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Unsymmetric diborane(4) compounds for small molecule activation and B-B coupling

Periodic Reporting for period 1 - DIBOR (Unsymmetric diborane(4) compounds for small molecule activation and B-B coupling)

Reporting period: 2016-05-01 to 2018-04-30

Organoboron complexes are ubiquitous reagents in synthesis due to their utility in functional group transformations including the Nobel Prize winning Suzuki-Miyaura reaction. Many routes to organoboranes use diboron(4) compounds (e.g. (RO)2B-B(OR)2, thus these compounds have received considerable interest, as have diboron(5) derivatives that possess two distinct boron moieties. Most unsymmetrical diboron(5) species are accessed by adding a neutral or anionic Lewis base to a (RO)2B-B(OR)2 precursor, quaternizing one boron center. This leads to the polarization of the B-B σ bond and these mixed sp2/sp3 species can then react as a source of nucleophilic boron that reacts with an array of carbon electrophiles, thus providing a powerful route for forming C-B bonds. Whilst the use of diborane(5) as a source of nucleophilic boron is now well established reports on the development of unsymmetrical diboron(4) and diboron(5) compounds that have significant electrophilicity are much less common. However, the limited reports to date have demonstrated that electrophilic diboron compounds exhibit unusual reactivity towards small molecules and C-H bonds for example, indicating that more need to be synthesised and further exploration of their reactivity performed.

Thus our initial objective was to synthesise a family of new unsymmetrical diboron(5) precursors that are readily amenable to further transformations, specifically:
(i) to readily access cationic diboron(4) electrophiles thereby enabling us to explore their use in small molecule and sigma-bond activation;
(ii) to undergo reductive coupling to form extended chains of homocatenated boron linked by electron precise B-B and B=B bonds.

The latter objective is of import as it seeks to develop the fundamental chemistry of electron precise boron compounds, particularly targeting systems containing both B-B single bonds and B=B double bonds. While the chemistry of boron’s near neighbour carbon exhibits a rich diversity of conjugated C-C, C=C and C≡C containing structures, before this work no boron compound containing a conjugated B-B=B-B structure had been reported. Thus this objective will develop our understanding of the fundamental properties of boron analogues of conjugated hydrocarbons and thus enable future developments in the highly topical area of electron precise boron compounds.
Overview of Results:
We successfully developed the high yielding synthesis of a family of sp2-sp3 diboron(5) species, e.g. CatB-BCl2(NHC) (1, Figure 1, NHC = N-heterocyclic carbene), in two steps from commercial materials. The synthesis consists of combing a NHC adduct of B2(cat)4 (cat=catechol) with BCl3 to form 1. This synthesis works with an array of different NHCs and with different boron trihalides (BCl3 and BBr3).

Our next objective was to access and assess the reactivity of borocations derived from 1 by halide abstraction. A range of halide abstracting agents were reacted with 1 including neutral halophilic Lewis acids (e.g. AlCl3) and Na salts of weakly coordinating borate anions. However, only decomposition products were observed (e.g. CatBCl). Attempts to trap the putative cation [CatB-B(X)(NHC)]+ by using pi nucleophiles or small molecules (e.g. H2) to react with it led to complex mixtures or again formation of CatBCl and other unidentified boron products. To overcome this, we performed the halide abstraction in the presence of Lewis bases (LB = 2-DMAP and PPh3). This strategy resulted in the formation of new Lewis base-stabilised diboron(5) monocations, e.g. CatB-B(Cl)(NHC)(LB) (2, Figure 1). However, these stabilised diboron(5) cations proved insufficiently electrophilic to activate pi nucleophiles.

Due to the instability of the diboron(4) cation derived from halide abstraction from 1, our next target was exchanging halide for an alternative substituent, specifically CatB-BH2(NHC), 3. The reaction of 1 with tin hydrides and a catalytic amount of halide abstractor resulted in the formation of the aforementioned dihydro-diboron compound, (3, Figure 1). Again in our hands it was not possible to achieve the synthesis and isolation of the putative 1-hydrodiboron(4) monocation, [CatB-B(H)(NHC)]+ derived from hydride abstraction due to its instability, with CatBH observed to form rapidly on hydride abstraction from 3. Again attempts to assess the reactivity of the cation derived from 3 by synthesising it in the presence of small molecules or pi nucleophiles led to complex intractable mixtures or the same outcome as in their absence (formation of CatBH).

The other key objective was using CatB-BX2(NHC) precursors to form B=B containing systems. Thus reaction of the dibromo congener, CatBBBr2(NHC) 4, with reducing agents (e.g. KC8) resulted in the reductive coupling of two diboron units forming the desired coplanar B4 chains (Figure 2). DFT calculations showed that the HOMO is strongly stabilized by pi delocalisation from the B=B bond to the outer boron atoms. These are the first boryl functionalized diborenes, thus are the first boron chain compounds showing extended pi conjugation along multiple (>2) boron centres.

Dissemination Summary:
This work has been presented by the Marie Curie Fellow, Dr. Jessica Cid, at the Summer School on Molecular Boron Chemistry (Wuerzburg, 2016), the 12th International Conference of Heteroatom Chemistry (Vancouver, 2017) and in the Annual Main Group Interest Group Meeting and AGM (London, 2017). Furthermore, this work has been published in two separate publications . In addition other team members (e.g. Dr Michael Ingleson and Prof. Holger Braunschweig) have presented elements of this work multiple times in their home countries and overseas).
During these two years, we have achieved the majority of our initial objectives and thus have made significant progress beyond the state of the art. For example we have synthesised a new family of NHC-coordinated unsymmetrical diboron(5) compounds. These compounds are extremely simple to access and we believe this novel methodology will be useful to access other unsymmetrical multiple boron containing systems in the future. Furthermore, B-H derivatives were accessed through the subsequent reaction with tin hydrides and Na[BArCl] as a catalyst.

When treated with a Lewis base and AlCl3 or K[B(C6F5)4] these unsymmetrical diboron(5) compounds led to the formation of Lewis base-stabilized di-boron(5) monocations supporting the intermediacy of electrophilic diboron intermediates. The number of these unsymmetrical and significantly electrophilic diboron compounds is still limited, and here we have presented several examples and studied their reactivity.

The reductions of these NHC-coordinated unsymmetrical diboron(5) compounds was more successful and we met out key objective, specifically the formation of diboryl diborenes species. These are the first examples of acyclic boron chains containing delocalized pi systems. The observation of pi-electron delocalization in these B4 chains is an exciting discovery which points to the possible future synthesis of extended -conjugated boron chains and polymers with interesting electronic properties. Thus this is significantly beyond the state of the art and will advance the field of electron precise boron compounds and conjugated boron chemistry. This is an area that is remarkably underdeveloped compared to its near neighbor carbon, thus this work will have significant impact on this field.
Figure 2. Reduction of diboron(5) compounds
Figure 1. Reactivity of diboron(5) compounds