with transition metal complexes resulted in metal insertion into the sigma bond, forming metallacyclic complexes. Thus reaction of the electron-poor triafulvene l,2-diphenyl-3-dicyanomethylenecyclopropene with (ethylene)bis(triphenylphosphine)platinum in refluxing benzene gave two crystalline products whose platinacyclobutene structure was confirmed by X-ray structure analysis (equation 364)429. 

Flood has recently published an excellent contribution which includes a review of the stereochemistry of insertion reactions of CO and S02 into metal-carbon sigma bonds. 

The first step in the sequence involves the dissociative loss of a neutral ligand from 19 to form 20 with no change in formal oxidation number of the metal. The second, and especially characteristic stage, involves the isomerization of 20 to 21. This reaction may be regarded as an oxidative addition of phenyl and H to the metal with a formal increase in oxidation number from 4-2 to 4-4 it may also be viewed as metal insertion into a C—H sigma bond. Such an isomerization is analogous to an earher established example 47) of this type of reaction ... 

Carbon Monoxide Insertion into Transition Metal-Carbon Sigma-Bonds. 

There are genuine examples where co-ordination-sphere dioxygen transfer is likely, but here again these involve heavier metal complexes such as those of Rh(I). Three typical examples are shown in which different mechanisms may be operating. For cyclic olefins (Scheme 4), an oxygen atom inserts into the conveniently positioned allylic carbon-hydrogen bond (37). For acyclic olefins (38, 39) oxidation initially occurs via a sigma complex, followed by reduction with triphenyl-... 

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