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Silylformamidine 1 exists in equilibrium with its carbenic form 1' due to an easy migration of the silyl group. The reaction of 1 with variously substituted fluorobenzenes proceeds as an insertion of the nucleophilic carbene 1' into the most acidic C-H bond upon mixing the reagents and does not require any catalyst. According to DFT calculations, the classical interpretation of the insertion reaction proceeding via a three-membered transition state structure requires high activation energy. Instead, low activation barriers are predicted for a transfer of the most acidic proton in the aromatic substrate to the carbene carbon. As the next step, a barrierless rearrangement of the formed ion pair toward the product completes the process. The reactivity of substituted benzenes in the reaction with silylformamidine can be roughly assessed by calculated pKa (DMSO) values for the C-H hydrogens. Benzene derivatives having pKa approx. less than 31 can undergo C-H insertion. The reaction provides aminals as the first products, which can easily be transformed into the corresponding aldehydes via acidic hydrolysis. As silylformamidine 1 is tolerant to many functional groups, the reaction can be applied to numerous benzene derivatives, making it a reliable strategy for application in organic synthesis.

The reaction of trimethyl(trifluoromethyl)silane-tetrabutylammonium difluorotriphenylsilicate (CF

A Cu-catalyzed, easily scalable one-pot synthesis of fused pyridines by the reaction of cyclic ketones with propargylamine is described. The protocol was optimized based on the results of more than 30 experiments. The highest product yields were achieved in i-PrOH as a solvent in the presence of 5.0 mol % CuCl2 in air. In contrast to the well-known Au-catalyzed protocol, our procedure is "laboratory friendly", cost-effective, and suitable for preparing dozens of grams of fused pyridine-based building blocks and does not require a high-pressure autoclave technique. Decreasing the catalyst amount in the reaction to 1.25 mol % CuCl2 provided a yield comparable to that achieved with 5 mol % catalyst, though a longer reaction time was required. A plausible reaction mechanism was proposed. The scope and limitation of the reaction were studied using 24 different cyclic ketones as starting materials. The fused pyridine yield decreased among cyclic ketones in the following order: six-membered ≫ eight-membered > five-membered ∼ seven-membered. The elaborated reaction conditions demonstrated tolerance to a number of protective functional groups in ketone such as ester, tert-butoxycarbonyl (Boc)-protected amine, and acetal moieties.

Physico-chemical properties important to drug discovery (pK

Using DFT and ab initio calculations, we demonstrate that noncyclic formamidines can undergo thermal rearrangement into their isomeric aminocarbenes under rather mild conditions. We synthesized the silylformamidine, for which the lowest activation energy in this process was predicted. Experimental studies proved it to serve as a very reactive nucleophilic carbene. The reactions with acetylenes, benzenes, and trifluoromethane proceeded via insertion into sp, sp

A comprehensive study on the synthesis of 5-fluoroalkyl-substituted isoxazoles starting from functionalized halogenoximes is reported. One-pot metal-free [3 + 2] cycloaddition of CF3-substituted alkenes and halogenoximes bearing ester, bromo, chloromethyl, and protected amino groups was developed for the preparation of 5-trifluoromethylisoxazoles. The target 3,5-disubstituted derivatives were obtained in a regioselective manner in good to excellent yield on up to 130 g scale. 5-Fluoromethyl- and 5-difluoromethylisoxazoles were synthesized by late-stage deoxofluorination of the corresponding 5-hydroxymethyl or 5-formyl derivatives, respectively, in turn prepared via metal-free cycloaddition of halogenoximes and propargylic alcohol. An alternative approach based on nucleophilic substitution in 5-bromomethyl derivatives was found to be more convenient for the preparation of 5-fluoromethylisoxazoles. Reaction of isoxazole-5-carbaldehydes with the Ruppert-Prakash reagent was used for the preparation of (β,β,β-trifluoro-α-hydroxyethyl)isoxazoles. Utility of described approaches was shown by multigram preparation of side-chain functionalized mono-, di-, and trifluoromethylisoxazoles, for example, fluorinated analogues of ABT-418 and ESI-09.

