Organic reactions at high pressure. Cycloadditions with enol and dienol derivatives

Abstract

Cycloaddition reactions between 1-acetoxybutadiene and various monofunctionalized (CHO, COCH3, C O~C H Y , and CN) ethylenes (i.e., acrylic dienophiles) or propylenes (Le., crotonic dienophiles) under the influence of 15 000 atm pressure and room temperature have been. studied. With the acrylic dienophiles only the ortho-cis cycloadducts were obtained in fair to good yields; with the crotonic dienophiles only crotonaldehyde gave the desired cyclized product in 5% yield. The diminished reactivity of the crotonic dienophiles compared to their acrylic counterparts is ascribed to the combination of unfavorable steric and electronic effects brought about by the introduction of the @-methyl group. Similar results were found when 1-methoxybutadiene was employed as the diene component. No cycloaddition products were generated from ethyl vinyl ether and 1-carboalkoxybutadienes (Diels-Alder reactions with inverse electron demand) under the high pressure conditions. The utilization of high pressures (15 000-20 000 atm) and moderate temperatures (24-110 "C) provides a valuable synthetic technique in preparation of 2-alkoxy-3,4-dihydro-2H-pyrans from a,@-unsaturated aldehydes or ketones and vinyl ethers (heterodiene synthesis) since the high-pressure reactions can be conducted at temperatures substantially lower than those required when the reactions are carried out at atmospheric pressure. The steric and electronic effects of methyl substitution on the diene and/or dienophile are described.

Theoretical Investigation on Regioselectivities of Diels‐Alder Reactions by Conjugated Effect

“…Experimentally, in the reaction of a monosubstituted olefin with a 1‐substituted diene, predominance of ortho cycloadduct is to be expected. [ 1‐22 ] In this respect, the regioselectivities of the D‐A reactions of 1‐substituted dienes with unsymmetrical dienophile ( to 58) will be predicted according to the local HSAB principle. The local softness and the GLS were obtained by ab initio method at the level of MP2/6‐311++G(d,p) with the finite difference approximation (only at atomic level) and ABEEMσπ model (at both atomic and π‐bond levels), respectively.…”

“…Because of good regio‐ and stereoselectivity, D‐A reactions between unsymmetrical dienes and dienophiles have been studied in various experimental [ 1‐25 ] and theoretical methods. [ 24,26‐33 ] Especially, D‐A reactions in pericyclic reactions were gained widespread investigations by the conceptual density functional theory (CDFT).…”

Selectivity Controls in the [4+2] Cycloadditions of 1-Oxygenated Dienes

“…19 Thus, HP's are well suited to cases where the dienophile is poorly reactive, 16 although limitations apply. 20 A good example in which a gem-dimethylcyclopropene plays this part is given below in the 'chiral dienophiles' section (Scheme 23 The regiocontrol observed in the cycloaddition of 1-alkoxydienes to unsymmetrical dienophiles is classically interpreted in terms of preferred interactions between the frontier orbitals with the largest coefficients. 22 It results that the transition states in which the alkoxy moiety of the diene and the electron-withdrawing group of the dienophile are 'ortho' to the other present lower activation barriers present.…”

ChemInform Abstract: Reactivity and Selectivity of Captodative Olefins as Dienes in Hetero‐Diels—Alder Reactions.

“…This suggests that, from the point of view of inductive and steric effects, there is no significant difference in reactivity between the heterodienes with an Et or Me at C(2). In contrast, in the presence of a Me group at C(4) of the heterodiene, such as in compound 3a, the reactivity was massively affected [21]. The reaction took place only after heating up to 1408 and required a longer reaction time ( Table 1, Entry 10), giving adduct 7g in low yield as a 78 : 22 mixture of trans/cis-forms resulting from the exo-and endo-approach.…”