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 sC3C4 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 sC3C4 bond to allow its heterolytic SN2 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 sC3C4 bond. The resultant species subsequently undergoes cisoid→transoid isomerization before losing the Lewis acid to form the products.