Alkenes, metalation

Figure I9.2S Schematic reprESeniaiion of the two formal exliemes of bonding in 1,3-diene complexes In (a) the bonding is considered as two almost independent ij -alkene-metal bonds, whereas in (b) diere are a bonds to C(l) and C(4) and an ij -alkene-metal bond from C(2)-C(3)...

Alkene-metal complexes are usually prepared by a process by which some other ligand is dissociated from the metal. Both thermal and photochemical reactions are used. 

Fig. 8.6. Representation of it bonding in a alkene-metal cation complex.

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

Carbocations formed through protonation of alkenes by proton acids are usually assumed as intermediates in alkylation with alkenes. Metal halides, when free of protic impurities, do not catalyze alkylation with alkenes except when a cocatalyst is present. It was shown that no neat conjugate Friedel-Crafts acids such as HA1C14 or HBF4 are formed from 1 1 molar compositions in the absence of excess HC1 or HF, or another proton acceptor.163-166 In the presence of a proton acceptor (alkene), however, the Lewis acid halides—hydrogen halide systems are readily able to generate carbocations ... 

The reactivity of alkenes increases with their nucleophilic nature in the order tetra-substituted>trisubstituted>disubstituted>monosubstituted. Further, the epoxidation rate V = /c2X[alkene][complex]/(l + J [alkene]) shows that decomposition of the alkene-metal complex represents the rate determining step in this reaction. 

Oxidation of Alkenes Metal Induced Formation of an Allylic Carbon-Oxygen Bond"... 

The selective and efficient metallation of alkenes in the allylic position by the Schlosser s base is certainly one of the major achievements in the field of superbase chemistry. A complete list of successful alkene metallations by superbases would be too long for the purpose of this chapter but some representative examples are shown in Tables 2 and 3 as a general trend for the unsubstituted and heterosubstituted alkenes, respectively. 

A more recent example of alkene metallation by superbase followed by reaction with electrophiles is the functionalization of isopulegol which can be performed without protection of the hydroxy group.81... 

Figure 16-26 The molecular orbital view of alkene—metal bonding according to Dewar.

Other synthetic routes to alkene-metal complexes reported in the literature involve bridge cleavage reactions of halogeno complexes (method F), facile intermolecular scrambling reactions involving metal-alkene and metal-PF3 complexes (18, 81) (method G) (Scheme 4), and the coupling together of two alkyne units to afford a metallo-cyclopentadiene derivative (method H). 

Scheme 8 Three selectivity issues in nucleophilic addition to alkene-metal complexes...

At the outset of the subsection, it should be emphasized that distinction between metallacyclopropenes and q -alkyne-metal complexes or that between metallacyclopropanes and q -alkene-metal complexes is largely a matter of semantics or convenience. Most or perhaps all of these species should probably be viewed as hybrids of three-membered metallacycles and ir-complexes. 

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