DLPNO-CCSD(T) and DFT study of the acetate-assisted C-H activation of benzaldimine at [RuCl2(p-cymene)]2: the relevance of ligand exchange processes at ruthenium(II) complexes in polar protic media

To gain mechanistic insights into the acetate-assisted cyclometallations of arylimines promoted by [RuCl2(p-cymene)]2 in polar protic media, DFT geometry optimizations (with M06 and ωB97X-D3 functionals and the cc-pVDZ-PP[Ru] basis set) followed by DLPNO-CCSD(T)/CBS energy evaluations were performed using benzaldimine as a model substrate and methanol as the solvent (with CPCM or SMD models). The calculation results show that coordination of the imine to an acetate ruthenium precursor is followed by anion (chloride or acetate) dissociation as the rate-determining step of the process. H-Bonding of two explicit MeOH to the anion reduces the calculated activation energy to ca. 23 kcal mol-1, in good agreement with the experimental half-life at room temperature. Subsequent AMLA/CMD C-H activation of the intermediate cationic complexes is a faster, reversible process. Alternative reaction pathways involving neutral diacetate ruthenium complexes offer AMLA/CMD transition state structures of lower energy but are precluded due to higher energy barriers for the initial ligand exchange processes at ruthenium. Solvent assistance accelerates the final chloride/acetate exchange processes on the cycloruthenate intermediates, particularly when compression in the condensed phase is taken into consideration. The performance of six DFT functionals (with the aug-pVTZ-PP[Ru] basis set) was assessed using the DLPNO-CCSD(T)/CBS reference energies. Neutral diacetate ruthenium complexes were incorrectly predicted as being kinetically relevant when using hybrid DFT methods (PBE0-D3(BJ), M06-2X or ωB97M-V). Good agreement between the calculated barrier heights and our benchmark energy results was obtained by using double-hybrid DFT methods. PWPB95 with D3(BJ) or D4 dispersion energy corrections was found to be the most accurate (ΔG≠ MUE of ca. 1 kcal mol-1). This study may aid our understanding of and help with further experimental investigations of synthetically useful carboxylate-assisted C-H bond functionalizations involving (N,C)-cyclometallated (p-cymene)Ru(II) intermediate complexes in sustainable polar protic solvents.

Preparation and characterization of terdentate [C,N,N] acetophenone and acetylpyridine hydrazone platinacycles: a DFT insight into the reaction mechanism

The reaction of N-ortho-chlorophenyl-substituted acetylpyridine hydrazones (a and d) with K₂[PtCl₄] (n-butanol/water, 100 °C) gave mononuclear complexes 1a and 1d with the ligands as [N,N] bidentate. In contrast, the reaction of N-phenyl or N-meta-chlorophenyl hydrazones (b and c, respectively) under analogous reaction conditions gave the cycloplatinated species 2b and 2c with the ligand as [C,N,N] terdentate. The treatment of the mononuclear complexes 1a and 1d with NaOAc (n-butanol, 100 °C) gave the corresponding cycloplatinated complexes 2a and 2d. Acetophenone hydrazone platinacycle 2e was prepared in a similar fashion and its reaction with tertiary mono- and triphosphines gave mono- or trinuclear species depending on the reaction conditions. The X-ray crystal structures of some of these complexes showed interesting π-π slipped stacking interactions between metallacyclic rings which, according to NCI analyses, showed an aromatic character. With an aim to rationalize the different reactivities shown by acetylpyridine hydrazones and the precise role of the acetate anion, the energy profiles for the three main steps of cycloplatination (iminoplatinum complex formation, chelation and cyclometallation) have been determined by using the DFT (M06) methods. Calculations indicate that the cycloplatination of 1b proceeds via electrophilic substitution, involving the direct replacement of the chloride anion at the Pt(ii) centre with the N-phenyl moiety as the rate-determining step, to give an agostic intermediate 5b+ that, subsequently, leads to the elimination of a proton as hydrogen chloride. When present as an "external" base, acetate enters the coordination sphere around the Pt(ii) centre and facilitates hydrazone N-H deprotonation and electrophilic C-H activation through a dissociative route, leading to a Wheland-type σ-complex intermediate 9ac.

Diastereoselective synthesis of 2-amino-4-phosphonobutanoic acids by electrophilic substitution and tin-Peterson olefination of bis-lactim ethers derived from cyclo-[L-AP4-D-Val]

Electrophilic substitutions on lithiated Schöllkopf's bis-lactim ethers derived from cyclo-[L-AP4-D-Val] take place regio- and stereoselectively at the alpha-position of the phosphonate ester. Subsequent olefination of alpha-silyl-, alpha-phosphoryl-, and alpha-stannyl-stabilized phosphonate carbanions give rise exclusively to vinylphosphonates. Both processes allow a direct and stereoselective access to a variety of 4-substituted and 3,4-disubstituted 2-amino-4-phosphonobutanoic acids (AP4 derivatives) in enantiomerically pure form that may be useful tools for characterizing the molecular pharmacology of metabotropic glutamate receptors (mGluRs) of group III. The relative stereochemistry was assigned from X-ray diffraction analyses or NMR studies of 1,2-oxaphosphorinane and other cyclic derivatives. In accordance to density functional theory (DFT) calculations, the syn-selectivity in the electrophilic substitutions may originate from the intervention of seven- and eight-membered chelate structures in which the bis-lactim ether moiety shields one of the faces of the phosphonate carbanion. DFT calculations for the tin-Peterson olefination of alpha-stannyl stabilized phosphonate carbanions indicate that rate and selectivity are determined in the initial carbon-carbon bond formation step where the unlike transition structures leading to (Z)-vinylphosphonates are favored both in the gas phase and in THF solution.

