Asymmetric carbonyl bridge

A variety of factors can result in an asymmetric carbonyl bridge, including inherent asymmetry in molecules (such as CO bridging two or three different metals) and crowding (where moving of a CO away from a symmetrically bridging position would help reduce crowding in part of a molecule).5... 

By consequent use of these arguments, it was also possible to solve the problem of the metal distribution in [MnFe2Se2(CO)9] , a related Mn/Fe mixed-metal complex anion possessing the characteristic asymmetric carbonyl bridge identified for the first time in [Mn3Se2(CO)9j ". The metal positions in the base of the M3Se2 pyramid are most probably filled in an ordered fashion with manganese (position M(l)) and iron (position M(2)). 

Partial reduction of the carbonyl bridge of the iV,iV-asymmetrically disubstituted benzodiazocine 198e gave an hydroxyl group, which was reacted with 4-chlorobenzoyl chloride to afford the corresponding ester. 

The slightly isosceles metallic triangle (Ni-Ni, 2.33 A Ni-Co, 2.37 A) is asymmetrically triply-bridged, above and below, by carbonyl groups (mean Co-CO distance, 1.86 A mean Ni-CO distance, 2.01 A). 

Self-supported titanium complexes with linked bis-BINOL ligands were used as an alternative approach for the immobilisation of catalysts, as shown in enantioselective sulfide oxidation (see Section 7.2.2). The same ligands were used with success in asymmetric carbonyl ene reactions. The chiral metal-bridged polymer 76, derived from ent-lOa, titanium tetraisopropoxide and water (Scheme 7.45), catalysed the ene reaction between 68b and 71, to give R)-72 in 88% yield and 88% enantiomeric excess. The catalyst can be reused at least five times without affecting its efficiency. 

In the case of [Fe3(CO)n]2-, a triply bridging carbonyl group is asymmetrically positioned (as deduced by the bond distances) above the Fe3 triangle, which is not found for [Ru3(CO)i i]2-,18 and [Os3(CO)i i]2-.15 In all the three compounds there is a carbonyl group bridging two metal atoms. 

Comparison with the corresponding dianion reveals important structural differences. In fact, the triply bridging apical carbonyl is no longer present and one of the bonds of the bridging carbonyl (namely, Fe3-Cl) weakens (becoming half-bonding). This asymmetric disposition of the ligand leads to an inequivalence of all the three Fe-Fe distances. 

Fig. 18. Ru6C(CO) 4, 18, as in its Ph4As+ salt (40). The anion has approximate C2l) symmetry, with four asymmetrically bridging and twelve terminal carbonyls bound to an octahedral Ru6C core. Mean Ru-Ru distance is 2.89 A, the mean Ru - distance is 2.05 A.

Fig. 23. (CH3CN)2Cu2Ru6C(CO),6, 20 (50). The distorted octahedral Ru6C core is capped by two directly bonded copper atoms [Cu-Cu = 2.693(1) A], one on an Ru3 face, the second on the CuRu2 face so formed. The Ru-Ru distances range from 2.798(1) to 3.072(1) A (mean 2.89 A). Ru-Q.rtjrte distances range from 2.031(4) to 2.073(4) A (mean 2.05 A). There are thirteen terminal carbonyls, and three asymmetrically bridging Ru-Ru edges. No Cu-CO contacts are short enough to imply bonding interactions.

Giblin, G. M. P. Kirk, D. T. Mitchell, L. Simpkins, N. S. Bridgehead enolates substitution and asymmetric desymmetrization of small bridged carbonyl compounds by lithium amide bases. Org. Lett. 2003, 5, 1673-1675. 

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