Sigma-pi energy separation in homodesmotic reactions.

A well-established quantity for specifying the aromaticity or antiaromaticity of cyclic conjugated molecules is the so-called aromatic stabilization energy (ASE), which can be derived-either experimentally or theoretically-from appropriate homodesmotic reactions. To gain further insight into the origin of aromaticity, several schemes have been devised to partition ASE into nuclear and electronic as well as sigma and pi contributions, some of which have resulted in contradictory statements about the driving force of aromatic stabilization. Currently, these contradictions have not been resolved and have resulted in a confusing distinction between two different types of aromaticity: extrinsic and intrinsic aromaticity. By investigating different homodesmotic reactions we show that, in contrast to ASE itself, the individual contributions that enter the ASE can strongly depend on the type of reaction. Caution is therefore advised if conclusions or physical interpretations are derived from the individual components. The contradictions result from the fact that some reactions suffer from an imbalance in the number of interaction terms at the two sides of the reaction equation. The concept of isointeractional reactions is introduced and results in the elimination of the imbalance. For these reactions, the contradictions disappear and the distinction between intrinsic and extrinsic aromaticity becomes unnecessary. As far as the sigma-pi partitioning is concerned, several schemes proposed in the literature are compared. Contradictory results are obtained depending on the partitioning scheme and reaction used. In this context, it is demonstrated that for the partitioning of the electron-electron interaction, the scheme introduced by Jug and K?ster is the one that is most theoretically grounded.     

On the aromatic stabilization of corannulene and coronene

The application of set of homodesmotic reactions allowed us to estimate the aromatic stabilization energy (ASE) of corannulene and coronene. Appropriate reactions have been applied to balance syn/anti mismatches in di-, tetra- and hexamethylene substituted derivatives. Based on many different polycyclic reference structures that compensate the effect of strain in the corannulene moiety the value of ASE comes to 44.5 kcal mol(-1). Planar corannulene is more stabilized by cyclic π-electron delocalization by ca. 10.7 kcal mol(-1), as compared with a bowl-shaped system. A similar approach for coronene leads to an ASE equal to 58.4 kcal mol(-1).     

Aromaticity of caffeine,xanthine and the dimethyl xanthines

Xanthine, caffeine and three isomeric dimethyl xanthines (theobromine, theophylline and paraxanthine) are often described as aromatic on various criteria. Here we complete the picture by assessing these molecules for aromaticity on the ring-current criterion. Magnetic response calculations are performed at the B3LYP and CHF/6-31G^∗∗ ipsocentric levels of theory on structures optimised at the B3LYP/6-31G^∗∗ level. All five systems display consistent π-electron ring-currents delocalised around the imidazole moiety in all cases; these are accompanied by localised features on the six-membered rings attributed to nitrogen and oxygen ‘lone-pair’ π-electron circulations. All are therefore aromatic on the magnetic criterion, with a ‘locally delocalised’ ring current in the imidazole moiety, similar to those in the isolated imidazole and methylimidazole molecules.     

Aromaticity in Group 14 homologues of the cyclopropenylium cation

The nature of the bonding and the aromaticity of the heavy Group 14 homologues of cyclopropenylium cations E₃H₃⁺ and E₂H₂E′H⁺ (E, E′=C–Pb) have been investigated systematically at the BP86/TZ2P DFT level by using several methods. Aromatic stabilization energies (ASE) were evaluated from the values obtained from energy decomposition analysis (EDA) of charged acyclic reference molecules. The EDA‐ASE results compare well with the extra cyclic resonance energy (ECRE) values given by the block localized wavefunction (BLW) method. Although all compounds investigated are Hückel 4n+2 π electron species, their ASEs indicate that the inclusion of Group 14 elements heavier than carbon reduces the aromaticity; the parent C₃H₃⁺ ion and Si₂H₂CH⁺ are the most aromatic, and Pb₃H₃⁺ is the least so. The higher energies for the cyclopropenium analogues reported in 1995 employed an isodesmic scheme, and are reinterpreted by using the BLW method. The decrease in the strength of both the π cyclic conjugation and the aromaticity in the order C?Si>Ge>Sn>Pb agrees reasonably well with the trends given by the refined nucleus‐independent chemical shift NICS(0)_πzz index.     

