Martin Breugst

Farewell to the HSAB treatment of ambident reactivity.

The concept of hard and soft acids and bases (HSAB) proved to be useful for rationalizing stability constants of metal complexes. Its application to organic reactions, particularly ambident reactivity, has led to exotic blossoms. By attempting to rationalize all the observed regioselectivities by favorable soft-soft and hard-hard as well as unfavorable hard-soft interactions, older treatments of ambident reactivity, which correctly differentiated between thermodynamic and kinetic control as well as between different coordination states of ionic substrates, have been replaced. By ignoring conflicting experimental results and even referring to untraceable experimental data, the HSAB treatment of ambident reactivity has gained undeserved popularity. In this Review we demonstrate that the HSAB as well as the related Klopman-Salem model do not even correctly predict the behavior of the prototypes of ambident nucleophiles and, therefore, are rather misleading instead of useful guides. An alternative treatment of ambident reactivity based on Marcus theory will be presented.

N‐Heterocyclic Carbenes: Organocatalysts with Moderate Nucleophilicity but Extraordinarily High Lewis Basicity

Since the first isolation and characterization of stable Nheterocyclic carbenes (NHCs) by Arduengo and co-workers in 1991, these compounds have attracted great interest in various fields of chemistry. As molecules with divalent carbon atoms, NHCs (e.g., 1–3, Scheme 1) are not only of theoretical interest but also of practical relevance as ligands in metal complexes and as nucleophilic organocatalysts.

Redox-neutral α-oxygenation of amines: reaction development and elucidation of the mechanism.

Cyclic secondary amines and 2-hydroxybenzaldehydes or related ketones react to furnish benzo[e][1,3]oxazine structures in generally good yields. This overall redox-neutral amine α-C–H functionalization features a combined reductive N-alkylation/oxidative α-functionalization and is catalyzed by acetic acid. In contrast to previous reports, no external oxidants or metal catalysts are required. Reactions performed under modified conditions lead to an apparent reductive amination and the formation of o-hydroxybenzylamines in a process that involves the oxidation of a second equivalent of amine. A detailed computational study employing density functional theory compares different mechanistic pathways and is used to explain the observed experimental findings. Furthermore, these results also reveal the origin of the catalytic efficiency of acetic acid in these transformations.

Redox-Neutral α-Sulfenylation of Secondary Amines: Ring-Fused N,S-Acetals

Secondary amines react with thiosalicylaldehydes in the presence of catalytic amounts of acetic acid to generate ring-fused N,S-acetals in redox-neutral fashion. A broad range of amines undergo α-sulfenylation, including challenging substrates such morpholine, thiomorpholine, and piperazines. Computational studies employing density functional theory indicate that acetic acid reduces the energy barriers of two separate steps, both of which involve proton transfer.

Nucleophilic Reactivities of Imide and Amide Anions

The kinetics of the reactions of amide and imide anions 2a-o with benzhydrylium ions 1a-i and structurally related quinone methides 1j-q have been studied by UV-vis spectroscopy in DMSO and acetonitrile solution. The second-order rate constants (log k(2)) correlated linearly with the electrophilicity parameters E of 1a-q according to the correlation log k(2) = s(N + E) (Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957), allowing us to determine the nucleophilicity parameters N and the nucleophile-specific parameters s for these nucleophiles. The reactivities of all sulfonamide and diacylimide anions are found in a relatively small range (15 < N < 22). Comparison with structurally related carbanions revealed that amide and imide anions are less reactive than carbanions of the same pK(aH). These effects can be attributed to the absence of resonance stabilization of one of the lone pairs in the amide or imide anions. As amide and imide anions are exclusively attacked at nitrogen by benzhydrylium ions, Kornblums interpretation of the ambident reactivity of amide anions has to be revised.

Zwitterions and unobserved intermediates in organocatalytic Diels-Alder reactions of linear and cross-conjugated trienamines.

The Diels-Alder reactions of cyclic linear and cross-conjugated trienamines with oxindoles have been studied with density functional theory [M06-2X/def2-TZVPP/IEFPCM//B97D/6-31+G(d,p)/IEFPCM]. These reactions are found to proceed in a stepwise fashion. Computations revealed that these transformations involve complex mechanisms including zwitterionic intermediates and several unstable alternate cycloadducts arising from (2 + 2) cycloadditions and hetero-Diels-Alder reactions. The observed regio- and stereochemistry can be rationalized by a combination of kinetic and thermodynamic control.

Reaction Mechanism of Iodine-Catalyzed Michael Additions

Molecular iodine, an easy to handle solid, has been successfully employed as a catalyst in different organic transformations for more than 100 years. Despite being active even in very small amounts, the origin of this remarkable catalytic effect is still unknown. Both a halogen bond mechanism as well as hidden Brønsted acid catalysis are frequently discussed as possible explanations. Our kinetic analyses reveal a reaction order of 1 in iodine, indicating that higher iodine species are not involved in the rate-limiting transition state. Our experimental investigations rule out hidden Brønsted acid catalysis by partial decomposition of I2 to HI and suggest a halogen bond activation instead. Finally, molecular iodine turned out to be a similar if not superior catalyst for Michael additions compared with typical Lewis acids.

Synergistic effects between Lewis and Brønsted acids: application to the Prins cyclization.

Brønsted and Lewis acids can catalyze the Prins cyclization, an efficient method for the synthesis of tetrahydropyrans from homoallylic alcohols and carbonyl compounds. Synergistic effects between weak Brønsted and Lewis acids in these reactions have been analyzed by density functional theory [M06-L/def2-QZVP/IEFPCM(CH2Cl2)//M06-L/6-311+G(2df,2p)]. In order to characterize the reactivities of the employed Lewis acids, methyl anion and hydroxide affinities were determined. On the basis of our calculations, we found that the coordination of Lewis acids to carboxylic and sulfonic acids results in a significant increase in the Brønsted acidities of the latter.

Influence of the N-Substituents on the Nucleophilicity and Lewis Basicity of N-Heterocyclic Carbenes

The ability to modulate the nucleophilicity and Lewis basicity of N-heterocyclic carbenes is pivotal to their application as organocatalysts. Herein we examine the impact of the N-substituent on the nucleophilicity and Lewis basicity. Four N-substituents popular in NHC organocatalysis, namely, the N-2,6-(CH3O)2C6H3, N-Mes, N-4-CH3OC6H4, and N-tert-butyl groups, have been examined and found to strongly affect the nucleophilicity. Thus, the N-2,6-(CH3O)2C6H3 group provides the most nucleophilic imidazolylidene NHC reported and the N-tert-butyl group one of the least. This difference in nucleophilicity is reflected in the catalyst efficiency, as observed with a recently reported trienyl ester rearrangement.

Asymmetric Redox-Annulation of Cyclic Amines.

Cyclic amines such as 1,2,3,4-tetrahydroisoquinoline undergo regiodivergent annulation reactions with 4-nitrobutyraldehydes. These redox-neutral transformations enable the asymmetric synthesis of highly substituted polycyclic ring systems in just two steps from commercial materials. The utility of this process is illustrated in a rapid synthesis of (−)-protoemetinol. Computational studies provide mechanistic insights and implicate the elimination of acetic acid from an ammonium nitronate intermediate as the rate-determining step.