Dehydropericyclic Reactions: Symmetry-Controlled Routes to Strained Reactive Intermediates

Abstract

The conceptual dehydrogenation of pericyclic reactions yields dehydropericyclic processes, which usually lead to strained or reactive intermediates. This is a simple scheme for inventing new chemical reactions. Computational results on two novel dehydropericyclic reactions are presented here. Conjugated enynes undergo a singlet-state photoisomerization that transposes the methylene carbon. We previously suggested excited-state closure to 1,2-cyclobutadiene followed by thermal ring opening. CCSD(T)//DFT computations show two minima of similar energy corresponding to 1,2-cyclobutadiene, one chiral and closed shell and the second a planar diradical. The chiral structure has a low barrier to ring opening and may best explain results on enyne photoisomerization. The first examples of 1,3-diyne + yne cycloadditions to give o-benzynes were reported in 1997. Computations on intramolecular versions of this tridehydro (-3H2) Diels-Alder reaction support a concerted mechanism for the parent triyne (1,3,8-nonatriyne); however, a slight electronic advantage in the concerted path may be outweighed by the difference in entropy of activation for sequential vs simultaneous formation of two new ring bonds.

Direct observation ofo-benzyne formation in photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions

“…Aer the discovery of a [4 + 2]-cycloaddition reaction between a conjugated diene and an alkene by Diels and Alder in 1928, 1 it took almost 70 years until Johnson [2][3][4][5][6] and Ueda [7][8][9][10][11] explored the [4 + 2]-cycloaddition between a diyne and an alkyne. This latter reaction is now known as a "hexadehydro-Diels-Alder" (HDDA) reaction so named by the Hoye group in 2012 as a logical extension of the dehydro-Diels-Alder reaction.…”

Benzocyclobutadienes: An Unusual Mode of Access Reveals Unusual Modes of Reactivity

“…Conversion of 7 to 9 is seen to be exergonic by 47.7 kcal•mol −1 . [9,13] The reaction of 9 with a second molecule of 7 to form the BCB intermediate 10 was seen to be a stepwise net [2+2] cyclization proceeding through a diradical ( S17 ). [5],[14] Because an alkyne is a strong radical-stabilizing group, [15] the lowest energy transition structure (TS) was computed to be that implied by arrows “a” (Figure 2).…”

Benzocyclobutadienes: An Unusual Mode of Access Reveals Unusual Modes of Reactivity

“…Conversion of 7 into 9 was seen to be exergonic by 47.7 kcal mol À1 . [9,13] Ther eaction of 9 with asecond molecule of 7 to form the BCB intermediate 10 was seen to be as tepwise net [2+ +2] cyclization proceeding through ad iradical (S17). [5,14] As an alkyne is as trong radical-stabilizing group, [15] the lowest-energy transition structure (TS) was computed to be that implied by arrows "a" (Figure 2).…”