83026-00-0Relevant academic research and scientific papers
On the reduction of α,β-unsaturated (Group 6) carbene complexes by NaBH4
Gómez-Gallego, Mar,Manche?o, María J.,Ramírez, Pedro,Pi?ar, Carmen,Sierra, Miguel A.
, p. 4893 - 4905 (2007/10/03)
Chromium and tungsten styryl Fischer carbene complexes 12 and 15 were transformed into Z-vinyl ether 13 and E-allyl ether 14 by NaBH4 reduction in EtOH. Deuterium labeling experiments demonstrate that the reaction occurs by the initial addition of the hydride to the carbene carbon atom, followed by a 1,3-rearrangement of the M(CO)5 fragment. The process could involve the participation of an η-allyl chromium intermediate. The reaction is general and has been applied to a series of α,β-unsaturated alkoxy and aminocarbene complexes. In the case of chromium and tungsten alkynyl carbenes 38 and 39, NaBH4 reduction exclusively yields E-allyl ether 14. The intermediacy of an allenyl complex 41 obtained after the 1,3-rearrangement of the metal center is confirmed by deuterium labeling experiments. (C) 2000 Elsevier Science Ltd. All rights reserved.
Mechanistic Features of Allylic Hydrogen Abstraction by Alkoxy Radicals
Kwart, H.,Brechbiel, M.,Miles, W.,Kwart, L.D.
, p. 4524 - 4528 (2007/10/02)
A TS(excit) of angular H abstraction from allylbenzene in the course of the allylic acetoxylation reaction was previously invoked to explain a temperature-independent primary KIE; kH/kD = 2.90.This reaction geometry is now fully supported by the finding of inverse α-secondary deuterium isotope effects at both ends of the double bond in allylbenzene; (kH/kD)αC1 = 0.977 and (kH/kD)αC2 = 0.985.In keeping with these results an unsymmetrically structured, bridged radical intermediate, formed by the interaction of t-BuO with the allylic double bond, steers the reaction course.Such a complex is recognized to be unusual since most of the verified cases of radical bridging involve heteroatom centers capable of octet expansion.A discussion is also given of the factors determining the relative influence of benzene and double bond participation in the H-abstraction reactions of allylbenzene, which possesses both of these activating functions.
