Opportunities Using Boron to Direct Reactivity in the Organic Solid State

Supramolecular Matter Through Crystal Engineering: Covalent Bond Formation to Postsynthetic Modification

Supramolecular chemistry can transform organic synthesis by revealing that crystalline materials are not static but rather dynamic environments for controlled covalent bond formations and manipulations. This review focuses on how supramolecular chemistry can be developed to direct molecular synthesis in the organic solid state, directing reliable C─C bond formations to enable transformations difficult or impossible in solution. Special attention is given to postsynthetic modifications that serve to broaden the functional scope of solid-state reactivity allowing organic crystals to be developed as molecular flasks and a form of supramolecular matter.

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

Separations of azeotropic mixtures are typically carried out using energy-demanding processes (e.g., distillation). Here, we report the capacity of a self-assembled chlorinated boronic ester-based adduct to confine acetonitrile and benzene in channels upon crystallization. The solvent confinement occurs via a combination of hydrogen bonding and [π···π] interactions. Quantitative separation of benzene from an azeotropic 1:1 mixture of a benzene/acetonitrile (v/v), and methanol is achieved through crystallization with the chlorinated adduct by complementary [C-H···O] and [C-H···π] interactions. Inclusion behavior is rationalized by molecular modeling and crystallographic analysis. The chlorinated boronic ester adduct shows the potential of modularity via isosteric substitution for the separation of challenging chemical mixtures (e.g., azeotropes).

Emergent Organoboron Acid Catalysts

Organoboron acids are stable, organic-soluble Lewis acids with potential application as catalysts for a wide variety of chemical reactions. In this review, we summarize the utility of boronic and borinic acids, as well as boric acid, as catalysts for organic transformations. Typically, the catalytic processes exploit the Lewis acidity of trivalent boron, enabling the reversible formation of a covalent bond with oxygen. Our focus is on recent developments in the catalysis of dehydration, carbonyl condensation, acylation, alkylation, and cycloaddition reactions. We conclude that organoboron acids have a highly favorable prospectus as the source of new catalysts.

Mechanochemical Mediated Coexistence of B←N Coordination and Hydrogen Bonding

Mechanochemistry afforded a photoactive cocrystal via coexisting (B)O-H⋅⋅⋅N hydrogen bonds and B←N coordination. Specifically, solvent-free mechanochemical ball mill grinding and liquid-assisted grinding of a boronic acid and an alkene resulted in mixtures of hydrogen-bonded and coordinated complexes akin to mixtures of noncovalent complexes that can be obtained in solution in equilibria processes. The alkenes of the hydrogen-bonded assembly undergo an intermolecular [2+2] photodimerization in quantitative conversion, effectively reporting the outcome of the self-assembly processes. Our results suggest that interplay involving noncovalent bonds subjected to mechanochemical conditions can lead to functional solids where, in the current case, the structure composed of the weaker hydrogen bonding interactions predominates.

Structures and Reactivities of Cocrystals Involving Diboronic Acids and Bipyridines: In Situ Linker Reaction and 1D‐to‐2D Dimensionality Change via Crystal‐to‐Crystal Photodimerization

Cocrystallizations of diboronic acids [1,3‐benzenediboronic acid (1,3‐bdba), 1,4‐benzenediboronic acid (1,4‐bdba) and 4,4’‐biphenyldiboronic acid (4,4’‐bphdba)] and bipyridines [1,2‐bis(4‐pyridyl)ethylene (bpe) and 1,2‐bis(4‐pyridyl)ethane (bpeta)] generated the hydrogen‐bonded 1 : 2 cocrystals [(1,4‐bdba)(bpe)₂] (1), [(1,4‐bdba)(bpeta)₂] (2), [(1,3‐bdba)(bpe)₂(H₂O)₂] (3) and [(1,3‐bdba)(bpeta)₂(H₂O)] (4), wherein 1,3‐bdba involved hydrated assemblies. The linear extended 4,4’‐bphdba exhibited the formation of 1 : 1 cocrystals [(4,4'‐bphdba)(bpe)] (5) and [(4,4'‐bphdba‐me)(bpeta)] (6). For 6, a hemiester was generated by an in‐situ linker transformation. Single‐crystal X‐ray diffraction revealed all structures to be sustained by B(O)−H⋅⋅⋅N, B(O)−H⋅⋅⋅O, O_w−H⋅⋅⋅O, O_w−H⋅⋅⋅N, C−H⋅⋅⋅O, C−H⋅⋅⋅N, π⋅⋅⋅π, and C−H⋅⋅⋅π interactions. The cocrystals comprise 1D, 2D, and 3D hydrogen‐bonded frameworks with components that display reactivities upon cocrystal formation and within the solids. In 1 and 3, the C=C bonds of the bpe molecules undergo a [2+2] photodimerization. UV radiation of each compound resulted in quantitative conversion of bpe into cyclobutane tpcb. The reactivity involving 1 occurred via 1D‐to‐2D single‐crystal‐to‐single‐crystal (SCSC) transformation. Our work supports the feasibility of the diboronic acids as formidable structural and reactivity building blocks for cocrystal construction.

Isolation of elusive cyclobutane ligands via template assisted photochemical [2+2] cycloaddition reaction and their utility in engineering crystalline solids

Solid-state photochemical [2+2] cycloaddition reactionis one of the promising routes to isolate highly strained cyclobutane derivatives whichmay otherwise be very difficult to obtain via conventional synthetic methodology. Various methods have...