Four‐Membered Cycle Formation Challenge: GaCl₃‐Promoted Formal [2+2]‐Cycloaddition of Donor–Acceptor Cyclopropanes to Bicyclobutylidene

Abstract

The first selective process of formal [2+2]‐cycloaddition of donor–acceptor cyclopropanes (DAC) to multiple C–C bonds to form a four‐membered ring in the product has been developed. Bicyclobutylidene is used do demonstrate this reaction. The process is promoted by GaCl3 and occurs through generation of 1,2‐zwitterionic intermediates. A carbocationic rearrangement occurs in the bicyclobutylidene moiety. It results in expansion of both cyclobutane rings into five‐membered ones and formation of a [2.3.3]propellane skeleton in the final product.

Synthesis of the Cationic Gallium Phthalocyanines and Their Catalytic Application in Gallium(III)-Activated Processes for Donor–Acceptor Substrates

“…In recent years, simple gallium compounds made it possible to significantly expand the range of organic reactions. They showed themselves especially well in the range of donor–acceptor cyclopropane − (DAC) reactions − and extend them to related substrates, in particular, styrylmalonates, and donor–acceptor alkenes (DAA) as an alternative to DAC.…”

“…It should be noted that gallium was found to be a unique element for this purpose. − At the moment, it has almost no alternatives for performing a wide range of efficient reactions . Why gallium?…”

“…Nearly all reactions of this family (with rare exceptions) require equimolar amounts of gallium halides (GaHal 3 ), usually 1–2 equiv, − which is an obvious drawback. In addition, if equimolar amounts of a mediator are used, it is extremely difficult to perform enantioselective reactions.…”

“…In addition, if equimolar amounts of a mediator are used, it is extremely difficult to perform enantioselective reactions. Yet another drawback lies in the high hydrolytic instability of GaHal 3 , which requires special glovebox techniques to be used, and sometimes results in poor reproducibility of the experiments. − An obvious solution to this problem is to use a catalytic scheme. But how?…”

Synthesis of (Het)aryl 2-(2-hydroxyaryl)cyclopropyl Ketones

“…In these reactions D-A cyclopropanes serve typically as synthetic equivalents of 1,3-dipoles providing approach to compounds, which are not easily accessible by other methods. In addition, D-A cyclopropanes can isomerize to alkenes conjugated to either electron-releasing [29][30][31] or electron-withdrawing groups [29,32,33] and exhibit reactivity of substituted styrenes (or their heterocyclic analogues) [31,[34][35][36][37][38] or Michael acceptors [39][40][41][42][43][44][45], respectively (Scheme 1b). Moreover, D-A cyclopropanes can provide their acceptor [4,[46][47][48] or donor [5,40,42,49] substituents for new bond formations.…”