Synthesis and Thermal Rearrangements of 4-Allyl-4-arylcyclobutenones

The synthesis and thermolysis of 4-allyl-4-arylcyclobutenones are detailed. Allylcyclobutenones were prepared through the trapping of the cyclobutenonium cation generated by treatment of 4-hydroxycyclobutenones with Lewis acids. A study of the nucleophilic trapping revealed the following mechanistic highlights:  the most likely intermediate is the cyclobutenonium cation, the regioselectivity of the allylation reaction is dictated by attack at the site most suited to stabilize the cation, and the rate of allylation is dependent on carbocation formation. Thermolysis of 4-allyl-4-arylcyclobutenones made possible a study of the competitive [2 + 2] cycloaddition and 6π electrocyclization. A judicious choice of substituents allows some control of the selectivity between the two pathways. The substituents control this selectivity through a combination of imposed electronics in the dienylketene intermediate, the relative ketenophilicity of the 4-position substituent, and the relative contortion substituents in the 2- and 3-positions impose on the transition state of the respective ring-closure processes. The scope of the cyclobutenonium cation trapping with silylated carbon nucleophiles was explored, producing 4-allenylcyclobutenones from the use of propargylsilane and 4-spirocyclobutenones from triisopropylsilane. Thermolysis studies of these precursors are also detailed.