Sulfinylnitriles: Sulfinyl-Metal Exchange-Alkylation Strategies

Homologation chemistry with nucleophilic α-substituted organometallic reagents: chemocontrol, new concepts and (solved) challenges

The transfer of a reactive nucleophilic CH2X unit into a preformed bond enables the introduction of a fragment featuring the exact and desired degree of functionalization through a single synthetic operation. The instability of metallated α-organometallic species often poses serious questions regarding the practicability of using this conceptually intuitive and simple approach for forming C-C or C-heteroatom bonds. A deep understanding of processes regulating the formation of these nucleophiles is a precious source of inspiration not only for successfully applying theoretically feasible transformations (i.e. determining how to employ a given reagent), but also for designing new reactions which ultimately lead to the introduction of molecular complexity via short experimental sequences.

Copper-catalyzed selective radical–radical cross-coupling for C–S bond formation: an access to α-alkylthionitriles

Oxidative radical–radical cross-coupling: a new protocol for C–S bond formation was developed by selective cross-coupling between a thiyl radical and an isobutyronitrile radical.

C- and N-Metalated Nitriles: The Relationship between Structure and Selectivity

Metalated nitriles are exceptional nucleophiles capable of forging highly hindered stereocenters in cases where enolates are unreactive. The excellent nucleophilicity emanates from the powerful inductive stabilization of adjacent negative charge by the nitrile, which has a miniscule steric demand. Inductive stabilization is the key to understanding the reactivity of metalated nitriles because this permits a continuum of structures that range from N-metalated ketenimines to nitrile anions. Solution and solid-state analyses reveal two different metal coordination sites, the formally anionic carbon and the nitrile nitrogen, with the site of metalation depending intimately on the solvent, counterion, temperature, and ligands. The most commonly encountered structures, C- and N-metalated nitriles, have either sp³ or sp² hybridization at the nucleophilic carbon, which essentially translates into two distinct organometallic species with similar but nonidentical stereoselectivity, regioselectivity, and reactivity preferences. The hybridization differences are particularly important in S_Ni displacements of cyclic nitriles because the orbital orientations create very precise trajectories that control the cyclization selectivity. Harnessing the orbital differences between C- and N-metalated nitriles allows selective cyclization to afford nitrile-containing cis- or trans-hydrindanes, decalins, or bicyclo[5.4.0]undecanes. Similar orbital constraints favor preferential S_Ni displacements with allylic electrophiles on sp³ centers over sp² centers. The strategy permits stereoselective displacements on secondary centers to set contiguous tertiary and quaternary stereocenters or even contiguous vicinal quaternary centers. Stereoselective alkylations of acyclic nitriles are inherently more challenging because of the difficulty in creating steric differentiation in a dynamic system with rotatable bonds. However, judicious substituent placement of vicinal dimethyl groups and a trisubstituted alkene sufficiently constrains C- and N-metalated nitriles to install quaternary stereocenters with excellent 1,2-induction. The structural differences between C- and N-metalated nitriles permit a rare series of chemoselective alkylations with bifunctional electrophiles. C-Magnesiated nitriles preferentially react with carbonyl electrophiles, whereas N-lithiated nitriles favor S_N2 displacement of alkyl halides. The chemoselective alkylations potentially provide a strategy for late-stage alkylations of polyfunctional electrophiles en route to bioactive targets. In this Account, the bonding of metalated nitriles is summarized as a prelude to the different strategies for selectively preparing C- and N-metalated nitriles. With this background, the Account then transitions to applications in which C- or N-metalated nitriles allow complementary diastereoselectivity in alkylations and arylations, and regioselective alkylations and arylations, with acyclic and cyclic nitriles. In the latter sections, a series of regiodivergent cyclizations are described that provide access to cis- and trans-hydrindanes and decalins, structural motifs embedded within a plethora of natural products. The last section describes chemoselective alkylations and acylations of C- and N-metalated nitriles that offer the tantalizing possibility of selectively manipulating functional groups in bioactive medicinal leads without recourse to protecting groups. Collectively, the unusual reactivity profiles of C- and N-metalated nitriles provide new strategies for rapidly and selectively accessing valuable synthetic precursors.

Highly enantioselective metallation-substitution alpha to a chiral nitrile

We report the deprotonation of a chiral nitrile and reaction of the resulting chiral organometallic species with a variety of electrophiles to give highly enantiomerically enriched 2-substituted nitrile products. The nitrile was treated with TMPMgCl and the resulting anion, an asymmetric alpha cyano Grignard species, was found to be configurationally stable at low temperature for a short time (half-life several minutes at -104 °C).

Symmetric diarylsulfoxides as asymmetric sulfinylating reagents for dialkylmagnesium compounds

At −78 °C, primary dialkylmagnesium compounds reacted with diarylsulfoxides when 1.5 equiv of the dilithium salt of (S)-BINOL was added as a promotor. Alkyl aryl sulfoxides resulted in up to quantitative yield and with up to 97% ee. This demonstrates the feasibility of asymmetric sulfinylations by achiral sulfinylating agents (from the perspective of Alkyl2Mg) as well as the feasibility of asymmetric sulfoxide–magnesium exchanges (from the perspective of Ar2SO).