Mechanistic Aspects of the Crabtree-Pfaltz Hydrogenation of Olefins - An Interplay of Experimentation and Quantum Chemical Computation.

Introduction of Crabtree's iridium-based hydrogenation catalyst in 1977 marked a paradigm shift both with respect to the role of iridium in homogeneous catalysis as well as catalytic hydrogenation of olefins. In 1998, Pfaltz introduced an improved catalyst, by use of BARF- as anion, and established the first chiral variant of the Crabtree catalyst. This led to numerous practical highly enantioselective syntheses. Elucidation of mechanistic details posed great problems because of instability of the crucial intermediates. A remarkable breakthrough was achieved by Brandt, Andersson et al. in 2003, based on dft calculations. These authors replaced a previously assumed IrI /IrIII catalytic cycle by a novel IrIII /IrV cycle. The proposal was experimentally verified by Pfaltz in 2014 and corroborated by advanced quantum chemical calculations. This essay is an attempt to describe a fascinating interplay of experiments and quantum chemical calculations for an important synthetic method.

Definition of the term asymmetric synthesis-History and revision.

The widely used term asymmetric synthesis was introduced by Emil Fischer in 1894 by demonstration with examples. An early definition by Marckwald was successful in applications involving enantiomeric reaction products. The term caused difficulties in cases of diastereomeric products, because of a lack of a clear definition of the term diastereoisomers, which in turn affected the introduction of the term diastereoselectivity. Clarity was reached only after Wheland had revived a definition by Victor Meyer of 1907. Today, a definition codified by the International Union of Pure and Applied Chemistry (IUPAC) is in use, which is slightly modified here.

Iridium-Catalyzed Asymmetric Allylic Substitution Reactions.

In this review, we summarize the origin and advancements of iridium-catalyzed asymmetric allylic substitution reactions during the past two decades. Since the first report in 1997, Ir-catalyzed asymmetric allylic substitution reactions have attracted intense attention due to their exceptionally high regio- and enantioselectivities. Ir-catalyzed asymmetric allylic substitution reactions have been significantly developed in recent years in many respects, including ligand development, mechanistic understanding, substrate scope, and application in the synthesis of complex functional molecules. In this review, an explicit outline of ligands, mechanism, scope of nucleophiles, and applications is presented.

Dibenzothiophenesulfilimines: A Convenient Approach to Intermolecular Rhodium-Catalysed C-H Amidation.

A sulfilimine-based Group 9 transition-metal-catalysed C-H amidation procedure is reported. Dibenzothiophene-based sulfilimines were shown to constitute a class of novel amidation reagents which enable the transfer of a wide range of N-sulfonyl and N-acyl moieties. It was demonstrated that sulfilimines, which are easily accessible from cheap reagents, are safe-to-handle and represent broadly applicable amidation reagents. The dibenzothiophene can be recycled after use. The C-H amidation was shown to proceed with high selectivity and gave the mono-amidated products, mostly in good to excellent yields.

Applications of Iridium-Catalyzed Asymmetric Allylic Substitution Reactions in Target-Oriented Synthesis.

