Substituent effects on cyclonona‐3,5,7‐trienylidenes: a quest for stable carbenes at density functional theory level

Nine boat‐shaped cyclonona‐3,5,7‐trienylidenes are compared and contrasted with respect to their multiplicity, nucleophilicity, electrophilicity, band gap (ΔE_{HOMO ? LUMO}), Natural bond orbital (NBO) atomic charge, force constant, as well as the aptitude for dimerization, and rearrangement through proper isodesmic reactions at B3LYP/AUG‐cc‐pVTZ and B3LYP/6‐311++G**//B3LYP/6‐31+G* levels of theory. The nine cyclic carbenes include unsubstituted (1_CH2) plus eight α‐cyclopropylcyclonona‐3,5,7‐trienylidenes, which are substituted with ?‐SiMe₂, ?‐NMe, ?‐PMe, ?‐O, ?‐S, ?‐CH₂, ?‐cyclopropyl, and ?‐CMe₂ (2_SiMe2, 2_NMe, 2_PMe, 2_O, 2_S, 2_CH2, 2_cyclopropyl, and 2_CMe2, respectively). The latter eight species enjoy the stabilizing interaction of the occupied Walsh orbital of cyclopropyl with the vacant p_π orbital of the carbene center (Walsh_cyclopropyl → p_{π carbene}). Among them, the singlet closed shell 2_NMe appears the most promising for exhibiting the highest relative singlet–triplet energy gap (ΔE_{s ? t} = 27.1 kcal mol^?1). In contrast, the least stable derivative is triplet 2_SiMe2, which exhibits the lowest relative ΔE_{s ? t} of ?5.5 kcal mol^?1. The overall trend of ΔE_s‐t is 2_NMe > 2_PMe > 2_S > 2_O > 2_cyclopropyl > 2_CMe2 > 2_CH2 > 1_CH2 > 2_SiMe2. With one negative force constant, the unsubstituted 1_CH2 turns out to be a transition state, whereas the rest emerge as minima. Copyright © 2015 John Wiley & Sons, Ltd.     

Silabenzene through divalent precursors at theoretical levels

Abstract  

Based on geometries and relative energies, three different mechanisms are proposed for the rearrangements of five isomers of silacyclohexadienylidenes to silabenzene at B3LYP and MP2 levels: (1) [1,2]-hydrogen migration through a planar transition state, (2) [1,4]-hydrogen migration through a boat transition state, and (3) zip-zap mechanism, comprised of three successive [1,2]-hydrogen migrations. The above results are compared and contrasted to rearrangements of the corresponding cyclohexadienylidenes to benzene.     

Effects of fused benzene rings on tautomerizations and inversions of benzo, azabenzo, and oxabenzocycloheptatrienes at theoretical levels

Biologically important bicyclic species, including 6H-, 6H-6-aza-, and 6-oxabenzocycloheptatrienes (in which the benzene moiety is fused meta with respect to the tetrahedral constituents: –CH₂–, –NH–, and –O–, respectively), show strong shifts of tautomerizations in favor of the corresponding tricyclic benzonorcaradienes (with ΔH values of −11.49, −14.55, and −19.20 kcal mol⁻¹, respectively), at B3LYP/6-311++G**//B3LYP/6-31G*, and MP2/6-311++G**//MP2/6-31G* levels, and at 298 K. In contrast, such shifts are strongly disfavored by the isomeric bicyclic species in which the benzene moieties are fused ortho or para with respect to –CH₂–, –NH–, and –O–, respectively. Hence for species with ortho benzene rings including 5H-, 5H-5-aza- and 5-oxabenzocycloheptatrienes, tautomerization ΔH values are 30.76, 31.89, and 25.27 kcal mol⁻¹, respectively, while for species with para fused benzene moieties including 7H-, 7H-7-aza-, and 7-oxabenzocycloheptatrienes, tautomerization ΔH values are 24.12, 26.00, and 19.55 kcal mol⁻¹, respectively. NICS calculations are successfully used to rationalize these results. The calculated energy barriers for inversion of the seven-membered rings of bicyclic species predict a dynamic nature for all the structures except for the virtually planar 6H-6-aza- and 6-oxabenzocycloheptatrienes. Finally, our theoretical data are compared to the experimental results where available. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ylide character and stability of heterocyclic benzenes