[3+2] Cycloaddition reactions of polyfluoroalkanethioamides with nitrile imines, generated in situ by dehydrochlorination of the corresponding hydrazonoyl chlorides, in the presence of triethylamine, were studied. A new method was proposed on the basis of these reactions for the synthesis of 2-polyfluoroalkyl-2,3-dihydro-1,3,4-thiadiazoles.

Structural restriction of the sulfonamide bond was used to design sultams with abnormal geometric parameters. Based on analysis of tertiary aliphatic sulfonamides published in the Cambridge crystallographic database, Paquette's sultams (i.e., bridged bicyclic sultams with a bridgehead nitrogen atom) were outlined, and a number of these compounds (including the novel smallest representative, 2-thia-1-azabicyclo[2.1.1]hexane 2,2-dioxide) were synthesized by cyclization of the corresponding amino sulfonyl fluorides. A series of tertiary aliphatic sulfonamides was studied by crystallographic and quantum chemical methods. It was found that the s-character of the nitrogen lone pair is the most important factor defining properties of the S-N bond. Thus, going from the sp3-hybrid lone pair in common sulfonamides to the sp-like lone pair in the smallest Paquette's sultam resulted in an increase in S-N bond length by ca. 0.06 A. The strain energy of ca. 30 kcal/mol was predicted for the latter compound, which was higher than for any existing sulfonamides studied.

Structure of six-membered cyclic aminoiminocarbenes (pyrimidine ylidenes) and their group 13–15 homologues was studied by quantum chemical calculations. Isodesmic and dimerization reaction energies, NICS values, frontier molecular orbitals, as well as NBO, Bader’s “atoms in molecules” and Laplacian bond order (LBO) analyses were employed for estimating relative stability of the studied carbenes (carbene homologues). They show a low degree of aromatic delocalization and considerable variation of electronic structure depending on the nature of divalent element. The predicted stability for the series of interest is lower than that for the corresponding five-membered Arduengo carbenic structures, but comparable with Alder’s bis(dimethylamino)carbene.

The synthesis of a covalently bonded new photoresponsive molecular capsule formed from two resorcin [4]arene cavitands by fourfold ether linkages using azobenzene subunits is reported. The new molecular capsule undergoes trans to cis isomerization upon irradiation with 365 nm light and can be reversed back from cis to trans form by means of exposure to visible light (530 nm) in benzene solution. The photo-switchable behaviour was also investigated by H-1 NMR spectroscopy which further substantiated the UV-vis experiment. The quantum chemical calculations both at PM6 as well as at the DFT (B97-D) level of approximation for various stereoisomers starting from all-trans to all-cis indicated an increasing flexibility of these molecular capsules. (C) 2016 Elsevier B.V. All rights reserved.

All three possible inherently chiral (cS)-propyloxy-octyloxy-calix[4]arene acetic acids were synthesized and their absolute configuration was proved. Complexation of the calix[4]arene acetic acids with a number of chiral compounds have been studied by molecular modeling and 1H NMR spectroscopy and enantioselectivity of binding of R- and S-forms of 2-aminobutanol was found.

We developed enhanced method for low temperature and good yield preparation of aryl and heteryl sulfur pentafluorides from chlorotetrafluorides using mercury oxide - hydrogen fluoride and mercury oxide - pyridinium poly(hydrogen fluoride) media as the fluorinating reagents.

International audience; Herein we report on the synthesis of a set of transient N-(PIII)-phosphanyl and N-(PV)-phosphoroselenoyl acyclic diaminocarbenes by deprotonation of the corresponding formamidinium salts. It is shown that N-(PV)-substituted acyclic diaminocarbenes undergo 1,2-phosphorus shift. Transformations of N-(PIII)-substituted acyclic diaminocarbenes result in tetrahydroquinazolines as final products. The proposed mechanism includes the isomerization of the carbenes into transient azomethine ylides followed by their cyclization via electrophilic aromatic substitution. Introduction of the electron-donating dimetylamino group into the phenyl substituent markedly enhances the rate of the reaction and the yield of the final product. In the case of the morpholine derivative, the corresponding azomethine ylide undergoes opening the morpholine ring giving vinyloxazolidin-2-amine. The results are corroborated by DFT calculations.

