![]() ĭienes: There is a wide range of acyclic and cyclic dienes available for the nitroso hetero-Diels–Alder reaction. ![]() Additionally, acylnitroso compounds can be generated by the rearrangement of diazonitroalkanes 26, the photochemical cleavage of 1,2,4-oxadiazole-4-oxides 25 and the cycloreversion of 9,10-dimethylantracene adducts 27 ( Scheme 7). In 2015, the Whiting group reported an extensive study of acylnitroso compounds prepared in situ by the catalytic aerobic oxidation of hydroxycarbamate using CuCl 2 and 2-ethyl-2-oxazoline in methanol. In a recent work by Tusun dirhodium caprolactamate, and the aerobic oxidation in the presence of catalytic amounts of Cu(II) and pyridine were used for the preparation of acylnitroso compounds. These include the oxidation of nitrile oxides or the corresponding hydroxamic acids using, for example, periodate, Dess–Martin periodinane, Swern oxidation conditions, lead and silver oxide, and transition-metal oxidation with peroxide as the oxidant. Compounds 1a–c will be studied in more detail in this review.Īcylnitroso compounds are generally prepared and used in situ due to their extremely reactive nature and the conditions have been summarized previously. Nitroso compounds: Nitroso compounds are highly reactive dienophiles often used for the hetero-Diels–Alder reaction and the most frequently used representatives are depicted in Figure 1. The main aim of this review is to provide insight into the fundamental relationships between the structures of the reactants and the regio- and stereoselectivity of the hetero-Diels–Alder reaction as well as the possibility of controlling the regio- and stereoselectivity of 3,6-dihydro-2 H-1,2-oxazine formation. Special emphasis is drawn to the influence and control of the reaction under solution and solid-state reaction conditions. This review will thus focus on the regio- and stereoselectivity of the nitroso hetero-Diels–Alder reaction. The possibility to control nitroso HAD reactions is the most relevant fact for using the nitroso hetero-Diels–Alder reaction for the syntheses of biologically important molecules. These aspects were partially covered in a review by Yamamoto in 2006, which focused on asymmetric nitroso hetero-Diels–Alder reactions, following the review by Miller that was published in 1998. However, none of these reviews focused on general aspects of the regio- and stereoselectivity of the nitroso hetero-Diels–Alder reaction and the possibility of its control. The profiles of the several topological parameters along the reaction are affected by the nature of the substituents attached to the boron atom and by the mode of addition (endo and exo) in the DA reactions.Several excellent reviews on nitroso hetero-Diels–Alder reactions have been published in the past, including general reviews on hetero-Diels–Alder reactions and their applications in organic synthesis, applications of nitroso hetero-Diels–Alder reactions for the synthesis of azasugars, and the utilization of nitroso hetero-Diels–Alder reactions in natural product synthesis and the synthesis of bioactive molecules. We propose that the relative reactivities of the dienophiles can be estimated by the charge density (ρr) and its Laplacian (∇2 ρr) at the (3,+1) critical point in the topology of ∇2 ρr, evaluated at the reactant molecules in the ground state. Interestingly, the cooperative effect between the two hydrogen bonds in the exo TS of the DA reaction of vinyltrifluoroborate (12) determines the almost exclusive exo selectivity predicted for this DA reaction. ![]() The moderate exo selectivity calculated for the DA reactions of boronates 8−11 was attributed mainly to the hydrogen bond between the oxygen atom of boronate moieties and one of the acidic hydrogens of the methylene of cyclopentadiene in the exo TSs, which also reduces the ability of the oxygen lone pairs to donate electron density into the vacant boron orbital. In the TSs of 5 and 7, a combination of electronic and steric factors reduce the endo selectivity. The DA reactions of 1−7 were slightly endo-selective due to the stronger C6−B secondary orbital interaction in the endo transition structures (TSs) evaluated by the C6|B delocalization index. The calculated reactivity of the dienophiles decreases in the order vinylborane (1) > dihalovinylboranes (2−4) > dialkylvinylboranes (5−7) ≈ vinyl boronic acid (8) > vinylboronates (9, 10) > vinyl MIDA boronate (11) ≈ vinyltrifluoroborate (12). The effect of the nature of the boron moiety upon the reactivity and the selectivity of a variety of vinylboron dienophiles (1−12) in the Diels−Alder (DA) reaction was investigated using density functional theory and the quantum theory of atoms in molecules. ![]()
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