Lithium enolates bond lengths

Similar effects were observed in the structures of the lithium salts of ester enolates [43] studied by Seebach et al. (1985). Here too systematic differences in angles are observed compared with amide and ketone enolates, and there is a correlation between the bond angles and the difference in the two C-O bond lengths at the reaction centre for three compounds [43], consistent with incipient elimination of t-butoxide to give the ketene [44] (Ferretti et al., 1991). 

Aldol reactions of boron enolates are frequently more diastereoselective than aldol reactions of, for example, lithium or aluminium enolates. This is partly ascribed to the relatively short boron-oxygen bond length (B-O = 1.36-1.47 A, Li-0 = 1.92-2.00 A, Al-0 = 1.92 A) which exacerbates the unfavourable 1,3-diaxial interactions that occur between the boron substituent... 

The third type of C = CX2 olefin examined from the point of view of structure correlation was the ester enolate. Three crystal structures of lithium ester enolates, obtained in the course of synthetic studies by Seebach et al., provided a total of four fragments depicted in Scheme 6.13 [108]. The variation in the C-O bond lengths and bond angles provides a picture of the incipient stages of elimination of an alkoxy ion (alcoholate OR), yielding a ketene. The C-OR bond length varies between 1.379(3) and 1.412(5) A, and the C = C-0 angle increases from 125.4 to 128.2° (d and / i in Scheme 6.14), i.e. the picture is similar to the enamine one. The other... 

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