Regio- and Enantioselective Synthesis of N-Substituted Pyrazoles by Rhodium-Catalyzed Asymmetric Addition to Allenes

Heterobimetallic Catalysis: Platinum-Gold-Catalyzed Tandem Cyclization/C-X Coupling Reaction of (Hetero)Arylallenes with Nucleophiles

Heterobimetallic catalysis offers new opportunities for reactivity and selectivity but still presents challenges, and only a few metal combinations have been explored so far. Reported here is a Pt-Au heterobimetallic catalyst system for the synthesis of a family of multi-heteroaromatic structures through tandem cyclization/C-X coupling reaction. Au-catalyzed 6-endo-cyclization takes place as the first fast step. Pt-Au clusters are proposed to be responsible for the increased reactivity in the second step, that is, the intermolecular nucleophilic addition which occurs through an outer-sphere mechanism by hybrid homogeneous-heterogeneous catalysis.

Enantioselective and regiodivergent allylation of pyrimidines with terminal allenes: an approach to pyrimidine acyclic nucleosides

An atom-economic addition of pyrimidines to allenes has been developed for the diverse synthesis of branched or linear N-allylpyrimidine analogues.

Regio- and Enantioselective Synthesis of Chiral Pyrimidine Acyclic Nucleosides via Rhodium-Catalyzed Asymmetric Allylation of Pyrimidines

A direct route to branched N-allylpyrimidine analogues is herein reported via the highly regio- and enantioselective asymmetric allylation of pyrimidines with racemic allylic carbonates. With [Rh(COD)Cl]₂/chiral diphosphine as the catalyst, a range of chiral pyrimidine acyclic nucleosides could be obtained under neutral conditions in good yields (up to 95% yield) with high levels of regio- and enantioselectivities (15:1 to >40:1 B/L and up to 99% ee). Furthermore, chiral pyrimidine acyclic nucleoside bearing two adjacent chiral centers has been successfully synthesized by asymmetric dihydroxylation.

Enantioselective Terminal Addition to Allenes by Dual Chiral Primary Amine/Palladium Catalysis

We herein describe a synergistic chiral primary amine/achiral palladium catalyzed enantioselective terminal addition to allenes with α-branched β-ketocarbonyls and aldehydes. The reactions afford allylic adducts bearing acyclic all-carbon quaternary centers with high regio- and enantioselectivity. A wide range of allenes including those aliphatic or 1,1'-disubstituted could be employed, thus expanding the scope of typical asymmetric allylic alkylation reactions.

From Palladium to Brønsted Acid Catalysis: Highly Enantioselective Regiodivergent Addition of Alkoxyallenes to Pyrazolones

A highly enantioselective regiodivergent addition of alkoxyallenes to pyrazolones was developed to afford multiply functionalized alkylated products bearing a quaternary carbon stereocenter in high yields with excellent stereoselectivities. One approach is enabled by palladium catalysis, thus leading to branched allylic pyrazol-5-ones under mild reaction conditions. The other is catalyzed by a chiral Brønsted acid to give linear products exclusively. Moreover, the usefulness of this new method was highlighted by converting the allylic products into other interesting multifunctionalized pyrazolone derivatives which would be of great potential for the exploitation of pharmaceutically important molecules.

Rhodium-Catalyzed Diastereoselective Cyclization of Allenyl-Sulfonylcarbamates: A Stereodivergent Approach to 1,3-Aminoalcohol Derivatives

A diastereoselective and stereodivergent rhodium-catalyzed intramolecular coupling of sulfonylcarbamates with terminal allenes is described and it provides selective access to 1,3-aminoalcohol derivatives, scaffolds found in bioactive compounds. The reaction is compatible with a large range of different functional groups, thus furnishing products with high diastereoselectivities and yields. Moreover, multigram scale reactions, as well as the application of suitable product transformations were demonstrated.