Optimizations are carried out on heterocyclic benzenes including oxabenzene ( 1_O ), thiabenzene ( 1_S ), selenabenzene ( 1_Se ), tellurabenzene ( 1_Te ), 1,1‐dihydropyridinium ( 2_N ), 1,1‐dihydrophosphinium ( 2_P ), 1,1‐dihydroarsinium ( 2_As ), and 1,1‐dihydrostybinium ( 2_Sb ), using HF and DFT methods with 6‐311++G* basis sets. These partly aromatic compounds have 6π‐electrons showing ylide character. Nonplanar boat conformers appear as global minima for 1_X . In contrast, planar conformers are the most stable in 2_X series. The inversion barrier energy of heteroatom in 1_O appears the least in 1_X structures. Ylide character increases in series 1_X and 2_X with the orders of 1_Te > 1_O > 1_Se > 1_S and 2_As > 2_N > 2_Sb > 2_P , respectively. Homodesmic studies show stability order for series 1_X to be 1_S > 1_Se > 1_Te > 1_O , whereas the order of stability for series 2_X is 2_As > 2_P > 2_Sb > 2_N . © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:412–417, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20443     

A theoretical investigation into dimethylcarbene and its diamino and diphosphino analogs: effects of cyclization and unsaturation on the stability and multiplicity

High levels of ab initio and DFT calculations (B3LYP/6‐311++G**, B3LYP/AUG‐cc‐pVTZ, and CCSD(T)/6‐311++G** levels) coupled with isodesmic reactions are used to compare and contrast the multiplicities and relative stabilities of singlet (s) and triplet (t) acyclic carbenes, including: dimethylcarbene, diaminocarbene, and diphosphinocarbene along with their saturated and unsaturated cyclic ones. Cyclization is unfavorable for all acyclic carbenes while unsaturation of cyclic analogs appears favorable. The simultaneous cyclization and unsaturation of dimethylcarbene increases the singlet–triplet energy gap (ΔE_s–t), while for diphosphinocarbene the situation is reversed. For diaminocarbene the increase of ΔE_s–t is encountered only during cyclization. Copyright © 2009 John Wiley & Sons, Ltd.     

Singlet-triplet energy separations in divalent five-membered cyclic conjugated C5H3X, C4H3SiX, C4H3GeX, C4H3SnX, and C4H3PbX (X = H, F, Cl, and Br)

Singlet-triplet energy gaps in cyclopenta-2,4-dienylidene, as well as its 2- or 3-halogenated derivatives, are compared and contrasted with their sila, germa, stana, and plumba analogues; at HF/6-31G* and B3LYP/ 6-311++G(3df, 2p) levels of theory. Energy gaps (ΔG_t-s), between triplet (t) and singlet (s) states, appear linearly proportional to: (i) the size of the group 14 divalent element (M = C, Si, Ge, Sn and Pb), (ii) the angle ∠C-M-C, and (iii) the ΔG_(LUMO-HOMO) of the singlet state involved. The magnitude of ΔG_t-s, for each 2- and/or 3-substituted species studied, increases with an order of: carbenes < silylenes < germylenes < stanylenes < plumbylenes. This order reverses for the barriers of the ring puckering. The puckering occurs with more ease for every singlet, compared to its corresponding triplet form.Regardless of the group 14 element (M) employed, every 3-halo-substituted species is more stable than the corresponding 2-halo-substituted isomer. For M = Pb, Sn and/or Ge; 3-halo-substituted species have higher ΔG_t-s than their corresponding 2-halo-substituted analogues. For M = Si, similar ΔG_t-s are found for 2- and 3-halogenated isomers. For M = C, 3-halo-substituted species have lower ΔG_t-s than their corresponding 2-halo-substituted analogues.Every cyclic singlet has a larger ∠C-M-C angle, than its corresponding cyclic triplet state, except for 3-halosilacyclopenta-2,4-dienylidenes where triplet has a larger ∠C-M-C angle than its corresponding singlet state.     

Synthesis of novel 3,4‐diaryl‐1H‐pyrroles

The synthesis of a series of novel 3,4‐diaryl‐1H‐pyrroles and related arylalkenes from p‐bromo thiophenol, tosylmethyl isocyanide and commercially available materials is reported. Arylalkenes having electron‐withdrawing substituents gave higher yield of 3,4‐diaryl‐1H‐pyrroles.     

