Formation of a Stable Cyclopropenylium Betaine

摘要

Cyclopropenylium ions (1) are very important organic compounds since they represent the smallest carbocyclic aromatic ring systems. Dealing with these substances is often hampered by their ionic nature, which often makes Cyclopropenylium salts kinetically unstable in the absence of sufficiently stabilizing substituents. In principle, it would be highly desirable to have neutral compounds with Cyclopropenylium character available, i.e., Cyclopropenylium compounds that have their counterion covalently bonded to them. We have now prepared for the first time to our knowledge an example of such a Cyclopropenylium betaine (2), a substance that is very stable and can readily be characterized by conventional techniques. Our synthesis of the metal-free title compound makes use of a novel organometallic template reaction that we recently discovered and described. This allows for the preparation of a suitably substituted three-membered ring system by very simole means in the coordination sphere of a erouo 4 bent metallocene unit. The synthesis utilizes the combined action of a group 4 transition metal and a very Lewis-acidic boron center to circumvent the kinetic and thermodynamic restraints of methylenecyclopropene formation by means of dimerization of two acetylenic units.