On the Thermodynamic Control of Ring Opening of 4-Substituted 1,3,3-Tris-Carbethoxycyclobutene and the Role of the C-3 Substituent in Masking the Kinetic Torquoselectivity

The predominant transformations of 4-methyl- and 4-phenyl-1,3,3-tris-carbethoxycyclobutenes to s-trans-trans-1,1,3-tris-carbethoxy-4-methyl- and 4-phenyl-1,3-butadienes, respectively, proceed through pathways entailing heterolytic cleavage of the s_C3C4 bond rather than the usual four-electron conrotatory ring opening following the rules of torquoselectivity. The adventitious or in situ generated halogen acid from CDCl₃ catalyzes the reaction of 4-methyl-1,3,3-tris-carbethoxycyclobutene by protonation of one of the two ester groups on C3 and, thereby, weakening the s_C3C4 bond to allow its heterolytic S_N2 cleavage by the chloride ion. This is followed by cisoid→transoid isomerization and loss of the elements of the halogen acid to form the products. In the Lewis acid-catalyzed reaction of 4-phenyl-1,3,3-tris-carbethoxycyclobutene in CH₂Cl₂, coordination of the Lewis acid with one of two ester groups on C3 is followed by heterolytic cleavage of the s_C3C4 bond. The resultant species subsequently undergoes cisoid→transoid isomerization before losing the Lewis acid to form the products.