Abstract
Precatalysts active for the dehydropolymerisation of primary amine-boranes are generally based on mid or late transition metal. We have found that the activity of the precatalyst system formed from CpR2TiCl2 and 2nBuLi towards the dehydrogenation of the secondary amine-borane Me2NH·BH3, to yield the cyclic diborazane [Me2N-BH2]2, increases dramatically with increasing electron-donating character of the cyclopentadienyl rings (CpR). Application of the most active precatalyst system (CpR = η-C5Me5) to the primary amine-borane MeNH2·BH3 enabled the first synthesis of high molar mass poly(N-methylaminoborane), [MeNH-BH2] n , the BN analogue of polypropylene, by an early transition metal such as catalyst. Significantly, unlike other dehydropolymerization precatalysts for MeNH2·BH3 such as [Ir(POCOP)H2], skeletal nickel, and [Rh(COD)Cl]2, the Ti precatalyst system was also active towards a range of substrates including BzNH2·BH3 (Bz = benzyl) yielding high molar mass polymer. Moreover, in contrast to the late transition metal catalysed dehydropolymerisation of MeNH2·BH3 and also the Ziegler-Natta polymerisation of olefins, studies indicate that the Ti-catalyzed dehydropolymerization reactions proceed by a step-growth rather than a chain-growth mechanism.
Highlights
Catalysis plays a pivotal role in molecular and macromolecular C–C bond forming chemistry
In addition to our report of [CpFe(CO)2]2 as an example of an earth abundant transition metal catalyst, we described the use of the group 4 metallocene precatalysts Cp2TiCl2 (6a vide infra)/two equiv. of nBuLi or Cp2Ti(PMe3)[2] as reasonably efficient dehydrocoupling catalysts for the secondary amine–borane Me2NH$BH3 (1), yielding the cyclodiborazane [Me2N–BH2]2 (3) (Scheme 1).[14]
Our initial investigations were based on the in uence of cyclopentadienyl ligand substitution on the activity of a series of twocomponent precatalysts, which were formed by CpR2TiCl2 and 2nBuLi
Summary
In addition to our report of [CpFe(CO)2]2 (ref. 6b) as an example of an earth abundant transition metal catalyst, we described the use of the group 4 metallocene precatalysts Cp2TiCl2 (6a vide infra)/two equiv. of nBuLi or Cp2Ti(PMe3)[2] as reasonably efficient dehydrocoupling catalysts for the secondary amine–borane Me2NH$BH3 (1), yielding the cyclodiborazane [Me2N–BH2]2 (3) (Scheme 1).[14]. Of nBuLi or Cp2Ti(PMe3)[2] as reasonably efficient dehydrocoupling catalysts for the secondary amine–borane Me2NH$BH3 (1), yielding the cyclodiborazane [Me2N–BH2]2 (3) (Scheme 1).[14] Others11b,15,16 have reported the use of neutral TiII and ZrII, and cationic ZrIV precatalysts for the dehydrocoupling of 1. From these studies, two general reaction mechanisms have been proposed. We report structure-correlated kinetic studies of different titanium based precatalyst systems for the dehydrogenation of the secondary amine–borane Me2NH$BH3 (1), and based on these results, the rst successful dehydropolymerisation of primary amine–boranes, yielding high molecular weight polyaminoboranes, that proceeds by a step-growth rather than a chain-growth mechanism
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