Direct and Asymmetric Nickel(II)-Catalyzed Construction of Carbon-Carbon Bonds from N-Acyl Thiazinanethiones

Direct, Stereodivergent, and Catalytic Michael Additions of Thioimides to α,β-Unsaturated Aldehydes - Total Synthesis of Tapentadol

Direct and stereodivergent Michael additions of N-acyl 1,3-thiazinane-2-thiones to α,β-unsaturated aldehydes catalyzed by chiral nickel(II) complexes are reported. The reactions proceed with a remarkable regio-, diastereo-, and enantioselectivity, so access to any of the four potential Michael stereoisomers is granted through the appropriate choice of the chiral ligand of the nickel(II) complex. Simple removal of the heterocyclic scaffold furnishes a wide array of either syn or anti enantiomerically pure derivatives, which can be exploited for the asymmetric synthesis of biologically active compounds, as demonstrated in a new approach to tapentadol. In turn, a mechanism, based on theoretical calculations, is proposed to account for the stereochemical outcome of these transformations.

Protected syn-Aldol Compounds from Direct, Catalytic, and Enantioselective Reactions of N-Acyl-1,3-oxazinane-2-thiones with Aromatic Acetals

A direct and asymmetric syn-aldol reaction of N-acyl-1,3-oxazinane-2-thiones with dialkyl acetals from aromatic acetals in the presence of 2-5 mol % [DTBM-SEGPHOS]NiCl₂, TMSOTf, and lutidine has been developed. It has been established that the oxazinanethione heterocycle, used for the first time as a scaffold in asymmetric carbon-carbon bond-forming reactions, can be smoothly removed to give access to a variety of enantiomerically pure compounds with high synthetic value.

Direct and Asymmetric Aldol Reactions of N ‐Azidoacetyl‐1,3‐thiazolidine‐2‐thione Catalyzed by Chiral Nickel(II) Complexes. A New Approach to the Synthesis of β‐Hydroxy‐α‐Amino Acids

A direct and asymmetric triisopropylsilyltrifluoromethanesulfonate (TIPSOTf) mediated aldol reaction of N‐azidoacetyl‐1,3‐thiazolidine‐2‐thione with aromatic aldehydes catalyzed by a chiral nickel(II)‐Tol‐BINAP complex has been developed (BINAP=2,2’‐bis(diphenylphosphino)‐1,1’‐binaphthyl). The catalytic protocol gives the corresponding anti α‐azido‐β‐silyloxy adducts with outstanding stereocontrol and in high yields. Theoretical calculations account for the stereochemical outcome of the reaction and lay the foundations for a mechanistic model. In turn, the easy removal of the thiazolidinethione yields a wide array of enantiomerically pure derivatives in a straightforward and efficient manner. Such a noteworthy character of the heterocyclic scaffold together with the appropriate manipulation of the azido group open a new route to the synthesis of di‐ and tripeptide blocks containing a β‐aryl‐β‐hydroxy‐α‐amino acid.

Stereoselective Alkylation of Chiral Titanium(IV) Enolates with tert-Butyl Peresters

Here, we present a new stereoselective alkylation of titanium(IV) enolates of chiral N-acyl oxazolidinones with tert-butyl peresters from Cα-branched aliphatic carboxylic acids, which proceeds through the decarboxylation of the peresters and the subsequent formation of alkyl radicals to produce the alkylated adducts with an excellent diastereoselectivity. Theoretical calculations account for the observed reactivity and the outstanding stereocontrol. Importantly, the resultant compounds can be easily converted into ligands for asymmetric and catalytic transformations.

Direct and Enantioselective Aldol Reactions Catalyzed by Chiral Nickel(II) Complexes

A direct and asymmetric aldol reaction of N‐acyl thiazinanethiones with aromatic aldehydes catalyzed by chiral nickel(II) complexes is reported. The reaction gives the corresponding O‐TIPS‐protected anti‐aldol adducts in high yields and with remarkable stereocontrol and atom economy. Furthermore, the straightforward removal of the achiral scaffold provides enantiomerically pure intermediates of synthetic interest, which involve precursors for anti‐α‐amino‐β‐hydroxy and α,β‐dihydroxy carboxylic derivatives. Theoretical calculations explain the observed high stereocontrol.

Non-Classical Amide Bond Formation: Transamidation and Amidation of Activated Amides and Esters by Selective N–C/O–C Cleavage

In the past several years, tremendous advances have been made in non-classical routes for amide bond formation that involve transamidation and amidation reactions of activated amides and esters. These new methods enable the formation of extremely valuable amide bonds via transition-metal-catalyzed, transition-metal-free, or metal-free pathways by exploiting chemoselective acyl C–X (X = N, O) cleavage under mild conditions. In a broadest sense, these reactions overcome the formidable challenge of activating C–N/C–O bonds of amides or esters by rationally tackling nN → π*C=O delocalization in amides and nO → π*C=O donation in esters. In this account, we summarize the recent remarkable advances in the development of new methods for the synthesis of amides with a focus on (1) transition-metal/NHC-catalyzed C–N/C–O bond activation, (2) transition-metal-free highly selective cleavage of C–N/C–O bonds, (3) the development of new acyl-transfer reagents, and (4) other emerging methods.

1 Introduction

2 Transamidation of Amides

2.1 Transamidation by Metal–NHC Catalysis (Pd–NHC, Ni–NHC)

2.2 Transition-Metal-Free Transamidation via Tetrahedral Intermediates

2.3 Reductive Transamidation

2.4 New Acyl-Transfer Reagents

2.5 Tandem Transamidations

3 Amidation of Esters

3.1 Amidation of Esters by Metal–NHC Catalysis (Pd–NHC, Ni–NHC)

3.2 Transition-Metal-Free Amidation of Esters via Tetrahedral Intermediates

3.3 Reductive Amidation of Esters

4 Transamidations of Amides by Other Mechanisms

5 Conclusions and Outlook