Primary phosphanes, unlike their nitrogen counterparts, have a reputation as being toxic, volatile compounds that often spontaneously ignite in air. [1] Accordingly, despite an impressive range of applications as asymmetric ligand precursors, [2] biomedicines, [3] polymers, [4] carbohydrates, [5] and macrocycles, [6] as well as possessing two reactive PÀH bonds for functionalization, they remain under-used. Some of the few primary phosphanes reported to be air stable [7] are shown in Scheme 1. There appears to be more than one factor that can confer stability. Thus supermesitylphosphane (1 a), [8] triptycylphosphane (1 b), [9] and various related compounds [7] undoubtedly owe their stability to steric factors. More puzzling is 1 c, which is stable in air indefinitely, whereas FcPH 2 and 1,1'-Fc'(PH 2 ) 2 (Fc = [(h 5 -C 5 H 4 )Fe II (h 5 -C 5 H 5 )], Fc' = [Fe II (h 5 -C 5 H 4 ) 2 )]) are oxidized in three-five days. [10a] Clearly the alkyl spacer groups are important, but their significance is not currently understood. The reasons for the air-stability of the compounds [R 2 R 1 N(CH 2 ) n PH 2 ]I, [11] S(CH 2 SCH 2 CH 2 PH 2 ) 2 , [3] and the related 1 d [12a] are also unclear; the authors suggested that the remote heteroatom might confer stability through negative hyperconjugation. [11, 12] Herein we report a potentially larger class of airstable phosphanes, which possess extended p conjugation, and we suggest that stability may be more the norm than the exception.
As part of our research into P-stereogenic 2-methoxy-2'diphenylphosphanyl-1,1'-binaphthyl (mop) ligands, [13] we required precursor chiral primary phosphanes, which have rarely been used in the design of asymmetric ligands. [14, 2c] Our initial target was (R)-2-phosphanyl-2'-methoxy-1,1'binaphthyl (4). We also recognized that 4 might provide a test of whether the local heteroatom conferred air stability.
Compound (R)-4 was prepared in a straightforward manner (Scheme 2) in high yield as a white powder, and its 31 P NMR (d = À126.3 ppm, average 1 J P-H = 202.6 Hz) and IR data (n(P-H) = 2291 cm À1 ) were as expected.
More gratifying was that (R)-4 did indeed show remarkable stability towards air oxidation: no oxidation was observed for (R)-4 after seven-days exposure to air. Suitable crystals of (R)-4 for X-ray crystallography [15] were obtained from acetonitrile, and the resulting structure (Figure 1) represents, to the best of our knowledge, the first such characterization of an optically pure primary phosphane. The PÀC bond length (1.811(6) ) correlates with the few other crystal structures reported for uncoordinated primaryphosphanes. [9, 10b, 11, 12a, 16, 17] On closer examination of the solid-state molecular structure we noted that the environment around the P atom is not crowded, nor does there appear to be any interaction between the P atom and the O atom of the methoxy substituent, apparently ruling out the ether group as being a Scheme 1. Phosphanes possessing a high degree of air stability.