Application of the semilocalized approach to chemical reactivity of butadiene

The semilocalized approach to chemical reactivity (J. Mol. Struct. (Theochem) 588 (2002) 99; Int. J. Quant. Chem. 94 (2003) 302) is applied to study the addition reaction of an electrophile or nucleophile to the butadiene molecule. In accordance with the classical concept of the reaction center and its neighborhood (substituent), only one of the two H₂C=CH-fragments of butadiene is supposed to be under a direct attack of the reagent, whereas the remaining H₂C=CH-group is assumed to play the role of the substituent and thereby to participate in the process indirectly by exerting certain electron-donating or accepting effect upon the former group and/or the reagent. The main aim of the study consists in revealing the role of the H₂C=CH-substituent in the formation of the known higher reactivity of the terminal carbon atom of the attacked C=C-bond (as compared to the internal atom) irrespective of the nature of the reagent. To this end, we seek to obtain an explicit algebraic representation of the interdependence between the direction and the extent of the total influence of the H₂C=CH-substituent, on the one hand, and the nature of the reagent, on the other hand. The expressions for electron density and bond order redistributions among separate fragments of contacting molecules derived previously in the form of power series are shown to yield the above-anticipated representation. On this basis, it is demonstrated that the electron-donating effect of the initially occupied (bonding) orbital of the substituent and the electron-accepting effect of its initially vacant (antibonding) orbital upon the remaining fragments of the whole reacting system may be considered independently whatever the nature of the reagent. However, a strong interdependence is established between the actual relative extents of these two components of the total effect of the H₂C=CH-group and the electron-donating (accepting) properties of the reagent. Moreover, this group of atoms is shown to manifest itself as an electron-donating (accepting) substituent under influence of an electrophilic (nucleophilic) attack. Using this principal result of the paper, the actual reactivity of butadiene with respect to electrophile (nucleophile) is interpreted by invoking a model system of a substituted ethene containing a simple (one-orbital) electron-donating (accepting) substituent, and a terminal addition easily follows for both types of the reagent.     

Oxidative Addition at a Carbene Center: Synthesis of an Iminoboryl–CAAC Adduct

The reaction of a cyclic (alkyl)(amino)carbene (CAAC) with dichloro‐ and dibromobis(trimethylsilyl)aminoborane results in the formation of haloiminoborane–CAAC adducts. When the iodo analogue is used, an oxidative addition at the carbene center affords a cationic iminoboryl–CAAC adduct, featuring a boron–nitrogen triple bond. Similar salts are also obtained by halide abstraction from the chloro‐ and bromoiminoborane–CAAC adducts. The reactivity of all of these compounds towards CO₂ is discussed.     

Synthesis and Transformations of 5‐Chloro‐2,2′‐Dipyrrins and Their Boron Complexes, 8‐Chloro‐BODIPYs

Symmetric dipyrrylketones 1 a , b were synthesized in two steps from the corresponding α‐free pyrroles, by reaction with thiophosgene followed by oxidative hydrolysis under basic conditions. The dipyrrylketones produced the corresponding 5‐chloro‐dipyrrinium salts or 5‐ethoxy‐dipyrrins on reaction with phosgene or Meerwein’s salt, respectively. Boron complexation of the dipyrrins afforded the corresponding 8‐functionalized BODIPYs (borondipyrromethenes) in high yields. The 5‐chloro‐dipyrrinium salts reacted with methoxide or ethoxide ions to produce monopyrrole esters, presumably via a 5,5‐dialkoxy‐dipyrromethane intermediate. In contrast, 8‐chloro‐BODIPYs underwent a variety of nucleophilic substitutions of the chloro group in the presence of alkoxide ions, Grignard reagents, and thiols. In the presence of excess alkoxide or Grignard reagent, at room temperature or above, substitution at the boron center also occurred. The 8‐chloro‐BODIPY was a particularly useful reagent for the preparation of 8‐aryl‐, 8‐alkyl‐, and 8‐vinyl‐substituted BODIPYs in very high yields, using Pd⁰‐catalyzed Stille cross‐coupling reactions. The X‐ray structures of eleven BODIPYs and two pyrroles are presented, and the spectroscopic properties of the synthesized BODIPYs are discussed.     

