4.8 Article

Accessing unusual heterocycles: ring expansion of benzoborirenes by formal cycloaddition reactions

Journal

CHEMICAL SCIENCE
Volume -, Issue -, Pages -

Publisher

ROYAL SOC CHEMISTRY
DOI: 10.1039/d3sc03433

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In this study, it was shown that a kinetically stabilized benzoborirene 1 can react with trimethylphosphine oxide, acetaldehyde, and tert-butyl isonitrile. The reaction with trimethylphosphine oxide resulted in the formation of benzo[c][1,2,5]oxaphosphaborole, which exhibited a long apical PO bond. The reactivity of benzoborirene 1 was consistent with computational analyses.
Benzoborirenes are a very rare class of strained boron heterobicyclic systems. In this study a kinetically stabilized benzoborirene 1 is shown to react with multiple bonds of trimethylphosphine oxide, acetaldehyde, and tert-butyl isonitrile. The (2 + 2) cycloaddition product with trimethylphosphine oxide, benzo[c][1,2,5]oxaphosphaborole, has a long apical PO bond (194.0 pm) that must be considered on the border line between ionic and covalent according to the natural bond orbital, quantum theory of atoms in molecules, and compliance matrix approaches to the description of chemical bonding. The coordination compound between the benzoborirene and phosphine oxide was observed by NMR spectroscopy at 213 K. The Lewis acidity of 1 is similar to that of B(OCH2CF3)3 and B(C6F5)3 based on the 31P{1H} NMR chemical shift of the Lewis acid base complexes with trimethylphosphine oxide at 213 K. Benzoboriene 1 does not react with acetone, but forms a (2 + 2) cycloaddition product, an oxaborole, with acetaldehyde. In contrast, it undergoes a double (2 + 1) reaction with tert-butyl isonitrile to yield a boro-indane derivative under mild conditions. The observed reactivity of 1 is in agreement with computational analyses of the respective potential energy surfaces. A kinetically stabilized benzoborirene releases its strain by ring enlargement reactions with multiply bonded small molecules by (2 + 2) or (2 + 1) reactions.

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