Metal-olefin interaction, intramolecular

Whereas the interpretation of the NMR parameters of di(5-hexenyl)zinc do not suggest any metal-olefin interaction, probably because the chain length is not appropriate to accommodate the latter in the ground state, this interaction may play a crucial role in the reactive complex leading to the intramolecular carbozincation process. Conversely, the chain length is not sufficiently long in the case of di(4-pentenyl)zinc (28) to achieve the requisite conformation for addition to the double bond, but it is favorable for zinc-olefin interaction24,25. 

The chelation between zinc and heteroatoms allows a diastereoselective allyl- or crotylzincation of secondary y-heterosubstituted vinyl metals. However, the intramolecular interaction between the metal center and an olefinic site was also described in the case of di(4-pentenyl)zinc [114]. This weak dipolar chelation is the result of the inherent polarity of the Zn - C " bond and the polarity of the double bond (having a partial negative charge on the terminal carbon). Such interactions were evidenced by NMR investigations and the spiro structure of di(4-pentenyl)zinc was postulated to interpret this phenomenon (Fig. 7-1). The oxygen chelation described in Scheme 7-95 is now replaced by a jr-chelation between a vinyl organozinc and an cu-olefinic residue. 

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

Similar it - metal interactions have been suggested for the intramolecular interaction of an olefin with aluminum in terminal olefin derivatives of the type AI[(CH2) CH=CH2]3, which have been shown to be monomeric in solution even though the corresponding AL[(CH2) +1CHs]3 alkyl derivatives are predominantly dimeric with bridging alkyl groups under similar conditions (37). 

Many different routes are available for the synthesis of vinylboranes and several of them are shown in Scheme 7. Hydroboration and diboration reactions of alkynes and borylated alkynes provide access to the frill series of mono-, di-, tri-, and tetraborylated olefins. 1,2-Diborylated olefins (33) are obtained via diboration of alkynes and 1,1-diborylated olefins (34) are accessible through hydroboration of borylalkynes. An alternative route to 1,1-disubstituted products involves the diboration of carbenoids formed in situ from vinylhalides and butyl hthium. In certain cases, metal-catalyzed dehydrogenative borylation of olefins may be used. Borylalkynes serve as precursors to triborylated (35) and tetraborylated (36) olefins. Thus, the sparingly soluble tetraborylethylene derivative (36) forms in good yield through platinum-catalyzed diboration of diborylacetylene in toluene at 40 °C if the base-free catalyst [Pt(cod)2] is used. If the reaction, however, is performed at higher temperature, ftnther diboration of (36) leads directly to the hexaborylated ethane (23) shown above. Intramolecular B-O interactions were postulated for (36) based on HF-SCF calculations. ... 

Transient carbenes display a rich and diverse chemistry as stoichiometric reagents, for example, in reactions such as olefin cyclopropanation, C-H insertion, dimerization, 1,2-migration, and so on. Carbenes are important in several synthetic methods and are growing in importance, especially the intramolecular versions. Carbenes are electron deficient, and unless strong resonance interaction is possible the reactions will be electrophilic. The chemical behavior of a carbene depends to some extent on its method of preparation, electronic state, and also on the presence or absence of certain metals or metallic salts. The state in which the carbene is produced depends on the method of generation, that is, singlets... 

The initial association between an alkene and an aluminium alkyl may involve interaction between the Ti-orbitals of the olefin and the Lewis acidic metal centre. Intramolecular association of this type is indicated by the lowering in the v(C=C) stretching frequency in Bu2AlCH CH CH CH=CH compared with that in its ether complex or in the free alkene. 

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