Alkenes insertions into metal-alkyl bonds

If a group, such as (C(0)0CH2CH2CH2CH=CH2), was used, the olefin was conforma-tionally constrained to bind perpendicular to the Pd—C bond and insertion-cyclization did not occur. Alkene insertion reactions into metal-acyl bonds usually occur more rapidly than insertions into metal-alkyl bonds. 

Some f/3-pentadienyl complexes of nickel, [CpNi(>j3-H2CCMeCHCMe= CH2)] (syn and anti isomers) (18 and 19) (150) have arisen in the course of work on the insertion of alkenes into metal-alkyl bonds, relevant to Ziegler... 

Sulfur dioxide reacts generally with transition metal alkyl, aryl, and a-allyl complexes to give sulfinate complexes. The reaction, first described in 1964 by Wojcicki and Bibler, resembles well-known insertion reactions of CO, C2F4, SnCl2, tetracyanoethy-lene, and other unsaturated species into metal-alkyl bonds, but there are important stereochemical and mechanistic differences Sulfur dioxide insertion into metal-alkene and metal-alkyne bonds have not been reported. However, PdCl2 has been used as a catalyst for copolymerization of ethylene and SO2 to polysulfones and insertion into a Pd-ethylene bond is a conceivable reaction step. 

Two examples of 1,2 insertions have been shown in Figure 14-7. An important application of 1,2 insertions of alkenes into metal-alkyl bonds is in the formation of polymers. 

Many examples of insertions of alkenes into metal-alkyl bonds of both early and late transition metal complexes have been observed. In general, the insertion process occurs more readily into metal-alkyl bonds of complexes containing electron-poor metal centers than into metal-alkyl bonds of complexes containing electron-rich metal centers. 

Insertion of alkenes into metal-alkyl bonds... 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

Figure 2.7 cis insertions of alkene and CO into metal-hydrogen and metal-alkyl bonds, respectively. 

The Cossee-Arlman mechanism proposes direct insertion of alkene into the metal-alkyl bond (see Section 2.3.2) without the formation of any intermediate. In the solid catalyst anion vacancies at the crystal edges are formed by simple... 

Norbornadiene complexes of platinum and palladium react with silver carboxylates to form the carboxymetallation products 29 which upon treatment with triphenylphosphane undergo intramolecular alkene insertion into the metal-alkyl bond to form nortricyclene 30. Reductive decomplexation leads to acylated nortricyclenol 31 (exu-tricyclo[2.2.1.0 ]heptan-3-ol). ... 

Kinetic data have been reported for cyclohexene reduction with a 1 6 Cr(acac)3- Bu3Al catalyst in heptane at 30 C, which showed a first-order dependence on catalyst and H2. Hydrogenation rates generally decrease with increasing substitution of the alkene substrate. Similar kinetic results were independently obtained for the Cr(acac)3- Bu3Al catalyst. A proposed mechanism involves alkylation of the metal-halide [equation (a)], hydride formation [equation (b)], followed by reversible insertion of the olefin substrate into the metal-hydride bond [equation (c)], and hydrogenolysis of the resulting metal-alkyl bond [equation (d)]. ... 

It is believed that the mechanism of halo alkylation involves oxidative addition of CCI4 to the metal followed by alkene insertion into the Cr-CCl3 bond (Scheme 4). The cycle is then completed by reaction with another molecule of CCI4. 

Metal allyl complexes are analogous to alkyls due to the generally easy interconversion, and they insert alkenes as shown in the preceding example, and for a number of Pd-allyl complexes [121]. However, some formal insertions into M-allyl bonds do not proceed through the four-center transition state formed... 

The general mechanism of insertion of an alkene into a metal-hydrogen or a metal-alkyl bond (reaction (91) R = H or alkyl) seems well established. It consists of a coordination of the olefin to the metal, followed by the hydride or the alkyl migration to the olefin, yielding an alkyl complex. The process, which involves a planar four-centered transition state, requires that the metal center is coordinatively unsaturated. 

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