Dimethylphosphine sulfides

The main difference between phosphine boranes and phosphine sulfides is that for the latter compounds -BuLi rather than i-BuLi is generally used as the 

Comparison between Table 5.18 and Table 5.1 reveals that although phosphine sulfides provide slightly better yields, the enantioselectivities are somewhat lower. It also seems that primary alkyl-lithiums are more enantioselective than the secondary ones (compare entries 5 and 7 with entry 6). 

The standard protocol provides 105, as seen throughout this chapter. The other enantiomer, 105, could in theory be obtained by functionalisation of 103 with a removable group A to afford 106. Then a regioselective lithiation into the less hindered methyl group (a kinetically controlled process) would furnish 107, which should provide 105 after removal of the group A. 

The trimethylsilyl derivative 109 was chosen because its optical purity can be easily increased by recrystallisation until a virtually enantiopure compound is obtained in 56% yield. Some screening showed that polyamines such as PMDETA in combination with. s-BuLi constitute the optimum conditions in order to minimise undesired functionalisation of the methylene group. Under these conditions, 109 was regioselectively lithiated and the carbanion further elaborated by known methods to afford, after a standard desilylation step, a-hydroxyphosphine sulfide 110 (55% yield, 94% ee) and p-hydroxyphosphine sulfide 111 (50% yield over two steps, 96% ee). The method was also adapted to yield diphosphine sulfides 112 (69% yield, 90% ee) and 113 (46% yield, 99% ee). The latter compound is indeed the disulfide of (5, 5)-t-Bu-MiniPHOS. During its synthesis it was observed that no meso compound was formed. 

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A full spectral characteristics including /cp couplings has been reported by Schmutzler and co-workersfor ( )-2,2-diethyl-3,4-dihydro-2/f-l,4-ben-zothiazin-3-yl dimethylphosphine oxide and ( )-2,2-diethyl-3,4-dihydro-2H-l,4-benzothiazin-3-yl dimethylphosphine sulfide. 

The same reactant, diphosphine disulfide was used in reaction with aldehydes and ketones to provide (dimethylthiophosphinoyloxymethyl-) dimethylphosphine sulfide derivatives in the presence of RhH(PPh3)4 and l,2-bis(diphenylphosphino)ethane (Scheme 20) ... 

The enantioselective deprotonation protocol has also been applied with certain success to dimethylphosphine sulfides (Scheme 5.43). 

Dimethylphosphine has been prepared by alkylation of phospho-nium iodide with methjd iodide and by reductive cleavage of phosphorus sulfides with methyImagnesium bromide. Diethyl-, dibutyl-, and dicyclohexylphosphines have been prepared by reduction of the corresponding tetraalkyldiphosphine disulfides with lithium hydroaluminate in ether. This latter method gives dialkylphosphines uncontaminated by mono- and trialkylphosphines. 

This method is not only restricted to dimethylphosphine boranes and sulfides. It has been also applied to cyclic diphosphine derivatives by deprotonation of one of the enantiotopic methylene groups a to the phosphorus atom (Scheme 5.3). In this case, a second stereogenic centre is created and therefore it is necessary to control also the diastereoselectivity of the reactions. 

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