Hydrogen intermolecular elimination

Noteworthy, the instability of HCo(CO)4 under the formation of Co2(CO)g can be attributed in part to the fast intermolecular elimination of H2- In this manner, also the formation of alkanes can be explained as a key step in the hydrogenation of olefins. On the other hand, the acidic properties of HCofCO) allow the convenient separation of product and catalyst after hydroformylation by conversion into water-soluble Co salts ( decobalting ) [10]. 

The above reaction is a convincing example of an intermolecular hydrogen abstraction leading essentially to the same result as obtained in the pyrolysis of alkyl-substituted thiirane oxides through an intramolecular /(-elimination of hydrogen. 

The first palladium-catalyzed formation of aryl alkyl ethers in an intermolecular fashion occurred between activated aryl halides and alkoxides (Equation (28)), and the first formation of vinyl ethers occurred between activated vinyl halides and tin alkoxides (Equation (29)). Reactions of activated chloro- and bromoarenes with NaO-Z-Bu to form /-butyl aryl ethers occurred in the presence of palladium and DPPF as catalyst,107 while reactions of activated aryl halides with alcohols that could undergo /3-hydrogen elimination occurred in the presence of palladium and BINAP as catalyst.110 Reactions of NaO-/-Bu with unactivated aryl halides gave only modest yields of ether when catalyzed by aromatic bisphosphines.110 Similar chemistry occurred in the presence of nickel catalysts. In fact, nickel catalysts produced higher yields of silyl aryl ethers than palladium catalysts.108 The formation of diaryl ethers from activated aryl halides in the presence of palladium catalysts bearing DPPF or a CF3-subsituted DPPF was also reported 109... 

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

The photoinduced -elimination of 1,2,3-triazole from 1-(A,A-bisacyl)amino-l,2,3-triazoles (142), itself formed from the photochemical isomerization of triazoles (141), proceeds either via an intra-or intermolecular hydrogen abstraction or electron-transfer mechanism followed by homolytic cleavage of the A,A-bond (path a) or via t -assisted )8-cleavage of the same weak bond (path b). The composition of the products suggests that in all cases a c-type 1,2,3-triazolyl radical (143) is eliminated which is further quenched by hydrogen abstraction as shown in Scheme 24 <93JHC1301>. 

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