Metal crotyl complexes

Table I. Structure of Metal-Crotyl Complexes and Composition of Their Hydrolysis Products"...

The essential steps in the nickel-catalyzed 1 1 codimerization reaction, which involve hydride addition to butadiene and ethylene coordination to the metal atom, were first proposed by Kealy, Miller, and Barney (35) and were later demonstrated by Tolman (40) using a model complex. Tolman prepared the complex H—Ni+L PFe [L = (EtO)3P] and showed that, after prior dissociation to form H—NiL3, it can react with butadiene to form a 7r-crotyl complex 19. 

Other conjugated diene systems readily react with either H4Ru4-(CO)i2 or Ru3(CO)i2, to yield 7r-allyl complexes. Thus, butadiene yields the crotyl derivative HRu3(CO)9C4H5. The X-ray structure of the related adduct formed from cis-trans- or trans-trans-2,4-diene is shown in Fig. 12. The hydride is considered to bond to the Ru(l)-Ru(2) edge, as this is the longest metal-metal bond in the structure and in the... 

Remarkably, the heterogenisadon process can confer dramatically enhanced activity on the metal complex. Thus, the dimolybdenum complex [Mo2(MeCN)8][BF4]-Si02 prepared by direct reaction of silica with the metal complex is unusual in its ability to catalyse the polymerisation of norbomene in the absence of an aluminium co-catalyst and at moderate temperatures.165 Sol-gel processing of the ruthenium complex m-Cl(H)Ru(CO)(P)3 (where P is a coordinated ether-phosphine) with tetraethoxysilane and [Al(0-i-Pr)3] gives a stable material with moderate surface area which is active in the hydrogenation of fraras-crotylaldehyde with reasonable chemoselectivity to the carbonyl reduced products, cis- and trans-crotyl alcohol.166... 

The aldol addition reaction, and the related crotyl metal additions (section 5.1), have figured prominently in the total synthesis of a number of complex natural products (reviews [48,140-142]). Figure 5.8 illustrates those mentioned in the preceding discussion, along with others selected from the recent literature, with the stereocenters formed by stereoselective aldol addition indicated ( ). For the Prelog-Djerassi lactone and ionomycin, recall (Figure 3.8) that most of the other stereo-centers were formed by asymmetric enolate alkylation. 

Table 8 gives the data on the electron structure of valence shells of atoms of the corresponding transition metals [86]. Taking into account the fact that the total number of electrons provided by three allyl (crotyl) ligands to the molecular orbitals of the complex is nine, it may be concluded that the complexes of Cr (chromium), Nb (niobium) and Ti (titanium) should be paramagnetic and those of Rh (rhodium) should be diamagnetic. In this case the total... 

Besides aUylsUanes or stannanes, allyl boron species have found widespread apphcations in synthesis. The Roush crotylation is a very well-estabhshed method with a broad substrate scope, and is therefore commonly used in organic synthesis [73]. The following example depicts a reversal in enantioselectivity induced by a cobalt complex present in the substrate. Roush et al. reported that the use of metal carbonyl complexes as substrate surrogates led to an improvement of enantioselectivity in the asymmetric crotylation of the respective aldehydes. These results were attributed to electronic effects exerted by the metal complexes that stabilize the transition state of the crotylation reaction (Scheme 3.47) [74]. 

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