Photoinduced Alcohol and Ketone Generation from Alkoxyaroylsilanes: Mechanistic Insights into Competing Radical Pathways

Acylsilanes represent a unique class of organosilicon compounds with distinctive photochemical reactivities, including hydrogen atom transfer (HAT) and silyl shift pathways. Recently, a new photolabile protecting group (PPG) benzoyldiisopropylsilane (BDIPS) featuring an acylsilane functionality was introduced for alcohol protection. While the photomediated deprotection of aliphatic silyl ethers in methanol provided the free alcohols, BDIPS-protected benzyl or allyl alcohols resulted in rearranged ketones upon photoexcitation in acetonitrile. In this work, we systematically investigate the photochemical behavior of different BDIPS-ethers, focusing on the mechanistic divergence leading to either alcohol release as PPG or rearranging ketone formation. Through a combined approach of femtosecond transient absorption spectroscopy and density functional theory calculations, we elucidate the competing reaction pathways for model compounds 1a, 1b, and 1c. Our results reveal that the presence of an α-hydrogen adjacent to an olefinic moiety kinetically favors the HAT pathway, yielding rearranged ketone products, while its absence promotes a silyl shift mechanism that results in efficient alcohol photodeprotection. Furthermore, solvent-dependent studies demonstrate distinct photoreaction behaviors for 1c in methanol and acetonitrile, underscoring the role of the local chemical environment in steering reaction outcomes. This study provides fundamental insights into the structure-reactivity relationships of acylsilane-based PPGs and offers a strategic basis for the rational design of photoresponsive systems with programmable release properties.