Structure of 1,3,2-diazaphosphinine heterocyclic system was studied using quantum chemical calculations and X-ray diffraction method for one derivative. It was found that two P–N single bonds were different in their nature: one of them turned out to be significantly shorter and stronger, whereas the other one is longer and more diffused. The found effect is strengthened by π-donor substituents. The effect was interpreted by equilibrium of Lewis structures and analyzing the charge distribution by the NBO approach. The observed effect can be considered as the special form of π-conjugation in the heterocycles with the phosphorus (V) atom included.

The facile method of generating internal chirality into the calixarene with two hydroxy groups at the lower rim via an attached chiral substituent has been proposed. The reaction with acylisocyanates, catalyzed by a small amount of triethylamine, proceeds forming predominantly one of the two possible calixarene carbamates. The best diastereomeric excess (60 %) has been achieved in the reaction of trichloroacyl isocyanates with 1,3-hydroxycalixarene substituted with the chiral phenylethyl amide moiety. The individual diastereomers of trichloroacetyl-carbamoylcalix[4]arenas were isolated by crystallization, and their absolute configuration was determined by X-ray diffraction study. The most favored conformations predicted for 1,3-dihydroxy calixarene structures by quantum chemical calculations possess very similar stability. However, the triethylamine molecule preferably connects to the one hydroxyl group of the two available ones, providing the most favorable adduct, which predominantly participates in the reaction with acylisocyanates. This gives rise to the observed diastereomeric excess. The subsequent treatment of the formed carbamoyl with n-propyl bromide in presence of NaH and hydrolysis of the product of alkylation easily provide a persistent internally chiral calixarene.

Cyclic zwitterionic phosphoranides 2a,b were found to be intermediate products in the reaction of N,N-dimethyl-N′-(aryl)formamidines with PBr3. The structure of phosphoranide 2a was determined by means of the X-ray and quantum chemistry investigations. Mechanism of its formation was proposed based on Density functional theory (DFT) calculations. Reactions of 2 with amines and selenium yielded either C-phosphorylated formamidines or benzazaphospholes. The first example of intramolecular heterocyclization of a pentavalent phosphorus derivative 15b into 3Н-1,3-benzazaphosphole has been demonstrated.

All six possible stereoisomers of calix[4]arenes with four different substituents on the lower rim of the macrocycle have been synthesized by stepwise alkylation of the hydroxyl groups of tert-butylcalix[4]arene using (R)- or (S)-1-phenylethylamide groups as chiral auxiliaries. They form three enantiomeric pairs, as confirmed by the values of their specific rotation.

Synthesis and deprotonation of N-phosphanylated[1,2,4]triazolo[4,3-a]pyridinium triflates were investigated. Phosphanylation by t-Bu2PCl proceeds at N1 of starting triazolo-pyridines. According to density functional theory calculations, deprotonation of the formed salt gives abnormal carbenes, which decompose by opening of the triazole cycle. Treatment by chlorodiphenylphosphane leads to an equilibrium mixture of N1- and N2-phosphanylated salts, which under addition of base yields normal carbene that rapidly rearranges to form C-phosphanylated product. Mechanisms of the reactions are discussed.

