Tertiary phosphine boranes

Nucleophilic Attack at Other Atoms. A highly efficient general synthesis of secondary- and tertiary- phosphine-borane adducts is afforded by treatment of the phosphine with sodium borohydride in the presence of acetic acid in THF. Any carbonyl groups present in the phosphine undergo concomitant... 

Chiral phosphines are widely used as auxiliaries for various metal-catalyzed asymmetric reactions and can be prepared from stable phosphine-borane complexes. Secondary P-chiral phos-phine-boranes can be prepared by reductive lithiation of the corresponding tertiary phosphine-borane using LN (eq Likewise, P-chiral tertiary phosphine ligands can be produced by the reductive lithiation of phosphinite-boranes followed by alkylation, both proceeding with retention of configuration (eq 18). ... 

The transformation of readily available enantiopure //-menthylphosphinates 2 into chiral phosphinous acid boranes 5 permits the elaboration of bulky P-stereogenic secondary phosphine boranes. Taking advantage of the synthetic potential of these compounds, abroad range of hindered P-chiral tertiary phosphine boranes 6 were prepared with excellent enantiomeric excesses [12,13]. Phosphinous acid 5 can easily be converted into one or the other enantiomer of the secondary phosphines boranes (Sp)- or (/ p)-6 by stereoselective reduction or substitution of the phosphinite borane derivatives, respectively (Scheme 4 and Table 1). 

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). 

Methylphosphinite boranes react smoothly with organolithium reagents to afford the corresponding tertiary phosphine boranes, as will be discussed in the next section. However, phosphinite boranes are not electrophilic enough to react with other weaker nucleophiles such as alcohols, amines or thiols. More reactive precursors, capable of producing a wide variety of phosphorus compounds, were needed. In phosphorus chemistry halophosphines, and chlorophosphines 27 in particular, are essential synthons (Scheme 4.12) as nucleophilic (after transformation into metal phosphides 28) and electrophilic building blocks. 

Since this report, a small number of chlorophosphine boranes have been prepared. They are very moisture-sensitive and still prone to racemisation in spite of the borane protection. Their optical purities can only be assessed after transformation into other less reactive compounds, such as tertiary phosphine boranes, assuming that the substitution reaction is completely stereoselective. Table 4.3 lists the compounds reported so far, along with the best conditions for their preparation. As depicted in Scheme 4.16, in all the reported cases the phosphorus atom bears a phenyl group. 

The reaction of methylphosphinite boranes with organolithiums at low temperature affords the corresponding P-stereogenic tertiary phosphine boranes by substitution of the methoxy group and inversion of configuration at the phosphorus atom (Scheme 4.17). 

Enantiomerically pure phosphine borane 26 was deprotonated at low temperature and halogenated with 1,2-dibromo- or 1,2-diiodoethane to furnish compounds 27, which could be isolated as crystalline solids in good yields. In analogy to chlorophosphine boranes (see Section 4.3.2.2), they are not configurationally stable and racemise slowly at room temperature. Therefore, compounds 27 were not isolated but used in situ with several carbanionic reagents to afford the corresponding tertiary phosphine borane with inversion (28) or retention of configuration (28 ) (Table 5.5). 

More recently, Livinghouse and co-workers studied the effect of Cu(I) on Pd-catalysed arylation of secondary phosphine boranes. Apart from a marked beneficial effect on the coupling elSciency, they discovered that Cu(I) is able to suppress the base-mediated racemisation of the phosphide borane intermediate, to generate a configurationally stable metallophosphide 47 (Scheme 6.22). Consequently, tertiary phosphine boranes 48 were obtained with retention of configuration, in good yields and excellent enantioselectivities. 

While the formation of a variety of carbon stereocentres has been effectively achieved, stereoselective approaches for the preparation of P-stereogenic phosphines have, until recently, been non-existent. In this context, Bergman s group has developed several chiral ruthenium catalysts to be investigated for the enantioselective ruthenium-catalysed alkylation of secondary phosphines into the corresponding chiral air- and moisture-tolerant tertiary phosphine-borane products after a subsequent treatment with BHs.THF. " These reactions proceeded through the intermediacy of nucleophilic phosphido species, which had low barriers to pyramidal inversion, allowing a DKR process. 

The initially discovered ruthenium catalyst, [((i )-i-Pr-PHOX)2Ru(H)][BPh4], was found to be effective in the reaction of methylphenylphosphine with benzylic chlorides, providing the corresponding tertiary phosphine-borane products with high yields and moderate to high enantioselectivities, as shown in Scheme 2.44. 

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