exo/endo Selectivity Control in Diels-Alder Reactions of Geminal Bis(silyl) Dienes: Theoretical and Experimental Studies

Exo-Selective Diels-Alder Reactions

The Diels-Alder reaction stands as one of the most pivotal transformations in organic chemistry. Its efficiency, marked by the formation of two carbon-carbon bonds and up to four new stereocenters in a single step, underscores its versatility and indispensability in synthesizing natural products and pharmaceuticals. The most significant stereoselectivity feature is the "endo rule". While this rule underpins the predictability of the stereochemical outcomes, it also underscores the challenges in achieving the opposite diastereoselectivity, making the exo-Diels-Alder reactions often considered outliers. This review delves into recent examples of exo-Diels-Alder reactions, shedding light on the factors inverting the intrinsic tendency. We explore the roles of steric, electrostatic, and orbital interactions, as well as thermodynamic equilibriums in influencing exo/endo selectivity. Furthermore, we illustrate strategies to manipulate these factors, employing approaches such as bulky substituents, s-cis conformations, transient structural constraints, and innovative control physics. Through these analyses, our aim is to provide a comprehensive understanding of how to predict and design exo-Diels-Alder reactions, paving the way for new diastereoselective catalyst systems and expanding the chemical scope of Diels-Alder reactions.

Origins of Regioselectivity in CuH-Catalyzed Hydrofunctionalization of Alkenes

Factors controlling the regioselectivity in alkene hydrocupration were computationally investigated using energy decomposition analysis. The results demonstrate that the Markovnikov-selective hydrocupration with electronically activated mono-substituted olefins is mostly affected by the destabilizing Pauli repulsion, which is due to the electron delocalization effect. The anti-Markovnikov-selective hydrocupration with 1,1-dialkyl-substituted terminal olefins is dominated by the repulsive electrostatic interactions, which is because of the unequal π electron distribution caused by the induction effect of alkyl substituents.

Inducing high exo selectivity in Diels-Alder reaction by dimethylborane substituent: a DFT study

In this work, the role of Lewis acid-base (LAB) interaction on the stereoselectivity of the Diels-Alder (DA) reaction has been studied by DFT in gas and solution (dichloromethane) phases. The calculations were performed at the B3LYP/6-311G++ (d, p) level. Two different series of DA reactions were investigated: (1)-three mono-substituted cyclopentadienes + dimethyl(vinyl)borane; (2)-five α,β-unsaturated carbonyl compounds + cyclopenta-2,4-dien-1-yldimethylborane. The reacting diene and dienophile pairs were chosen to restrict LAB interaction to the exo reaction pathway. It was found that in some of the examined cases, the favorable LAB interaction is so strong that it can lead to a completely exo-selective DA reaction. Furthermore, a novel multistep synthetic method was hypothesized for preparing exo cycloadduct with near 100% stereoselectivity. Our results can open up new avenues toward the rational design of exo-selective DA reactions for synthesizing novel bioorganic compounds.

A mechanistic study on the regioselective Ni-catalyzed methylation–alkenylation of alkyne with AlMe3 and allylic alcohol

The recently reported Ni-catalyzed methylation–allylation of alkynes with allylic alcohols and AlMe3 reagents delivers valuable tetrasubstituted alkene units in a highly regioselective fashion.

Endo Selectivity in the (4 + 3) Cycloaddition of Oxidopyridinium Ions

The (4 + 3) cycloaddition of 2-trialkylsilyl-4-alkylbutadienes with an N-methyloxidopyridinium ion affords cycloadducts with high regioselectivity and excellent endo selectivity.

A Direct Organocatalytic Enantioselective Route to Functionalized trans-Diels-Alder Products Having the Norcarane Scaffold

An enantioselective methodology to construct trans-Diels-Alder scaffolds by organocatalysis with excellent selectivity, high yield and up to five contiguous stereocenters is presented. The reaction concept integrates the halogen effect and a novel discovered pseudo-halogen effect to direct an endo-selective, secondary-amine catalyzed Diels-Alder reaction allowing for the subsequent formation of trans-Diels-Alder cycloadducts featuring the norcarene scaffold. The methodology relies on the reaction between an in situ generated trienamine and an α-brominated or α-pseudo-halogenated enone to form a fleeting cis-Diels-Alder intermediate. The endo-transition state-enhanced by the (pseudo-)halogen effect-sets the stereochemistry that allows for a subsequent S_N 2-like reaction at a tertiary center to obtain the trans-Diels-Alder scaffold. The mechanism was investigated and supported by experimental results as well as computational studies.

Theoretical investigation of the effect of hydrogen bonding on the stereoselectivity of the Diels–Alder reaction

Here, we report the computational prediction of high exo selectivities in a series of Diels–Alder reactions with H-bonding interaction.

The simplest Diels-Alder reactions are not endo-selective

There is a widespread perception that the high level of endo selectivity witnessed in many Diels-Alder reactions is an intrinsic feature of the transformation. In contrast to expectations based upon this existing belief, the first experimental Diels-Alder reactions of a novel, deuterium-labeled 1,3-butadiene with commonly used mono-substituted alkenic dienophiles (acrolein, methyl vinyl ketone, acrylic acid, methyl acrylate, acrylamide and acrylonitrile) reveal kinetic endo : exo ratios close to 1 : 1. Maleonitrile, butenolide, α-methylene γ-butyrolactone, and N-methylmaleimide behave differently, as does methyl vinyl ketone under Lewis acid catalysis. CBS-QB3 calculations incorporating solvent and temperature parameters give endo : exo product ratios that are in near quantitative agreement with these and earlier experimental findings. This work challenges the preconception of innate endo-selectivity by providing the first experimental evidence that the simplest Diels-Alder reactions are not endo-selective. Trends in behaviour are traced to steric and electronic effects in Diels-Alder transition structures, giving new insights into these fundamental processes.