Tandem Ring-Closing Metathesis/Transfer Hydrogenation: Practical Chemoselective Hydrogenation of Alkenes

Abstract

An operationally simple chemoselective transfer hydrogenation of alkenes using ruthenium metathesis catalysts is presented. Of great practicality, the transfer hydrogenation reagents can be added directly to a metathesis reaction and effect hydrogenation of the product alkene in a single pot at ambient temperature without the need to seal the vessel to prevent hydrogen gas escape. The reduction is applicable to a range of alkenes and can be performed in the presence of aryl halides and benzyl groups, a notable weakness of Pd-catalyzed hydrogenations. Scope and mechanistic considerations are presented.

In tandem or alone: a remarkably selective transfer hydrogenation of alkenes catalyzed by ruthenium olefin metathesis catalysts

“…Interestingly, carbonyl functions in ketones 10 and enones (Table 3, entries 3, 4, 7 and 8) do not undergo reduction under these conditions, which makes our system different from the previously described [Ru]/NaBH 4 couple. 6 As suggested by the experiment conducted with 3f, primary and secondary benzyl ethers and an allylic stereocenter survived the reaction untouched (Table 3, entry 5). Interestingly, compounds bearing unprotected OH group 3g (Table 3, entry 6) underwent transfer hydrogenation quantitatively and no esterification took place.…”

“…5 Recently a more convenient protocol, using NaBH 4 as a hydrogen source has been reported. 6 During our study on a new chelating ruthenium azinate complex (Az-II, Fig. 1), we have noticed its reactivity in numerous transformations.…”

“…It is noteworthy that the cyclohexene ring is easily reduced by the newly published system composed of NaBH 4 and Ru olefin metathesis catalysts. 6 We speculated that the observed high sensitivity of the Gru-II/HCOOH catalytic system towards the ring strain and the substitution pattern can be utilised for selective reduction of one C-C double bond in the presence of others. To explore this interesting possibility, a competition experiment, shown in Scheme 2, was performed, to demonstrate the level of control offered by our transfer hydrogenation system.…”

Tandem Catalysis Utilizing Olefin Metathesis Reactions

“…Peese and co‐workers recently reported a Ru‐4 ‐catalyzed tandem RCM/transfer hydrogenation (RCM/TH) of dienes by using a methanolic solution of NaBH 4 as a hydrogen donor. By reaction condition optimization, they established the appropriate solvent ratios to prevent rapid hydrogen evolution and premature NaBH 4 consumption.…”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…Several metathesis catalysts were employed in a tandem RCM transfer hydrogen process (e.g. the conversion of 418 to 419) where sodium borohydride and methanol were added to the system after the RCM event to transform the metathesis catalyst into a hydrogenation catalyst [881]. Grubbs catalysts I and II as well as Fischer carbene complex analogs (e.g.…”