Unexpected reaction pathways in the reaction of alkoxyalkynylchromium(0) carbenes with aromatic dinucleophiles

Thermal- or SiO(2)-induced reactions of the Michael adducts of 1,2-aromatic dinucleophiles and alkynylchromium(0) carbene complexes, compounds 7-10, form different products in good yields depending on the nature of the aromatic dinucleophile used. Thus, 1,2-diaminobenzene derivatives 7 and 8 rearrange to pentacarbonylchromium(0) isocyanide complexes 11, 12, 14, and 15 in a process that occurs through bicyclic intermediates 24. Adducts 9 derived from o-aminophenol give 2,3-dihydro-1,5-benzoxazepine derivatives 17 by intramolecular 1,2-addition, followed by protonation at the chromium center and reductive elimination. In contrast, base-promoted addition of the phenolic hydroxy group in compound 9 a affords 3-ethoxy-5-phenyl-5,6-dihydro-2H-1,6-benzoxazocin-2-one (18), together with the expected adduct 17 a. Compound 18 is formed by a nucleophilic addition to a CO ligand in a preformed carbene complex. This is a new example of the rare attack of a nucleophile on a CO ligand in a Fischer carbene complex. Adducts 10 form seven-membered-ring carbene complexes 19 and 20 by intramolecular aminolysis. In contrast, reaction of alkynyl carbene complexes with 1,8-diaminonaphthalene under very mild conditions leads to 2-substituted perimidines 33 together with the corresponding ethoxymethylmetal carbene complex 32 through an unprecedented fragmentation process in a formal retro-Aumann reaction.     

Stereoselective synthesis of phosphoranyl aryloxiranes through the addition of a nucleophilic stable carbene to aromatic aldehydes

The [2+1] addition of the stable (phosphanyl)(silyl)carbene 1 to aromatic aldehydes affords phosphoranyl aryloxiranes, a new class of polyfunctional epoxides, in high yields and excellent diastereoselectivity. No reaction is observed for aldehydes bearing strongly electron-donating groups. Theoretical calculations show a good correlation between Gibbs activation energy and the electronic nature of the substituent on the phenyl ring.     

Transformations of bis(2-chloroethyl) arylphosphonites in the presence of haloacetic acid esters

The reaction of aryl(dichloro)phosphines with (2-chloroethoxy)trimethylsilane in the presence of haloacetic acid esters was studied with the goal of developing a new method for the synthesis of ethyl [(2-chloroethoxy)(4-dimethylaminophenyl)phosphoryl]acetate. The effect of various factors, such as substituent in the para position of the aromatic ring in the initial aryl(dichloro)phosphine, halogen nature in haloacetic acid ester, and chloro(trimethyl)silane, on the reaction course was analyzed.     

Study on the Heck reaction promoted by carbene adduct of cyclopalladated ferrocenylimine and the related reaction mechanism

Carbene adduct of cyclopalladated ferrocenylimine has been successfully applied to Heck reaction of various aryl bromides with olefins. On the basis of kinetic studies, in situ ¹³C NMR spectra investigations and Hg poisoning experiments, it was proposed that the Heck reaction catalyzed by carbene adduct of cyclopalladated ferrocenylimine proceeded through a classical Pd(0)/Pd(II) cycle and such palladacycle was only a reservoir of the catalytically active Pd(0) species.     

