Metalated Nitriles: Halogen−Metal Exchange with α-Halonitriles

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.

Metalated nitriles: SNi' cyclizations with a propargylic electrophile

A rare 5-exo-dig SNi' cyclization with magnesiated and lithiated nitriles affords a cis-fused hydrindane bearing an exocyclic allene. The cyclization of the dilithiated nitrile pits a stereoelectronic preference for a trans-hydrindane against a cyclization through a less strained transition structure to the corresponding cis-hydrindane. Computational modeling suggests that the dilithiated nitrile cyclizes to a cis-hydrindane because the preferred transition structure positions the lithium cation in a cone of electron density that bridges the nitrile-bearing carbon, an alkoxide, and an electron-rich alkyne functionality.

Organo-manganese η2-auxiliary directed reactions: a diastereoselective approach to 2,3-allenols

Propargyl aldehydes underwent isomerization to allenyl aldehydes under mildly basic conditions when complexed to an organo-manganese auxiliary using methylcyclopentadienyl manganese tricarbonyl (MMT). This traceless auxiliary magnifies the axial chirality of the allene moiety, allowing for highly diastereoselective additions to the aldehyde carbonyl and subsequent access to an array of 2,3-allenols. Using this strategy, a nitrile-substituted 2,3-allenol was prepared and efficiently converted to Hagen's gland lactone.

Chemoselective efficient synthesis of functionalized β-oxonitriles through cyanomethylation of Weinreb amides

Homologation of Weinreb amides with cyanomethyllithium: a new route to β-oxonitriles.

Overcoming challenges in the palladium-catalyzed synthesis of electron deficient ortho-substituted aryl acetonitriles

Highly electron deficient monoaryl, di-aryl and bis-diaryl acetonitriles were effectively synthesized using either a nucleophilic aromatic substitution (NAS) or a palladium-mediated coupling pathway. Synthesis of di-aryl acetonitriles most conveniently proceeded via NAS--palladium-mediated coupling was not required. This reaction, however, results in a product that is more acidic than the reactants. Facile deprotonation of the product prevents efficient formation of the bis-diaryl acetonitrile through a NAS pathway. Thus, palladium-mediated coupling is required to prepare the bis-diaryl acetonitrile efficiently. In the palladium-catalyzed coupling, choice of base and solvent (and thus the counter cation for the benzylic anion nucleophile) is important. Also, choice of the supporting ligand is important, indicating the sensitivity of the reaction to steric and ligand electronic effects.

The First Enantioenriched Metalated Nitrile Possessing Macroscopic Configurational Stability

[structure: see text]. Magnesium-bromine exchange on enantiopure cyclopropyl bromonitrile 5 at -100 degrees C for 1 min followed by a D2O quench gives the deuterionitrile in 81% ee (retention); additional trapping experiments establish t(1/2)(rac) = 11.4 h at -100 degrees C. These experiments provide the first glimpse into the stereochemical aspects of Mg-Br exchange. The intermediate formed is the first metalated nitrile demonstrated to possess macroscopic configurational stability.

Utilization of achiral alkenyl amines for the preparation of high affinity Grb2 SH2 domain-binding macrocycles by ring-closing metathesis

A family of previously reported ring-closing metathesis (RCM)-derived macrocycles that exhibit potent Grb2 SH2 domain-binding affinity is characterized by stereoselectively-introduced upper ring junctions that bear bicyclic aryl substituents. However, the synthetic complexity of these macrocycles presents a potential limit to their therapeutic application. Therefore, the current study was undertaken to simplify these macrocycles through the use of achiral 4-pentenylamides as ring-forming components. A series of macrocycles (5a-f) was prepared bearing both open and cyclic constructs at the upper ring junction. The Grb2 SH2 domain-binding affinities of these macrocycles varied, with higher affinities being obtained with cyclo-substituents. The most potent analogue (5d) contained a cyclohexyl group and exhibited Grb2 SH2 domain-binding affinity (K(D) = 1.3 nM) that was nearly equal to the parent macrocycle (2), which bore a stereoselectively-introduced naphthylmethyl substituent at the upper ring junction (K(D) = 0.9 nM). The results of this study advance design considerations that should facilitate the development of Grb2 SH2 domain-binding antagonists.

Mild Palladium-Catalyzed Selective Monoarylation of Nitriles

Two new palladium-catalyzed procedures for the arylation of nitriles under less basic conditions than previously reported have been developed. The selective monoarylation of acetonitrile and primary nitriles has been achieved using alpha-silyl nitriles in the presence of ZnF2. This procedure is compatible with a variety of functional groups, including cyano, keto, nitro, and ester groups, on the aryl bromide. The arylation of secondary nitriles occurred in high yield by conducting reactions with zinc cyanoalkyl reagents. These reaction conditions tolerated base-sensitive functional groups, such as ketones and esters. The combination of these two methods, one with alpha-silyl nitriles and one with zinc cyanoalkyl reagents, provides a catalytic route to a variety of benzylic nitriles, which have not only biological significance but utility as synthetic intermediates. The utility of these new coupling reactions has been demonstrated by a synthesis of verapamil, a clinically used drug for the treatment of heart disease, by a three-step route from commercial materials that allows convenient variation of the aryl group.

Cyclic Nitriles: Diastereoselective Alkylations

[reaction: see text] Diastereoselective alkylations of metalated conformationally locked 4-tert-butylcyclohexanecarbonitrile are highly diastereoselective with magnesium and copper counterions but only modestly diastereoselective with lithium as the counterion. Selective generation of diverse metalated nitriles is readily achieved through bromine-magnesium, -copper, and -lithium exchange reactions of the corresponding bromonitrile or, for lithium, by deprotonating the parent nitrile with lithium diethylamide. Collectively, high alkylation stereoselectivities correlate with the retentive alkylations of C-metalated nitriles, whereas N-lithiated nitriles alkylate with modest selectivity, reflecting minimal steric differences in the corresponding axial and equatorial electrophile trajectories.

Metalated Nitriles: Organolithium, -magnesium, and -copper Exchange of α-Halonitriles

[reaction: see text] alpha-Halonitriles react with alkyllithium, organomagnesium, and lithium dimethylcuprate reagents generating reactive, metalated nitriles. The rapid halogen-metal exchange with alkyllithium and Grignard reagents allows selective exchange in the presence of reactive carbonyl electrophiles, including aldehydes, providing a high-yielding alkylation protocol. Lithiated and magnesiated nitriles react with propargyl bromide by S(N)2 displacement whereas organocopper nitriles react by S(N)2' displacement, correlating with the formation of a C-metalated nitrile.