Metal-free dehydropolymerisation of phosphine-boranes using cyclic (alkyl)(amino)carbenes as hydrogen acceptors

The divalent carbene carbon centre in cyclic (alkyl)(amino)carbenes (CAACs) is known to exhibit transition-metal-like insertion into E–H σ-bonds (E = H, N, Si, B, P, C, O) with formation of new, strong C–E and C–H bonds. Although subsequent transformations of the products represent an attractive strategy for metal-free synthesis, few examples have been reported. Herein we describe the dehydrogenation of phosphine-boranes, RR’PH·BH3, using a CAAC, which behaves as a stoichiometric hydrogen acceptor to release monomeric phosphinoboranes, [RR’PBH2], under mild conditions. The latter species are transient intermediates that either polymerise to the corresponding polyphosphinoboranes, [RR’PBH2]n (R = Ph; R’ = H, Ph or Et), or are trapped in the form of CAAC-phosphinoborane adducts, CAAC·H2BPRR’ (R = R’ = tBu; R = R’ = Mes). In contrast to previously established methods such as transition metal-catalysed dehydrocoupling, which only yield P-monosubstituted polymers, [RHPBH2]n, the CAAC-mediated route also provides access to P-disubstituted polymers, [RR’PBH2]n (R = Ph; R’ = Ph or Et).


Synthesis of 3a
Method A: PhPH2·BH3 (74 mg, 0.60 mmol) and CAAC Me (171 mg, 0.60 mmol) were dissolved in THF (0.5 mL) in a quartz J. Young NMR tube. P-H activation occurred instantly at 22 °C to give two diastereomers. This was immediately followed by a degree of formation of [PhHPBH2]n and (CAAC Me )H2, hence isolation of these compounds has not been achieved and there are traces of (CAAC Me )H2 visible in the 1 H NMR spectrum. The initial ratio of diastereomers observed after ten minutes in solution was 1:12.4.

Method B:
3a can also be synthesised through a stepwise procedure PPhH2 (used as a ca. 10 % weight solution in hexanes, 4.24 g of hexanes solution, 3.90 mmol) was added to a solution of CAAC Me (1.00 g, 3.50 mmol) in THF (10 mL) and stirred for 20 minutes at 22 °C. The volatiles were removed in vacuo to leave a pale yellow powder. NMR S11 spectroscopic data for CAAC Me (H)(PPhH) obtained was directly comparable to those described in the literature for CAAC Cy (H)(PPhH). 7 The powder was redissolved in THF (5 mL) and BH3·THF (3.50 mL of a 1M in THF solution, 3.50 mmol) was added. Volatiles were immediately removed in vacuo, however as with method A above it was not possible to isolate the product as a degree of formation of [PhHPBH2]n and (CAAC Me )H2 immediately occurs.

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No detection of separate peaks for the different isomers, postulated to be due to broad peaks overlapping. 31

Mechanistic studies
Proposed and subsequently discounted mechanisms for phosphine-borane dehydrogenation mediated by CAAC Me i) P-H activation followed by -bond metathesis P-H activation to give 3a followed by a -bond metathesis step, where simultaneous B-H and C-P bond cleavage occurs, releasing the reactive monomer, . A similar hydride transfer mechanism has been reported previously for the conversion of an N-heterocyclic phosphinophosphine-borane to a mixture of N-heterocyclic phosphine and cyclic phosphinoboranes. 9 This mechanism was ruled out using DFT as no concerted transition state leading to (CAAC Me )H2 and [PhHP-BH2] could be located.

ii) P-H activation followed by H2 elimination
P-H activation to give 3a followed by the formation of a five-centre transition state where the 4atm' below).

iii) Concerted hydrogen transfer
The P-H activation to give 3a is reversible and there is an irreversible concerted hydrogen transfer from PhPH2·BH3 to CAAC Me to give (CAAC Me )H2 and [PhHP-BH2]. This mechanism was ruled out using DFT as no concerted transition state (TS) leading to (CAAC Me )H2 and [PhHP-BH2] in one step could be located.

iv) B-H activation followed by -bond metathesis
The P-H activation to give 3a is reversible and the products are achieved via a B-H activation compound and undergoing σ-bond metathesis step where simultaneous P-H and C-B bond cleavage occurs. This mechanism was ruled out using DFT as the TS for the B-H activation step has a significantly higher energy barrier than for the P-H activation step (34.2 kcal mol -1 vs 4.9 kcal mol -1 ).