Competition between cyclisation and bisimine formation in the reaction of 1,3-diaminopropanes with aromatic aldehydes

Condensation of 1,3-diamines with aldehydes or ketones gives rise to two major products, the hexahydropyrimidine and the bisimine. Experimental studies of the reaction between a range of aromatic aldehydes and 1,3-diaminopropane or 1,3-diamino-2-propanol establish that the hexahydropyrimidine is favoured by the less nucleophilic amine and by the presence of electron withdrawing groups on the aryl ring of the aldehyde. Calculations indicate that the electronic nature of this aryl ring substituent influences both the relative thermodynamic stability of the final products and the reactivity of the aldehyde as an electrophile.     

Post-Hartree-Fock studies on the structure of bis(ortho-substituted phenyl)methylenes

Long-lived triplet bisarylmethylenes are now well-known. Experimental data (primarily ESR hyperfine parameters) suggest that the carbon framework of long-lived bisarylmethylenes approaches D(2)(d)() symmetry, as ortho-substitution forces the central angle to approach 180 degrees . According to DFT modeling, the approach of the central angle to 180 degrees is accompanied by a dramatic shortening of the central CC bonds and severe quinoid distortion of the phenyl rings. In contrast, X-ray investigation of bis(2,4,6-trichlorophenyl)methylene shows a structure closer to the carbene valence bond representation with less seriously distorted phenyl rings, a more acute central angle, and a longer bond from the methylene carbon to the aryl substituent. We address the difficulty of achieving a balance of cumulene and carbene character, treating the model systems diethynylmethylene, dicyanomethylene, and diisocyanomethylene by post-Hartree-Fock methods CAS and CCSD as well as DFT models, and applying the perturbation-corrected CAS methods to the chloro and methyl ortho-substituted bisphenyl carbenes.     

Single and Consecutive Cyclization Reactions of Alkynyl Carbene Complexes and 8‐Azaheptafulvenes: Direct Access to Polycyclic Pyrrole and Indole Derivatives

The reactivity of alkynyl and enynyl Fischer carbene complexes towards 8‐azaheptafulvenes is examined. Alkynyl carbenes 1 a – f undergo regioselective [8+2] heterocyclization with 8‐aryl‐8‐azaheptafulvenes 2 a , b providing cycloheptapyrroles 3 and 4 with metal carbene or ester functionality at C3. Moreover, consecutive cyclization reactions are involved when enynyl carbenes are used. Thus, the cyclopenta[b]pyrrole framework 7 is formed by the consecutive [8+2] cyclization and cyclopentannulation reactions. The initially formed cyclopentannulation adduct can be intercepted through a Diels–Alder reaction with classic dienophiles to afford increasing structural complexity (compounds 8 and 9 ). More importantly, the construction of the indole skeleton is accomplished with a high degree of substitution and functionalization (compounds 11 – 15 ) by a one‐pot sequence that involves [8+2] cyclization, R? NC or CO insertion, and ring closure.     

β 沸石中铝的状态及归属

The states of aluminum in zeolites β were investigated with FTIR spectroscopy. Three kinds of aluminium, framework, nonframework and transient state aluminium, were detected in template-removed zeolite β. The states of aluminium in our samples strongly depends on the nature of the compensating cation, consistent with Brougeat-Lami et al's conclusion. But some new assignments of aluminum species and new opinions about the states of aluminum and their transformation are proposed. From the experimental results, we suggest that, the distortion and tension of zeolite lattice occur when protons, which possess high electron affinity , are located at cationic sites, leading to the breakage of Al-- O bond and the removal of aluminium. The distortion is released when H+ is replaced by other cations (e.g. Na+ ) so that transient state aluminium can go back into the framework, while the cationic nonframework aluminium species enter the solution upon the ion-exchange.     

Silyldefluorination of Fluoroarenes by Concerted Nucleophilic Aromatic Substitution

The reaction of readily generated silyl lithium reagents with various aryl fluorides to provide the corresponding aryl silanes is reported. DFT calculations reveal that the nucleophilic aromatic substitution of the fluoride anion by the silyl lithium reagent proceeds through concerted ipso substitution. In contrast to the classical nucleophilic aromatic substitution, this concerted ionic silyldefluorination also occurs on more electron‐rich aryl fluorides.     

