Borane bonds oxidative cleavage

While oxidative cleavage of a single borane B—H bond at a transition metal center is well estabhshed, activation of two B—H bonds at the same... 

The result of theoretical investigations have suggested that cleavage of a B—H bond occurs to initiate ammonia borane dehydrogenation [38]. Alternatively, the oxidative N—H addition of ammonia to the dehydrogenated intermediate C may constitute a feasible reaction pathway due, in particular, to the fact that ammonia and aniline oxidative addition to la and related iridium-PCP systems has been reported experimentally [39]. 

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). 

A process that involves an unprecedented B—B bond cleavage in B2cat2 promoted by two Ni(I) centers in the dimer [Ni(p -PNP)]2 has been described by Meyer and Mindiola to proceed plausibly via a binuclear oxidative addition reaction (Scheme 20A). The B NMR spectrum of [Ni(Bcat)(PNP)] clearly reveals the formation of a rare example of a nickel-boryl (47 ppm). Alternatively, the [Ni(Bcat)(PNP)] complex can be prepared by o-bond metathesis from [Ni(0 Bu)(PNP)] and B2cat2 (Scheme 20B) A proposed intermediate could be detected by NMR spectroscopy and confirmed by DFT calculations as the isomer that conducts the CT-bond metathesis. The weak interaction between the nickel center and the tethered borane fragment in B2cat2 is rationaHzed as a consequence of the high energy required to access the empty dj(2 y2-orbital in a stericaUy encumbered square planar environment. 

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