Diels-Alder Reactivity of Allenylboronic Acid Pinacol Ester and Related Dienophiles: Mechanistic Studies and Distortion/Interaction-Activation Strain Model Analysis.

A DFT study of the Diels-Alder reactions of vinylboronic acid pinacol ester, allenylboronic acid pinacol ester, methyl acrylate, and methyl 2,3-butadienoate with cyclopentadiene has been performed. Competitive mechanisms for the reactions of the carboxylic esters have been fully investigated, and similar results to those previously found for the organoboron compounds were obtained. Moreover, reactivity and selectivity patterns were correctly reproduced.

Allenylboronic Acid Pinacol Ester: A Selective Partner for [4 + 2] Cycloadditions.

We have studied the reaction of allenylboronic acid pinacol ester with cyclopentadiene with experimental and computational methods. The reaction occurred efficiently with complete Diels-Alder periselectivity and regioselectivity at the proximal double bond. The concerted mechanism for the observed transformation was computed to be favored over competitive addition to the distal double bond, [3,3]-sigmatropic rearrangements, and stepwise radical mechanism.

Theoretical Study of the Borono-Mannich Reaction with Pinacol Allenylboronate.

A density functional theory study of the mechanism of the Borono-Mannich reaction using benzylamine and piperidine as representative examples of primary and secondary amines with pinacol allenylboronate is presented. The study shows that both reactions progress through coordination between the boron and the phenolic oxygen. Ring size strain and hydrogen bond activation appear to determine the observed divergent regioselectivity.

Remarkable Reactivity of Boron-Substituted Furans in the Diels-Alder Reactions with Maleic Anhydride.

The reactivity of boron-substituted furans as dienes in the Diels-Alder reaction with maleic anhydride has been investigated. Gratifyingly, the furans with boryl substituents at C-3 gave the exo cycloadduct exclusively with excellent yields. In particular, the potassium trifluoroborate exhibited outstanding reactivity at room temperature.

Alkylhalovinylboranes: a new class of Diels-Alder dienophiles.

The Diels-Alder reactions of alkylhalovinylboranes have been investigated theoretically and experimentally. Alkylhalovinylboranes presented higher reactivity than the corresponding dialkylvinylboranes. Although / selectivities were high for the reactions with cyclopentadiene, facial selectivities for the chiral analogues were low.

Theoretical Study of the BF-Promoted Rearrangement of Oxiranyl N-Methyliminodiacetic Acid Boronates.

The mechanism of the rearrangement of oxiranyl N-methyliminodiacetyl (MIDA) boronates in dicholoromethane has been extensively investigated with density functional theory. Several reaction pathways were examined. Our results revealed that the most-favorable mechanisms for the BF-promoted rearrangement of 2-phenyl oxiranyl MIDA boronate (1) and 1-phenyl oxiranyl MIDA boronate (24) comprise two steps: ring opening of the epoxide to a carbocation intermediate followed by migration of a MIDA-boryl group (for the reaction of 1) and hydrogen (for the reaction of 24), to give the same BF-coordinated α-boryl aldehyde in both cases.

A hydrogen bond rationale for the enantioselective β-alkenylboration of enones catalyzed by O-monoacyltartaric acids.

DFT calculations suggest that O-monoacyl L-tartaric acids catalyze the asymmetric conjugate alkenylboration of enones through transition structures that are stabilized by hydrogen-bonding interactions. Formation of a five-membered acyloxyborane is proposed. The hydrogen of the free carboxy group derived from the catalyst interacts with the carbonyl group of the cyclic acyloxyborane, stabilizing the transition structure and reducing the flexibility of the system.

Reactivity and selectivity of boron-substituted alkenes in the Diels-Alder reaction with cyclopentadiene. A study of the electron charge density and its Laplacian.

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. 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 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.

[4 + 3] and [4 + 2] mechanisms of the Diels-Alder reactions of vinylboranes: an analysis of the electron charge density distribution.

The Diels-Alder (DA) reactions of isoprene with vinylborane, dimethylvinylborane and dichlorovinylborane have been studied using density functional theory and the quantum theory of atoms in molecules. We evaluated the topological properties of the transition structures (TSs) and the evolution of such properties along the reaction paths. In accordance with previous studies, our results indicate that the endo TSs of the reaction with vinylborane present high [4 + 3] character, while the exo TSs and all the TSs of the reactions with dimethylvinylborane and dichlorovinylborane have [4 + 2] character.

Theoretical investigation of the Diels-Alder reactions of unsaturated boronates.

The Diels-Alder reactions of simple unsaturated boronates have been investigated using computational methods and the results were compared with those for the analogue dihalo- and dialkylboranes. Our results indicate that the activating effect of the boronate moiety is small. All the studied reactions are concerted normal electron-demand Diels-Alder reactions with asynchronous transition structures and weak [4 + 3] C-B secondary orbital interactions, which explains the low experimental reactivity.

Application of the multi-standard methodology for calculating 1H NMR chemical shifts.

Gauge including atomic orbitals (GIAO) (1)H NMR chemical shift calculations have been performed for 66 organic compounds at 72 different levels of theory using the multi-standard approach (MSTD) previously developed for (13)C NMR. This straightforward computational technique involves the combination of methanol and benzene as standards. The studied methodology has been shown to predict (1)H NMR chemical shifts efficiently at different levels of theory.

Mechanistic insights into the BINOL-derived phosphoric acid-catalyzed asymmetric allylboration of aldehydes.

