Rhenium complexes Lewis acid

Gladysz has investigated the solution behaviour and configurational stability of these chiral rhenium complexes further, he extensively studied the chiral recognition process that results from the enantioface binding of alkenes, aldehydes and ketones on these rhenium chiral Lewis acids 3.7 and/or 3.8. There is a huge body of papers and reviews on this chemistry, and the reader may consult the referenced review. ... 

A remarkable aspect of the rhenium oxo complex chemistry is the ability of terminal oxo functions to act as nucleophiles. This has been used to incorporate dioxo complexes such as [Re02(py)4] or [Re02(CN)4] between the negatively charged interlayers of silicates like hector-ite (pyridine complex) or between the positive layers of a hydrotalcite-like Mg/Al double hydroxide. " The same concept has been applied for the isolation of molecular units containing Re-O-M bridges where M is a Lewis acid. ... 

Notably, as illustrated in Scheme 124, enantiofaces of aldehydes can be differentiated by a chiral rhenium template to result in stereoselective reaction with cyanide ion (297). The rhenium Lewis acid element forms stereoisomeric it complexes with aldehydes, which are convertible via... 

The chemistry of rhenium complexes with dinitrogen is quite extensive. It was earlier shown (73) that complexes such as [Re-Cl(N2)(Ph2PCH2CH2PPh2)2] contain a rather basic dinitrogen and that this can react with Lewis acids to form adducts, although few of... 

One means of stereoselective cleavage of biaryl lactones [53] is activation of the carbonyl group with a Lewis acid and subsequent attack with a chiral nucleophile. Conversely, activation can be effected with a chiral Lewis acid followed by attack of an achiral nucleophile. Complexation of a biaryl lactone to the chiral fragment [CpRe (NO)(PPh3)j then reduction with K(s-Bu)3BH (K-selectride) and ring opening of the intermediate rhenium lactolate gives the metalated aldehyde (dr = 75 25) which is converted to the alcohol without essential loss of optical purity (Sch. 6) [54]. 

Compared to manganese, little work has been carried out on analogous alkyl complexes of rhenium, although that which has may be divided into the same categories, namely, (1) reaction with transition metal hydrides, for example [CpW(H)(CO>3] (2) reaction with Lewis acids, for example, AlCUEt (3) orthometallation reactions (4) reaction with cationic species, for example, (5) reaction with anionic nucleophiles, for example, BF4" (6) reaction with neutral nucleophiles, for example, PPh3-... 

In conclusion, it can be noted that high valent transition metals seem perfectly capable of serving as effective Lewis acids. Many of the systems discussed here exhibit exceptional robusmess, stability and a propensity to form crystalline complexes. This would facilitate the task of crystallization and structural analysis, and one can imagine that transition metal complexes can be used as structural probes of Lewis acid-carbonyl interactions. In this vein, the first glimpse of the origins of Cram selectivity in a-chiral aldehydes may have been obtained from the crystal structure of a rhenium aldehyde complex. Lastly, the... 

Finally, electrophilic addition of H(C6F5)3, a strong Lewis acid, to a Cp ligand yields an T 4—Cp Re complex 38. Oxidative addition of a C-H bond (intramolecular C-H bond activation) from the r 4-Cp ring results in a zwitterionic rhenium dihydride, as shown in equation 8.72.101... 

Furthermore, several chiral Lewis acid complexes of titanium [90-95], tin [96], rhenium [97,98], boron [99], and magnesium [100] have been employed for the preparation of chiral cyanohydrins. However, only in some cases are the enantiomeric excesses satisfactory. 

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