The structures of six dimeric copper(II) formates and acetates with picoline as an axial ligand, (I)-(VI), have been determined at room temperature and electron density distributions in lithium acetate dihydrate, (VII), and copper(II) formate diurea dihydrate, (VIII), have been studied at 120K. (VI) show no important geometrical differences in the Cu2(COO)4 cage between the copper acetates and formates. The observed deformation-density distributions in the CH3COO-(VII) and HCOO-(VIII) ions are in accord with theoretical expectations. Mulliken population analysis of eight carboxylate ions using STO-6G basis sets indicates that the electron population of the carboxylate group of the bridging ligand correlates with the -2J values of the binuclear copper(II) carboxylates. The key point that determines the strength of the spin-exchange interaction is the 2px orbital population of the carboxylate C atom (x is parallel to the C--R bond axis in the RCOO-ion). The electron population on a carboxylate O atom is shown to exhibit a roughly linear correlation with the pKa value of the parent carboxylic acid. The conundrum of why formate and fluoroacetate are exceptions to the correlation of -2J with pKa value has been solved in this paper.

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. Alkylating these C-magnesiated nitriles with alkyl halides, sulfonates, and unstrained ketones occur with retention of the C-Mg configuration, whereas aldehyde and acyl cyanide acylations proceed with inversion of 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.

Linear correlations with positive slopes have been found for the zero-field splitting parameters (D) and the anisotropic exchange parameters (Dex) vs the singlet–triplet separations (-2J) for [Cu2(O2CR)4(L)2] (R = F3C, FCH2, Cl3C, ClCH2, CH3, H, and C6H5CO) for a wide range of -2J values, ca. 240–650 cm-1. The values of D and Dex for benzoylformate were found to be the highest ever studied. The quality of the linear relations of -2J, Dex and D with the square of the coefficient of 2px(C) of the symmetric HOMO of the bridging carboxylate ions, coeff,2 were found to be in the order, -2J vs coeff2 > Dex vs coeff2 > D vs coeff2. A roughly linear relationship with a negative slope was found for D vs -2J for a system, [Cu2(O2CCMenPh3-n)4(L)2], in which the -2J values cover a relatively small range, ca. 320–370 cm-1.

[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.

Alkylations of conformationally constrained acyclic nitriles containing vicinal dimethyl groups and an adjacent phenyl group or trisubstituted alkene are exceptionally diastereoselective. Probing the alkylation stereoselectivity with a series of C- and N-metalated nitriles implicates a reactive conformation in which an sp2-hybridized substituent projects over the metalated nitrile to avoid allylic strain. Steric screening thereby directs the electrophilic attack to the face of the metalated nitrile opposite the projecting substituent. Excellent stereoselectively is maintained in a diverse range of alkylations that efficiently install quaternary centers, even with isopropyliodide in which a contiguous array of tertiary-quaternary-tertiary stereocenters is created! Screening the conformational requirements with a series of acyclic nitriles and esters reveals the key structural requirements for high selectivity while providing a robust, predictive model that accounts for comparable ester alkylations affording the opposite diastereomer! The intensive survey of metalated nitrile alkylations identifies the key structural features required for high 1,2-asymmetric induction, addresses the long-standing challenge of asymmetric alkylations with acylic metalated nitriles, and provides a versatile method for installing hindered quaternary centers with excellent stereocontrol.