Metal complex catalysts main group metals

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

Finally, chain polymerisation can occur via coordination, as is the case for polymerisation involving Ziegler-Natta catalysts. These catalysts are complexes formed between main-group metal alkyls and transition metal salts. Typical components are shown in Table 2.1. 

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

Many organometallic compounds that have main group metal-hydrogen or metal-metal bonds undergo 1,2-hydrometallation or 1,2-dimetallation of alkynes. Pd complexes are good catalysts for these processes [118]. Since the resulting products contain one or two reactive carbon-metal bonds they are well suited for further transformations, particularly in a sequential fashion. 

These complexes are the first examples of multifunctional catalysts and demonstrate impressively the opportunities that can reside with the as yet hardly investigated bimetallic catalysis. The concept described here is not limited to lanthanides but has been further extended to main group metals such as gallium [31] or aluminum [32]. In addition, this work should be an incentive for the investigation of other metal-binaphthyl complexes to find out whether polynuclear species play a role in catalytic processes there as well. For example, the preparation of ti-tanium-BINOL complexes takes place in the presence of alkali metals [molecular sieve ( )]. A leading contribution in this direction has been made by Kaufmann et al, as early as 1990 [33], It was proven that the reaction of (5)-la with monobromoborane dimethyl sulfide leads exclusively to a binuclear, propeller-like borate compound. This compound was found to catalyze the Diels-Alder reaction of cyclopentadiene and methacrolein with excellent exo-stereoselectivity and enantioselectivity in accordance with the empirical rule for carbonyl compounds which has been presented earlier. 

Both homogeneous and heterogeneons catalysts are effective for this reaction. Homogeneons catalysts are nsnally composed of a transition metal compound combined with a main group metal alkyl cocatalyst, or they consist solely of a well-defined transition metal carbene complex. The most common transition metals nsed in these catalysts are Mo, W, and Re, although other metals from groups 4-9 have also been used. Literally thousands of different... 

The only known metal catalyst for the asymmetric catalytic Strecker reaction is the aluminum salen catalyst 465 (Sch. 65) recently reported by Sigman and Jacobsen [97]. They prepared 11 different chiral salen complexes from different transition and main group metals and screened these complexes for the addition of trimethylsilyl cyanide to imine 460 at room temperature. The aluminum catalyst 465 was optimum in terms both of asymmetric induction and rate. This constitutes the first aluminum salen complex successfully developed for an asymmetric catalytic reaction. 

Most of transmetalation between main group metal compounds and transition metal complexes leads to the formation of a transition metal-carbon bond. The reaction which causes alkyl or aryl ligand transfer from transition metal to main group element is much less common. Olefin polymerization catalyzed by a metallocene catalyst is sometimes accompanied by chain transfer caused by the transfer of the growing polymer end from Ti or Zr to an A1 compound that is used as the cocatalyst (Scheme 5.23) [139,140]. 

Hydroamination is an atom-economical process for the synthesis of industrially and pharmaceutically valuable amines. The hydroamination reaction has been studied intensively, including asymmetric reactions, and a variety of catalytic systems based on early and late transition metals as well as main-group metals have been developed." However, Group 5 metal-catalysed hydroaminations of alkenes had not been reported until Hultzsch s work in 2011. Hultzsch discovered that 3,3 -silylated binaphtho-late niobium complex 69 was an efficient catalyst for the enantioselective hydroaminoalkylation of iV-methyl amine derivatives 70 with simple alkenes 71, giving enantioselectivities up to 80% (Scheme 9.30). Enantiomerically pure (l )-binaphtholate niobium amido complex 69 was readily prepared at room temperature in 5 min via rapid amine elimination reactions between Nb(NMe2)5 and l,l-binaphthyl-2-ol possessing bullqr 3,3 -silyl substituents. Since the complex prepared in situ showed reactivity and selectivity identical... 

A quite new class of catalysts based on early main group metals (Ca, Sr, and K) was recently reported to promote general conversion of conjugated double bond (137). The catalytic reaction is initiated by the formation of a highly reactive metal hydride that adds either to an alkene or to a silane. The regiochemistry for the hydrosilylation of 1,1-diphenylethylene (DPE) catalyzed by calcium complex can be completely controlled by the polarity of the solvent. Amine borane and phosphine borane complexes were successfully used as effective catalysts for hydrosilylation of organic compounds with internal unsaturated bond (138) that cannot be selectively hydrosilylated in the presence of Pt catalysts. 

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