Several possible reaction pathways are analyzed for the recently studied experimental reaction of diaminocarbenes with aroylimines, where the carbene acted as an oxygen-abstracting agent. A number of structures corresponding to local minima and transition states are located by geometry optimization. In contrast to the more recent interpretation of the mechanism of this process, the reaction does not proceed via the direct formation of the corresponding carbonyl ylide resulted from the electrophilic addition of diaminocarbene to the carbonyl oxygen atom. Two other, more favorable pathways were predicted instead: the nucleophilic attack of the carbene lone pair on the imino nitrogen (pathway "a") or on the carbon atom in the C═N moiety of aroylimine (pathway "b"), in agreement with predictions of the frontier molecular orbital (FMO) theory. Both intermediate adducts undergo a subsequent decomposition onto nitrile ylide and urea. Which of the two pathways becomes preferential depends on the nature of the substituents: pathway "a" is more favored for the experimentally studied species, whereas pathway "b" is thermodynamically preferable for the small-sized model structures.

Potential atropisomeric ligands, diphenylphosphane derivatives of bis-triazoles, were synthesized by direct phosphanylation of the starting bis-triazoles with chlorodiphenylphosphane. Their crystal structure was elucidated by X-ray diffractional analysis. The conformations of these compounds were modeled by DFT calculations.

The deprotonation of N-di-tert-butylphosphanyl-N-aryl-N′-diisopropylformamidinium salts led to a new type of stable acyclic N-phosphanyl-diaminocarbene (PADC). Carbenes 8a–8d were separated as single compounds. The molecular structure of 8c was determined by X-ray diffraction analysis. The PADC 8 possess an optimal substitution pattern at the nitrogen atoms, and the N–Ccarbene–N bond angle is 120.7°. Structural changes near the carbenic carbon atom, such as substitution of phenyl with mesityl, phosphanyl with selenophosphoryl, or the diisopropylamino group with 2,2,6,6-tetramethylpiperidino, rendered them unstable. The PADCs underwent N,C-phosphorus shifts to afford new C-phosphanylformamidines.

An approach to the synthesis of the novel cyclic sulfonimidoyl fluorides was elaborated based on the electrophilic fluorination of 1-oxo-2H-1,2,4-benzothiadizines with Selectfluor®. Crystal structure of 1-oxo-1-fluoro-3-phenyl-1,2,4-benzothiadiazine was determined.

A series of trifluoromethyl-substituted carbenes R-C(:)-CF3 (R = NMe2, OMe, F, PMe2, P(NMe2)(2), P(N(Pr-i)(2))(2), SMe, Cl); (dimethylamino)-(perfluoroalkyl)carbenes Me2N-C(:)-R (R = CF3, C2F5, n-C3F7, i-C3F7, and t-C4F9) and symmetrically substituted carbenes R-C(:)-R (R = NMe2, OMe, F, PMe2, SMe, Cl) have been investigated by means of quantum chemistry methods. Different levels of approximation were used, including the CCSD(T) approach also known in quantum chemistry as the "golden standard", in combination with three different basis sets (TZVP, cc-pVDZ, -cc-pVTZ). Relative stabilities of carbenes have been estimated using the differences between the singlet and triplet ground state energies (Delta E-ST) and energies of the hydrogenation reaction for the singlet and triplet ground states of the carbenes. The latter seem to correlate better with stability of carbenes than the Delta E-ST values. The C-13 NMR chemical shifts of the methylidene carbon indicate the more high-field chemical shift values in the known, isolable carbenes compared to the unstable ones. This is the first report on the expected chemical shifts in the highly unstable singlet carbenes. Using these criteria, some carbene structures from the studied series (as, for instance, Me2N-C(:)-CF3, Me2N-C(:)-C3F7-i) are proposed as good candidates for the experimental preparation.

