Mechanism of Phosphine Borane Deprotection with Amines: The Effects of Phosphine, Solvent and Amine on Rate and Efficiency

Abstract

The kinetics of borane transfer from simple tertiary phosphine borane adducts to a wide range of amines have been determined. All data obtained, including second-order kinetics, lack of cross-over, and negative entropies of activation for reaction of triphenylphosphine borane with quinuclidine and triethylamine, are consistent with a direct (SN 2-like) transfer process, rather than a dissociative (SN 1-like) process. The identities of the amine, phosphine, and solvent all impact substantially on the rate (k) and equilibrium (K) of the transfer, which in some cases vary by many orders of magnitude. P-to-N transfer is more efficient with cyclic amines in apolar solvents due to reduced entropic costs and ground-state destabilisation. Taken as a whole, the data allow informed optimisation of the deprotection step from the stand-point of rate, or synthetic convenience. In all cases, both reactants should be present at high initial concentration to gain kinetic benefit from the bimolecularity of the process. Ultimately, the choice of amine is dictated by the identity of the phosphine borane complex. Aryl-rich phosphine boranes are sufficiently reactive to allow use of diethylamine or pyrrolidine as a volatile low polarity solvent and reactant, whereas more alkyl-rich phosphines benefit from the use of more reactive amines, such as 1,4-diaza[2.2.2]bicyclooctane (DABCO), in apolar solvents at higher temperatures.

Isolable Diaminophosphide Boranes

“…Scattered reports on template syntheses of diphosphino–disilane transition metal complexes [23] imply that such species can in principle, like their lighter congeners, act as chelating ligands. Aiming at making an aminofunctionalized 1,2‐diphosphino‐disilane available for complexation studies, we set out to study de‐protecting 7 by sequestering the borane units with 1,4‐diazabicyclo[2.2.2]octane (DABCO) [24] . Heating a hexane solution of both reactants to 60 °C produced a colorless precipitate that was identified by 11 B NMR spectroscopy as the expected bis‐borane adduct of DABCO ( δ 11 B −10.9 ppm, 1 J BH =99 Hz [25] ).…”

“…Scattered reports on template syntheses of diphosphino-disilane transition metal complexes [23] imply that such species can in principle, like their lighter congeners, act as chelatingl igands.A iming at making an aminofunctionalized 1,2-diphosphino-disilane available for complexation studies, we set out to study de-protecting 7 by sequestering the borane units with 1,4-diazabicyclo[2.2.2]octane (DABCO). [24] Heating a hexane solution of both reactants to 60 8Cp roduced ac olorless precipitate that was identified by 11 BNMR spectroscopy as the expected bis-borane adduct of DABCO( d 11 B À10.9 ppm, 1 J BH = 99 Hz [25] ). The 31 PNMR spectrum displayed am ain signal (d 31 P9 6.4 ppm with 56 %o ft he total integrated intensity) which lacks the characteristic broadening arising from spincoupling to aq uadrupolar 11 Bn ucleusa nd is tentatively assigned to the targeted borane-free diphosphino-disilane.…”

Tetrathia[7]helicene Phosphorus Derivatives: Experimental and Theoretical Investigations of Electronic Properties, and Preliminary Applications as Organocatalysts

“…Tetrathiahelicene phosphine–borane complexes 1 a–d and 2 a–d represent a convenient and versatile platform of stable helical phosphorus precursors, from which the corresponding 7‐TH mono and diphosphanes 3 a–d and 4 a–d could be obtained. We then investigated the most suitable conditions for the deprotection of adducts 1 and 2 , taking into account the well‐known procedures reported in the literature, including treatment with alcohols or amines …”

Which future for stereogenic phosphorus? Lessons from P* pincer complexes of iron(ii)

“…65 One advantage of the electrophilic chlorophosphine boranes is that they undergo umpolung to nucleophilic secondary lithium phosphide boranes feasible by transmetalation with t BuLi at À85 1C (Scheme 15). 65c Deboronation of the final product is achieved either by reaction with an excess of amine 71 or by treatment with strong acids such as HBF 4 etherate. 72 The latter method is mandatory for trialkylphosphines due to their electron-rich nature.…”