Metal catalyzed allylic substitution is a cornerstone of organometallic and synthetic chemistry. Enantioselective versions have been developed with catalysts derived from transition metals, most notably molybdenum, nickel, ruthenium, rhodium, iridium, palladium, and copper. The palladium- and the iridium-catalyzed versions have turned out to be particularly versatile in organic synthesis because of the very broad scope of the nucleophile and great functional group compatibility. Assets of the iridium-catalyzed reaction are the formation of branched, chiral products from simple monosubstituted allylic substrates, high degrees of regio- and enantioselectivity, and use of modular, readily available chiral ligands. The possibility to use carbon, nitrogen, oxygen, and sulfur compounds as well as fluoride as nucleophiles allows a wide range of chiral building blocks to be prepared. Our Account begins with the presentation of fundamental reaction schemes and chiral ligands. We will focus our discussion on reactions promoted by phosphoramidite ligands, though numerous chiral ligands have been employed. The subsequent section presents a brief overview of reaction mechanism and experimental conditions. Two versions of the iridium-catalyzed allylic substitution have emerged. In type 1 reactions (introduced in 1997), linear allylic esters are commonly used as substrates under basic reaction conditions. In type 2 reactions (introduced in 2007), environmentally friendly branched allylic alcohols can be reacted under acidic conditions; occasionally, derivatives of allylic alcohols have also been applied. A unique feature of the type 2 reactions is that highly electrophilic allylic intermediates can be brought to reaction with weakly activated alkenes. The subsequent text is ordered according to the strategies followed to transform allylic substitution products to desired targets, most of which are natural products or drugs. Syntheses starting with an intermolecular allylic substitution are discussed first. Some fairly complex targets, for example, the potent nitric oxide inhibitor (-)-nyasol and the drug (-)-protrifenbute, have been synthesized via less than five steps from simple starting materials. Most targets discussed are cyclic compounds. Intermolecular allylic substitution with subsequent ring closing metathesis is a powerful strategy for their synthesis. Highlights are stereodivergent syntheses of Δ9-tetrahydrocannabinols (THC), wherein iridium- and organocatalysis are combined (dual catalysis). The combination of allylic alkylation with a Diels-Alder reaction was utilized to synthesize the ketide apiosporic acid and the drug fesoterodine (Toviaz). Sequential allylic amination, hydroboration and Suzuki-Miyaura coupling generates enones suitable for conjugate addition reactions; this strategy was employed in syntheses of a variety of alkaloids, for example, the poison frog alkaloid (+)-cis-195A (pumiliotoxin C). Intramolecular substitutions offer interesting possibilities to build up stereochemical complexity via short synthetic routes. For example, in diastereoselective cyclizations of chiral compounds, substrate control can be overruled by catalyst control in order to generate cis- and trans-isomers selectively from a given precursor. This approach was used to prepare a variety of piperidine and pyrrolidine alkaloids. Finally, complex polycyclic structures, including the structurally unusual indolosesquiterpenoid mycoleptodiscin A, have been generated diastereo- and enantioselectively from olefins by polyene cyclizations and from electron-rich arenes, such as indoles, in dearomatization reactions.

The 50th Anniversary of the Cahn-Ingold-Prelog Specification of Molecular Chirality.

"… Just over 50 years ago, Robert S. Cahn, Sir Christopher Ingold, and Vladimir Prelog published the Review 'Specification of Molecular Chirality' in Angewandte Chemie. The article would change the everyday language of the chemist by bringing the term chirality to their attention. Today, the CIP system is the specification tool in organic chemistry …" Read more in the Editorial by Günter Helmchen.

Immobilized catalysts for iridium-catalyzed allylic amination: rate enhancement by immobilization.

The first immobilized catalyst for Ir-catalyzed asymmetric allylic aminations is described. The catalyst is a cationic (π-allyl)Ir complex bound by cation exchange to an anionic silica gel support. Preparation of the catalyst is facile, and the supported catalyst displayed considerably enhanced activity compared with the parent homogeneous catalyst. Up to 43 consecutive amination runs were possible in recycling experiments.

Enantio- and regioselective iridium-catalyzed allylic esterification.

A highly enantioselective and regioselective Ir-catalyzed allylic esterification is described, in which branched allylic esters are synthesized directly. Carboxylates were used as nucleophiles and linear allylic phosphates as electrophiles. In some cases the allylic substitution reaction was found to be accompanied by a kinetic resolution process, which causes a change of the enantiomeric excess.

Addition of organometallic reagents to chiral N-methoxylactams: enantioselective syntheses of pyrrolidines and piperidines.

