Low-Valent Tungsten Redox Catalysis Enables Controlled Isomerization and Carbonylative Functionalization of Alkenes

Abstract

Tungsten catalysis has played an instrumental role in the history of organometallic chemistry, with electrophilic, fully oxidized W(VI) catalysts featuring prominently in olefin polymerization and metathesis reactions. Here, we report that the simple W(0) precatalyst, W(CO)<sub>6</sub>, catalyzes the isomerization and hydrocarbonylation of alkenes via a W(0)/W(II) redox couple. The 6- to 7-coordinate geometry changes associated with this redox process are key in allowing isomerization to take place over multiple positions and stop at a defined unactivated internal site that is primed for <i>in situ</i> functionalization. DFT studies and crystallographic characterization of multiple directing-group-bound W(II) model complexes illuminate potential intermediates of this redox cycle and showcase the capabilities of the 7-coordinate W(II) geometry to facilitate challenging alkene functionalizations.

Low-Valent Tungsten Catalysis Enables Site-Selective Isomerization–Hydroboration of Unactivated Alkenes

“…These results support the notion that alkene isomerization proceeds via a 1,3-hydride shift. This is in contrast with our previous method, which gave terminal D-incorporation consistent with a 1,2-hydride shift pathway. Conducting the reaction without HBpin corroborated previous studies that showed the ability of W(0) to promote isomerization via 1,3-H shift without external hydride sources. , In addition, the lack of deuterium incorporation at the β-C­(sp 3 ) site argues against a 1,1-hydroboration of a Fischer-type W-carbene.…”

“…Although Koh recently leveraged the stabilizing features of π-benzyl intermediates to promote a homobenzylic borylation, the means to enable C–B bond formation through alkylmetal stabilization at less-activated sp 3 sites via chain-walking still remain elusive. On the basis of recent findings by our group, where a W-catalyst and bidentate directing group were used to control an isomerization–hydrocarbonylation reaction, we wondered whether we could expand this strategy to enable a W-catalyzed C­(sp 3 )-borylation controlled by native directing groups at C­(sp 3 )–H sites that are beyond reach in conventional chain-walking events (Scheme b) . Such a method would complement existing approaches toward β-boryl carbonyl compounds in terms of scope and chemoselectivity .…”

“…Specifically, we believed that the ability of low-valent W catalysts to adopt multiple coordination geometries ,, combined with their high Lewis acidity when compared to later transition metals would be particularly critical for success. We anticipated that such a technology would not only expand the boundaries of chain-walking reactions by offering a complementary site-selectivity profile but also stimulate the adoption of low-valent W catalysts in alkene functionalization. , As part of our interest in alkene functionalization and chain-walking reactions, we describe the successful implementation of this goal, culminating in the development of the first W-catalyzed hydroboration of alkenes …”

“…Next, we turned our attention to studying the mechanism of our C­(sp 3 ) β-borylation. A priori , one might expect that both 1,2- and 1,3-hydride shifts might come into play for the initial alkene isomerization. ,, To this end, we conducted the borylation of 1a and 1i with DBpin. Deuterium incorporation at the γ-position was anticipated for a pathway consisting of 1,3-hydride shift; on the contrary, labeling at the terminal δ sp 3 site would indicate a mechanism via 1,2-hydride shift. As shown in Scheme , deuterium was incorporated exclusively into the γ-position of 2a- d and 2i- d (66–67%), with the H-containing products likely arising from residual HBpin in the DBpin solution.…”