Advanced Organic Chemistry Practice Problems |top|

A thermal [4+4] cycloaddition is under thermal conditions because the suprafacial/suprafacial interaction of 4π components (each diene) leads to an anti-aromatic transition state (4n electrons in a cyclic array). Under photochemical excitation , one diene is promoted to its ππ* excited state, which inverts its frontier orbital symmetry (now HOMO has opposite phase). This allows a suprafacial/suprafacial interaction via the excited state HOMO of one and the ground state HOMO of the other, which is thermally allowed.

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Excellent for mastering the "logic" of electron flow. advanced organic chemistry practice problems

Never stick to 2D drawings. Use chair conformations and Newman projections to see steric clashes. A thermal [4+4] cycloaddition is under thermal conditions