Investigations on 2,3'-Biquinolyls. 12. Alkylation and Arylation of 2,3'-Biquinolyl Dianion with Organometallic Compounds. New Type of Reaction for Dianions of Aromatic Compounds

2,3'-Biquinolyl dianion reacts with organolithium and organomagnesium compounds with the formation, after treatment of the reaction mixture with water, of 2'-alkyl(aryl)-1',2'-dihydro-2,3'-biquinolyls, and, after treatment of the reaction mixture with alkyl halides, of 1'-alkyl-2'-alkyl(aryl)-1',2'-dihydro-2,3'-biquinolyls. The reaction includes attack of the nucleophilic reagent with an electron transfer to a molecule of solvent.     

Highly efficient N‐Heterocyclic carbene–palladium complex‐catalyzed multicomponent carbonylative Suzuki reaction: novel practical synthesis of unsymmetric aryl ketones

Highly efficient and chemoselective N‐heterocyclic carbene palladium complexes‐catalyzed multicomponent carbonyltive Suzuki reaction with sodium tetraphenylborate used as phenylating reagent has been demonstrated in this article. Both electron‐rich and electron‐deficient aryl iodides gave unsymmetric aryl ketones in excellent yields. Copyright © 2007 John Wiley & Sons, Ltd.     

Substituent stereochemistry effects on diastereoselective methylation reaction of 4-chloroadamantan-2-ones

π-Facial diastereoselectivity effects of the substituent in 4-position on the nucleophilic addition of substituted adamantan-2-ones were observed for the methylation reaction of 4-chloroadamantan-2-ones. NMR study revealed that when chlorine atom is in axial stereochemical position, exclusively anti-addition occurs, whereas selective preference for syn-addition was observed with stereochemical equatorial position for chloro substituent. The success of this strategy can be attributed to the important role that CeCl₃ plays in increasing the nucleophilicity and decreasing the basicity of the methylorganometallic reagent.     

The unprecedented reaction of dimethylsulfonium methylide with Michael acceptors: synthesis of 1-substituted vinyl silanes and styrenes

Rather than the usual cyclopropanation, conditions for an unprecedented elimination reaction from the adduct of dimethylsulfonium methylide and various Michael acceptors have been established leading to functionalized 1-substituted alkenes. In silyl substituted substrates (2a and 2h), where a facile Peterson-type olefination is possible from the adduct; elimination took place instead to give functionalized 1-substituted vinyl silanes. Aryl substituted Michael acceptors (2b-e, 2g and 2i-k) also underwent a similar kind of olefination to give 1-substituted styrene derivatives with moderate yields along with a side product, which arose by nucleophilic demethylation from the adduct of dimethylsulfonium methylide and arylidene malonates. Hammett studies revealed that selectivity for olefination vs. demethylation increases as the aryl substituent becomes more electron deficient. Alkylidene malonates (2f and 2l) with a beta-alkyl substituent did not favour the olefination process. Sequential addition of Michael acceptors and alkyl halides to a mixture of dimethylsulfonium methylide and sodium dimsylate provided olefination followed by alkylation on the active methine group. A mechanistic pathway has been formulated from the studies of a few sulfonium methylides.     

Five-to-six membered ring-rearrangements in the reaction of 5-perfluoroalkyl-1,2,4-oxadiazoles with hydrazine and methylhydrazine