The reactions of (di-tert-butylphosphanyl)amines and P,P-di-tert-butylphosphinoselenoic amides with Alder's dimer were studied. For di-tert-butylphosphanylamines, the reaction proceeds by primary electrophilic attack of Alder's dimer at the phosphorus atom to afford a dicationic salt 3. The deprotonation of 3 led to N-phosphanylformamidine 5 (“phosfam”). Alkyl(di-tert-butylphosphanyl)amines reacted with Alder's dimer in a 2:1 molar ratio to give N-phosphanylformamidinium salts; the second equivalent of (alkylamino)phosphane acts as a base. (Arylamino)phosphanes reacted with Alder's dimer to give benzazaphospholium derivatives. To direct the electrophilic attack of Alder's dimer at the nitrogen atom, phosphinoselenoic amides were used. They reacted with Alder's dimer at the selenium atom followed by a selenium–phosphorus shift to give N-(di-tert-butylphosphoroselenoyl)formamidinium salts. The phosphinoselenoic amides with bulky substituents (adamantyl, tBu) underwent cleavage of the N–alkyl bond to afford phosfams. Various key intermediates such as 3 and 22b were isolated and characterized. A convenient method for the synthesis of carbene precursor PIII and PV N-substituted formamidinium salts was developed.

The reactions of perfluoroalkyl thioamides with trimethyl phosphine, trimethyl phosphite, and tris(dimethylamino)phosphine have been analyzed by means of quantum chemical (DFT and MP2) calculations. The reaction seems to proceed via the nucleophilic attack of the electrophilic carbon atom by the phosphorus lone pair with the formation of cyclic or acyclic adducts. The latter releases the thiophosphate molecule forming perfluoroalkylaminocarbene as the short-lived intermediate. The reaction of the carbene with the second molecule of trialkyl phosphite yields phosphorus ylide. The ylide undergoes a migration of fluorine from carbon to phosphorus. The reactions of perfluoroalkyl thioamides with phosphines and tris(dimethylamino)phosphine probably proceeds differently. Using alkyl thioamides or amides instead of perfluoroalkyl thioamides also makes the reaction less favorable. The only combination of perfluoroalkyl thioamides with trialkyl phosphite fulfills both the kinetic requirements (moderate activation energies and relative energies for intermediates) and the thermodynamic aspects (higher stabilities of the reaction products compared with the starting materials). © 2013 Wiley Periodicals, Inc.

The tetramethyl ester of (hydroxy)(phenyl)methylenebis(phosphonic acid) reacts with morpholinosulfur trifluoride (MOST) to provide, along with the expected product of the substitution of a hydroxy group by fluorine, the adduct of (morpholino)(phenyl)carbene with PF5, compound 3a. Fluorination of dimethyl α-aroylphosphonates with MOST yields adducts of (amino)(aryl)carbene with PF5 (p-XC6H4)(Morph)C:PF5 [3a–f, where Morph is morpholyl, X = H (a), Me (b), OEt (c), F (d), Cl (e), CF3 (f)] as the major reaction products. The reaction mechanism proposed is based on experimental data and DFT calculations. The structure of adduct 3a was determined by X-ray studies. Hydrolysis of compounds 3e and 3f, bearing substituents Cl and CF3 on the phenyl ring, respectively, yields previously unknown benzoylpentafluorophosphates [p-XC6H4–C(=O)PF5]–MorphH+.

Calixarenes substituted at the upper rim by electron-deficient pentafluorophenyl groups have been synthesized by reaction of hexafluorobenzene with organolithium derivatives of calixarenes. The single crystal structures indicate stacking interactions between the two perfluorinated aromatic rings. The thermodynamic characteristics of the flattened cone-flattened cone inversion process have been studied using dynamic NMR and quantum chemical methods. The calculated activation free energy for tetra(pentafluorophenyl)-substituted species is higher than that derived for tetraphenyl calixarene. The calixarenes substituted at the upper rim by electron-deficient pentafluorophenyl groups are potential hosts for anion recognition.

The interaction of inherently chiral calix[4]arene carboxilic acids and their amides with amino acids in the organic phase has been studied using electron spectroscopy. It was found that the chiral calix[4]arenes are able of enantioselective recognition of L- and D-forms of amino acids. Stability constants of the calixarene – amino acid supramolecular complexes were determined and mechanism of the host-guest interaction was examined by molecular modeling method.

Molecular and electronic structure of triethylammonium salt of formally conjugated N-[(1-acetyl- 2-oxopropyl)(phenyl)-λ4-sulfanylidene]ethanesulfonamide anion, synthesized by reaction of N-(ethylsulfonyl)benzenesulfinimidoyl chloride with acetylacetone in the presence of triethylamine, has been studied theoretically and by X-ray diffraction.

