A General, Rhodium‐Catalyzed, Synthesis of Deuterated Boranes and N‐Methyl Polyaminoboranes

Abstract

The rhodium complex [Rh(Ph PCH CH CH PPh )(η -FC H )][BAr ], 2, catalyzes BH/BD exchange between D and the boranes H B⋅NMe , H B⋅SMe and HBpin, facilitating the expedient isolation of a variety of deuterated analogues in high isotopic purities, and in particular the isotopologues of N-methylamine-borane: R B⋅NMeR 1-d (R=H, D; x=0, 2, 3 or 5). It also acts to catalyze the dehydropolymerization of 1-d to give deuterated polyaminoboranes. Mechanistic studies suggest a metal-based polymerization involving an unusual hybrid coordination insertion chain-growth/step-growth mechanism.

Amine–Borane Dehydropolymerization: Challenges and Opportunities

“…Such processes have been modelled in related phosphine–borane model systems . In principle, such a β‐hydride transfer pathway could be probed by deuterium labelling of the substrate, as reported recently for N‐methylamine–borane . Although no strong trend was observed, polymers with N−D substituents yielded slightly higher molecular weight material, as measured by GPC, which could indicate that N−D cleavage is the rate‐determining step of termination.…”

“…For Manners’ TiCl 2 Cp R 2 /2× n BuLi catalyst system, high molecular weight polymer is only observed at high conversions (i.e., after 8 hours, Figure A), monomer is consumed early on, and increased catalyst loading produces higher degrees of polymerization. In contrast, Weller's [Rh{Ph 2 P(CH 2 ) 3 PPh 2 }(η 6 ‐C 6 H 5 F)][BAr F 4 ] catalyst displays an unusual hybrid chain‐growth/step‐growth mechanism (Figure B). Relatively long chain polymers were produced very early on in the reaction, polymer molecular weights decreased with higher catalyst loadings, monomer was only consumed gradually and the system is sensitive to hydrogen, signaling a coordination/insertion/chain‐growth process for the first ca 90 % of reaction.…”

“…In the context of amine–borane dehydropolymerization, very little is known about chain‐transfer/termination events (Scheme ). As mentioned, in some systems hydrogen has been found to act as a chain‐modifying agent, which causes additional complexity as H 2 is a product of dehydropolymerization . Such modification is proposed to operate during a coordination/dehydrogenation/insertion chain‐growth mechanism (Scheme A), where hydrogenolysis of a Rh−N or Rh−B bond by H 2 leads to termination of the growing chain (Scheme A).…”

“…Possible step‐growth coupling motifs have been proposed for the Rh‐system, as shown in Figure . 11 B NMR data point towards a linear head‐to‐tail coupling as the most likely linkage, that is, Figure C . As with chain‐growth processes, model system designed to probe step‐growth coupling processes in amine–borane dehydropolymerization would be beneficial.…”

“…Dehydrogenation of secondary amines and boranes, or bulky primary examples (e.g., H 3 B⋅N t BuH 2 ) yields oligomers, monomeric aminoboranes (e.g., H 2 B=N i Pr 2 ) or cyclic borazanes [e.g., (H 2 BNMe 2 ) 2 ]. Deuterated polyaminoboranes have been recently prepared from H 3 B⋅NMeD 2 , D 3 B⋅NMeH 2 and D 3 B⋅NMeD 2 to form the isotopologues (R 2 BNMeR′) n (R, R′=H or D, Figure ) that were found to possess physical properties similar to (H 2 BNMeH) n . Interestingly, the same catalyst that was used for dehydropolymerization, [Rh{Ph 2 P(CH 2 ) 3 PPh 2 }(η 6 ‐C 6 H 5 F)][BAr F 4 ], was also used to prepare the deuterium‐labeled on boron precursor amine–boranes by H/D exchange with D 2 .…”