Nickel-Catalyzed Ring-Opening Allylation of Cyclopropanols via Homoenolate

We report herein a nickel-catalyzed ring-opening allylation of cyclopropanols with allylic carbonates that occurs under mild and neutral conditions. The reaction displays linear selectivity for both linear and branched acyclic allylic carbonates and is also applicable to cyclic allylic carbonates, affording a variety of δ,ε-unsaturated ketones in moderate to good yields. Mechanistic experiments are in accord with a catalytic cycle involving decarboxylative oxidative addition of allylic carbonate to Ni(0), alkoxide exchange with cyclopropanol, cyclopropoxide-to-homoenolate conversion on Ni(II), and C-C reductive elimination.

Enantioselective Conjugate Addition of Catalytically Generated Zinc Homoenolate

We report herein an enantioselective conjugate addition reaction of a zinc homoenolate, catalytically generated via ring opening of a cyclopropanol, to an α,β-unsaturated ketone. The reaction is promoted by a zinc aminoalkoxide catalyst generated from Et₂Zn and a chiral β-amino alcohol to afford 1,6-diketones, which undergo, upon heating, intramolecular aldol condensation to furnish highly substituted cyclopentene derivatives with good to high enantioselectivities. The reaction has proved applicable to various 1-substituted cyclopropanols as well as chalcones and related enones. The chiral amino alcohol has proved to enable ligand-accelerated catalysis of the homoenolate generation and its conjugate addition. Positive nonlinear effects and lower reactivity of a racemic catalyst have been observed, which can be attributed to a stable and inactive heterochiral zinc aminoalkoxide dimer.

Nickel-catalyzed reductive coupling of homoenolates and their higher homologues with unactivated alkyl bromides

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.

Developments in the construction of cyclopropanols

The ring-opening of cyclopropanols is one of the most active areas of research and it has been well documented in recent years owing to subsequent coupling with various partners, thus providing the facile syntheses of a large number of multifunctional compounds that may otherwise be difficult to access. Evidently, the useful cascade reaction requires easy access to diversely functionalized cyclopropanol substrates. However, developments in the construction of cyclopropanols have not received adequate attention. Herein, recent reports on the formation of cyclopropanols are summarized, and the highly stereoselective production of new promising substrates for the cyclopropanol ring-opening/cross-coupling reactions are introduced and improved syntheses of known cyclopropanols are depicted. This review may facilitate more interesting applications of the cyclopropanol ring-opening/coupling reaction in the synthesis of pharmaceutical compounds, natural products, and structurally more diversified organic synthetic intermediates.

Synthesis of trans-2-Substituted Cyclopropylamines from α-Chloroaldehydes

Cyclopropylamines are prevalent in pharmaceuticals and agrochemicals. Herein, we report the synthesis of trans-2-substituted cyclopropylamines in high diastereoselectivity from readily available α-chloroaldehydes. The reaction proceeds via trapping of an electrophilic zinc homoenolate with an amine followed by ring closure to generate the cyclopropylamine. We have also observed that cyclopropylamine cis/trans-isomerization occurs in the presence of zinc halide salts and that this process can be turned off by the addition of a polar aprotic cosolvent.

Efficient approaches to the stereoselective synthesis of cyclopropyl alcohols

Cyclopropanes occur in a diverse array of natural products, including pheromones, steroids, terpenes, fatty acid metabolites, and amino acids, and compounds that contain cyclopropanes exhibit interesting and important pharmacological properties. These valuable synthetic intermediates can be functionalized, or their rings can be opened, and the synthetic utility and unique biological activity of cyclopropanes have inspired many investigations into their preparation. One of the most powerful methods to generate cyclopropanes is the Simmons-Smith cyclopropanation. Since the original studies in the late 1950s reported that IZnCH(2)I could transform alkenes into cyclopropanes, researchers have introduced various modifications of the original procedure. Significantly, Furukawa demonstrated that diethylzinc and CH(2)I(2) react to generate carbenoids, and Shi described more reactive zinc carbenoids that contain electron-withdrawing groups on zinc (XZnCHI(2)). Despite these advances, the development of catalytic asymmetric Simmons-Smith reactions remains challenging. Although researchers have achieved catalytic asymmetric cyclopropanation of allylic alcohols, these reactions have had limited success. One attractive approach to the synthesis of cyclopropanes involves tandem reactions, where researchers carry out sequential synthetic transformations without the isolation or purification of intermediates. Such a synthetic strategy minimizes difficulties in the handling and purification of reactive intermediates and maximizes yields and the generation of molecular complexity. This Account summarizes our recent effort in the one-pot enantio- and diastereoselective synthesis of cyclopropyl alcohols. In one approach, an asymmetric alkyl addition to α,β-unsaturated aldehydes or asymmetric vinylation of aliphatic or aromatic aldehydes generates allylic zinc alkoxide intermediates. Directed diastereoselective cyclopropanation of the resulting alkoxide intermediates using in situ generated zinc carbenoids provides cyclopropyl or halocyclopropyl alcohols with high enantio-, diastereo-, and chemoselectivity. Other strategies employ bimetallic reagents such as 1-alkenyl-1,1-heterobimetallics or CH(2)(ZnI)(2) and provide access to di- and trisubstituted cyclopropyl alcohols. These methods enable facile access to skeletally diverse chiral cyclopropyl alcohols in high yields and stereoselectivities without the isolation or purification of the intermediates.

Gold-catalyzed regioselective oxidation of terminal allenes: formation of α-methanesulfonyloxy methyl ketones

Synthetically useful α-methanesulfonyloxy methyl ketones are readily prepared in one-step from terminal allenes in fair to good yields. The chemistry relies on a gold-catalyzed intermolecular oxidation of the 1,2-diene unit using 3,5-dichloropyridine N-oxide as the oxidant. The reaction tolerates a range of functional groups and shows excellent regioselectivity.

Diasteroselective preparation of cyclopropanols using methylene bis(iodozinc)

A diastereoselective synthesis of trans-2-substituted cyclopropanols is outlined. Bimetallic CH(2)(ZnI)(2) was found to react with α-chloroaldehydes to give cyclopropanols in yields of 64-89% and dr's ≥ 10:1. The high trans-selectivity resulted from equilibration of the cyclopropoxide intermediates.