Steric effects on the dialkyl substituted X2C2Si silylenes: A theoretical study

With the aim of recognizing the steric effects on the silylenic H₂C₂Si structures, ab initio and DFT calculations are carried out on 24 structures of X₂C₂Si (where X is hydrogen (H), methyl (Me), isopropyl (i‐pro), and tert‐butyl (tert‐Bu)). These species are at either triplet (t) or singlet (s) states. They are confined to the following three sets of structures ( 1 _X, 2 _X and 3 _X). Structures 1 _X include silacyclopropenylidenes ( 1 _s‐H and 1 _t‐H) and their 2,3‐disubstituted derivatives ( 1 _t‐Me, 1 _s‐Me; 1 _t‐i‐pro, 1 _s‐i‐pro; 1 _{t‐tert‐Bu}, 1 _{s‐tert‐Bu}). Structures 2 _X include vinylidenesilylenes ( 2 _s‐H and 2 _t‐H) and their 3,3‐disubstituted derivatives ( 2 _t‐Me, 2 _s‐Me; 2 _t‐i‐pro, 2 _s‐i‐pro; 2 _{t‐tert‐Bu}, 2 _{s‐tert‐Bu}). Structures 3 _X include ethynylsilylenes ( 3 _s‐H and 3 _t‐H) and their 1,3‐disubstituted derivatives ( 3 _t‐Me, 3 _s‐Me; 3 _t‐i‐pro, 3 _s‐i‐pro; 3 _{t‐tert‐Bu}, 3 _{s‐tert‐Bu}). Singlet–triplet energy separations (Δ E_{s‐t, X}) and relative energies for the above structures are acquired at HF/6‐31G*, B₁LYP/6‐31G*, B3LYP/6‐31G*, MP2/6‐31G*, HF/6‐31G**, B1LYP/6‐31G**, B3LYP/6‐31G**, and MP2/6‐31G** levels of theory. The highest Δ E_{s‐t, X} is encountered for 1 _X. All singlet states of X₂C₂Si, are more stable than their corresponding triplet states. Linear correlations are found between the LUMO–HOMO energy gaps of the singlet 1 _s‐X and 2 _s‐X with their corresponding singlet–triplet energy separations calculated at B3LYP/6‐31G**. The seven structures 2 _s‐Me, 2 _t‐Me, 3 _s‐Me, 1 _t‐Me, 1 _s‐Me, 1 _{s‐tert‐Bu}, and 3 _{t‐tert‐Bu} do not appear to be real isomers. Different stability orders are obtained as a function of the substituents (X). The order of stability for six isomers of H₂C₂Si is 1 _s‐H > 2 _s‐H > 3 _s‐H > 2 _t‐H > 3 _t‐H > 1 _t‐H. Replacing hydrogen atoms by methyl group (X = Me) presents a new stability order: 1 _s‐Me > 3 _s‐Me > 2 _s‐Me > 3 _t‐Me > 2 _t‐Me > 1 _t‐Me; and for (i‐pro)₂C₂Si is 1 _s‐i‐pro > 2 _s‐i‐pro ≈ 3 _s‐i‐pro > 3 _t‐i‐pro ≈ 2 _t‐i‐pro > 1 _t‐i‐pro. Using the larger tert‐butyl group as a substituent (X), yet it offers a more different stability order for six structures of (tert‐Bu)₂C₂Si: 1 _{s‐tert‐Bu} > 3 _{s‐tert‐Bu} > 2 _{s‐tert‐Bu} > 3 _{t‐tert‐Bu} > 1 _{t‐tert‐Bu} > 2 _{t‐tert‐Bu}. Among eight levels employed, B3LYP/6‐31G** appears as the method of choice. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:619–633, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20204     

Nickel−Copper bimetallic mesoporous nanoparticles: As an efficient heterogeneous catalyst for N‐alkylation of amines with alcohols

A bimetallic catalyst (Ni/Cu‐MCM‐41) is prepared via co‐condensation method. The latter is characterized by Fourier transform infrared (FT‐IR), X‐ray powder diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDX), diffuse reflectance spectroscopy (DRS), and nitrogen adsorption–desorption analysis. Catalytic performance of Ni/Cu‐MCM‐41 is probed in N‐alkylation of amines with alcohols through a hydrogen autotransfer process. Noteworthy, this catalytic system appears very efficient for synthesis of a range of secondary and tertiary amines in good to excellent isolated yields. Moreover, the catalyst is successfully recovered and reused four times without notable decrease in its activity.