Branching Out: Rhodium-Catalyzed Allylation with Alkynes and Allenes

We present a new and efficient strategy for the atom-economic transformation of both alkynes and allenes to allylic functionalized structures via a Rh-catalyzed isomerization/addition reaction which has been developed in our working group. Our methodology thus grants access to an important structural class valued in modern organic chemistry for both its versatility for further functionalization and the potential for asymmetric synthesis with the construction of a new stereogenic center. This new methodology, inspired by mechanistic investigations by Werner in the late 1980s and based on preliminary work by Yamamoto and Trost, offers an attractive alternative to other established methods for allylic functionalization such as allylic substitution or allylic oxidation. The main advantage of our methodology consists of the inherent atom economy in comparison to allylic oxidation or substitution, which both produce stoichiometric amounts of waste and, in case of the substitution reaction, require prefunctionalization of the starting material. Starting out with the discovery of a highly branched-selective coupling reaction of carboxylic acids with terminal alkynes using a Rh(I)/DPEphos complex as the catalyst system, over the past 5 years we were able to continuously expand upon this chemistry, introducing various (pro)nucleophiles for the selective C-O, C-S, C-N, and C-C functionalization of both alkynes and the double-bond isomeric allenes by choosing the appropriate rhodium/bidentate phosphine catalyst. Thus, valuable compounds such as branched allylic ethers, sulfones, amines, or γ,δ-unsaturated ketones were successfully synthesized in high yields and with a broad substrate scope. Beyond the branched selectivity inherent to rhodium, many of the presented methodologies display additional degrees of selectivity in regard to regio-, diastereo-, and enantioselective transformations, with one example even proceeding via a dynamic kinetic resolution. Many advances presented in this account were driven by detailed mechanistic investigations including DFT-calculations, ESI-MS and in situ IR experiments and enabled the application of our chemistry for target-oriented syntheses demonstrated by several examples shown herein. In general, this research topic has matured over the past years into a viable option when synthesizing chiral compounds, from small molecules such as quercus lactones to complex target structures such as Homolargazole or Clavosolide A. This demonstrates the importance and utility of these coupling reactions, especially considering the ease with which carbon-heteroatom bonds can be built stereoselectively, with many of the product classes displaying motifs common in modern APIs.

Enantioselective, Stereodivergent Hydroazidation and Hydroamination of Allenes Catalyzed by Acyclic Diaminocarbene (ADC) Gold(I) Complexes

The gold-catalyzed enantioselective hydroazidation and hydroamination reactions of allenes are presented herein. ADC gold(I) catalysts derived from BINAM were critical for achieving high levels of enantioselectivity in both transformations. The sense of enantioinduction is reversed for the two different nucleophiles, allowing access to both enantiomers of the corresponding allylic amines using the same catalyst enantiomer.

Asymmetric synthesis of allylic amines via hydroamination of allenes with benzophenone imine

Rhodium-catalyzed highly regio- and enantioselective hydroamination of allenes is reported. Exclusive branched selectivities and excellent enantioselectivities were achieved applying a rhodium(i)/Josiphos catalyst. This method permits the practical synthesis of valuable α-chiral allylic amines using benzophenone imine as ammonia carrier.

Rh-Catalyzed Stereospecific Synthesis of Allenes from Propargylic Benzoates and Arylboronic Acids

An enantiospecific approach to the synthesis of optically active, trisubstituted allenes from chiral propargylic benzoates and arylboronic acids has been developed. The transformation is catalyzed by a Rh-(P,olefin) complex formed in situ from [{Rh(cod)Cl}2] and a readily available phosphoramidite ligand. The method furnishes an assortment of diverse allenes in high yields and excellent enantiospecificity under mild conditions.

Rhodium-catalyzed chemo- and regioselective decarboxylative addition of β-ketoacids to alkynes

A highly efficient rhodium-catalyzed chemo- and regioselective addition of β-ketoacids to alkynes is reported.

Chirality transfer in metal-catalysed intermolecular addition reactions involving allenes

This review covers chirality transfer in metal-catalysed intermolecular addition reactions involving allenes, including axial-to-central chirality transfer, asymmetric catalysis and racemization.

Regioselective Rhodium-Catalyzed Addition of 1,3-Dicarbonyl Compounds to Terminal Alkynes

A new method for the rhodium-catalyzed regioselective C-C bond formation using terminal alkynes and 1,3-dicarbonyl compounds to achieve valuable branched α-allylated 1,3-dicarbonyl products is reported. With a Rh(I)/DPEphos/p-CF3-benzoic acid as the catalyst system, the desired products can be obtained in good to excellent yields and with perfect regioselectivity. A broad range of functional groups were tolerated, and first experimental insights of a plausible reaction mechanism were obtained.