Alkene metal complex

Conjugated dienes can be dimerized or trimerized at their 1,4 positions (formally, [4 4- 4] and [4 4-4 4-4] cycloadditions) by treatment with certain complexes or other transition metal compounds. " Thus butadiene gives 1,5-cyclooctadiene and 1,5,9-cyclododecatriene. " The relative amount of each product can be controlled by use of the proper catalyst. For example, Ni P(OC6H4—o-Ph)3 gives predominant dimerization, while Ni(cyclooctadiene)2 gives mostly trimerization. The products arise, not by direct 1,4 to 1,4 attack, but by stepwise mechanisms involving metal-alkene complexes. " ... 

Stable transition-metal-alkene complexes can be obtained readily from their salts and alkenes in water.141... 

It is important to realize that there is a great deal of overlap in the topics covered in this chapter. For example, the chemistry of metal carbonyls is intimately related to metal alkene complexes, because both types of ligands are soft bases and many complexes contain both carbonyl and alkene ligands. Also, both areas are closely associated with catalysis by complexes discussed in Chapter 22, because some of the best-known catalysts are metal carbonyls and they involve reactions of alkenes. Therefore, the separation of topics applied is certainly not a clear one. Catalysis by metal complexes embodies much of the chemistry of both metal carbonyls and metal alkene complexes. 

Probably the first metal alkene complex was Zeise s salt, K[Pt(C2H4)Cl3] or the bridged compound [PtCl2(C2H4)]2. These compounds were first prepared by Zeise in about 1825. The palladium analogs of these compounds are also now known. A large number of metal alkene complexes are known, and some of the chemistry of these materials will be described here. 

A number of synthetic methods are useful for preparing metal alkene complexes. A few of the more general ones will be described here, but the suggested readings given at the end of the chapter should be consulted for more details. 

Being hybrid fields between organic and inorganic chemistry, the chemistries of metal alkene complexes and organometallic compounds have developed at a rapid rate. There is no doubt that this type of chemistry will be the focus of a great deal of research for some time to come. 

Metal-alkene complex formation was also necessary, but again was not ratedetermining. 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

Silver ions form similar alkene complexes which are soluble in aqueous solution and may be used to effect the separation of unsaturated hydrocarbons from alkanes. Catalysis for the polymerization of alkenes also form metal-alkene complexes which lead to polymerized product. 

A number of X-ray crystal structures of nickel(O) complexes containing both alkenes and phosphines have been reported to date with the aim of gaining more information on the bonding in metal alkene complexes. Structural data for the most relevant nickel(O) phosphine alkene complexes are reported in Table 9. 

Chemical and physical properties of metal alkene complexes have been rationalized on the basis of the Dewar-Chatt-Duncanson bonding model167,1 8 and a number of semiempirical and ab initio MO calculations have been performed for a more quantitative description of the bonding.16 "177... 

Figure 3.55 Dewar Chatt Duncanson bonding model in metal alkene complexes.

Alkylation then yields the trans alkene shown other alkynes can give rise to cis products. It seems likely that the metal-alkene complex (XVII) is similar to intermediates believed to occur in various catalytic processes involving nickel-bipyridyl complexes. 

The platinum catalyst is effective in very small amounts, and can be introduced as H2PtCl6 or as elemental platinum on an inert support. A particularly active catalyst is the soluble platinum complex of divinyltetramethyldisilox-ane, CH2=CHSiMe2-0-SiMe2CH=CH2. The hydrosilyla-tion reaction operates through the Chalk-Harrod mechanism or one of its variants. bz jn these mechanisms, the first step involves the conversion of a metal alkene complex to a metal alkene silyl hydride complex. In addition to platinum, recently ruthenium, rhodium, palladium, copper, and zinc complexes are being studied as hydrosilation catalysts. " ... 

While allylic oxidation products may arise by elimination of a metal hydride from an intermediate adduct or metal-alkene complex, allylmercury species (34) are thought to be intermediates in the case of mercury(II) acetate. A number of pathways have beien suggested, for example involving radical and carbenium ion intermediates, and addition-elimination and rearrangement inocesses. ... 

A quite different case is presented by the bent metallocene systems, [Cp2MH(alkene)] where M = Nb or Ta. Here the metal-alkene complex is relatively quite stable, so much so that the kinetics of formation of a stable alkyl metal complex cannot be easily studied. Nonetheless it is possible to get a handle on the hydrometallation kinetics by means of dynamic NMR methods (Table 1). The increased rate for propene versus ethylene results from both steric destabilization of the ground state and electronic stabilization of the transition state for the former. The first is typical of alkene complexes the second implies that some partial positive charge develops at the -carbon during the hydrometallation process, also seen in the trend for substituted styrenes. 

The nature of metal-alkene complexes is discussed in Chapter 48. 

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