Diastereoselective synthesis of 2-amino-4-phosphonobutanoic acids by conjugate addition of lithiated Schöllkopf's bislactim ethers to vinylphosphonates

Conjugate additions of lithiated bislactim ethers derived from cyclo-[Gly-Val] and cyclo-[Ala-Val] to alpha-, beta-, or alpha,beta-substituted vinylphosphonates allow direct and stereoselective access to a variety of 3- or 4-monosubstituted and 2,3-, 2,4-, or 3,4-disubstituted 2-amino-4-phosphonobutanoic acids (AP4 derivatives) in enantiomerically pure form. The relative stereochemistry was assigned by X-ray diffraction analysis or NMR study of 1,2-oxaphosphorinane derivatives. Competitive eight-membered "compact" and "relaxed" transition-state structures are invoked to rationalize the stereochemical outcome of the conjugate additions.

Templating Schiff-base lateral macrobicycles: an experimental and theoretical structural study of the intermediates

In this paper, we report a structural study both in the solid state and in solution of barium complexes with the diamine N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L(2)), that allows us to rationalize the template effect of the metal ion in the synthesis of Schiff-base lateral macrobicycles resulting from the condensation of L(2) with different dicarbonyl compounds. The X-ray crystal structures of [Ba(L(2))(ClO(4))](ClO(4)) (3) [triclinic space group P1 with Z = 2, a = 10.467(2) A, b = 10.4755(2) A, c = 16.9911(3) A, alpha = 85.075(1) degrees, beta = 80.907(1) degrees, and gamma = 61.627(4) degrees ] and [Ba(L(2))(NCS)(H(2)O)](SCN) (4) [monoclinic space group P2(1)/n with Z = 4, a = 9.954(5) A, b = 29.193(5) A, c = 11.313(5) A, and beta = 91.371(5) degrees ] demonstrate that in the solid state the barium(II) ion induces an anti conformation of the receptor in the complexes. Variable temperature (1)H and (13)C NMR data point out that in solution compounds 3 and 4 exist as a mixture of syn and anti isomers. The presence of the syn isomer in solution, independent of the counterion employed (perchlorate or thiocyanate), accounts for the effectiveness of the barium(II) ion as a template agent in the synthesis of the lateral macrobicycles resulting from the condensation of L(2) with different dicarbonyl compounds. Density functional theory calculations (at the B3LYP/LanL2DZ level) for [Ba(L(2))](2+) predict the syn conformation to be more stable both in vacuo and in solution (PCM model). In order to asses which of the two isomers predominates in acetonitrile solution, the (13)C NMR shielding tensors of the two isomers of [Ba(L(2))](2+) were calculated for the in vacuo optimized structures by using the GIAO method, and the results were compared with the experimental ones. According to these analyses, a syn stereochemistry is assigned to the major species in solution.

Stereoselective synthesis of alpha-monofluorinated phosphonate mimetics of naturally occurring phosphoserine and phosphothreonine, via electrophilic fluorination of lithiated bis-lactim ethers

Electrophilic fluorinations of lithiated bis-lactim ethers derived from cyclo-[L-AP4-D-Val] allow a direct access to alpha-monofluorinated phosphonate mimetics of naturally occurring phosphoserine and phosphothreonine, in enantiomerically pure form and suitably protected for solid-phase peptide synthesis.

Conjugate additions of 1-propenylphosphonates to metalated Schöllkopf's bis-lactim ether: stereocontrolled access to 2-amino-3-methyl-4-phosphonobutanoic acids

Diastereoselectivity in the conjugate addition of metalated Schöllkopf's bis-lactim ethers 5a-e to (E)- and (Z)-1-propenylphosphonates 4a,b was studied experimentally and theoretically and utilized to achieve a direct and stereocontrolled synthesis of all four diastereoisomers of 2-amino-3-methyl-4-phosphonobutanoic acid, 6a,b and their enantiomers. The relative stereochemistry was assigned from an NMR study of cyclic derivatives 13a,b. According to semiempirical calculations, both in vacuo (PM3) or a dielectric continuum (PM3/COSMO), initial lithium-phosphoryl coordination, without an energy barrier, to form a solvated chelate complex is followed by the rate-determining reorganization to the 1,4-addition product through an eight-membered transition state. The translation of the Z,E geometry into a syn, anti configuration at the adducts originates from an orientational preference in the transition state for a compact disposition of the reaction partners.