Introduction to the Chemistry of Donor–Acceptor Cyclopropanes: A Historical and Personal Perspective

The advancements in the effective synthesis of cyclopropane and its derivatives made it evident that the ring strain of this smallest carbocyclic system can model this molecule as an intriguing building block in synthetic organic chemistry. Vicinal substitution of the cyclopropane ring with an electron-donating and an electron-withdrawing substituent has evolved as the most intriguing approach for activation of the cyclopropane ring toward the carbon–carbon bond cleavage strategies. The term "Donor–Acceptor Cyclopropane (DAC)" was first put forward by Professor Hans-Ulrich Reissig for the siloxy-substituted cyclopropane carboxylates, and the idea behind the development of these cyclopropanes was to introduce electron-donating/donor group to the acceptor cyclopropanes, which would help in their ring opening. The reactivity of these DACs is based on the choice of donor and acceptor substituents that polarize the cyclopropane carbon–carbon bond toward heterolytic cleavage. A donor/electron-releasing substituent contributes to enhancing the electron density of the cyclopropane carboncarbon bond, while the vicinally substituted acceptor/electron-withdrawing substituent pulls out the electron density from this substituted carboncarbon bond of the cyclopropane. With this synergic push–pull effect of the two substituents, there is sufficient net polarization of the vicinally substituted carbon–carbon bond, enabling the cyclopropane to undergo diverse transformations, including cyclopropane ring-opening, isomerization, and cycloaddition reactions.