Secondary phosphine boranes deprotonation

Similarly to the P-CHj group, secondary phosphine-boranes react smoothly in the presence of a base (BuLi, NaH) under mild conditions to afford other kinds of functionalized phosphine-boranes in good to high yields, without racemi-zation. Yet the success of deprotonation/treatment with an electrophile process to afford substituted phosphine derivatives without any loss in optical purity may depend on the deprotonation agents employed. Use of butyllithium usually provides the products with high enantiomeric excess in good to high yields [73]. 

P-Chirogenic diphosphine 19, which rhodium-chelate complex forms a seven-membered ring (rare case for P-stereogenic ligand), was also prepared in reasonable yield (68%) using the wide chemistry of secondary phosphine borane [37]. Deprotonation of the enantiomerically enriched ferf-butylmethylphos-phine-borane 88 (Scheme 15) followed by quenching with a,a -dichloro-o-xylene and recrystallization afforded optically active diphosphine-borane 89 (precursor of free phosphine 19). 

Treatment of Cjq with lithiated secondary phosphine boranes or phosphinite boranes followed by removal of the BHj group afforded 1,2-hydrophosphorylated 27 in good yields (Scheme 3.14) [114, 117]. The phosphorus nucleophiles were generated by deprotonation of the corresponding borane complexes with Bull in THF-HMPA, and added to toluene solutions of Cjq at -78 °C. Complexes 26 are stable in air at room temperature for months. In the NMR spectrum, the proton of 26b appears... 

Many of the secondary phosphine boranes obtained with the menthol methodology have been deprotonated (usually with n-BuLi) and quenched with electrophiles leading to a variety of tertiary phosphine boranes (Table 2.13). 

Table 2.13 Phosphine boranes 54 prepared via deprotonation of secondary phosphine boranes 55 and electrophilic quenching (Scheme 2.20).

In this synthesis two cleavages of the phenyl-phosphorus bond by lithium afforded, after hydrolysis, the Aw(secondary) phosphine borane 109, which underwent cyclisation with dichloromethane upon deprotonation giving the... 

It was found that 14 was over-oxidised to phosphinous add borane 15, probably due deprotonation of the formed secondary phosphine borane and immediate oxidation of the phosphide anion. Interestingly, the analysis of the optical purity of 16, prepared by methylation of 16, showed that the reactions were enantioselective. 

The optically pure secondary phosphine boranes are easily deprotonated, giving extremely nucleophilic phosphide borane anions, which preserve the absolute configuration at the P atom at low temperatures. This fact prompted Danjo, Imamoto and co-workers to couple the phosphide boranes derived from 9 with aryl cation equivalents. In particular, they found that fluoroarene tricarbonylchromium complexes such as 17 were excellent substrates for the SNAr process due to the high electron deficiency of the arene ring and significant electrophilicity of the ipso carbon (Scheme 5.9). 

Several diphosphine boranes have been prepared by taking advantage of the availability of a few optically pure secondary phosphine boranes (Section 5.2.1.1). After deprotonation, the phosphide boranes were easily alkylated by Az5(benzylic) substrates (Scheme 5.22). 

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