Enantioselective iridium-catalyzed allylic substitutions were used to prepare N-allyl hydroxamic acid derivatives that were suitable for ring-closing metathesis, giving N-methoxylactams. Reactions of these derivatives with Grignard or organolithium compounds gave hemiaminals, which could be reduced diastereoselectively via acyliminium intermediates to give cis-piperidines or cis-pyrrolidines with substituents in the 2,6- or 2,5-positions, respectively. In addition, compounds with a quaternary carbon center could be synthesized by corresponding reactions with potassium cyanide/AcOH. The procedures were applied in the syntheses of alkaloids (-)-209D and (+)-prosophylline.

Syntheses of the hexahydroindene cores of indanomycin and stawamycin by combinations of iridium-catalyzed asymmetric allylic alkylations and intramolecular Diels-Alder reactions.

Short and concise syntheses of the hexahydroindene cores of the antibiotics indanomycin (X-14547 A) and stawamycin are presented. Key methods used are an asymmetric iridium-catalyzed allylic alkylation, a modified Julia olefination, a Suzuki-Miyaura coupling, and an intramolecular Diels-Alder reaction.

Iridium-catalyzed allylic substitutions with cyclometalated phosphoramidite complexes bearing a dibenzocyclooctatetraene ligand: preparation of (π-allyl)Ir complexes and computational and NMR spectroscopic studies.

(π-Allyl)Ir complexes derived from dibenzocyclooctatetraene and phosphoramidites by cyclometalation are effective catalysts for allylic substitution reactions of linear monosubstituted allylic carbonates. These catalysts provide exceptionally high degrees of regioselectivity and allow the reactions to be run under aerobic conditions. A series of (π-allyl)Ir complexes were prepared and characterized by X-ray crystal structure analyses. An allylic amination with aniline displayed different resting states depending on the presence of a strong base. DFT calculations were carried out on the mechanistic aspects of these reactions. The results suggest that for the (π-allyl)Ir complexes, the formation and reactions with nucleophiles proceed with comparable rates.

Enantioselective syntheses of the alkaloids cis-195A (pumiliotoxin C) and trans-195A based on multiple applications of asymmetric catalysis.

Short enantio- and diastereoselective syntheses of the decahydroquinoline alkaloids cis- (pumiliotoxin C) and trans-195A are presented. Key steps are an enantioselective iridium-catalyzed allylic amination, a Suzuki-Miyaura coupling, a catalyst-controlled copper-catalyzed 1,4-addition, and a reductive amination.

Enantioselective total synthesis and absolute configuration of apiosporic acid.

The first total synthesis of the polyketide apiosporic acid is presented. Key steps are a Julia-Kocienski olefination, a Suzuki-Miyaura cross-coupling, and an intramolecular Diels-Alder reaction. The absolute configuration of the natural product was determined.

Enantio- and regioselective iridium-catalyzed allylic hydroxylation.

The first enantioselective allylic hydroxylation to prepare branched allylic alcohols directly is described. Bicarbonate was used as nucleophile in conjunction with new single component Ir-catalysts, which are stable to air and water. Excellent regio- and enantioselectivities have been achieved with a representative set of substrates.

Iridium-catalyzed allylic vinylation and asymmetric allylic amination reactions with o-aminostyrenes.

An Ir-catalyzed allylic vinylation reaction of allyl carbonates with o-aminostyrene derivatives has been realized, providing skipped (Z,E)-diene derivatives. With (E)-but-2-ene-1,4-diyl dimethyl dicarbonate as the substrate, an efficient enantioselective synthesis of 1-benzazepine derivatives via an Ir-catalyzed domino allylic vinylation/intramolecular allylic amination reaction has been developed. Mechanistic studies of the allylic vinylation reaction have been carried out, and the results suggest that the leaving group of the allylic precursor plays a key role in directing the reaction pathway. Screening of various allylic precursors showed that Ir-catalyzed reactions of allyl diethyl phosphates with o-aminostyrene derivatives proceed via an allylic amination pathway. A subsequent ring-closing metathesis (RCM) reaction of the amination products led to a series of enantiomerically enriched 1,2-dihydroquinoline derivatives. Their utility is indicated by an asymmetric total synthesis of (-)-angustureine.

