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Dihydrogen Activation at Non-Metallic Centers

Final Report Summary - DANMC (Dihydrogen Activation at Non-Metallic Centers)

The primary goal of this project is to develop new systems for heterolytic dihydrogen activation and catalytic hydrogenation, which contain non-fluorinated-triarylboranes as hydride acceptor and a basic oxygen as proton acceptor. Systems of this type would considerably enhance the scope of nonmetal-based catalytic hydrogenation, because the electrophilic centers involved are less Lewis acidic than the fluorinated arylboranes used so far. Our proposed alternative to FLPs is based on the formation of a strained benzo-1,2-oxetane, in which the energy released upon ring-opening will be the driving force to effect the heterolytic splitting of dihydrogen (Scheme 4). We propose that the energetic cost of the H-H splitting will be compensated by the synergistic combination of three factors: (1) liberation of the ring strain in the four-membered ring, coupled with (2) the energy released as a result of the partial localization of the π-cloud in the aromatic ring (SIBL), and finally (3) the inherent Lewis acidity of boron, which should facilitate formation of the corresponding borohydride.
Specifically, we have explored the use of a triarylboron compound in which one of the aryl groups bears a hydroxyl group ortho to the boron atom (Scheme 1, 5). Upon basic treatment, the phenol is deprotonated, and interaction of the anion with the nearby electron-deficient boron results in formation of a tetracoordinate 1,2-oxaboretanide. This borate 6 should be thermodynamically unstable because of two main reasons: first, the high strain of the four-membered ring. Second, participation of the aryl group in the four-membered ring implies a partial localization of the π-cloud, which results in destabilization of the ground state of the molecule. Our system posseses two features that should facilitate the process: variation of the substituents on boron R to tune the Lewis acidity of the system, thus facilitating formation of the desired borohydride, and potential introduction of bulky ortho substituents in the aromatic ring to help prevent dimerization (which might result in diminished reactivity). A simplified mechanistic cycle for the proposed dihydrogen activation is outlined in Scheme 1 (right). Treatment of 7 with a base results in formation of a phenolate, and subsequent formation of borate 8. Interaction of the antibonding σ* orbital of H2 with a non-bonding lone pair of oxygen, facilitated by the basicity of oxygen. H2 cleavage, where the lone pair of oxygen populates the antibonding σ* orbital of H2, consequently weakening both the H-H and the B-O bonds. At that point the H2 σ-bonding orbital donates into the vacant p orbital of boron, and heterolytic cleavage occurs. H2 extrusion to reform the borate.