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 cyclobutenon ium cation generated by treatment of 4-hydroxycyclobutenones with Lewis aci ds. A study of the nucleophilic trapping revealed the following mechanistic highlights: the most likely intermediate is the cyclobutenonium cation, th e regioselectivity of the allylation reaction is dictated by attack at the site most suited to stabilize the cation, and the rate of allylation is dep endent on carbocation formation. Thermolysis of 4-allyl-4-arylcyclobutenone s made possible a study of the competitive [2 + 2] cycloaddition and 6 pi e lectrocyclization. A judicious choice of substituents allows some control o f the selectivity between the two pathways. The substituents control this s electivity 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 S-positions impose o n the transition state of the respective ring-closure processes. The scope of the cyclobutenonium cation trapping with silylated carbon nucleophiles w as explored, producing 4-allenylcyclobutenones from the use of propargylsil ane and 4-spirocyclobutenones from triisopropylsilane. Thermolysis studies of these precursors are also detailed.