The effect of cyclochirality of rccc-2,8,14,20-tetra-n-decyl-4,10,16,22-tetra-O-methylresorcin[4]arene (C) on the enantiodiscrimination of a number of chiral bidentate and tridentate aromatic and aliphatic biomolecules (G) has been investigated by nano-electrospray ionization (nano-ESI)-Fourier transform ion cyclotron resonance mass spectrometry. The experimental approach is based on the formation of diastereomeric proton-bound [C center dot H center dot G](+) complexes by nano-ESI of solutions containing an equimolar amount of quasi-enantiomers (C) together with the chiral guest (G) and the subsequent measurement of the rate of the G substitution by the attack of several achiral and chiral amines. In general, the heterochiral complexes react faster than the homochiral ones, except when G is an aminoalcoholic neurotransmitter whose complexes, beyond that, exhibit the highest enantioselectivity. The kinetic results were further supported by both collision-induced dissociation experiments on some of the relevant [C-2 center dot H center dot G](+) three-body species and Density functional theory (DFT) calculations performed on the most selective systems. Copyright (C) 2012 John Wiley & Sons, Ltd.

This article summarizes some of our investigations of compounds featuring the structural fragment P‒C‒P. Phosphorus atoms in different oxidation states and coordination numbers display different chemical behaviors. This may lead to unique chemical properties when they are bridged by a methylene group in a P‒CH2‒P system. For example, the PIII‒CH2‒PVunit is able to add different unsaturated compounds to give zwitterionic products with oppositely charged phosphorus atoms. In addition, the two hydrogen atoms in P‒CH2‒P may also take part in different transformations, resulting in the formation of carbodiphosphoranes or in the formation of a direct P-P bond to give three-membered ring diphosphiranes.

The reactions of 1,3-diphosphaphenalene 3 with sulfur, borane-tetrahydrofuran, and transition metal carbonyls are reported.

A rare example of the addition of sulfur to a phosphaalkene double bond leading to 1,2λ3,σ3-thiaphosphirane 4 is described. The relative accessibility of this compound has allowed the study of some of the chemical properties of the thiaphosphirane ring. The thermodynamic stability of 1,2λ3,σ3-thiaphosphiranes compared with λ5,σ3-phosphoranes has been studied by DFT calculations.

The salt (η5-pentamethylcyclopentadienyl)silicon(II) tetrakis(pentafluorophenyl)borate (5) reacts at −78 °C with lithium bis(trimethylsilyl)amide in dimethoxyethane (DME) as solvent to give quantitatively the compound [bis(trimethylsilyl)amino][pentamethylcyclopentadienyl]silicon(II) 6A in the form of a colorless viscous oil. The reaction performed at −40 °C leads to the silicon(IV) compound 7, the formal oxidative addition product of 6A with DME. Cycloaddition is observed in the reaction of 6A with 2,3-dimethylbutadiene to give the silicon(IV) compound 8. Upon attempts to crystallize 6A from organic solvents such as hexane, THF, or toluene, the deep yellow compound trans-1,2-bis[bis(trimethylsilyl)amino]-1,2-bis(pentamethylcyclopentadienyl)disilene (6B), the formal dimer of 6A, crystallizes from the colorless solution, but only after several days or even weeks. Upon attempts to dissolve the disilene 6B in the described organic solvents, a colorless solution is obtained after prolonged vigorous shaking or...

