Ru-Catalyzed Hydroboration of Ynones Leads to a Nontraditional Mode of Reactivity.

Although hydroboration of simple ketones and alkynes have been well-established, little is known about the unique hydroboration reactivity for ynones, a family of important building blocks. Herein we report a new reaction mode of ynones leading to structurally novel and synthetically useful but previously inaccessible products, vinyl α-hydroxylboronates, under mild ruthenium-catalyzed hydroboration conditions. This reaction features high efficiency, a broad scope, and complete chemo-, regio-, and stereoselectivity, in spite of many possible competitive pathways. Both control experiments and detailed DFT studies suggested a two-step mechanism, involving initial rate-determining conjugate addition of hydroborane to form the key boryl allenolate intermediate followed by a fast second hydroboration of the enolate motif of the allenolate. Notably, direct 1,4-addition of hydroborane to carbonyl-conjugated alkynes also represents a new mode of reactivity. Despite the overwhelming complexity of this process, which involves selectivity control in almost every step, a thorough and detailed computation on a large set of possible transition states explained the unusual reactivity and intrinsic origin of selectivity.