Phosphane-borane adducts

Figure 18 Syntheses of poly(phenylphosphinoborane) [PhPH-BH2] (25) and poly(isobutylphosphinoborane) p -BuPH-BH2]K (26) from their respective phosphane-borane adducts. (Adapted from ref. 45.)...

The conjugate addition of phosphorus nucleophiles of various oxidation states and in neutral or metallated form constitutes an efficient and well-known method for C—P bond formation [30]. In the case of phosphanes as nucleophiles especially, the corresponding phosphane-borane adducts have been used in 1,4-additions to Michael acceptors. Following the idea to use a chirally modified phosphorus nucleophile in asymmetric Michael additions to aromatic nitroalkenes, we synthesized the new enantiopure phospite 45 starting from TADDOL (44) with nearly quantitative yield. Due to the C2 symmetry, of the... 

In continuation of our efforts to explore the utility of the SAMP/RAMP hydra-zone methodology, we developed the first asymmetric synthesis of a-phosphino ketones via formation of a carbon-phosphorus bond in the a-position to the carbonyl group [70]. The key step of this asymmetric C—P bond formation is the electrophilic phosphinylation of the ketone SAMP hydrazone 87, giving rise to the borane-adduct of the phosphino hydrazone 88 with excellent diastereoselectiv-ity (de = 95-98%). Since these phosphane-borane adducts are stable with respect to oxidation, the chemoselective cleavage of the chiral auxiliary by ozonolysis leading to the a-phosphino ketones (R)-89 could be accomplished with virtually no racemization. Using RAMP as a chiral auxiliary, the synthesis of the enantiomer (S)-89 was possible (Scheme 1.1.25). 

It is interesting to note that good correlations between the phosphorus chemical shift and the P-M bond strength have only been found for phosphane-borane adducts where zr-back bonding does not exist and steric factors are limited. 

Rippert J, Linden A, Hansen HJ (2000) Formation of diastereoisomerically pure oxazaphospholes and their reaction to chiral phosphane-borane adducts. Helv Chim Acta 83 311-321... 

P H -CP MAS measurements performed by Wiegand et al. for a series of phosphane-borane adducts, shown in Fig. 4, revealed significant B- P scalar spin-spin interactions, /bp 50 Hz, which provided evidence for covalent bonding between the reaction centres in these frustrated Lewis pairs . The authors have indicated that in view of the data collected by them it is reasonable to assume that this coupling belongs to a / rather than a coupling via the olefinic backbone. The experimental /bp values have been duly reproduced by the DFT computations. 

Whittell, G.R., Balmond, E.L, Robertson, A.P.M., Patra, S.K., Haddow, M.F. and Manners, L, Reactions of amine- and phosphane-borane adducts with frustrated Lewis pair combinations of group 14 triflates and sterically hindered nitrogen bases. Ear. J. Inorg. Chem. 2010 (25), 3967-3975 (2010). 

Faure et al92 reported the first enzymatic resolution of phosphane-borane complex. Thus, the borane adduct of (2-hydroxypropyl)diphenylphosphane 107 was resolved using the lipase CAL-B and vinyl acetate as acyl donor in organic solvent. The remaining unreacted substrate (.S )-107 was recovered with 91 % ee. 

The sign of /C( Sn, P) is negative in all tin-phosphorus compounds (dominant influence of the lone pair of electron at the phosphorus atom), except of phosphane adducts of tin(IV) halides. Even in transition metal complexes of stannylphosphanes or in borane complexes of stannylphosphanes the coupling sign does not change, although the phosphorus lone pair of electrons is engaged. 

We wonld expect a considerable downfield shift npon coordination of the phosphane, and we are therefore not snrprised to observe it in the ensning adducts. However, why do we observe an npfield shift upon coordination to the borane with the phosphanes in the lower part of the table The difference mnst lie in the behavior of the snbstituents on phosphorns, as this is the one parameter that changes as we look down the list. In the top part, the snbstituents are H, methyl, and phenyl, whereas in the lower part, the snbstitnents are flnoride, amide, and methoxide. The latter three (F, NMe and MeO) are capable of a 7r-bonding interaction toward phosphorus that increases as the electron density on phosphorus diminishes upon coordination. Since the P-NMR chemical shifts are more sensitive toward. ir-interactions than cr-interactions, the net result can very well be an upfield shift upon coordination of the phosphane, if substituents capable of . r-backbonding are present on phosphorus. 

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