Additive‐Free Isomerization of Allylic Alcohols to Ketones with a Cobalt PNP Pincer Catalyst

Abstract

Catalytic isomerization of allylic alcohols in ethanol as ag reen solventw as achieved by using air and moisture stable cobalt (II) complexes in the absence of any additives. Under mild conditions, the cobaltP NP pincer complex substituted with phenyl groups on the phosphorusa toms appeared to be the most active. High rates were obtaineda t1 20 8C, even thought he addition of one equivalent of base increases the speed of the reaction drastically.A lthough some evidencew as obtained supporting ad ehydrogenation-hydrogenation mechanism, it was proven that this is not the major mechanism. Instead, the cobalt hydride complex formed by dehydrogenation of ethanol is capable of double-bond isomerization through alkene insertion-elimination.Isomerization reactions comply with the principles of green chemistry. [1] The catalytic isomerization of allylic alcohols into the saturated carbonyl compounds is an elegant synthetic process that eliminates the more conventional two-step oxidation and reduction pathway. [2] During the last half century, many preciousm etal catalysts for allylic alcoholi somerization, based on Ir,R u, Rh or Pd have been developed. [2, 3] Some of these even work well at ambient temperature. [4] Although noble metals playa ni ndispensable role in catalysis, they are rather expensive, less abundant and generally not used in the last step of drug synthesis. Thus, catalysts based on cheap metals like iron [5] and nickel [6] have been used for the isomerization of allylic alcohols recently.N evertheless, the requirement of either activateds ubstrates [5b] or the need for activation of the catalyst make those systemsl ess desirable. Recently,r eports have appeared on the sole use of base as catalystfor the isomerization of allylic alcohols to the corresponding ketones. [7] To the best of our knowledge,t here is only one report on the use of a cobalt catalyst[ HCo(CO) 4 ]t hat can promote the isomerization of an allylic alcohol to an aldehyde or ak etone, but harsh conditions including the use of toxic carbon monoxide were required to achievel ow conversionsa nd selectivities towards the desired products. [8] Recently,c obalt pincer complexes have been discovered to be effective catalysts [9] for hydrogenation, [10] dehydrogenation, [10g, 11] transfer hydrogenation, [12] hydrosilylations-hydroborations, [13] alkene isomerization [14] and N 2 activation. [15] Hanson and co-workersh ave reportedacationic cobalt(II) alkyl complex, which was appliedf or the hydrogenation of aldehydes, ketones and imines,a sw ell as for the isomerizationo fa lkenes. [16] Furthermore, Liu andc o-workers have recently reported impressive findings about pincerc obalt complexest hat are suitable catalysts for regioselective olefin isomerizationb yu sing ammonia borane as the activation source. [13] Althoughm any catalytic reactions have been performed with pincerc obalt catalysts, isomerization reactions of allylic alcohols with this type of complexes are still unknown.Herein, we report the first cobalt(II) catalyzed isomeri...

Erbium-Catalyzed Regioselective Isomerization–Cobalt-Catalyzed Transfer Hydrogenation Sequence for the Synthesis of Anti-Markovnikov Alcohols from Epoxides under Mild Conditions

“…29,30 Ammonia borane is an attractive alternative to commonly used hydrogen transfer agents such as alcohols since it is nonvolatile and harmless. 31−35 We started our study by investigating the catalytic activities of a series of well-known cobalt complexes 36,37 in the transfer hydrogenation of styrene oxide (1a) (Table 1, entries 1−4). Notably, Co-I has been reported by the groups of Luo and Liu for the transfer hydrogenation of alkynes using ammonia borane as the hydrogen source.…”

Stereospecific Isomerization of Allylic Halides via Ion Pairs with Induced Noncovalent Chirality

“…Different transition-metal catalysts have been used, in particular, ruthenium, rhodium, palladium, and iridium. 6 …”

“…A relevant example of this class of reaction is the isomerization of allylic alcohols into carbonyl compounds, a synthetic transformation in organic chemistry . In recent decades, significant work has been put into the development of methods for performing this isomerization efficiently, − in some instances even under mild reaction conditions, and with a large substrate scope. Different transition-metal catalysts have been used, in particular, ruthenium, rhodium, palladium, and iridium …”

“…In recent decades, significant work has been put into the development of methods for performing this isomerization efficiently, − in some instances even under mild reaction conditions, and with a large substrate scope. Different transition-metal catalysts have been used, in particular, ruthenium, rhodium, palladium, and iridium …”

β-Amino Phosphine Mn Catalysts for 1,4-Transfer Hydrogenation of Chalcones and Allylic Alcohol Isomerization

“…Typically, isomerization of allylic alcohols requires a precious-metal catalyst . Nickel, molybdenum, iron, and cobalt have been utilized, , and only one manganese catalyst has been reported . The allylic alcohol isomerization capability of the HL Ph Mn­(CO) 3 Br and base system in toluene was tested for the three different allyl alcohols 7b , 10b , and 11b and gave quantitative conversion (Scheme ).…”