Hydrogen borrowing

Whittlesey, Williams and co-workers fnrther developed the catalytic indirect Wittig reaction and fonnd that the more electron-rich NHC present in complex 18 provided a more reactive catalyst [8]. Catalyst 18 was used to convert benzyl alcohol 8 and phosphoninm ylide 19 into the product 20 under slightly milder reaction conditions and in a shorter time than in previous work (Scheme 11.4). Other C-C bond-forming reactions from alcohols using a borrowing hydrogen approach have been reported, with Peris and co-workers using Ir-NHC complexes for the C-3 alkylation of indoles with alcohols [9]. 

Madsen and co-workers have reported an important extension to the amine alkylation chemistry, in which oxidation takes place to give the amide product [13]. A ruthenium NHC complex is formed in situ by the reaction of [RuCl Ccod)] with a phosphine and an imidazolium salt in the presence of base. Rather than returning the borrowed hydrogen, the catalyst expels two equivalents of H. For example, alcohol 31 and benzylamine 27 undergo an oxidative coupling to give amide 32 in good isolated yield (Scheme 11.7). 

Scheme 13.15 Tandem Borrowing Hydrogen Wittig reaction with Ru-NHC complexes...

Scheme 26 Alkylation of amines with alcohols by borrowing hydrogen...

The activation of alcohols by iridium-catalyzed borrowing hydrogen reactions has also been applied to the formation of C-C bonds [113]. Reactions proceed by temporary removal of hydrogen from an alcohol to give an aldehyde (or ketone), which is transformed into an alkene with subsequent return of the hydrogen to provide a new C-C bond. Ishii and coworkers have used [lr(cod)Cl]2 with... 

Saidi O, Blacker AJ, Lamb GW et al (2010) Borrowing hydrogen in water and ionic liquids iridium-catalyzed alkylation of amines with alcohols. Org Process Res Dev 14(4) 1046-1049... 

There has been significant interest and significant developments in the use of Ru—NHC complexes for the so-called borrowing hydrogen reactions, which brings about temporary oxidation of alcohols to aldehydes and ketones, which are then utilized for the formation of new C-C and C-N bonds (Scheme 12.3) [59],... 

Scheme 12.3 "Borrowing hydrogen" pathways leading to the formation of alkanes (pathway a), alcohols (pathway b), imines/amines (pathway c), and amides (pathway d).

Scheme 12.4 Borrowing hydrogen reaction with C H activated complex 34.

Scheme 3 General mechanism for alcohol-based borrowing hydrogen (a) or hydrogen autotransfer reactions (b)...

In 2015, Zhao and co-workers described the first dynamic kinetic asymmetric amination of alcohols via borrowing hydrogen methodology under the cooperative catalysis of iridium complex 25 and chiral phosphoric acid 27 (Schemes 31, 32) [179]. The authors proposed that, initially, the two stereocenters in the alcohols were both racemized to ketone by the first oxidation, followed by tautomerization of the iminium intermediates 28 and 30 through enamine intermediate 29. Then, the... 

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