Metalated Nitriles: Electrophile-Dependent Alkylations

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.

Cyclic nitriles: stereodivergent addition-alkylation-cyclization to cis- and trans-abietanes

Diverse cyclic hydroxy nitriles are readily synthesized through sequential 1,2-1,4-Grignard addition-methylations to 3-oxo-1-cyclohexene-1-carbonitrile. Acid-catalyzed intramolecular cyclizations of the cyclic hydroxy nitriles reveal fundamental stereoselectivity trends in Friedel-Crafts cyclizations to cis- and trans-abietanes. In contrast to previous assumptions, comparative cationic cyclizations with electron-rich and electron-poor aromatic nucleophiles exhibit similar preferences for cyclization to cis-abietanes. Optimizing the cyclizations for trans-abietanes has identified ZrCl 4 as an exceptional Lewis acid which, for cyclizations of iminolactones, favors trans-abietanes as the only observable diastereomer. The sequential oxonitrile addition-Friedel-Crafts cyclization strategy provides a rapid, stereodivergent synthesis of cis- or trans-abietanes, demonstrates the dramatic influence of ZrCl 4 in promoting cationic cyclizations, and in contrast to previous assumptions suggests that the cyclization stereoselectivity is not correlated with the electronic nature of the aromatic nucleus.

Alkenenitriles: Conjugate Additions of Alkyl Iodides with a Silica-Supported Zinc−Copper Matrix in Water

A new silica-supported zinc-copper matrix reagent promotes the conjugate addition of alkyl iodides to cyclic and acyclic alkenenitriles in water. X-ray diffraction and electron microscopy techniques suggest that the active copper species generated from elemental zinc and copper(I) iodide is finely dispersed, zerovalent copper. Alkyl iodides react with the silica-supported reagent to generate putative radicaloid intermediates that efficiently add to alkenenitriles to provide beta-substituted nitriles. Conjugate additions to acyclic and cyclic 5-7-membered alkenenitriles are most effective for primary alkyliodides, although secondary and tertiary alkyliodides are viable reaction partners. The strategy addresses the challenge of performing conjugate additions to disubstituted alkenenitriles and demonstrates the beneficial role of the silica-supported reagent.

Cyclic oxonitriles: stereodivergent grignard addition-alkylations

Sequential carbonyl addition-conjugate addition of Grignard reagents to cyclic 5-7-membered oxoalkenenitriles efficiently generates cyclic magnesiated nitriles. Alkylations of these magnesiated nitriles exhibit diastereoselectivities that depend intimately on the size of the carbocyclic ring: 5-membered oxonitriles generate magnesiated nitriles whose alkylations are controlled by steric constraints whereas 6- and 7-membered oxonitriles generate internally coordinated, C-magnesiated nitriles whose alkylations are controlled by stereoelectronic effects. Reversing the alkylation selectivity of 6-membered C-magnesiated nitriles is achieved by conversion to an N-metalated nitrile in which steric, rather than electronic, effects direct the electrophile trajectory. Collectively, the conjugate addition-alkylation generates highly substituted, cyclic 5-7-membered nitriles containing three new stereocenters with selective access to diastereomers at the quaternary nitrile-bearing carbon.

Oxonitriles: a grignard addition-acylation route to enamides

[reaction: see text] Sequential addition of three different Grignard reagents and pivaloyl chloride to 3-oxo-1-cyclohexene-1-carbonitrile installs four new bonds to generate a diverse array of cyclic enamides. Remarkably, formation of the C-magnesiated nitrile intermediate is followed by preferential acylation by pivaloyl chloride rather than consumption by an in situ Grignard reagent. Rapid N-acylation of the C-magnesiated nitrile generates an acyl ketenimine that reacts readily with Grignard reagents or a trialkylzincate, effectively assembling highly substituted, cyclic enamides.

Diastereoselective and Enantiospecific Synthesis of γ-Substituted α,β-Unsaturated Nitriles from O-Protected Allylic Cyanohydrins

gamma-Functionalized alpha,beta-unsaturated nitriles are prepared diastereoselectively and enantiospecifically from enantioenriched cyanohydrin-O-phosphates and carbonates derived from alpha,beta-unsaturated aldehydes, either by palladium or iridium-catalyzed nucleophilic allylic substitution reactions with different nucleophiles. Appropriate reaction conditions for dibenzylamine, benzylamine, sodium azide, NaOAc, tetra-n-butylammonium acetate (TBAA), the corresponding sodium salts of phenol and N-hydroxysuccinimide and the carbonucleophile sodium dimethyl malonate are described. Different substituted O-protected cyanohydrins, such as carbonates and phosphates, derived from crotonaldehyde, (E)-hex-2-enal, oct-2-enal, 2-methylbut-2-enal, and cinnamaldehyde are used as allylic substrates. The substitution takes place with total retention of the configuration for the (E)-gamma-functionalized nitriles and with inversion of the configuration for the Z-isomers. In general, cyanohydrin-O-phosphates are the materials of choice to get the highest E-diastereoselectivity. Dibenzylamine is the best nucleophile for the synthesis of gamma-nitrogenated alpha,beta-unsaturated nitriles in the presence of either palladium or iridium catalysts when aliphatic compounds and cinnamaldehyde derivative are used (up to 98% dr). For the synthesis of gamma-oxygenated alpha,beta-unsaturated nitriles sodium or TBAA the reagents are selected to avoid epimerizations in up to 76% dr. Finally, the Tsuji-Trost reaction with sodium malonate works only under palladium catalysis in up to 70% dr.

C-metalated nitriles: electrophile-dependent alkylations and acylations

Sequential carbonyl addition-conjugate addition of Grignard reagents to 3-oxocyclohex-1-ene-1-carbonitrile generates C-magnesiated nitriles whose alkylation stereoselectivities intimately depend on the nature of the electrophile. The alkylation of these C-magnesiated nitriles with alkyl halides, sulfonates, and unstrained ketones occurs with the retention of the C-Mg configuration, whereas aldehyde and acyl cyanide acylations proceed with inversion of the stereochemistry. Mechanistic probes indicate that the stereoselectivity is controlled by stereoelectronic effects for most electrophiles, except allylic, benzylic, and cyclopropyl halides where single-electron-transfer processes intervene. Screening numerous alkylations of C-magnesiated nitriles with a diverse range of electrophiles reveals the reaction scope and delineates the fundamental stereoelectronic effects responsible for the highly unusual electrophile-dependent alkylations.

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.