Novel α-spirocyclic (alkyl)(amino)carbenes at the theoretical crossroad of flexibility and rigidity

Spirocyclic (alkyl)(amino)carbenes (CAACs) with reasonable conformations of cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl substituents in the position α to the carbenic center are investigated at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G*. The singlet–triplet energy separations (?E _s?t), HOMO–LUMO energy gaps (?E _HOMO?LUMO), hydrogenation energies (?E _H), heats of methylene formation (?E) as well as philicity indices (N and ω) of these carbenes are compared and contrasted. The highest ?E _s?t is encountered for the cyclopropyl substituted CAAC (54.2 kcal/mol), while the other ones lay in a narrow range of 48.2–51.8 kcal/mol. Assignment of the barriers for ring inversions is carried out in order to probe the “flexible steric bulk” of cyclic substituents, showing negligible differences with those of the unsubstituted rings. The calculated N and ω indices indicate that in contrast to the popular belief, CAACs appear less electrophilic than N-heteorcyclic carbenes (NHCs).     

Novel silylphenol antioxidants by density functional theory

We have scrutinized five novel silylphenol antioxidants, including 2-silylphenol ( 1 ), 4-silylphenol ( 2 ), 2,6-disilylphenol ( 3 ), 2,4-disilylphenol ( 4 ), and 2,4,6-trisilylphenol ( 5 ), at M06/6–311++G** level of theory. To evaluate the antioxidant efficiency, the electronic effects on O─H bond dissociation energy (BDE) and vertical ionization potential (IP_v) of 1 – 5 are investigated, which are mainly governed by electronic effects. The conductor-like polarized continuum model (CPCM) is applied to measure the antioxidant capacity in the solution phase. The results show that antioxidants with the lowest BDE and IP_v values can efficiently act via hydrogen atom transfer (HAT) and single electron transfer (SET) mechanisms, respectively. The stability of resulting radicals is measured by nucleus independent chemical shift (NICS) index, natural bond orbital (NBO) analysis, and nucleophilicity (N) index. The BDE shows lower values in the gas phase with respect to water, while water exhibits lower IP_v values than gas. Structure 5 turns out as the most efficient antioxidant. The overall order of antioxidant efficiency in both gas and water phases is 5 > 2 > 3 > 4 > 1 .     

Novel Triplet Ground State Silylenes: H–N = C = Si, CN–N = C = Si, and MeO–N = C = Si at DFT Levels

Summary.  DFT calculations predict the existence of three new triplet ground state silylenes: [(imino)-methylene]silylene, [(cyanoimino)methylene]silylene, and [(methoxyimino)methylene]silylene, with CNSiX formula (X = H, CN, and OMe, respectively). Discrepancies are found between DFT and some ab initio results.     

A theoretical study on C2HXSi silylenes (X = H,CN, NH2, and OMe)

Relative stabilities and structural characters of 30 silylenic C₂HXSi species (X = H, NH₂, CN, and OMe), with singlet (s) and/or triplet (t) states, are calculated at six levels of theory: HF/6‐311++G**, MP3/6‐31G*, B1LYP/6‐311++G**, B3LYP/6‐311++G**, MP2/6‐311++G**, and MP4(SDTQ)/6‐311++G**. The four possible isomers considered for C₂SiHX are (i) 3‐X‐1‐silacyclopropenylidene ( 1 _s‐X and 1 _t‐X ), (ii) X‐vinilydensilylene ( 2 _s‐X and 2 _t‐X ), (iii) ethynyl‐X‐silylene ( 3 _s‐X and 3 _t‐X ), and (iv) (X‐ethynyl)silylene ( 4 _s‐X and 4 _t‐X ). The GIAO–NICS calculations show that singlet cyclic structures, 1 _s‐X , are considerably more aromatic than benzene. Conversely, triplet cyclic C₂HCNSi breaks down through optimization, and transforms into a novel high‐spin acyclic carbenosilylene minimum ( 1 _t‐CN ). Singlet 3 and triplet 3 cross at a divalent angle (|XSiC) of 152°. This angle narrows to 137° for crossing of singlet 3 _s‐CN and triplet, 3 _t‐CN . The smallest |XSiC occurs at 132° for crossing of 3 _s‐H and 3 _t‐H . © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:283–293, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20297     

