Guanidine–Amide-Catalyzed Aza-Henry Reaction of Isatin-Derived Ketimines: Origin of Selectivity and New Catalyst Design

Density functional theory (DFT) calculations were performed to investigate the mechanism and the enantioselectivity of the aza-Henry reaction of isatin-derived ketimine catalyzed by chiral guanidine–amide catalysts at the M06-2X-D3/6-311+G(d,p)//M06-2X-D3/6-31G(d,p) (toluene, SMD) theoretical level. The catalytic reaction occurred via a three-step mechanism: (i) the deprotonation of nitromethane by a chiral guanidine–amide catalyst; (ii) formation of C–C bonds; (iii) H-transfer from guanidine to ketimine, accompanied with the regeneration of the catalyst. A dual activation model was proposed, in which the protonated guanidine activated the nitronate, and the amide moiety simultaneously interacted with the ketimine substrate by intermolecular hydrogen bonding. The repulsion of CPh3 group in guanidine as well as N-Boc group in ketimine raised the Pauli repulsion energy (∆EPauli) and the strain energy (∆Estrain) of reacting species in the unfavorable si-face pathway, contributing to a high level of stereoselectivity. A new catalyst with cyclopropenimine and 1,2-diphenylethylcarbamoyl as well as sulfonamide substituent was designed. The strong basicity of cyclopropenimine moiety accelerated the activation of CH3NO2 by decreasing the energy barrier in the deprotonation step. The repulsion between the N-Boc group in ketimine and cyclohexyl group as well as chiral backbone in the new catalyst raised the energy barrier in C–C bond formation along the si-face attack pathway, leading to the formation of R-configuration product. A possible synthetic route for the new catalyst is also suggested.

Synthesis, characterization, and crystal structures of organonickel(II) complexes coordinated to novel 1-bromo-2,6-bis{[(λ5-phosphanylidene)imino]methyl}benzene NCN-pincer ligands

A novel family of four 1-bromo-2,6-bis{[(λ5-phosphanylidene)imino]methyl}benzene ligands has been synthesized and characterized. The phosphiniminomethyl substituents are decorated with either three phenyl groups, two phenyl and one cyclohexyl group, one phenyl and two cyclohexyl groups, or three cyclohexyl groups. Each ligand was metallated using zero-valent nickel through an oxidative addition to form a family of organonickel(II) complexes, namely (2,6-bis{[(triphenyl-λ5-phosphanylidene)imino]methyl}phenyl-κ3 N,C 1,N′)bromidonickel(II) dichloromethane hemisolvate, [NiBr(C44H37N2P2)]·0.5CH2Cl2, (2,6-bis{[(cyclohexyldiphenyl-λ5-phosphanylidene)imino]methyl}phenyl-κ3 N,C 1,N′)bromidonickel(II) diethyl ether hemisolvate, [NiBr(C44H49N2P2)]·0.5C4H10O, (2,6-bis{[(dicyclohexylphenyl-λ5-phosphanylidene)imino]methyl}phenyl-κ3 N,C 1,N′)bromidonickel(II), [NiBr(C44H61N2P2)], and (2,6-bis{[(tricyclohexyl-λ5-phosphanylidene)imino]methyl}phenyl-κ3 N,C 1,N′)bromidonickel(II), [NiBr(C44H73N2P2)]. This family of complexes represents a useful opportunity to investigate the impact of incrementally changing the steric characteristics of a complex on its structure and reactivity.

Crystal structure of 7,8,15,16,17-pentathiadispiro[5.2.59.36]heptadecane

The title compound, C12H20S5, crystallizes in the monoclinic space group P21/c with four molecules in the unit cell. In the crystal, the asymmetric unit comprises the entire molecule with the three cyclic moieties arranged in a line. The molecules in the unit cell pack in a parallel fashion, with their longitudinal axes arranged along a uniform direction. The packing is stabilized by the one-dimensional propagation of non-classical hydrogen-bonding contacts between the central sulfur atom of the S3 fragment and the C—H of a cyclohexyl group from a glide-related molecule [C...S = 3.787 (2) Å].

