Recently, however, we have shown that this rationale is incomplete. The catalytic ability of Lewis acids, such as hydrogen, halogen, chalcogen, and pnictogen bonded species, is traditionally attributed to the lowering of the LUMO of the activated dienophile, resulting in a smaller, i.âe., more favorable, HOMO dieneâLUMO dienophile energy gap and hence stronger orbital interaction. The DielsâAlder reaction between cyclopentadiene (diene) and 2âbutenone (dienophile) catalyzed by a bifunctional halogenâbonded catalyst, as shown by Jungbauer etâ al. Thus, in general, weakly interacting Lewis acids play a crucial role in the reactivity of organic reactions, and hence it is highly important to understand the driving force behind this class of catalysis. Moreover, in our previous work, we have shown that Lewis acidic alkali cations can also exert a remarkable catalytic effect on the generally slow aromatic DA reaction: the archetypal aromatic cycloaddition reaction between benzene and acetylene can be effectively accelerated by up to 5 orders of magnitude. Similar catalytic effects were also reported using catalysts featuring similar weak interactions such as pnictogen, chalcogen, and halogen bonds (Scheme 1). For example, hydrogenâbonded solvents can activate ketones to undergo a DA reaction, or bifunctional hydrogen bond donor organocatalysts, such as differently substituted thioureas, are able to increase both the reaction rate as well as the endo/ exoâselectivity of DA reactions between a diene and α, βâunsaturated carbonyl compounds. In analogy with conventional Lewis acids, hydrogenâbonded organocatalysts are also able to significantly modify both the reactivity and selectivity of the DA reaction. on the total synthesis of the tetrodotoxin. Interestingly, LAs can also reverse the regiochemical course of the DA reaction leading to products that otherwise would be impossible to synthesize: a pioneering example has been shown by Kishi etâ al. For example, Lewis acids (LAs) are able to greatly accelerate the DA via binding to the dienophile. A striking number of organocatalysts have been developed over the years to increase the reactivity and selectivity of the DA reaction. Since its discovery in 1928, it has paved the way for a convenient procedure to create sixâmembered rings, with up to four stereocenters, becoming the gold standard for many applications ranging from the synthesis of natural products to the industrial production of relevant compounds in the pharmaceutical field. The DielsâAlder (DA) cycloaddition reaction is of paramount importance in synthetic organic chemistry. These findings again demonstrate the generality of the Pauli repulsionâlowering catalysis concept. Notably, the reactivity can be further enhanced on going from a Period 3 to a Period 5 LA, as these species amplify the asynchronicity of the DielsâAlder reaction due to a stronger binding to the dienophile. Our detailed activation strain and KohnâSham molecular orbital analyses reveal that these LAs lower the DielsâAlder reaction barrier by increasing the asynchronicity of the reaction to relieve the otherwise destabilizing Pauli repulsion between the closedâshell filled Ïâorbitals of diene and dienophile. The reaction barriers systematically increase from halogen The catalytic effect of various weakly interacting Lewis acids (LAs) across the periodic table, based on hydrogen (Groupâ 1), pnictogen (Groupâ 15), chalcogen (Groupâ 16), and halogen (Groupâ 17) bonds, on the DielsâAlder cycloaddition reaction between 1,3âbutadiene and methyl acrylate was studied quantum chemically by using relativistic density functional theory.
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