Isotropic C-13 nuclear magnetic shielding values (sigma(iso)) and corresponding chemical shift magnitudes (delta(C)) have been calculated for the large series of the thiocarbonyl compounds using DFT (RI-BP86 and B3LYP) and ab initio (RHF and MP2, with or without resolution of the identity approximation) levels of theory and different basis sets. As expected, the Hartree-Fock method and hybrid B3LYP approach essentially overestimate deshielding of the C=S carbon nuclei. The RI-BP86 level of approximation provides a noticeably better agreement with the experiment and can be used for calculations of the theoretical C-13 NMR spectra for the most classes of the thiocarbonyl compounds. Geometry optimizations and the nuclear magnetic shielding calculations using the MP2 level of theory provide the most reliable results. The resolution of the identity algorithm implemented into the TURBOMOLE program set does not affect the calculated sigma(iso) and delta C values compared to the strict MP2 nuclear magnetic shielding calculations (using the GAUSSIAN-03 program packet). A poor agreement with the experiment found in the case of the thiourea and thiosemicarbazide derivatives is referred to the different conformations of these species in the gas phase, solid state, and solution.

The reaction of potassium fluoride with the corresponding acid chlorides results in the formation of dialkylamido sulfoxylic acid fluorides Alk 2 N-S-F. Stereo- and regioselectivity of dimethylamido and diisopropylamido sulfoxylic acid fluorides addition reactions to the C,C double bond in cyclohexene, 3,4-dihydro-2H-pyrane, and vinyl ethyl ether has been investigated. This reaction is a convenient approach to the β -fluoroalkyl sulfenic acid derivatives.

The 1.4-diphospha-2-azol-5-ylidene is a homologue to the Ender's type carbene. It is a possible candidate for a ligand in the metathesis reaction of olefins. Based on density functional calculations the differences between the electronic structures of both systems are evaluated. The NHC (N-heterocyclic carbene) possesses a larger singlet-triplet energy separation than the PHC (P-heterocyclic carbene) analogue. Thus the latter exerts a larger Lewis acidity than the former. In comparison with, the donor-ability (a-basicity) in both systems is similar. As a consequence for the PHC carbene a Ru-fragment as a ligand for catalysis is stronger bound. This causes in the olefin metathesis a lower dissociation energy (compared to the NHC analogue) with respect to the formation of the catalyst active 14el species. As a consequence, the olefin will be weaker bound as well. This can be overcome by attaching sterically demanding substituents such as mesityl or super-mesityl to the phosphorus atoms. They induce mutual steric hindrance with concomitant increase of the S-T separation of the free carbene. Thus the Lewis acidity of the carbene is reduced. On this basis for the PHC's with larger S-T energy separations the dissociation energy of the phosphine fragment is raised and the adding olefin fragment will be stronger bound to the transition metal. While these effects describe the electronic situation in the reactive species, steric effects at the ligand carbene mediate the stabilities of the individual intermediates in the metathesis reaction by exertion of inter- and intra-ligand repulsion. (c) 2005 Elsevier B.V. All rights reserved.

A variety of conformations for three model calix[4]arenes with 8 or 12 OH groups have been investigated by calculations at density functional (RI-BP86) and RI-MP2 level of approximation. The calixarenes form stable complexes with the alkali metal cations of lithium up to caesium. For the investigations all-valence electron basis sets as well as various effective core potentials were probed. The stabilities of complexes were analysed in comparison with the corresponding benzene complexes, M(+)center dot C6H6. The formation of the calixarene metal complexes is considered in two steps, (a) in a distortion from the equilibrium conformation of the free calixarenes and (b) subsequent complexation. The distortion energies are small for the 'crown' and larger for the 'boat' conformations. On the other hand the latter are more stabilized by significant interaction energy of the cation with two adjacent pi-systems of the aromatic rings. As a result, these two conformations are of similar stabilities for K+ to Cs+ complexes with resorc[4]arenes, with a slight advantage for the 'boat' structure. The most stable conformation for the coordination products of these cations with the calix[4]arene with 12 OH groups is a slightly flattened 'crown' that is derived by maximum hydrogen bonding of the OH-groups and the most effective cation-pi interactions. Special cases are complexes of Li+ and Na+ which in most instances prefer the coordination on the oxygen atoms of the upper rim of the calixarene cavities and thus form 'boat'-like structures. (c) 2005 Published by Elsevier B.V.