Steric control of supramolecular association in structures of Zn(S2COR)2 with N,N′-bis(pyridin-4-ylmethyl)oxalamide

The crystal and molecular structures of the one-dimensional coordination polymer [Zn(S2COEt)2(4LH2)]n (1) and binuclear [Zn(S2COCy)2]2(4LH2) (2) are described, where 4LH2 is N,N′-bis(pyridin-4-ylmethyl)ethanediamide. In 1, the Zn(S2COEt)2 entities are linked by bidentate bridging 4LH2 ligands through the pyridyl-N atoms to generate a twisted supramolecular chain. As a result of monodentate xanthate ligands, the N2S4 donor set defines a distorted tetrahedral coordination geometry and, crucially, allows the participation of the non-coordinating sulfur atoms in supramolecular association. Thus, in the crystal amide-N–H···O(amide) and amide-N–H···S(thione) hydrogen bonds link chains into a three-dimensional architecture. The substitution of the ethyl group in the xanthate ligand with a cyclohexyl group results in very different structural outcomes. In 2, a binuclear molecule is observed with the coordination geometry for zinc being defined by chelating xanthate ligands and a pyridyl-N atom with the NS4 donor set defining a highly distorted geometry. In the molecular packing, amide-N–H···S(thione) hydrogen bonds stabilise a supramolecular chain along the a-axis and these are connected into a three-dimensional arrangement by methylene-C–H···O and methylene-C–H···π(pyridyl) interactions. The relative importance of the specified intermolecular interactions and weaker, contributing contacts has been revealed by an analysis of the calculated Hirshfeld surfaces of 1 and 2.

Crystalline state photochromism of 3-furylfulgides: impact of size and bond flexibility of the non-aromatic alkylidene group

3-Furylfulgides are photochromic compounds showing high thermal stability in their closed forms. However, their photochromic properties in the solid state should be improved further to fabricate molecular devices. Understanding how the size and the flexibility of the non-aromatic alkylidene moiety alter the crystalline state photochromic properties is also important here, as the alkylidene group is directly involved in the photochromic ring closing and opening reactions. The synthesis of four 3-furylfulgides composed of different alkylidene groups (rigid isopropyl, flexible 2-butyl, rigid cyclopentyl and flexible cyclohexyl), their crystal structures and structure–photochromic property correlation in the crystalline state are reported here. Crystallographic data along with reaction cavity volumes calculated using the programCAVITY[Ohashiet al.(1981),J. Am. Chem. Soc.103, 5805–5812] disclosed that fulgides with flexible groups at the ring closing site have more free volume around the reactive area in the crystal lattice, which can provide more space for the atomic movements in the reaction and flexibility can reduce the strain built up in the closedC-isomers by making conformations. According to UV–vis spectroscopic data, a higher yield ofC-isomers and a better fatigue resistance were obtained for the 3-furylfulgide with the largest and flexible cyclohexyl group showing greater photochromic properties in the crystalline state than the fulgide containing the smallest and rigid isopropyl group.

Crystal structure of 4-cyclohexyl-1-(propan-2-ylidene)thiosemicarbazide

In the title compound, C10H19N3S, the cyclohexyl group adopts a chair conformation and adopts a position approximatelysynto the thione S atom. The CN2S thiourea moiety makes dihedral angle of 13.13 (10)° with the propan-2-ylideneamino group. An intramolecular N—H...N hydrogen bond is noted. In the crystal, inversion dimers linked by pairs of N—H...S hydrogen bonds generateR22(8) loops.

Crystal structure ofN-[4-amino-5-cyano-6-(methylsulfanyl)pyridin-2-yl]-2-(cyclohexylsulfanyl)acetamide

In the title molecule, C15H20N4OS2, the acetamido fragment is nearly coplanar with the pyridyl ring [C—N—C—C torsion angle = −4.1 (2)°], while the cyclohexylsulfanyl portion protrudes from this plane [N—C—C—S torsion angle = −40.8 (6)°]. In the crystal, alternating pairwise N—H...O and N—H...N hydrogen bonds across inversion centres form chains along [101], which are associated into stepped layersviaoffset π–π stacking between pyridyl rings [centroid–centroid distance = 3.566 (1) Å]. The cyclohexyl group and the two atoms of the S—C bond attached to it are disordered over two sets of sites with site-occupancy factors of 0.8845 (18) and 0.1155 (18).

5-Bromo-3-cyclohexylsulfinyl-2,4,6-trimethyl-1-benzofuran

In the title compound, C17H21BrO2S, the cyclohexyl ring adopts a chair conformation and the arylsulfinyl unit is positioned equatorially relative to the cyclohexyl group. The benzofuran unit is essentially planar, with an r.m.s. deviation of 0.016 (2) Å. In the crystal, molecules are linked by weak C—H...O, C—H...π and Br...π [3.663 (2) Å] interactions, resulting in a three-dimensional network. A Br...Br [3.6838 (6) Å] contact is observed. The O atom of the sulfinyl group is disordered over two orientations with an occupancy ratio of 0.863 (5):0.137 (5).