Photoinduced Deprotection of 2-Nitrophenylneopentyl Glycol Boronates Enables Light-Triggered Polycondensation of Siloxanes.

Abstract

Various protecting groups have been developed for boronic acids, mostly based on diols. Alternatives include trifluoroborates and amine complexes, which offer easier synthesis and release under milder conditions. We present here a new strategy involving photocleavable protecting groups for boronic and borinic acids, based on the 2-nitrobenzyl motif. A screening of several 1,2- and 1,3-diols bearing a 2-nitrobenzyl group led us to identify 1-(2-nitrophenyl)neopentyl glycol (npnp) for the protection of boronic acid derivatives. This diol is easily prepared in a single step on gram scale, and the corresponding npnp boronates are stable in wet acetonitrile at 90 °C for several days, and under under acidic conditions. Irradiation at 365 nm in acetonitrile allows for the controlled liberation of boronic acids in good to excellent yields, a method also applied to a dimesitylborinic ester bearing a 2-nitrobenzylalcohol moiety. 1-(2-Nitrophenyl)neopentyl glycol boronates ArB(npnp) demonstrated their utility in light-triggered siloxane crosslinking. We showed that catalysts incorporated into a polymer matrix, irradiated, and then incubated at 50 °C for 7 days resulted in efficient polymerization, forming solid materials in some cases.