Coordination compounds intramolecularly coordinating substituents

It is rather surprising that only a few diorganomagnesium compounds have been reported in which intramolecular coordination of a heteroatom-containing substituent is present. The synthesis of (2-Me2NCH2C6H4)2Mg (67) was reported. It has been used in a study on the infiuence of the presence of potentially intramolecular coordinating substituents on Schlenk equilibria However, it has never been structurally characterized. 

C. Diorganozinc Compounds Containing Intramolecularly Coordinating Substituents... 

The second structural motif, B (Figure 72), involves also a dimeric structure in which two alkoxy oxygen atoms bridge between two zinc atoms, but now an additional intramolecular coordinating substituent is present, resulting in the formation of four-coordinate zinc atoms. Most of these strucmral investigations are aimed at the elucidation of the mechanism of the enantioselective addition of organozinc compounds to aldehydes, a topic that will be discussed in Chapter 13. 

However, the meta isomer is not necessarily the thermodynamically favored product. For substituents with which intramolecular O Tl coordination can take place, the ortho isomer is typically the most thermodynamically stable e.g., 1 1 Tl(III) trifluoroacetate and methylphenyl or ethylphenyl ether after short times (< 1 h) produces the kinetically favored para-thallated products. However, after long times (> 50 h) the thermodynamically more stable ortho-thallated products are produced. Even under kinetic conditions, compounds with basic substituents that can form a five- or six-membered ring chelated to the ortho position lead to predominant or exclusive ortho substitution e.g., benzoic acid undergoes ortho substitution. ... 

Compounds such as phenylacetate esters and phenylethyl ethers, which have oxygen substituents that can serve as directing groups, show high ortho. para ratios under these conditions.12 These reactions are believed to involve coordination of the N02+ at the substituent oxygen, followed by intramolecular transfer. 

In the case of neutral systems the geometry corresponds to a bicapped tetrahedron or, in other words, to a tetrahedron which undergoes two nucleophilic coordinations. They have been observed [5] to be stable in solution. A careful H variable temperature NMR study shows that these compounds undergo an intramolecular isomerization which takes place without cleavage of Si-N bonds. The AGt of this so-called "Baylar Twist" isomerization has been estimated at between 12 and 18 kcal/mole depending on the substituents at the silicon. We can conclude that this work supports strongly hexacoordinated silicon structures as possible intermediates in the usual course of nucleophilic reactions at silicon. 

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