A nickel-catalyzed reductive cross-coupling
reaction of aryl cyclopropyl
ketones with easily accessible unactivated alkyl bromides to access
aryl alkyl ketones has been developed. This strategy facilitates access
to various of γ-alkyl-substituted ketones via ring opening of
cyclopropyl ketones (26 examples, 50–90% yield). Initial mechanistic
studies revealed that the reaction proceeds via radical cleavage of
the alkyl bromide.
A direct and convenient method for the palladiumcatalyzed reductive cross-coupling of aryl iodides or alkenyl bromides and secondary benzyl halides under ambient CO pressure to generate a diverse array of aryl/alkenyl alkyl ketones has been developed. This strategy successfully achieves a three-component carbonylative reaction with Zn as the reducing agent for CÀ C bond formation, overcoming the well-known homocoupling of aryl or alkenyl halides, direct cross-coupling between two different electrophiles and other carbonylative coupling reactions. In addition, this method avoids use of preformed organometallic nucleophiles, such as organo-magnesium, zinc and boron reagents. This approach enables the construction of valuable aryl alkyl/ alkenyl ketone derivatives (60 examples, 56-95% yields).Reactivity studies indicate that in situ formed benzylic zinc reagents are intermediates in the catalytic system.
The catalytic generation of homoenolates and their higher homologues has been a long-standing challenge. Like the generation of transition metal enolates, which have been used to great affect in synthesis and medicinal chemistries, homoenolates and their higher homologues have much potential, albeit largely unrealized. Herein, a nickel-catalyzed generation of homoenolates, and their higher homologues, via decarbonylation of readily available cyclic anhydrides has been developed. The utility of nickel-bound homoenolates and their higher homologues is demonstrated by cross-coupling with unactivated alkyl bromides, generating a diverse array of aliphatic acids. A broad range of functional groups is tolerated. Preliminary mechanistic studies demonstrate that: (1) oxidative addition of anhydrides by the catalyst is faster than oxidative addition of alkyl bromides; (2) nickel bound metallocycles are involved in this transformation and (3) the catalyst undergoes a single electron transfer (SET) process with the alkyl bromide.
A Ni-catalyzed desymmetrizing cross-electrophile coupling of cyclic meso-anhydrides with aryl triflates has been successfully demonstrated. This is the only example using cyclic meso-anhydrides in cross-electrophile coupling reactions. A diverse array of valuable γ-keto acid building blocks can be generated under these conditions with excellent functional group tolerance and stereochemical fidelity.
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