Aromatic stabilization energy calculations for the antiaromatic fluorenyl cation. Issues in the choice of reference systems for positively charged species

Aromatic stabilization energy (ASE) calculations for the fluorenyl cation show substantial destabilization in comparison to suitable reference systems (16.3 +/- 1.6 kcal/mol), supporting its categorization as an antiaromatic species. The choice of appropriate reference systems is exacting for cationic systems because of the need to match strain energies, convolved with allylic-type resonance terms and other potential structural effects that stabilize charge. Several homodesmotic ASE reaction systems are examined to demonstrate the role played by these factors in the calculation of an ASE value for the fluorenyl cation. The magnitudes of the derived ASE are quite similar for four very different determinative, homodesmotic reaction systems, giving strong support to the inherent accuracy of the final derived ASE value. The results of nucleus independent chemical shift calculations for the components of each one of the ASE reactions add additional weight to this conclusion.     

Quantum chemical study of alternating N/B and N/C π bonds in (un)charged even‐membered 4n and 4n+2 cyclic systems and their related open systems

With semi‐empirical MO and ab initio calculations at different levels, we investigated the π conjugation of alternating X? Y bonds with X? Y for N/B and N/C combinations in an open and cyclic arrangement. Although the intrinsic symmetry is lost for the acyclic even‐membered compounds, the alternation is still reflected in its geometry and electron‐density transfer. For the cyclic π compounds, we focused our attention on borazine N₃B₃H₆ (D_3h symmetry), which is isoelectronic with benzene (D_6h symmetry). Specific attention is given to the electrophilic behavior of borazine with respect to CH and SiMe. The dynamics based on the results of FT‐ICR mass spectrometry was studied in more detail. In addition, the results of the cyclic systems with 4n and 4n+2 π electrons concerning their geometries are compared with the corresponding carbon compounds. Attention is also given to the dication of borazine, because of the corresponding triplet ground state of the benzene dication. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

‘Push-Pull’ and spirobicyclic structures by reacting N-methyl cyclic ketene-N,X (X=S, O)-acetals with isocyanates and isothiocyanates

Nucleophilic N-methyl cyclic ketene-N,X (X=S, O)-acetals can react with electrophilic aryl isocyanates and aryl isothiocyanates to form ‘push-pull’ mono-adducts, di-adducts and spirobicyclic 6/5 ring compounds.     

Shape persistent hybrid dendrimers from benzene and triazole via ‘click chemistry’

Fully aromatic rigid dendrimers have been synthesized at room temperature based on two different ‘click’-reactions: The dendronization was carried out via the copper catalyzed azide–alkyne Huisgen cycloaddition or Diels–Alder cycloaddition, respectively. The feasible combination of both reactions leads to dendrimer hybrids. Stiff aromatic dendrimers, whose highly nitrogen containing scaffolds are solely based on interconnected benzene and triazole rings, were created.     

Regio- and π-facial selective Lewis acid interceded Diels-Alder reactions of α-dienyl-β-lactams: an indepth analysis

The regio-, diastereo-, and π-facial selective Lewis acid mediated Diels-Alder reactions of cis/trans-3-butadienyl-2-azetidinones with unsymmetrical dienophiles viz. methyl acrylate, dimethyl fumarate, and acrolein leading to the synthesis of diastereomerically pure and biologically potent 1,3,4-trisubstituted-2-azetidinones are reported. Theoretical calculations at HF/6-31G^∗∗ and 6-31G^∗∗/DFT levels have been performed to support the observed π-facial selectivity. The formation of diastereomerically pure ‘endo’ adducts is supported by the X-ray diffraction studies.     