v) Homolytic cleavage of P-C bond
After 3a is formed homolytic cleavage of the P-C bond occurs to give a [CAAC Me (H)] • and a [PhPH(BH3)] • radical. The mechanism was ruled out using DFT as the energy for P-C bond homolysis is very high (55.8 kcal mol -1 ) and significantly higher than the heterolytic cleavage of the P-C bond (35.3 kcal mol -1 ) S26

vi) Intermolecular hydride abstraction with an iminium ion leading to a borenium ion
We considered an intermolecular hydride abstraction from 3a, Supplementary discussion of the polymerisation mechanism from phosphinoborane monomers Studies on the mechanism of polymerisation of phosphinoborane monomers, [RR'P-BH2], generated from the reaction of a CAAC with a phosphine-borane substrate, are complicated due to several key factors: 1) The monomers generated in situ are highly reactive and readily polymerise and thus have not been isolated in their pure monomeric form.
2) A number of different polymer architectures are obtained from the polymerisation reaction including cyclic and linear oligomers, and linear polymers.

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3) There is not a single well-defined initiator-derived end-group, and there are limitations in determining the identity of the various end-groups present in samples of the polymers based on the experimental techniques available.
Here we propose a polymerisation mechanism which involves several non-mutually exclusive pathways which likely occur in parallel, and discuss our experimental observations and limitations.
We propose that the ambipolar phosphinoborane monomer can self-initiate and spontaneously undergo an addition head-to-tail polymerisation sequence, see pathway a) shown above. This mechanism is reminiscent of the metal-free thermolysis of amine-stabilised phosphinoboranes reported by Scheer and Manners. 10 Figure 40). The milder conditions required to generate the phosphinoborane using CAAC likely do not facilitate branching or cross-linking that could inhibit high molar mass material being formed as opposed to when the monomer is thermally generated from the phosphine-borane under more harsh thermal conditions. 8 In addition, in concert with the proposed self-initiated pathway a) above, trace Lewis acids or bases present in the reaction mixture could engage in chain-end capping of polymer chain or monomer at any point during the polymerisation. These chain-end capping reactions may favour linear propagation by preventing the back-biting reaction, see pathway b) above.
Traces of free phosphine and borane can be generated thermally from dissociation of the phosphine-borane adduct, and traces of free CAAC can also be present in the reaction mixture.
It is noteworthy that the electronic effect of the neutral Lewis acid or base capping group on the reactive P or B chain-end diminishes as the polymer chain lengthens, this is in contrast to radical or ionic chain polymerisations. In well-defined chain polymerisations the radical or charged reactive site consistently migrates to the end of polymer chain with each monomer addition, leading to solely unidirectional chain growth.  in C-1'pair differs from the orientation in TS1' and is similar to that found for C-1pair, indicating that the relative orientation present in C-1pair is favoured (See Supplementary Table 3). In addition, attempts to structurally optimise C-1'pair with toluene as solvent only furnished only the P-H activation product F'.
The P-H activation product was calculated with either a SP,S configuration (F) or a RP,S configuration (F'), the latter being slightly higher in energy. A slightly higher activation barrier    Gibbs free energies are given below the structures in kcal mol −1 in the gas phase.   Oldroyd.

Synthesis of 3c
rac-PhEtHP·BH3 (50 mg, 0.33 mmol) and CAAC Me (94 mg, 0.33 mmol) were dissolved in toluene (2 mL) in a J. Young Schlenk tube. Immediate conversion to the product (as two diastereomers 3c' and 3c'') was observed through 31 P NMR spectroscopy. The initial ratio of diastereomers observed after 10 minutes in solution was 1 : 1.14. Crystals of 3c suitable for X-ray  Oldroyd.

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Supplementary Tables X-ray crystallography X-ray diffraction experiments on 1b, 3c, 4a, 4b and CAAC Me H2 were carried out at 100(2) K on a Bruker APEX II CCD diffractometer using Mo-Kα radiation (λ = 0.71073 Å). Intensities were integrated in SAINT 28 and absorption corrections based on equivalent reflections were applied using SADABS 29 . The structures were solved using SHELXT 30 and refined against all F 2 in SHELXL 31 using Olex2. 32 All the non-hydrogen atoms were refined anisotropically. While all of the hydrogen atoms were located geometrically and refined using a riding model, apart from the B-H protons in 1b, 3c, 4a and 4b which were located in the difference map and refined freely. In 3c H9 and H26 were also located in the difference map with isotropic displacement parameters Uiso(H) = 1.2Ueq(C). Disordered portions of a THF solvent molecule in the structure of 1b·(THF) were successfully modelled over two positions and refined with the sum of the occupancies set to 1. Residual electron density from a disordered solvent molecule in the lattice of 4b was removed using a solvent mask in Olex2, and the contributions were excluded from the formula. Crystal structure and refinement data are given in Supplementary