Spirophosphoranes, pentacoordinated

A New Approach to Activation of Hydroxy Compounds Using Pentacoordinated Spirophosphoranes... 

During last year the majority of researches in this area has been focused on the synthesis and structural determination of novel hypervalent phosphorus compounds as well as on the stereochemistry of pentacoordinated chiral spirophosphoranes. In these studies, Mironov et al. obtained tricyclic pentacoordinated spirophosphoranes containing a phosphorus-carbon bond with high regio- and stereo-selectivity whereas Kawashima presented the synthetic route to perfectly anti-apicophilic carba-phosphatranes. ... 

While the chemistry of pentacoordinated spirophosphoranes has been so far widely explored, cyclic hexacoordinate phosphorus compounds are much less studied because very often they are transient species and difficult to detect. However, an interesting example of a stable hexacoordinate phosphoranate bearing Martin ligands was recently described by Yamamoto. Another approach involved the participation of donor-acceptor nitrogen-phosphorus" and sulphur-phosphorus bonds in the formation of stable hexacoordinated compounds. 

Interestingly, oxidative addition of DIAD to other sulfur containing P compounds (18, 19), gave pentacoordinated spirophosphoranes (20) and (21) instead of hexacoordinated species. Plausibly,the -OCH2CH2SH moiety may not be able to render the phosphorus sufficiently acidic to give hexacoordination, thus the pentacoordination of phosphorus was assigned to structures (20) and (21), by comparison with compound (22) and other related compounds (Scheme 6). 

Enzymatic phosphoryl transfer reactions are ubiquitous in nature and play significant roles in ATP hydrolysis and protein phosphorylation processess. As previously described, pentacoordinate phosphorus species have been assumed as transient intermediates or transition states in these pathways and their structural and electronic properties are undoubtedly related to the outcome of the process. Therefore, to aid understanding of the phosphorus-catalyzed biological routes, many model pentacoordinated phosphoranes have been synthesized. While most studies have focused on aspects of apicophilicity, anti-apicophilicity or Berry pseudorotation, there have been limited investigations on the stereochemistry of pentacoordinated spirophosphoranes with a chiral phosphorus atom. In the past year, much attention has been paid to the synthesis and determination of absolute configuration of several chiral pentacoordinate spirophosphoranes derived from D- and L-aminoacids. Some significant achievements in this area will be discussed in this section. 

The chiral pentacoordinated spirophosphoranes were synthesized using method outlined in Scheme 1 ... 

To investigate the stereochemistry of pentacoordinated spirophosphoranes with chiral chelate ligands such as amino acids, the diastereoi-somers could be differentiated by reverse-phase high-performance liquid chromatography (HPLC), solution or solid-state circular dichroism (CD),... 

A variety of pentacoordinated spirophosphoranes undergo olefination with aldehydes in the presence of t-BuOK to give a,p-unsaturated esters, amides, and nitriles with high Z selectivity (Scheme 46) [212, 217], This method has recently been extended to the Z selective olefination of ketones by modifying the pentacoordinated spirophosphoranes, which are readily prepared through the reaction of the corresponding P-H phosphoranes with a-halo carbonyl compounds in the presence of DBU [218],... 

A good illustration of these concepts is provided by the three spirophosphoranes A, B and C in Figure 11. Since all of them are apt to be involved in a P(III) P(V) equilibrium, the proportion of P(III) can be taken as a probe of the stability of the pentacoordinated compound. It was thus found85 that C, with three different substituents in the e subset, is the one for which the proportion of P(III) is highest. 

The following example is a good illustration of these two facts the spirophosphorane 70 (Scheme 9), with a TBP structure, is a racemate of the enantiomers x and x, which can both be in tautomeric equilibrium with 70a, a derivative of tricoordinated phosphorus. The pentacoordinated P is electrophilic and can therefore undergo a number of reactions with nucleophilic reagents, whereas the tricoordinated P atom of 70a is nucleophilic or even biphilic, and 70a itself, with its OH group, has a nucleophilic protic centre with all its specific chemical properties. 

One particularly fascinating class of pentacoordinate phosphorus compounds is the class of chiral spirophosphoranes with amino acid residues as chiral chelate ligands since they can serve as important structural features relevant to chiral phosphoryl transfer pathways. Therefore, their synthesis and stereochemistry were recently widely explored." ... 

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