Push-pull vs captodative aromaticity

Vinylogs of fulvalenes with cyclopropenyl and cyclopentadienyl moieties attached either to different carbon atoms ( c-C 3H 2CHCHC 5H 4- c, 7) or to the same carbon atom [XC( c-C 3H 2)( c-C 5H 4), 10] [X = CH 2; C(CN) 2; C(NH 2) 2; C(OCH 2) 2; O; c-C 3H 2; c-C 5H 4; SiH 2; CCl 2] of the double bond inserted between the two rings are examined theoretically at the B3LYP/6-311G(d,p) level. Both types of compounds are shown to possess aromaticity, which was called "push-pull" and "captodative" aromaticity, respectively. For the captodative mesoionic structures XC( c-C 3H 2)( c-C 5H 4), the presence of both the two aromatic moieties and the CC double bond is the necessary and sufficient condition for their existence as energetic minima on the potential energy surface. Aromatic stabilization energy (ASE) was assessed by the use of homodesmotic reactions and heats of hydrogenation. Spatial magnetic criteria (through space NMR shieldings, TSNMRS) of the two types of vinylogous fulvalenes 7 and 10 have been calculated by the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept of Paul von Rague Schleyer, and visualized as iso-chemical-shielding surfaces (ICSS) of various sizes and directions. TSNMRS values can be successfully employed to visualize and quantify the partial push-pull and captodative aromaticity of both the three- and five-membered ring moieties. In addition, the push -pull effect in compounds 7 and 10 could be quantified by the occupation quotient pi* CC/pi CC of the double bond inserted between the two rings.     

Platinum-catalyzed highly selective thiocarbonylation of acetylenes with thiols and carbon monoxide

A novel carbonylative addition of thiols (RSH) to terminal acetylenes (R′-CCH) takes place successfully in the presence of platinum catalysts under the pressure of carbon monoxide, providing α,β-unsaturated thioesters (R′-C(C(O)SR)CH₂) in good yields regioselectively. This ‘hydrothiocarbonylation’ reaction of acetylenes may include the formation of the platinum sulfide complex as key species.     

The effect of substitutents on the strain energies of small ring compounds

The effect of substitutents on the strain energy (SE) of cyclic molecules is examined at the CBS, G2, and G2(MP2) levels of theory. Alkyl substitutents have a meaningful effect upon the SE of small ring compounds. gem-Dimethyl substitution lowers the strain energy of cyclopropanes, cyclobutanes, epoxides, and dimethyldioxirane (DMDO) by 6-10 kcal/mol relative to an unbranched acyclic reference molecule. The choice of the reference compound is especially important for geminal electronegative substitutents. The SE of 1,1-difluorocyclopropane is estimated to be 20.5 kcal/mol relative to acyclic reference molecule 1,3-difluoropropane but is 40.7 kcal/mol with respect to the thermodynamically more stable (DeltaE = -20.2 kcal/mol) isomeric reference compound 2,2-difluoropropane. The SE of dioxirane (DO) is estimated to be approximately 18 kcal/mol while the SE of DMDO is predicted to be approximately equal to 11 kcal/mol by using homodesmotic reactions that maintain a balanced group equivalency. The total energy (CBS-APNO) of DMDO is 2.6 kcal/mol lower than that of isomeric 1,2-dioxacyclopentane that has an estimated SE of 5 kcal/mol. The thermodynamic stability of DMDO is a consequence of its relatively strong C-H (BDE = 102.7 kcal/mol) and C-CH(3) (BDE = 98.9 kcal/mol) bonds. By comparison, the calculated sec-C-H and -C-CH(3) G2 bond dissociation energies in propane are 100.3 and 90.5 kcal/mol.     

Synthesis of Rhenabenzenes from the Reactions of Rhenacyclobutadienes with Ethoxyethyne

Treatment of Na[Re(CO)₅] with RC?CCO₂Et (R=phenyl, naphthalen‐1‐yl, phenanthren‐9‐yl and pyren‐1‐yl) followed by reaction with acetyl chloride and ethanol afforded the rhenacyclobutadienes Re{‐C(R)?C(CO₂Et)C(OEt)?}(CO)₄. Reactions of these rhenacyclobutadienes with HC?COEt produced rhenabenzenes Re{‐C(R)?C(CO₂Et)C(OEt)?CHC(OEt)?}(CO)₄. Except for R=Ph, new rhenacyclobutadienes with pendant alkenyl substituents Re{‐C(R)?C(C(OEt)?CH(CO₂Et))C(OEt)?}(CO)₄ were also isolated from these reactions. The NMR spectroscopic and X‐ray structural data, as well as the aromatic stabilization energy (ASE) values suggest that the rhenabenzenes are aromatic, with extensive delocalized π character.     

