The Bonding and Reactivity of α-Carbonyl Cyclopropanes

Abstract

The cyclopropane functionality has been exploited in a myriad of settings that range from total synthesis and methodological chemistry, to medical and materials science. While it has been seen in such a breadth of settings, the typical view of the cyclopropane moiety is that its reactivity is derived primarily from the release of ring strain. While this simplified view is a useful shorthand, it ignores the specific nature of cyclopropyl molecular orbitals. This review aims to present the different facets of cyclopropane bonding by examining the main models that have been used to explain the reactivity of the functionality over the years. However, even with advanced theory, being able to precisely predict the reactivity of an exact system is nigh impossible. Specifically chosen, carbonyl-bearing cyclopropyl species act as so-called acceptor cyclopropanes and, if correctly derivatised, donor–acceptor cyclopropanes. By undertaking a case study of the history of carbonyl cyclopropanes in organic synthesis, this review highlights the relationship between the understanding of theory and pattern recognition in developing new synthetic methods and showcases those successful in balancing this critical junction.1 Cyclopropanes2 The Strain Model3 The Forster–Coulsin–Moffit Model4 The Walsh Model5 Acceptor, Donor, and Donor–Acceptor Cyclopropanes6 Reactions of Carbonyl Cyclopropanes

Au Nanoparticle‐Catalyzed Silaboration of Aryl‐Substituted Cyclopropyl Aldehydes Forming Rearranged β‐Boronate Silyl Enol Ethers

“…It is reasonable that the initially formed α‐silyloxy cyclopropyl radical (intermediate II , Scheme ) from the cleavage of I undergoes ring‐opening rearrangement into a benzyl radical (intermediate III ), that is captured by the chemisorbed Bpin moiety forming the final silaboration product. It's also possible that coupling may occur in intermediate I , via the radical‐type bond‐forming and breaking concerted‐like route shown in the bottom part of Scheme , as the σ cyclopropane bonds are rather loose …”

A Vinyl Cyclopropane Ring Expansion and Iridium‐Catalyzed Hydrogen Borrowing Cascade

“…We also attempted to gather evidence for nucleophilic opening of the cyclopropane ring by homo‐conjugate addition . Interestingly, when we added KOH to a mixture of ( E / Z )‐ 6 and 2 a , without iridium catalyst, both starting ketone 1 and reduced product 3 a were formed, and no ring opened or rearranged products were observed (Scheme B) .…”

Catalytic Enantioselective Cyclopropylalkynylation of Aldimines Generated In Situ from α-Amido Sulfones

“…On the other hand, the presence of the cyclopropyl group in organic compounds has attracted enormous attention due to its exceptional steric, electronic, and conformational properties as a consequence of its tensioned structure [13][14][15]. Its presence in nature (terpenes, antibiotics, pheromones, or fatty acids) has led to the search for new biologically active compounds incorporating this ring.…”