Alkoxide substituent

Further studies have shown that the degree of aggregation, and hence the number of active sites, is a function of the alkoxide substituent.813 For example, in (270) one of the i.vo-propoxide groups bridges two aluminum centers the other three are terminal ligands and all three initiate the ROP of CL. Less aggregated species such as (270) generally exhibit simple first order kinetics. 

In a related publication, Kobayashi and his team reported on Zr-catalyzed asymmetric Mannich reactions that utilize the more electron-rich oxygenated ketene acetals shown in Scheme 6.28 [93], A noteworthy aspect of this study was that the levels of syn/anti diaste-reocontrol proved to be dependent on the nature of the alkoxide substituent whereas the (3-TBS acetals predominantly afforded the syn isomer, the OBn derivatives afforded a larger amount of the anti isomer. As before, the presence of an additive, this time 1,2-dimeth-ylimidazole (DMI), proved to be important with regard to the level of Ti-facial selectivity. The phenol activating group can be removed by the same procedure as reported previously, with essentially identical degrees of efficiency (see Scheme 6.27). 

However, in a comprehensive study, Seebach and coworkers have shown the usefulness of titanium and zirconium alkyls as selective nucleophilic reagents.106,107 Typically compounds of the type (RO)3MR (M = Ti, Zr) are reacted with aldehydes or ketones to yield asymmetric alkoxides (equation 32),107 The method has been extended to allow enantioselective addition using chiral organotitanium reagents where the chirality is imposed by the initial alkoxide substituents.107,108... 

Cinnamyl ethers are carbolithiated to give intermediates similar to 37, but without the additional lithium cation of the alkoxide substituent they presumably exist in a structure closer to 46. The products of deuteration and carboxylation of this organolithium have anti... 

Reaction of the component metal alkoxides afforded the species Tl2Zr[OCH(CF3)2]6. X-ray data confirmed a hexacoordinate Zr center while the Tl atoms were coordinated to alkoxide O and F of the alkoxide substituents.400 Variable-temperature 19F and 205T1 NMR spectroscopy indicated the F- -Tl interactions persisted in solution although the structure was fluxional. 

Compound 6 does not rearrange, as the two jr-termini in C are not in adequate proximity. In D accessibility to the norcaradiene double bond is diminished and the solvated alkoxide substituent is not easily surmounted. 

The second column consists of halide-substituted carbene complexes [14]. In these cases, the bonding scheme is similar to Fischer carbenes in that the singlet halocarbene fragments interact with singlet metal centers, but the halide substituents do not contribute as much electron density into the carbon p orbital as the more strongly jt-donating alkoxide substituents. Presumably, this electronic deficit... 

All the carbon atoms in the 12-membered ring are now present, and they are sorted out by the two steps that follow. The first is a Cope rearrangement a [3,3]-sigmatropic rearrangement, accelerated as we have described (p. 914) by the presence of an alkoxide substituent. 

There are some important differences between metal alkoxides and Si(OR)4. (i) Owing to their lower electronegativity, metal alkoxides are stronger Lewis acids than silicon alkoxides. Nucleophilic attack at the metal is thus facilitated, and the hydrolysis rates are strongly increased. For example, the hydrolysis rate of Ti(OR)4 is about 10 times faster than that of Si(OR)4 with the same alkoxide substituents, (ii) Most metals have several stable coordination numbers, or the expansion of the coordination sphere in transition states is easier. Owing to both effects, the reactivity of some tetravalent isopropoxides in hydrolysis reactions increases in the order Si(OPr% < Sn(OPr%, Ti(OPr% < Zr(OPr% < Ce(OPr%. 

Evans, D. A., Golob, A. M. (1975). [3,3]Sigmatropic rearrangements of 1,5-diene alkoxides. Powerful accelerating effects of the alkoxide substituent. Journal of the American Chemical Society, 97(16), 4765-4766. 

The alternative mode of reaction of diazoketones, that of carbene formation, has been used to prepare a hydrindane derivative (Scheme 7). The second step of this sequence, a vinylcyclopropane - cyclopentane rearrangement, can be carried out under much milder conditions if the cyclopropane carries an alkoxide substituent. ... 

The rate-constant results [62,86] suggest that as the size of the alkoxide substituents increases or, as the reacting species become more highly condensed, steric effects become increasingly important as observed, for example, in organoalkoxysilanes [47]. 

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