A complementary strategy of utilizing ππ* excited state of alkene instead of nπ* excited state of the carbonyl chromophore in a "transposed Paternò-Büchi" reaction is evaluated with atropisomeric enamides as the model system. Based on photophysical investigations, the nature of excited states and the reactive pathway was deciphered leading to atropselective reaction. This new concept of switching of excited-state configuration should pave the way to control the stereochemical course of photoreaction due to the orbital approaches required for photochemical reactivity.
Gaining predictable control over various forms of selectivities, such as enantio- and/or regio-selectivities, has been a long-standing goal in chemical catalysis. Although a number of factors such as the molecular features of the reactants and catalysts, as well as the reaction conditions, can influence the outcome of a reaction, it is not quite conspicuous as to what combinations of these parameters would offer a desired form of selectivity. We use machine learning tools, such as the neural network (NN), decision tree (DT), logistic regression (LR) and Random forest algorithms, to (a) analyze the outcome of an important catalytic regio-selective difluorination reaction of alkenes, and (b) decipher the complex interplay of various molecular parameters and their non-linear dependencies. The connection between what features of alkenes will yield 1,1-difluorination and how subtle changes would steer the reaction to 1,2-difluorination under identical conditions is enunciated. The NN was able to accurately predict whether a given alkene would yield a 1,1- or 1,2-difluorinated product. A combination of DT and the random forest classifier offered important chemical insights, which could be used in making a more rational choice of the reactant alkene for the desired regioisomeric product. The results could have far reaching implications in predicting which regioisomer is likely to be formed under a given set of conditions, and thus this technique is capable of expediting the development of catalytic transformations.
Mild and environment friendly hypercoordinate iodine compounds exhibit promising reactivities resembling that of transition metal catalysts. Hypercoordinate iodine reagents or catalysts are increasingly been employed in contemporary organic synthesis. However, mechanistic insights on such reactions continue to remain rather limited. Recent advances in the mechanistic understanding on a selected set of reactions involving hypercoordinate iodine form the main theme of this review. An overview of bonding, reactivity, and mechanistic insights on iodine(III) reactions such as α-functionalization of carbonyl compounds, alkynylation, amination, C H functionalization, phenol dearomatization, and trifluoromethylation have been described. In keeping with the current practices in mechanistic studies, we have maintained an interdisciplinary flavor in this compilation by providing a balanced view of computational and experimental understanding on the burgeoning domain of hypercoordinate iodine mediated reactions and catalysis. FIGURE 1 | Molecular orbital (MO) diagram for three-center-fourelectron (3c-4e) bond in hypercoordinate iodine compound RIL 2 . SCHEME 1 (a) General representation of α-tosyloxylation ketone catalyzed by Koser's reagent. (b) General mechanism for the α-tosyloxylation ketone. (c) Commonly employed iodoarenes in α-tosyloxylation. Overview wires.wiley.com/compmolsci SCHEME 2 (a) Effect of different substituents on the catalytic activity of iodoaryl in α-tosyloxylation. (b) Effect of different substituents on dihedral angle θ(C1-C2-I3-O4). (c) Formation of iodono intermediate 4 and the related ΔG rxn . WIREs Computational Molecular Science Hypercoordinate iodine(III) promoted reactions and catalysis Volume 7,
The need for metal-free environmentally benign catalysts has provided a strong impetus toward the emergence of hypercoordinate iodine reagents. At this stage of development, molecular insights on the mechanism and origin of stereoselectivity are quite timely. In this study, the origin of stereoinduction in a class of iodoresorcinol-based chiral hypercoordinate iodine-catalyzed synthesis of biologically important spirocyclic bisoxindoles from aryl dianilides has been established by using density functional computations. Formation of an interesting helical fold by the 2,6-chiral amide arms on the resorcinol framework is found to be facilitated by a network of noncovalent interactions. In the chiral environment provided by the helical fold, enantioselectivity is surprisingly controlled in a mechanistic event prior to the ring closure to the final spirocyclic product, unlike that commonly found in spirocyclic ring formation. A vital 1,3-migration of the chiral aryl iodonium (Ar*-I(CF3COO)) in an O-iodonium enolate to the corresponding C-iodonium enolate, which retains the chiral memory, holds the key to the enantiocontrol in this reaction and thus renders ring closure to be stereospecific.
The mechanism of a metal-free, phenyliodine(III) bis(trifluoroacetate) promoted, dual aryl C-H activation of an anilide to a spirocyclic bis-oxindole is examined using density functional theory (M06-2X). The most preferred pathway proceeds through the involvement of a novel iodonium ion intermediate and a pivotal trifluoroacetate counterion. The two sequential aryl C-H activations, assisted by trifluoroacetate as well as the superior leaving group ability of PhI, facilitate the formation of spirocyclic bis-oxindole.
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