Action d'organometalliques sur des dialcoxysilanes cycliques et acycliques

The reaction of various organometallics with cyclic and acyclic dialkoxysilanes has been studied.In the case of the acyclic compounds, phenyl-α-naphthylmethoxyborneoxy- and phenyl-α-naphthylmethoxycyclohexyloxy-silane, our results confirm those previously observed with phenyl-α-naphthylmethoxymethoxysilane. The reactions are selective. In ether, aromatic and saturated organomagnesium compounds substitute only the methoxy group with retention, but allylic and benzylic organomagnesium substitute the bulky alkoxy group with inversion.In THF and DME, irrespective of the type of organomagnesium compound, the methoxy group alone is substituted with retention of configuration.With the cyclic compounds, 2-methoxy-2-silaindane, both methoxy and menthoxy groups are substituted and our results confirm the tendency of cyclic derivatives to be substituted with retention of configuration.     

Highly efficient one-pot, three-component Mannich reaction catalysed by boric acid and glycerol in water with major ‘syn’ diastereoselectivity

Boric acid and glycerol efficiently catalysed the one-pot, three-component Mannich reaction of aldehydes, aromatic amines and cyclic ketones in water at ambient temperature to afford the corresponding β-amino carbonyl compounds in good yields. All but one reaction proceeded with moderate ‘syn’ diastereoselectivity. This observation is just the reverse of the major anti diastereoselectivity obtained in most of the earlier reported procedures. The methodology is mild and efficient using minute quantities of catalyst with no side products and a very simple work-up procedure.     

Nonsimple relationships between the P-chiral diamidophosphite and the arylphosphine moieties in Pd-catalyzed asymmetric reactions: combinatorial approach and P,P-bidentate phosphine-diamidophosphites

A small family of P,P^∗-bidentate C₁-symmetric ligands containing 1,3,2-diazaphospholidine rings with stereogenic phosphorus atoms has been prepared. Palladium catalytic systems with these phosphine-diamidophosphites afforded 95% and 63% ees in asymmetric allylic substitution and desymmetrization processes, respectively. The influence of the nature of both the phosphine and diamidophosphite moieties of these compounds on the enantioselectivity is discussed. The ‘mixed-ligand approach’ in Pd-catalyzed asymmetric allylation with participation of some new P^∗-monodentate diamidophosphites and PPh₃ is also considered.     

Fluorination of activated aromatic systems with Selectfluor™ F-TEDA-BF4 in ionic liquids

Selectfluor™ was shown to be soluble in ionic liquid, thus allowing the ‘green’ electrophilic fluorination of activated aromatic systems compounds in high chemoselectivity and yields.     

‘One-flask’ synthesis to 3,5-disubstituted 1,2,4-triazoles from aldehydes with hydrazonoyl hydrochlorides via 1,3-dipolar cycloaddition

A new ‘one-flask’ synthesis of 3,5-disubstituted 1,2,4-triazoles has successfully been developed to synthesize a series of 3,5-disubstituted 1,2,4-triazoles. The transformation involves the 1,3-dipolar cycloaddition reaction of hydrazonoyl hydrochlorides with oxime intermediates prepared from aldehydes with hydroxylamine hydrochloride in the presence of excess amount of triethylamine. In this ‘one-flask’ 1,3-dipolar reaction, hydrazonoyl hydrochlorides was concerned as the masked 1,3-dipole nitrilimine under basic condition. Furthermore, this newly developed methodology can be applied to various aldehyde substrates including aliphatic, cyclic aliphatic, aromatic, and heterocyclic aldehydes.     

Cyclic vinylogous triflate hemiacetals as new surrogates for alkynyl aldehydes

Cyclic vinylogous triflate hemiacetals can serve as ‘synthetic equivalents’ for alkynyl aldehydes: treatment of a vinylogous triflate hemiacetal with excess amounts of Grignard reagents produces acyclic alkynyl alcohols in good to high yields. This transformation likely involves the Grob-type C-C bond cleaving fragmentation to form the alkynyl aldehyde in situ. Subsequent nucleophilic attack of the Grignard reagent furnishes secondary alkynols. Vinylogous triflate hemiacetals are easily prepared by DIBALH reduction of vinylogous acyl triflates, which are derived from cyclic 1,3-diketones.