Opportunities Using Boron to Direct Reactivity in the Organic Solid State

Abstract

This Account describes work by our research group that highlights opportunities to utilize organoboron molecules to direct chemical reactivity in the organic solid state. Specifically, we convey a previously unexplored use of hydrogen bonding of boronic acids and boron coordination in boronic esters to achieve [2+2]-photocycloadditions in crystalline solids. Organoboron molecules act as templates or ‘shepherds’ to organize alkenes in a suitable geometry to undergo regio- and stereoselective [2+2]-photocycloadditions in quantitative yields. We also provide a selection of publications that served as an inspiration for our strategies and offer challenges and opportunities for future developments of boron in the field of materials and solid-state chemistry.1 Introduction1.1 Template Strategy for [2+2]-Photocycloadditions in the Solid State2 Boronic Acids as Templates for [2+2]-Photocycloadditions in the Solid State2.1 Supramolecular Catalysis of [2+2]-Photocycloadditions in the Solid State Using Boronic Acids3 Boronic Esters as Templates for [2+2]-Photocycloadditions in the Solid State3.1 Application of Photoproducts: Separation of Thiophene from Benzene through Crystallization3.2 Crystal Reactivity of B←N-Bonded Adducts: The Case of Styryl­thiophenes4 Conclusions and Perspectives

Emergent Organoboron Acid Catalysts

“…In the solid state, boronic acids have been used to template reactants through hydrogen bonding, heteroatom coordination, or the formation of borate esters to allow for selective and highyielding [2 + 2] cycloadditions upon irradiation. 153,154 Thus, boronic acids present a relatively untapped stratagem for cycloaddition catalysis.…”

“… They proposed that the boronic acid promotes the heterolytic cleavage of the hydroxy C–O bond of the benzofuran through the formation of a borate ester and that the resulting stabilized cation undergoes a cycloaddition with the diene to produce the desired seven-membered ring under mild conditions. In the solid state, boronic acids have been used to template reactants through hydrogen bonding, heteroatom coordination, or the formation of borate esters to allow for selective and high-yielding [2 + 2] cycloadditions upon irradiation. , Thus, boronic acids present a relatively untapped stratagem for cycloaddition catalysis.…”

Confinement and Separation of Benzene from an Azeotropic Mixture Using a Chlorinated B←N Adduct

“…Our group and others have employed boronic ester coordination with pyridines (B←N) to generate H-shaped and T-shaped adducts. The adducts have enabled the confinement and separation of petrochemicals, and the design of electronic and dynamic materials .…”

Mechanochemical Mediated Coexistence of B←N Coordination and Hydrogen Bonding

“…[9][10][11] Boronic acids (BAs) and related derivatives (e.g., boronic esters, boroxines) are unique and intriguing platforms from which to study self-assembly. As a building block in a supramolecular synthesis, the B(OH) 2 group of an aryl BA can react to participate in hydrogen bonds [12][13][14][15][16][17][18] (i.e., OÀ H•••X) and/or B-coordination (i.e., BÀ X), [5,[18][19][20][21][22][23] depending on reaction conditions. Thus, hydrogen-bonded cocrystals of boronic acids will typically form from solution at room temperature [14][15][16][17] while boroxines typically form with addition of heat [23][24][25] or added reaction time.…”