Differential effects of the brefeldin A analogue (6R)-hydroxy-BFA in tobacco and Arabidopsis.

The effects of two brefeldin A (BFA) analogues (BFA lactam; 6(R)-hydroxy-BFA) on plant cells were tested. Although these two compounds elicited BFA-like effects in mammalian cells, the lactam analogue failed to elicit a response in plant cells. By contrast, while the 6(R)-hydroxy-BFA analogue gave rise to a classic BFA response in tobacco mesophyll protoplasts and true leaves of Arabidopsis (redistribution of Golgi enzymes into the ER), it failed to cause the formation of BFA-compartments in Arabidopsis root cells and cotyledonary leaves. Even when the GNL1-LM mutant of Arabidopsis, which has a cis-Golgi located BFA-sensitive ARF-GEF, was used, the 6(R)-hydroxy analogue failed to elicit a response at conventional BFA concentrations. Only at concentrations of over 200 µM did 6(R)-hydroxy-BFA elicit a BFA-like effect. These differences are interpreted in terms of the different properties of the respective TGN- (Arabidopsis roots) and cis-Golgi- (tobacco mesophyll) localized BFA-sensitive ARF-GEFs.

Enantioselective syntheses of 2,5-disubstituted pyrrolidines based on iridium-catalyzed allylic aminations--total syntheses of alkaloids from amphibian skins.

A broadly applicable route to trans-2,5-disubstituted pyrrolidines has been developed. Key steps are an asymmetric iridium-catalyzed allylic amination, a Suzuki-Miyaura coupling, and an intramolecular aza-Michael addition. Enantiomeric excesses in the range of 93-99 % ee have been achieved. Total syntheses of the alkaloids (-)-225 C, (+)- and (-)-223 H (xenovenine), (+)-223 AB, (+)-195 B, and (+)-223 R have been carried out as applications.

Ir-catalysed asymmetric allylic substitutions with cyclometalated (phosphoramidite)Ir complexes--resting states, catalytically active (pi-allyl)Ir complexes and computational exploration.

Mechanistic aspects of allylic substitutions with iridium catalysts derived from phosphoramidites by cyclometalation were investigated. The determination of resting states by (31)P NMR spectroscopy led to the conclusion that the cyclometalation process is reversible. A novel, one-pot procedure for the preparation of (pi- allyl)Ir complexes was developed, and these complexes were characterised by X-ray crystal structure analyses and spectral data. They are fully active catalysts of the allylic substitution reaction. DFT calculations on the allyl complexes, transition states of the allylic substitution and product olefin complexes gave further mechanistic insight.

Platinum(II) chloride-catalyzed stereoselective domino enyne isomerization/Diels-Alder reaction.

Chiral 1,6-enynes were prepared via Ir-catalyzed allylic substitutions. Their platinum(II) chloride-catalyzed domino enyne isomerization/Diels-Alder reaction provided stereoselective access to complex heterocycles. Very high diastereoselectivity was induced by a chirality center of the enyne.

A configurational switch based on iridium-catalyzed allylic cyclization: application in asymmetric total syntheses of prosopis, dendrobate, and spruce alkaloids.

A method for the stereoselective synthesis of 2,6-disubstituted piperidines has been developed that is based on the use of an intramolecular iridium-catalyzed allylic substitution as a configurational switch. The procedure allows the preparation of 2-vinylpiperidines with enantiomeric excesses (ee) of greater than 99%. As applications, total syntheses of piperidine alkaloids have been elaborated, most often by using Ru-catalyzed cross-metatheses as a key step for introduction of a side chain. Asymmetric total syntheses of the prosopis alkaloids (+)-prosopinine, (+)-prosophylline, (+)-prosopine, and of the dendrobate alkaloid (+)-241D and its C6 epimer are described.