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
<p>The predominant transformations of 4-methyl- and 4-phenyl-1,3,3-<i>tris</i>-carbethoxycyclobutenes to s-<i>trans</i>-<i>trans</i>-1,1,3-<i>tris</i>-carbethoxy-4-methyl- and 4-phenyl-1,3-butadienes, respectively, proceed through pathways entailing heterolytic cleavage of the s<sub>C3C4</sub> bond rather than the usual four-electron conrotatory ring opening following the rules of torquoselectivity. The adventitious or in situ generated halogen acid from CDCl<sub>3</sub> catalyzes the reaction of 4-methyl-1,3,3-<i>tris</i>-carbethoxycyclobutene by protonation of one of the two ester groups on C3 and, thereby, weakening the s<sub>C3C4</sub> bond to allow its heterolytic S<sub>N</sub>2 cleavage by the chloride ion. This is followed by <i>cisoid</i><b>→</b><i>transoid</i> 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-<i>tris</i>-carbethoxycyclobutene in CH<sub>2</sub>Cl<sub>2</sub>, coordination of the Lewis acid with one of two ester groups on C3 is followed by heterolytic cleavage of the s<sub>C3C4</sub> bond. The resultant species subsequently undergoes <i>cisoid</i><b>→</b><i>transoid</i> isomerization before losing the Lewis acid to form the products.<br></p>