Hydrogen bonding pentacoordinate

To account for stereochemical results for the epoxidation of allyl alcohols, a slightly different intermediate has been proposed as shown in Fig. 6.9.16 The authors propose an intermediate (A) analogous to the intermediate in peracid oxidations. A small molecule of alcohol or water is coordinated to Ti with deprotonation and another is coordinatively ligated to Ti without deprotonation to achieve a pentacoordinated ligand sphere. During epoxidation, the allyl alcohol substrate is held in position by a hydrogen bond. 

Other mechanisms with the involvement of an incipient methyl cation were also proposed.418,460,462 An attack of methyl cation released from extensively polarized methoxy groups on the carbon-hydrogen bond forms pentacoordinated carbocation intermediates that, in turn, yields ethyl methyl ether after the loss of a proton ... 

A zwitterionic hexacoordinate pentafluorosilicate (175) was recently reported by Tacke193.175 was prepared in the same manner as pentacoordinate zwitterionic complexes (cf Section II.A.3), as shown in equation 45. The crystal structure of 175 showed that one water molecule is present for each molecule, and that the structure is dominated by N—H-F and O—H-F hydrogen bonds. The Si—C bond length (1.943-1.946 A) is distinctly longer than in the corresponding pentacoordinate zwitterion (9), while the Si—F bonds (1.685-1.740 A) are comparable to those in the pentacoordinate analog. The solid state 29 Si NMR of 175 is also compatible with hexacoordination, with 529Si= -177.2 ppm. 

Fig. 29 The elongated pentacoordinate phosphorus species observed in the X-ray structure of /1-PGM from Lactococcus lactis. Distances shown are in angstroms. In addition to the interactions shown, the P03 moiety also makes hydrogen-bonding contacts with two backbone amide N-H groups, and the hydroxyl of Ser-114.

Furthermore, tert-butylation of adamantane (26) to 1 -/ert-buty I adamantane (54) has also been achieved. During the process, the rert-butyl cation 4 inserts itself into the bridgehead carbon-hydrogen bond generating the pentacoordinate cation 53, which, via deprotonation, yields alkylated product 54 (Scheme 6.17). 

Hydroxylic solvents are able to promote heterolytic Si-X bond cleavage (X =F or Cl) (1, 268). The kinetics have been studied extensively and the activation of the reaction by an additional nucleophile has been observed (268, 269, 341, 342). A mechanism, involving an increase of the nucleophilicity of the alcohol by hydrogen bonding, has been proposed (Scheme 63) (103,341). Alternatively, a pentacoordinate intermediate has been invoked (1,342) (Scheme 64). Discrimination between the two above interpretations has recently been provided good evidence for the nucleophilically induced process at silicon has been presented (268,269,343). 

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