Alkoxy complexes group 6 metals

The surface alkoxy complex was then decomposed by calcination of the catalyst in a stream of dry air at 623 K (see eq. 2). During the reaction, the evolution of stoichiometric amount of appropriate alkene was observed. Secondary hydroxyl groups appeared as a result of the reaction (2). Their presence created the possibility of anchoring the subsequent layer of the same or the other transition metal ions. 

The molecular complexity of metal alkoxides also depends on the steric hindrance of alkoxy groups. Bulky secondary or tertiary alkoxy groups tend to prevent oligomerization. Trimeric species [Ti(OEt)4]3 have been evidenced in pure liquid titanium ethoxide (Fig. lb) whereas titanium iso-propoxide Ti(OPr )4 remains monomeric (Fig.la). This is no more the case for zirconium iso-propoxide which is dimeric because of the larger size of Zr(rV). Moreover solvent molecules can also be used for coordination expansion leading to solvated dimers [Zr(OPri)4(Pr OH)]2 when the alkoxide is dissolved in its parent alcohol (Fig.lc). 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

For ketones and aldehydes in which adjacent substituents permit the possibility of chelation with a metal ion, the stereochemistry can often be interpreted in terms of the steric requirements of the chelated TS. In the case of a-alkoxyketones, for example, an assumption that both the alkoxy and carbonyl oxygens are coordinated with the metal ion and that addition occurs from the less hindered face of this chelate correctly predicts the stereochemistry of addition. The predicted product dominates by as much as 100 1 for several Grignard reagents.120 Further supporting the importance of chelation is the correlation between rate and stereoselectivity. Groups that facilitate chelation cause an increase in both rate and stereoselectivity.121 This indicates that chelation not only favors a specific TS geometry, but also lowers the reaction barrier by favoring metal ion complexation. 

Bifunctional ligands containing alkoxy groups can be used for anchoring metal complexes to surface OH groups through a longer alkyl chain (Scheme 7.16).249-251... 

In contrast, Fe-Hg-X complexes show little tendency to form halide bridged species and less is known about complexes containing Zn. We first reported the formation of Fe-Si-O-M four membered ring systems with soft metals M = Ag, Rh, Pd, and Pt, and then prepared bimetallic complexes with more oxophilic metals in order to better understand the conditions for the occurrence of this unusual (t-alkoxy-silyl bridging mode. We have expanded our studies on Cd-containing complexes [3b-d] to Group 13 elements and we report here about the synthesis and reactivity of new, stable heterometallic Fe-M (M =... 

A similar reaction mechanism was proposed by Chin et al. [32] for the hydrolysis of the biological phosphate monoester adenosine monophosphate (AMP) by the complex [(trpn) Co (OH2)]2+ [trpn = tris(ami-nopropyl)amine]. Rapid cleavage is observed only in the presence of 2 equiv metal complex. It is evident from 31P NMR spectra that on coordination of 1 equiv (trpn)Co to AMP a stable four-membered chelate complex 4 is formed. The second (trpn)Co molecule may bind to another oxygen atom of the substrate (formation of 5) and provide a Co-OH nucleophile which replaces the alkoxy group. The half-life of AMP in 5 is about 1 h at pD 5 and 25 °C. 

The catalytic cycle of the reaction is depicted in Scheme 20.6 [31]. After the initial ligand exchange, the ketone (10) is coordinated to the metal ion of 11 (a), yielding complex 12. A direct hydride transfer from the alkoxide to the ketone takes place via a six-membered transition state (b) in which one alkoxy group is oxidized (13). The acetone (14) and the newly formed alcohol (15) are released... 

A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes [436 -439] (Figure 3.11). These carbene complex precursors can be prepared from alk-oxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-445] or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) [391,446-452]. 

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