BINOL-derived phosphoric acids catalyze the asymmetric allylboration of aldehydes. DFT and QM/MM hybrid calculations showed that the reaction proceeds via a transition state involving both a hydrogen-bonding interaction from the catalyst hydroxyl group to the pseudoaxial oxygen of the cyclic boronate and a stabilizing interaction from the phosphoryl oxygen of the catalyst to the formyl hydrogen of the aldehyde. These interactions lower the energy of the transition structure and provide extra rigidity to the system.

A facile microwave-assisted Diels-Alder reaction of vinylboronates.

The Diels-Alder reaction of vinylboronates can be easily performed using microwave irradiation giving excellent yields of the cycloadducts. Pinacol vinylboronate was the reagent of choice due to its stability towards hydrolysis, operational simplicity and yields of Diels-Alder products. To the best of our knowledge, this is the first example of microwave-assisted Diels-Alder reaction of boron-substituted dienophiles.

Synthetic and computational studies on the tricarboxylate core of 6,7-dideoxysqualestatin H5 involving a carbonyl ylide cycloaddition-rearrangement.

Reaction of diazodiketoesters 17 and 28 with methyl glyoxylate in the presence of catalytic rhodium(II) acetate generates predominantly the 6,8-dioxabicyclo[3.2.1]octanes 29 and 30, respectively.

A multi-standard approach for GIAO (13)C NMR calculations.

The influence of the reference standard employed in the calculation of (13)C NMR chemical shifts was investigated over a large variety of known organic compounds, using different quantum chemistry methods and basis sets. After detailed analysis of the collected data, we found that methanol and benzene are excellent reference standards for computing NMR shifts of sp(3)- and sp-sp(2)-hybridized carbon atoms, respectively. This multi-standard approach (MSTD) performs better than TMS in terms of accuracy and precision and also displays much lower dependence on the level of theory employed.

Asymmetric allylboration reactions with Soderquist's chiral 10-substituted-9-borabicyclo[3.3.2]decanes: a theoretical study.

Chiral B-allyl-10-substituted-9-borabicyclo[3.3.2]decanes are highly efficient reagents for the enantioselective allylboration of adehydes and ketones.

Mechanistic insights into the catalytic asymmetric allylboration of ketones: Brønsted or Lewis acid activation?

Binaphthol ligands promote the enantioselective addition of allylboronates to ketones. In this study, we use DFT calculations to establish the identity of the reacting chiral species. Our results show that a cyclic Lewis acid-activated boronate is the most reactive species on the basis of calculated energy barriers, and it is only this species that leads to the correct enantiomer.

Theoretical study of the asymmetric conjugate alkenylation of enones catalyzed by binaphthols.

To rationalize the experimental results observed in the asymmetric conjugate addition of alkenylboronates to enones catalyzed by binaphthols and shed light into the factors controlling the rate, the selectivity, and the substituent effects of this process, a theoretical DFT study has been performed. The calculations suggest the catalytic cycle is finely balanced. Reversible exchange of methoxy ligands gives rise to the binaphthol-derived alkenylboronate, which is highly Lewis acidic and strongly coordinates to the enone carbonyl in a reversible fashion, lowering the energy barrier for the subsequent conjugate addition step.

Studies on the intramolecular cyclizations of bicyclic delta-hydroxynitriles promoted by triflic anhydride.

Studies have been conducted to investigate the reactivity of several bicyclic delta-hydroxynitriles with triflic anhydride in dichloromethane. The reactions of the analogues derived from 1-indanone and 1-tetralone lead to annulated enones. These products arise from an initial elimination reaction that generates an alkene, followed by the addition of the carbon-carbon double bond to the activated cyano group.

Asymmetric conjugate addition of alkynylboronates to enones: rationale for the intriguing catalysis exerted by binaphthols.

The conjugate addition of alkynylboronates to enones catalyzed by binaphthols has been studied theoretically with DFT methods. The high reactivity of the alkynylboronate derived from binaphthol seems to arise from electronic effects since its acidic boron atom binds tightly to the enone carbonyl and lowers the activation energy of the alkynylboration step. Steric clashes between the atoms of the ligands on boron and the enone can be invoked to account for the observed facial diastereoselectivity.

A promising enantioselective Diels-Alder dienophile by computer-assisted rational design: 2,5-diphenyl-1-vinyl-borolane.

Several chiral vinylboranes have been theoretically evaluated as enantioselective Diels-Alder dienophiles. Theoretical results suggest that optically pure 2,5-diphenyl-1-vinyl-borolane should be the reagent of choice for performing such transformations efficiently.

A DFT study on the regioselectivity of the reaction of dichloropropynylborane with isoprene.

This theoretical study deals with the reaction of isoprene and dichloropropynylborane. We report the results of the DFT calculations applied to the two processes involved, Diels-Alder cycloaddition and 1,4-alkynylboration. The boron influences both the chemoselectivity and the regioselectivity of this reaction through secondary orbital interactions (SOI hereafter) that give rise to transition structures with strong [4 atom + 3 atom] character.

A theoretical study of the reaction of alkynylboranes with butadiene: competition between cycloaddition and alkynylboration.

The reactions of alkynyldihaloboranes and alkynyldialkylboranes with butadiene have been explored by using DFT methods at the B3LYP level with the 6-31G basis set. Transition structures for the concerted [4+2] cycloaddition have been found for the alkynylborane derivatives. Along with these, another reactive pathway has been found for the cycloaddition process with transition structures of high [4+3] character.