Detours for Reaching at New Germylenes,Silylenes, Carbenes,and Carbenogermylenes through Substituted Cyclopropenylidenes at <Emphasis Type="Italic">Ab initio</Emphasis> and DFT Levels

Summary. Heavier atom containing X ₂SiC₂, and X ₂GeC₂ cyclopropenylidenes transform into new silylenes, germylenes, and carbenogermylenes at DFT and ab initio levels (X = H, CN, NH₂, and OMe). The number of transformations or rearrangements appear roughly proportional to the covalent radii of the group 14 elements (Ge > Si > C).     

1,2,4,6-Cycloheptatetraenes racemizations: substituent effects via ab initio

Racemizations of three series of non-planar 1-, 4- and 5-X-C₇H₅ cyclic conjugated allenes (1_x, 2_x, and 3_x to 1′_x, 2′_x and 3′_x, respectively) are investigated using ab initio HF, B3LYB and MP2 methods with a 6-31G* basis set (X=H, F, Cl, Br, I, OH, NH₂, CN and NO₂). These three topologically different series of allenes are found to go through their corresponding planar 2-, 3- and 4-X-C₇H₅ singlet carbenic transition states, 1_x–TS, 2_x–TS and 3_x–TS (Series 1: 1_x1_x–TS1′_x; Series 2: 2_x2_x–TS2′_x; Series 3: 3_x3_x–TS3′_x).Frequency calculations show negative force constants for all planar and singlet carbeneic transition states: 1_x–TS, 2_x–TS and 3_x–TS.Based on HF/6-31G*, B3LYP/6-31G* and MP2/6-31g* calculations, the order of substituent effects on energy barrier (ΔG^#) of racemization, increase in the order of: NO₂>CN>H>I>ClBr>OH>F>NH₂ for all three Series 1–3. The height of the energy barrier related to each substituent (X) is highest in Series 3 and lowest in Series 1.Hammet free energy relationships (ρ) found via B3LYP, for Series 1, 2 and 3, are −5.8, −3.4 and −4.1, respectively. Rather similar ρ values are found using HF (−4.3, −4.4 and −4.3). These data indicate that electron-releasing groups highly increase the rate of enantiomeric interconversions.Linearity of the free energy relationships is found to be a function of steric effects. Hence, higher correlation coefficients (R) are found for Hammet free energy relationships for Series 3. This is followed by Series 2, which in turn has a more linear free energy relationship than Series 1 (Series 3>Series 2>Series 1).     

From halo-azasilylenes to halo-phosphasilylenes (X-CNSi vs. X-CPSi) at ab initio and DFT levels

The higher the electropositivity of the substituents has been associated with the higher possibility of encountering a triplet ground state silylene [P.P. Gaspar, M. Xiao, D. Ho Pae, D.J. Berger, T. Haile, T. Chen, D. Lei, W.R. Winchester, P. Jiang, J. Organomet. Chem. 646 (2002) 68]. Here, going from the reported halo-azasilylenes X-CNSi [M.Z. Kassaee, S.M. Musavi, H. Hamadi, M. Ghambarian, S.E. Hosseini, J. Mol. Struct. (Theochem) 730 (2005) 33] to the analogues more electropositively substituted halo-phosphasilylenes X-CPSi has reduced the chances of encountering triplet ground state silylenes (X = H, F, Cl and Br). Even though all singlet isomers seem to be more stable than their corresponding triplets, singlet-triplet cross over diagrams may help the future design of new acyclic triplet state silylenes.     

Synthesis and reactions of N-methylbenzylammonium fluorochromate(VI) on silica gel, a selective and efficient heterogeneous oxidant

N-Methylbenzylammonium fluorochromate(VI) (MBAFC) is easily synthesized by addition of N-methylbenzylamine to an aqueous solution of CrO3 and HF. MBAFC shows selectivity in the oxidation of aryl alcohols to their corresponding aldehydes and ketones under mild conditions. The durability, ease of work up and efficiency of MBAFC are considerably increased upon its absorption on silica gel.