Rhodium‐Catalyzed Enantioselective [4+2] Cycloadditions of Vinylcarbenes with Dienes

The reaction of 2‐siloxycyclo‐1,3‐dienes with E‐vinyldiazoacetates in the presence of the bulky chiral dirhodium tetracarboxylate catalyst, Rh2(R‐p‐PhTPCP)4 results in an enantioselective [4+2] cycloaddition, in which three new stereogenic centers are formed. The [4+2] cycloadducts are generated as single diastereomers with high enantiocontrol (95–98 % ee). When the diene contains an additional stereogenic center, effective kinetic resolution can be achieved. Selective [4+2] cycloaddition: A mechanistically unusual approach for a highly enantioselective [4+2] cycloaddition between rhodium‐stabilized vinylcarbenes and siloxydienes was developed. The reaction is initiated by attack of the diene at the vinylogous position of the vinylcarbene and the outcome is dependent of the reacting conformation of the vinylcarbene intermediate.