Cationic Rhodium(I) Complex-Catalyzed [Sigma]- and [Pi]-Bond Activation Cascade: Isomerization of Allyl Propargyl Ethers to Allenic Aldehydes and Dienals (Report‪)‬

INTRODUCTION Cascade reactions can eliminate a number of reaction steps and reduce hazardous waste and solvents, therefore cascade reactions have attracted much attention in current organic synthesis [1]. If a single catalyst can catalyze multiple reaction steps without using multiple catalysts, convenient operation can be realized (Scheme 1). When a single catalyst can act as multiple roles and catalyze multiple reaction steps without loss of catalytic activity, we can realize such an attractive process. It is well known that transition-metal complexes are able to activate [sigma]-bonds by oxidative addition or electrophilic substitution pathway [2]. [pi]-Bonds can also be activated through the formation of a complex between an electrophilic transition metal and n-electrons of alkene or alkyne multiple bonds [3]. Sequential activation of these bonds by a single transition-metal complex would allow developing a novel cascade reaction. We anticipated that cationic rhodium(I) complexes would be promising catalysts for this purpose.