Nucleophilic Substitution on Phosphinite Boranes

These reactions proved to be quite general, providing a wide range of phosphine boranes. Most of them are listed in Table 4.4. 

The optical purity of the compounds, determined by HPLC, is usually very high and can be brought to perfection with a simple recrystallisation (entries 3, 4 and 42). In a few cases, the expected absolute configuration of the phosphorus atom has been confirmed by X-ray analysis.  

This can be done using either (-)- or (+)-ephedrine in the synthesis of 7 or, more interestingly, reverting the order of addition of the organolithium reagents from one of the precursors. An example is found in Table 4.4 (entries 1 and 12) in the synthesis of both enantiomers of PAMP BH3 from (4-)-7 (derived from (—)-ephedrine). Although promising, this approach has rarely been used, i.e. in most cases only one of the enantiomers of the phosphine borane has been obtained. 

Although the substitution of the methoxy group by organolithiums has a broad scope, some limitations have been found. Mezzetti and co-workers suggested that 0,0-substituted aryllithiums do not react with (5)-P(BH3)Me(OMe)Ph. In contrast, the bulky r-BuLi reacts smoothly (Table 4.4, entries 3 and 4), probably due to its higher nueleophilieity. 

Phosphine borane 37 has three stereogenic centres, but a thorough analysis, ineluding the X-ray erystal strueture, indicated that it was present as a raeemie mixture. Its formation was interpreted via the formation of an unde-teeted intermediate 36 (Seheme 4.19) with a P-C double bond, which sulfers a second attack of i-PrIi to form an anionic phosphine borane and the final 

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