The hydrazinolysis reaction of 5-perfluoroalkyl-1,2,4-oxadiazoles with hydrazine or methylhydrazine as bidentate nucleophiles has been investigated. The reaction occurred through the addition of the bidentate nucleophile to the C(5)-N(4) double bond of the 1,2,4-oxadiazole followed by ring-opening and ring-closure (ANRORC) involving the second nucleophilic site of the reagent. This ring-closure step could involve either the original C(3) of the 1,2,4-oxadiazole (giving a five-to-five membered ring rearrangement) or an additional electrophilic center linked to it (exploiting a five-to-six membered ring rearrangement). An alternative initial nucleophilic attack may involve the additional electrophilic center linked at C(3), that is the carbonyl group, leading to the formation of the hydrazones which undergo the Boulton-Katritzky rearrangement (BKR). The chosen reaction path is a function of the used nucleophile and of the nature of the substituent at C(3). At variance with previous hypotheses, when methylhydrazine was used, the observed regiochemistry always showed the preferred initial attack by the less hindered NH(2) end of the nucleophile on C(5). Moreover, new spectroscopic evidence allowed the assignment of correct structures to the products formed by reaction of 5-perfluoroalkyl-3-phenyl-1,2,4-oxadiazoles with methylhydrazine.     

Unexpected Vulnerability of DPEphos to C−O Activation in the Presence of Nucleophilic Metal Hydrides

C−O bond activation of DPEphos occurs upon mild heating in the presence of [Ru(NHC)₂(PPh₃)₂H₂] (NHC=N-heterocyclic carbene) to form phosphinophenolate products. When NHC=IEt₂Me₂, C−O activation is accompanied by C−N activation of an NHC ligand to yield a coordinated N-phosphino-functionalised carbene. DFT calculations define a nucleophilic mechanism in which a hydride ligand attacks the aryl carbon of the DPEphos C−O bond. This is promoted by the strongly donating NHC ligands which render a trans dihydride intermediate featuring highly nucleophilic hydride ligands accessible. C−O bond activation also occurs upon heating cis-[Ru(DPEphos)₂H₂]. DFT calculations suggest this reaction is promoted by the steric encumbrance associated with two bulky DPEphos ligands. Our observations that facile degradation of the DPEphos ligand via C−O bond activation is possible under relatively mild reaction conditions has potential ramifications for the use of this ligand in high-temperature catalysis.     

Synthesis and analgesic‐antiinflammatory activities of ethyl 2‐[3‐(1‐phenoxy(methoxy)carbonyl‐4‐aryl‐(alkyl)‐1,4‐dihydropyridyl)]acetates

Reaction of ethyl 2‐(3‐pyridyl)acetate 4a or ethyl 2‐methyl‐2‐(3‐pyridyl)acetate 4b , with phenyl chloroformate or methyl chloroform ate, afforded the intermediate pyridinium salt 5 which undergoes regioselective nucleophilic attack at C‐4 upon reaction with a Grignard reagent in the presence of a cuprous iodide catalyst at ?23° to yield the corresponding ethyl 2‐[3‐(1‐phenoxy(methoxy)carbonyl‐4‐aryl(alkyl)‐1,4‐dihydropyridyl)]acetates 6a‐f in 64–96% chemical yield. No product arising from reaction of the ester substituent of the pyridinium salt 5 with the Grignard reagent was observed. The ¹H nmr spectra of 6a‐f exhibited dual resonances for the 1,4‐dihydropyridyl H‐2, H‐5 and H‐6 protons at 25° in deuteriochloroform. These dual resonaces were attributed to two different rotameric configurations resulting from restricted rotation about the nitrogen‐to‐carbonyl carbamate bond due to its double bond character. Compound 6 generally exhibited superior analgesic and antiinflammatory activities, compared to the reference drugs aspirin and ibuprofen, respectively. These structure‐activity correlations indicate the 1,4‐dihydropyridyl ring system present in 6 is a suitable bioisostere for the aryl (heteroaryl) ring present in aryl(heteroaryl)acetic acid non‐steroidal antiinflammatory drugs.     

Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6F5)3 Towards Carbene Formation

Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N₂ release. Recently, tris(pentafluorophenyl)borane [B(C₆F₅)₃] has been shown to be an alternative transition metal-free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α-aryl α-diazocarbonyl compounds using catalytic amounts of B(C₆F₅)₃ is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions.