Chlorophosphine boranes

The chlorophosphine boranes 97 are efficient starting reagents for the synthesis of various classes of P-chiral phosphorus compounds. Reactions of chlorophosphine boranes 97 with nucleophiles, such as carbanions, phenoxides, phenylthiolates, or amides, leads to the formation of corresponding organophos-phorus compounds 100-103 in yields of 53-99% and with up to 99% ee. This method was also used for the preparation of various classes of symmetric and asymmetric P-chiral ligands useful for asymmetric reactions, catalyzed by complexes of transition metals (Scheme 31) [52, 60, 61]. 

Scheme 31 Chlorophosphine boranes 97 as chiral starting reagents...

Bauduin C, Moulin D, Kaloun EB, Darcel C, Juge S (2003) Highly enantiomerically enriched chlorophosphine boranes synthesis and applications as F-chirogenic electrophilic blocks. J Org Chem 68 4293-4301... 

The first report of an optically pure chlorophosphine borane appeared in 1997 (Scheme 4.15). ... 

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. 

Chlorophosphine boranes are more reactive than phosphinite boranes and therefore are easily attacked by carbanionic reagents such as organolithiums, organosodiums or Grignard reagents. " However, they can lead to... 

Scheme 4.24 Utility of a chlorophosphine borane in the synthesis of a ferrocenyldiphosphine borane.

More recently Juge reported an unexpected reaction of chlorophosphine boranes with t-BuLi (Scheme 4.29). 

After ring opening of 7 by an organolithium, the opened aminophosphine alkoxide 12 can be trapped with an equivalent of a chlorophosphine to afford 66 after boronation. Alternatively, attack of 12 on a highly enantioen-riched chlorophosphine borane 31 affords the doubly P-stereogenic compounds 67. Both 66 and 67 can be easily deprotected with DABCO in toluene, for example, with complete retention of configuration at the phosphorus atom. 

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). 

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