Deprotonation methylphosphine boranes

Double deprotonation of methylphosphine borane and addition of 111 produced an equimolar mixture of phospholane boranes 112. They were separated by meticulous column chromatography, yielding 38% yield of (/ p)-112, which was oxidatively dimerised to afford 113 in 50% yield. Although (/ p,/ p)-113 can obviously be prepared from the other enantiomer of 111, it was prepared by different protocols (see Scheme 2.40 in this chapter and Chapter 5, Section 5.3.2). 

The deprotonation with i-BuLi of diethyl ether solutions of several aryldi-methylphosphine boranes 4 in the presence of (-)-sp generated the corresponding carbanions 5, which were quenched with benzophenone to afford derivatives 6 in good yield and optical purity (Table 5.1). 

Scheme 5.4 Enantioselective deprotonation-electrophilic quenching of aryldi-methylphosphine boranes.

The enantioselective deprotonation protocol has been expanded to alkyldi-methylphosphine boranes. Kann and co-workers subjected t-butyl- and... 

Enantioselective deprotonation followed by oxidative coupling with Cu(II) salts has been employed to prepare C2-symmetric 1,2-Z)Zj (methylphosphine borane)ethanes, described in the following paragraphs and in Section 5.2.1.3. 

Imamoto and co-workers used a few optically pure alkynyl(t-butyl)-methylphosphine boranes 47, obtained by enantioselective deprotonation, to prepare the corresponding 1,2-diphosphinoethanes 48 (Scheme 5.21). 

The practical unavailability of (+)-sparteine means that the standard enantioselective deprotonation protocol can not be used to prepare (i ,f )-BisP diphosphines (see, however. Section 5.4.1). Some work has been performed to overcome this limitation. Imamoto and co-workers were able to prepare both enantiomers of t-Bu-BisP (BH3)2 by using both enantiomers of mrt-butyl-methylphosphine borane. Unfortunately, the preparation of the latter compounds involved a preparative HPLC separation step and therefore the strategy cannot be easily adapted for large-scale synthesis. 

Alkyldimethylphosphine-boranes 74 underwent enantioselective deprotonation employing (-)-sparteine/s-BuLi, followed by oxidation with molecular oxygen [91, 92] in the presence of triethyl phosphite (Scheme 12) to afford moderate yields of enantiomerically enriched alkyl(hydroxymethyl)methylphosphine-bo-ranes 76, with 91-93% ee in the case of a bulky alkyl group and 75-81% ee in the case of cyclohexyl or phenyl groups [93]. Except for the adamantyl derivative (in which the ee increased to 99%), no major improvement in the ee was observed after recrystallization. 

The methods described in this chapter are based on the enantioselective deprotonation of alkyl groups in - usually achiral - phosphine derivatives, which provide highly enantioenriched a-carbanions that are versatile precursors of a variety of mono- and diphosphines. This method was developed by combining two known facts firstly, that the methyl group in methylphosphines and their oxides, boranes and sulfides can be deprotonated with strong bases and secondly common organolithium reagents ( -, s- or r-BuLi) can exert enantioselective deprotonations in prochiral substrates in the presence of certain chiral auxiliaries. ... 

---