Supports Bearing Alkenes

As shown in Table 1, a remarkable variety of alkene complexes bearing metal centers in a low oxidation state have been isolated and their structures have been determined by X-ray analysis. All the C-C bond distances in olefins coordinated to early transition metals at low oxidation states are more or less elongated compared to free ethylene. These structural data, together with those from NMR studies [14], indicate a major contribution of the metallacyclo-propane structure (2), a fact which is also supported by calculation studies [15]. In the case of ethylene bridging two metal centers such as [ Cp2ZrX 2(iu-f/-C2H4)] (3), the M-C bond could be characterized as a er-bond and there is a little contribution from the / -ethylene canonical structure [16-18]. 

Insight into the mechanism involved was obtained in two labeling studies, as shown in Eqs. 5.24 and 5.25. The former indicates that the carbon bearing the hydroxyl group preferentially forms the new C-C bond to the terminal alkyne carbon. The latter indicates that the alkene geometry is largely retained. These studies support the intervention of a jt-allyl species in which rotation around the ruthenium-allyl axis is slow relative to the rate of reductive elimination and the absence of a n-allyl intermediate. 

Dimethyl-2-phenylspiro[2.5]nonane-4,8-dione (17) was obtained with high enantioselectiv-ity from styrene and 2-diazo-5,5-dimethylcyclohexane-l,3-dione (diazodimedone) in the presence of a copper complex bearing chiral 1,3-dioxo ligands derived from 3-trifluoroacetyl-( + )-camphor. The catalysts retain their activity even if linked to a polymeric support. Despite the impressive optical induction for this transformation, no other successful applications of these catalysts have so far been reported with a trisubstituted alkene, only very low enan-tioselectivity was observed. ... 

A special type of alkene metathesis is the reaction of an alkene with an alkyne to give a diene. Thus, a reaction between a monoalkylated acetylene and a mono-alkylated ethylene leads to a 1,3-diene bearing a methyhdene and a mixture of (Z)-and ( )-alkene residues [213]. If both unsaturated residues are structurally confined, as in a propargyl-allylamide, cychzation to a vinyl pyrrohne occurs [236]. This reaction has become a useful tool in the generation of sohd support-bound dienes for [2 -i- 4]-cycloadditions [237]. 

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