Concerted vs Stepwise Mechanisms in Dehydro-Diels–Alder Reactions

Abstract

The Diels-Alder reaction is not limited to 1,3-dienes. Many cycloadditions of enynes and a smaller number of examples with 1,3-diynes have been reported. These "dehydro"-Diels-Alder cycloadditions are one class of dehydropericyclic reactions which have long been used to generate strained cyclic allenes and other novel structures. CCSD(T)//M05-2X computational results are reported for the cycloadditions of vinylacetylene and butadiyne with ethylene and acetylene. Both concerted and stepwise diradical routes have been explored for each reaction, with location of relevant stationary points. Relative to 1,3-dienes, replacement of one double bond by a triple bond adds 6-6.5 kcal/mol to the activation barrier; a second triple bond adds 4.3-4.5 kcal/mol to the barrier. Product strain decreases the predicted exothermicity. In every case, a concerted reaction is favored energetically. The difference between concerted and stepwise reactions is 5.2-6.6 kcal/mol for enynes but diminishes to 0.5-2 kcal/mol for diynes. Experimental studies on intramolecular diyne + ene cycloadditions show two distinct reaction pathways, providing evidence for competing concerted and stepwise mechanisms. Diyne + yne cycloadditions connect with arynes and ethynyl-1,3-cyclobutadiene. This potential energy surface appears to be flat, with only a minute advantage for a concerted process; many diyne cycloadditions or aryne cycloreversions will proceed by a stepwise mechanism.

Inquiry of the reaction paths in thermal retro-Diels–Alder reactions in the gas phase: Theoretical study on the concerted and stepwise elimination mechanisms of cyclohexenes

“…A considerable amount of experiments and theoretical studies have shown that the activation energy for both mechanisms is nearly similar, and it is the nature of the molecule itself, or the substituents presents in the substrate, that may encourage one reaction path over the other [1][2][3][4][5][6][7][8]. Conversely, the story of the reverse process is quite different.…”

Tactics for probing aryne reactivity: mechanistic studies of silicon–oxygen bond cleavage during the trapping of (HDDA-generated) benzynes by silyl ethers

“…50 kcal mol −1 (Δ) 1 (cf. ΔE rxn = −51.4 kcal mol −1 for the parent ethyne + 1,3-butadiyne to benzyne 4 ); (ii) this thermal mode of benzyne generation is complementary to all previously reported, preparatively practical methods in two important ways—the triyne precursors are not, themselves, benzene derivatives and the aryne is formed in the absence of reagents (e.g., bases or reducing agents) and byproducts (e.g., metal salts, amines, or halide ions) that can affect or, in some instances, interfere with the benzyne trapping reactions; and (iii) a beneficial consequence of point (ii) is that inherent reactivity of benzynes can sometimes be probed more fundamentally and/or that new types of aryne trapping reactions can be uncovered. 5,6 An example of this latter feature is the net addition of a silyl ether (like that in 1 ) across the pair of strained benzyne sp-hybridized carbons to give an o -(trialkylsilyl)aryl ether moiety (like that in 2 ).…”

Cycloaddition Reactions of Azide, Furan, and Pyrrole Units with Benzynes Generated by the Hexadehydro-Diels–Alder (HDDA) Reaction

“…50 kcal•mol −1 more stable than the triyne precursor (! ), 14, 7 yet this intermediate, albeit aromatic, is rendered quite reactive because of its large strain energy (ca. 54 kcal•mol −1 15 ).…”