P-alkoxide

This section concludes with a discussion of the reactions of metal haloge-nides with other alkoxylating agents (besides M OR) and the reactions of the salts MXj with alkaline alkoxides, used in the synthesis of M(OR) . In this connection the synthesis of Mo and W alkoxides and akoxohalogenides of ortho-series, using alkylalkoxoorthosilicates or B, Nb, P alkoxides, are described ... 

The higher nucleophilicity of a p alkoxide can be attributed to unfavourable dipole-dipole interactions resulting from repulsion of the... 

These considerations suggested that pyranoses and furanoses should undergo base-catalyzed addition directly and in a stereocontrolled manner to suitable triple bond systems AsB (or compounds containing cumulative double bond systems A—B=C, Scheme 1, path c). Only instability of aldehydic intermediates in basic media and insufficient or undifferentiated reactivities of the a- and p-alkoxides seemed to lower the expectations for a stereocontrolled anomeric O-activation. 

The nucleophilic addition of enol silanes with aldehydes to produce P-silyloxy carbonyl adducts 47 is an example of a group-transfer process (Scheme 2), for applications in polymer synthesis, see [64a, 64b, 64cj. In its simplest mechanistic rendition the reaction proceeds upon coordination of the aldehyde to Lewis acid MX4 to afford an activated electrophilic species 42. Addition of the nucleophilic enol silane 43 to 42 leads to C-C bond formation and generation of the aldol adduct. Various intermediate structures 44,45,46 have been postulated to be formed concomitant with or following C-C bond formation. The generation of intermediates 45 and 46 necessitates subsequent silylation of the P-alkoxide furnishing aldol adduct 47 and regenerating catalyst MX4. 

As shown in Scheme 3, classic approaches to p-hydroxy ketones are often ineffective for the formation of hydroxypropionates. Standard aldol reactions are complicated by the presence of the P-alkoxide of 4. Enolization of 4 typically results in facile P-elimination, thus preventing aldol addition and formation of hydroxypropionate 7. This approach was later successfully applied to the myriaporone problem by Loh through the use of boron enolates. Likewise, relatively few allylation reactions are appropriate for hydroxypropionate synthesis as the required organometallic species 8 would also be prone to elimination. 

Scheme 7 Euautioselective coupliug, P-alkoxide elimiuatiou sequeuce...

Waymouth and coworkers used chiral zirconocene complexes such as 56 with EtsAl as the stoichiometric reductant to enantioselectively desymmeter-ize oxabicychc compounds (Scheme 9) [29]. A reductive coupling mechanism to give 57 followed by P-alkoxide ring opening and transmetallation is consistent with the experimental results. Neither direct insertion of the alkene into the M - C bond nor nucleophilic attack mechanisms can be ruled out, however [12],... 

In a reaction similar to the P-alkoxide elimination reactions seen with zir-conocenes, catalytic Rh(OH)(cod)2 and 2 eq. of arylboronic acids gave cyclic products 165 from enynes 166 (Scheme 35) [100]. In this reaction, transmet-allation of Rh - OR with B - Ph gave Rh - Ph species 167, which inserted into the alkyne, cyclized to 168, and finally underwent P-alkoxide elimination to provide Rh-OCHa. This reaction is limited to the formation of five-membered rings, but it can also undergo cascade type reactions of enediynes to give multicyclic products [100]. 

John, L. Sobota, P. Alkoxide Molecular Precursors for Nanomaterials A One Step Strategy for Oxide Ceramics, in Ceramic Materials Wunderlich, W. Ed. Sciyo Croatia, 2010. 

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