Reactivity apical

A chiral titanium complex with 3-cinnamoyl-l,3-oxazolidin-2-one was isolated by Jagensen et al. from a mixture of TiCl 2(0-i-Pr)2 with (2R,31 )-2,3-0-isopropyli-dene-l,l,4,4-tetraphenyl-l,2,3,4-butanetetrol, which is an isopropylidene acetal analog of Narasaka s TADDOL [48]. The structure of this complex was determined by X-ray structure analysis. It has the isopropylidene diol and the cinnamoyloxazolidi-none in the equatorial plane, with the two chloride ligands in apical (trans) position as depicted in the structure A, It seems from this structure that a pseudo-axial phenyl group of the chiral ligand seems to block one face of the coordinated cinnamoyloxazolidinone. On the other hand, after an NMR study of the complex in solution, Di Mare et al, and Seebach et al, reported that the above trans di-chloro complex A is a major component in the solution but went on to propose another minor complex B, with the two chlorides cis to each other, as the most reactive intermediate in this chiral titanium-catalyzed reaction [41b, 49], It has not yet been clearly confirmed whether or not the trans and/or the cis complex are real reactive intermediates (Scheme 1.60). 

The reason for the high reactivity of 13a is ambiguous. The reductive elimination may proceed through a transition state like 15 (Scheme 32), where the ring oxygens occupy apical and equatorial positions, which allows relief of the ring strain of the five-membered ring. 

The chemical reactivities of groups in the apical and equatorial positions of pentacovalent intermediates are different.664 In particular, elimination of a nucleophilic group to form a tetrahedral phosphate is easier from an apical position than from an equatorial position. For the in-line displacement of Eq. 12-27 elimination of RO should be easy. However an adjacent attack would leave - OR in an equatorial position. Before it could be eliminated, the intermediate would probably have to undergo a permutational rearrangement by which -OR would be transferred from an equatorial to an apical position. 

Pentafluorophenylation of iminium cations has been studied by DFT methods, and the experimental conditions have also been optimized.57 Silane reagents, (FsC6)4-nSiF (n = 1, 3) have been investigated, particularly with reference to their activation by weak Lewis bases. Pentafluorophenyl significantly stabilizes pentacoordinate silicon species, but as a group it is much more reactive in an apical than an equatorial position. 

It seems clear that cluster-substituted carbonium ions are easily generated, are very stable thermally, and are of sufficient electrophilic reactivity to be useful in the synthesis of many new functional cluster compounds. Before we consider further their exceptional stability and the mode of their stabilization, it is of interest to mention some chemical consequences of the high stability of a positive charge generated a to the apical carbon atom of the (OC)9Co3C cluster. For instance, one might expect the addition of the positive part of an ionic or polar reagent, X+Y-, to the... 

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