An Ylide Transformation of Rhodium(I) Carbene: Enantioselective Three‐Component Reaction through Trapping of Rhodium(I)‐Associated Ammonium Ylides by β‐Nitroacrylates

The chiral Rh^I–diene‐catalyzed asymmetric three‐component reaction of aryldiazoacetates, aromatic amines, and β‐nitroacrylates was achieved to obtain γ‐nitro‐α‐amino‐succinates in good yields and with high diastereo‐ and enantioselectivity. This reaction is proposed to proceed through the enantioselective trapping of Rh^I‐associated ammonium ylides by nitroacrylates. This new transformation represents the first example of Rh^I‐carbene‐induced ylide transformation.     

Rhodium(I)-Catalyzed ‘Metallo-Ene’ Cyclizations/β-Eliminations

Octadienyl carbonates 5 provide cyclic 1,4-dienes 6 when treated with Rh¹ complexes (1–10 mol-%) at 80°. Similar cyclization of cyclohexenyl acetate 8 affords cis- fused hexahydroindene 9 . Analogous ring closures of nonadienyl carbonate 10 yield preferably the cis-divinypyrrolidine 11 with Rh¹ catalysis but the trans-isomer 12 when catalyzed by Pd⁰. Azaoctadienyl carbonate 5a undergoes elimination with [RhH(PPh₃)₄] (5 mol-%, 80°) in MeCN giving acyclic triene. 7 .     

The Stereochemical Course of the α‐Hydroxyphosphonate–Phosphate Rearrangement

The phosphonate–phosphate rearrangement is an isomerisation of α‐hydroxyphosphonates bearing electron‐withdrawing substituents at the α‐carbon atom. We studied the stereochemical course of this rearrangement with respect to phosphorus. A set of four diastereomeric α‐hydroxyphosphonates was prepared by a Pudovik reaction from two diastereomeric cyclic phosphites. The hydroxyphosphonates were separated and rearranged with Et₃N as base. In analogy to trichlorphon, which was the first reported compound undergoing this rearrangement. All four hydroxyphosphonates could be rearranged to 2,2‐dichlorovinyl phosphates. Single‐crystal X‐ray structure analyses of the α‐hydroxyphosphonates and the corresponding phosphates allowed us to show that the rearrangement proceeds with retention of configuration on the phosphorus atom.     

A Mononuclear η2‐Bonded Phosphaalkyne Nickel(0) Complex

The synthesis and characterization of the new complex [Ni(Im^iPr)₂(η²‐P≡C‐tBu)] ( 1 ) is reported. Compound 1 represents the first structurally characterized example of a mononuclear nickel(0) complex with a side on coordinated phophaalkyne ligand.     

Asymmetric Conjugate Reduction of α,β‐Unsaturated Ketones and Esters with Chiral Rhodium(2,6‐bisoxazolinylphenyl) Catalysts

New asymmetric conjugate reduction of β,β‐disubstituted α,β‐unsaturated ketones and esters was accomplished with alkoxylhydrosilanes in the presence of chiral rhodium(2,6‐bisoxazolinylphenyl) complexes in high yields and high enantioselectivity. (E)‐4‐Phenyl‐3‐penten‐2‐one and (E)‐4‐phenyl‐4‐isopropyl‐3‐penten‐2‐one were readily reduced at 60 °C in 95 % ee and 98 % ee, respectively, by 1 mol % of catalyst loading. (EtO)₂MeSiH proved to be the best hydrogen donor of choice. tert‐Butyl (E)‐β‐methylcinnamate and β‐isopropylcinnamate could also be reduced to the corresponding dihydrocinnamate derivatives up to 98 % ee.     

