Unexpected Propargylic Retro-Brook Rearrangements in Alkynes

Abstract

Retro-Brook rearrangements refer to the intramolecular migration of a silyl group from oxygen to carbon. In this study, we report a novel propargylic retro-Brook rearrangement observed in terminal alkynes bearing a silyl ether moiety. Retro-Brook rearrangements involving [1,2]-, [1,4]-, and [1,5]-migrations are described, affording propargylsilanes in reasonable yield. The reaction mechanism was investigated experimentally by deuterium quenching and rationalized by density functional theory calculations. The terminal alkyne and the subsequent propargyl/allenyl dianion were shown to be crucial for the reaction favoring the retro-Brook rearrangement product over the Brook rearrangement. The second deprotonation at the propargylic position was determined to be the rate-limiting step. In addition, a gas-phase Brook-type rearrangement of the propargylsilanes was observed under GC–MS conditions. This observation was also further confirmed by DFT calculations.

1,2-Silyl Shift-Induced Heterocyclization of Propargyl Silanes: Synthesis of Five-Membered Heterocycles Containing a Functionalized Olefin Side Chain

“…We began our research by developing the synthesis of tetrahydrofuran derivatives 2a–h (Scheme ). Reaction conditions were optimized on propargyl silane 1a , which was obtained by retro -Brook rearrangement of O -silyl-protected hex-5-yn-1-ol, as described previously by the Wang group . Reaction optimization was performed by running reactions in NMR tubes and observing the conversion in the presence of a diphenylmethane internal standard (Figure .…”

“…Commercially available reagents were used as received. Compounds 1a and 1d were prepared according to the procedures described in the literature . All reactions were followed by TLC on E. Merck Kieselgel 60 F254, with detection by UV Full Paper light or developed using generic KMnO 4 or I 2 stain, GC analysis, and NMR analysis.…”

Cation Bridge Mediating Homo- and Cross-Coupling in Copper-Catalyzed Reductive Coupling of Benzaldehyde and Benzophenone

“…Notably, MO t Bu (M = Na, K, and Li) is irreplaceable in the reaction and has a significant influence on the Brook rearrangement under otherwise identical conditions. The Brook rearrangement − is a highly popular tool for the synthesis of organic compound intermediates, − and it has been used in several transition metal-catalyzed C–C coupling reactions. ,− Thus far, the specific mechanism details of how organobases influence the Brook rearrangement and mediate C–C coupling reactions remain unclear. , Herein, we conducted a detailed theoretical insight into the mechanism of the reaction, particularly the role of three organobases in mediating Brook rearrangement and C–C coupling (Scheme b). The novel mechanism of the Brook rearrangement and C–C coupling between benzaldehyde and benzophenone mediated by organobases provides an unprecedented strategy for the organic synthesis of pinacol, and it is important in the designing of highly selective C–C functionalization reaction systems.…”