EPR evidence for α-triphenylstannylvinyl radicals in the O-directed hydrostannation of dialkylacetylenes with Ph3SnH/cat. Et3B/O2.

Here we provide definitive EPR evidence for the existence of α-triphenylstannylvinyl radicals in the low temperature O-directed free radical hydrostannation of dialkyl propargylic alcohols with Ph3SnH/cat. Et3B and O2 in PhMe. Isotropic hyperfine splitting patterns and spectral simulations confirm the assignments made. In the case of the α-triphenylstannylvinyl radical (Z)-2, an isotopic 119/117Sn hyperfine coupling constant of 9.5 mT (95 G) was measured along with a 1Hß hyperfine coupling constant of 1.1 mT.

Fast ring-opening of an intermediary α-stannyl-ß-cyclopropylvinyl radical does not support formation of an α-stannylvinyl cation in the O-directed free radical hydrostannation of dialkyl acetylenes.

O-directed hydrostannation of ß-cyclopropyl propargyl alcohol 22 with stannanes and cat. Et3B in THF/H2O or PhMe/MeOH fails to deliver any detectable products of α-stannylvinyl cation capture. Instead only α-stannyl-ß-cyclopropylvinyl radical intermediates can be detected, which undergo fast H-atom abstraction and/or cyclopropane ring-opening as a result of eliminative ß-scission.

The O-Directed Free Radical Hydrostannation of Propargyloxy Dialkyl Acetylenes with Ph3 SnH/cat. Et3 B. A Refutal of the Stannylvinyl Cation Mechanism.

In this Personal Account, we will give an overview of the room temperature O-directed free radical hydrostannation reaction of propargylically-oxygenated dialkyl acetylenes with Ph3 SnH and catalytic Et3 B/O2 in PhMe. We will show how this excellent reaction evolved, and how it has since been used to stereoselectively construct the complex trisubstituted olefin regions of three synthetically challenging natural product targets: (+)-pumiliotoxin B, (-)-(3R)-inthomycin C, and (+)-acutiphycin. Throughout this Account, we will pay special attention to highlighting important facets of the I-SnPh3 exchange processes that have so far been used in the various different steric settings that we have addressed, and we will document the range of cross coupling protocols that have critically underpinned the first successful applications of this method in complex natural product total synthesis. Last, but not least, we will comment on various aspects of the O-directed free radical hydrostannation mechanism that have been published by ourselves, and others, and we will discuss all of the factors that can contribute to the observed stereo-and regio-chemical outcomes. We will also challenge and refute the recent non-directed stannylvinyl cation mechanism put forward by Organ, Oderinde and Froese for our reaction, and we will show how it cannot be operating in these exclusively free radical hydrostannations.