Preparation of (S,S)‐Fmoc‐β2hIle‐OH, (S)‐Fmoc‐β2hMet‐OH,and (S)‐Fmoc‐β2hTyr(tBu)‐OH for Solid‐Phase Syntheses of β2‐ and β2/β3‐Peptides

The preparation of three new N‐Fmoc‐protected (Fmoc=[(9H‐fluoren‐9‐yl)methoxy]carbonyl) β²‐homoamino acids with proteinogenic side chains (from Ile, Tyr, and Met) is described, the key step being a diastereoselective amidomethylation of the corresponding Ti‐enolates of 3‐acyl‐4‐isopropyl‐5,5‐diphenyloxazolidin‐2‐ones with CbzNHCH₂OMe/TiCl₄ (Cbz=(benzyloxy)carbonyl) in yields of 60–70% and with diastereoselectivities of >90%. Removal of the chiral auxiliary with LiOH or NaOH gives the N‐Cbz‐protected β‐amino acids, which were subjected to an N‐Cbz/N‐Fmoc (Fmoc=[(9H‐fluoren‐9‐yl)methoxy]carbonyl) protective‐group exchange. The method is suitable for large‐scale preparation of Fmoc‐β²hXaa‐OH for solid‐phase syntheses of β‐peptides. The Fmoc‐amino acids and all compounds leading to them have been fully characterized by melting points, optical rotations, IR, ¹H‐ and ¹³C‐NMR, and mass spectra, as well as by elemental analyses.     

Rhodium(II)-Catalyzed Decomposition of β,γ-Unsaturated Diazo Compounds

The Rh^II-catalyzed decomposition of β,γ-unsaturated diazo ketones 1 in the presence of MeOH leads via vinylogous Wolff rearrangement to γ,δ-unsaturated esters 6 (Schemes 1 and 2). A modest asymmetric induction is achieved when the reaction is carried out with chiral tetrakis(pyrrolidinecarboxylato)- or tetrakis(oxazolidinonato)dirhodium(II) complexes. Vinyl and phenyl diazoacetates 11 and 20 , respectively, or 1-diazo-3-phenyl-propan-2-one ( 25 ), when subjected to the same reaction conditions, react by OH insertion with MeOH (Schemes 3–5). In the absence of MeOH, phenyl diazoacetates 20 and 25 undergo intramolecular CH insertion to 22 and 26 , respectively. Intramolecular CH insertion occurs with N-aryldiazoamides 23 even in the presence of MeOH (Scheme 5).     

Rhodium(III)/Amine Synergistically Catalyzed Enantioselective Alkylation of Aldehydes with α,β‐Unsaturated 2‐Acyl Imidazoles

A synergistic catalysis combination of chiral‐at‐metal rhodium complex and amine catalyst was developed for enantioselective alkylation of aldehydes with α,β‐unsaturated 2‐acyl imidazoles. The corresponding adducts were obtained in good yields with excellent enantioselectivities (up to 99% ee).     

Microwave‐Assisted,Rhodium(III)‐Catalyzed N‐Annulation Reactions of Aryl and α,β‐Unsaturated Ketones with Alkynes

New Rh^III‐catalyzed, one‐pot N‐annulation reactions of aryl and α,β‐unsaturated ketones with alkynes in the presence of ammonium acetate have been developed. Under microwave irradiation conditions, the processes lead to rapid formation of the respective isoquinoline and pyridine derivatives with efficiencies that are strongly dependent on the steric nature of the aryl ring and enone substituents. By employing this protocol, a variety of isoquinoline and pyridine derivatives were prepared in high yields. In addition, a new one‐pot approach to the synthesis of pyridines, involving four‐component reactions of ketones, formaldehyde, NH₄OAc, and alkynes, has been uncovered. This process takes place through a route involving initial aldol condensation of the ketone with formaldehyde to generate a branched α,β‐unsaturated ketone that then undergoes Rh^III‐catalyzed N‐annulation with NH₄OAc and the alkyne.     

Copper(I)‐Catalyzed Asymmetric [3+2]‐Cycloaddition of α‐Substituted Iminoesters with α‐Trifluoromethyl α,β‐Unsaturated Esters

Reported herein is an example of highly regio‐, diastereo‐ and enantioselective Cu(I)‐catalyzed intermolecular [3+2] cycloaddition reaction of α‐substituted iminoesters with α‐trifluoromethyl α,β‐unsaturated esters. This novel strategy provided a facile access to pyrrolidines with two skipped (aza)quaternary stereocenters including a CF₃ all‐carbon quaternary stereocenter. A broad substrate scope was observed and high yields (up to 94%) with excellent diastereoselectivity (up to >20 : 1 d.r.) and enantioselectivity (up to 98% ee) were obtained.