Cycloadditions of adamantanethione S-methylide to CC multiple bonds

Adamantanethione S-methylide (12) is a nucleophilic 1,3-dipole which easily combines with electrophilic acetylenic and ethylenic bonds affording dihydrothiophene and thiolane derivatives, usually in high yields. The S-methylide 12, generated by extrusion of nitrogen from the 2,5-dihydro-1,3,4-thiadiazole 11, can not be isolated, but is intercepted in situ by dipolarophiles; otherwise, 12 furnishes irreversibly the spirothiirane 13. The ¹H nmr spectra and mass spectra establish the regiochemistry for the adducts of methyl propiolate, acrylonitrile, methyl acrylate, benzylidenemalononitrile and methyl α-cyanocinnamate. The 1,3-dipole does not react with common alkenes; the highly strained trans-cyclooctene gives rise to a cycloadduct.     

Three-membered ring formation reaction—III : Mechanism of the reaction of α-halogenoacrylic ester with organozinc compounds

The stereochemistry of ring formation in the reaction of α-halogenoacrylic ester with organozinc compounds was studied using β-deuterated α-halogenoacrylic ester. A quantitative study was made on the steric course of every step of the reactions involved in the synthesis of methyl β-deuterio-α-bromoacrylate-cis-d₁ starting from methyl propiolate. The mode of the CC double bond opening of methyl β-deuterio-α-bromoacrylate-cis-_d1 to form dimethyl 1 -bromo-2-propyl-cis-1,2- cyclopropanedicarboxylate-d₂ was confirmed to be cis and trans in a 50 to 50 ratio. Asymmetric syntheses for the cyclopropanedicarboxylic ester were possible, especially under the influence of chiral organozinc alkoxide system. A stepwise mechanism was postulated for the ring formation reaction.     

Insertion Reactions of [ReH(CO)5–n(PMe3)n] Complexes (n = 2–4) with aldehydes,CO2, and activated acetylenes

The complexes of the type [ReH(CO)_5–n(PMe₃)_n] (n = 4, 3) were reacted with aldehydes, CO₂, and RC?CCOOMe (R = H, Me) to establish a phosphine-substitutional effect on the reactivity of the Re–H bond. In the series 1–3 , benzaldehyde showed conversion with only 3 to afford a (benzyloxy)carbonyltetrakis(trimethylphosphine)rhenium complex 4 . Pyridine-2-carbaldehyde allowed reaction with all hydrides 1–3 . With 1 and 2 , the same dicarbonyl[(pyridin-2-yl)methoxy-O, N]bis(trimethylphosphine)rhenium 5b was formed with the intermediacy of a [(pyridin-2-yl)methoxy-O]-ligated species and extrusion of CO or PMe₃, respectively. The analogous conversion of 3 afforded the carbonyl[(pyridin-2-yl)methoxy-O,N]tris(trimethylphosphine)rhenium ( 1 ) 7b . While 1 did not react with CO₂, 2 and 3 yielded under relatively mild conditions the formato-ligated [Re(HCO₂)(CO)(L)(PMe₃)₃] species ( 8 (L = CO) and 9 (L = PMe₃)). Methyl propiolate and methyl butynoate were transformed, in the presence of 1 , to [Re{C(CO₂Me)?CHR}(CO)₃(PMe₃)₂] systems ( 10a (R = H), and 10b (R = Me)), with prevailing α-metallation and trans-insertion stereochemistry. Similarly, HC≡CCO₂Me afforded with 2 and 3 , the α-metallation products [Re{C(CO₂Me)?CH₂}(CO)(L)(PMe₃)₃] 11 (L = CO) and 12 (L = PMe₃). The methyl butyonate insertion into 2 resulted in formation of a mixture of the (Z)- and (E)-isomers of [Re{C(CO₂Me)?CHMe} (CO)₂(PMe₃)₃] ( 13a , b ). In the case of the conversion of 3 with MeC?CCO₂Me, a Re–H cis-addition product [Re{(E)-C(CO₂Me)?CHMe}(CO)(PMe₃)₄] ( 14 ) was selectively obtained. Complex 11 was characterized by an X-ray crystal-structure analysis.     

The Interaction of Thioethynyl Esters of Thiophosphoric Acids with Cytochrom P 450

Abstract

We have shown that thioesters of phosphoric acids of general formula (RO)₂P(X)C[tbnd]CR¹, were X=O or S, containing an acetylenic bond in α-position of the S-ester group has unugually high destructive effect on some isoforms of cytochrome P 450 of arthropods and mammals. These isoforms probably, participate in the process of inactivation of the acetylenic thioesters. We assume that this process may be thought as being a rupture of rhe P-S bond, which leads to the formation of killer particles - alkylthioketenes.     

N‐Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns

Depending on the amount of methanol present in solution, CO₂ adducts of N‐heterocyclic carbenes (NHCs) and N‐heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][OCO₂Me] and [EMMIm][OCO₂Me]. The reactivity pattern of representative 1‐ethyl‐3‐methyl‐NHO–CO₂ adduct 4 has been investigated and compared with the corresponding NHC–CO₂ zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO–CO₂H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO–CO₂ zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]₂ cyanomalonate salts formed exemplify the first C?C bond‐forming carboxylation reactions with NHO‐activated CO₂. The reaction of the free NHO with dimethyl carbonate selectively led to methoxycarbonylated NHO, which is a perfect precursor for the synthesis of functionalized ILs [NHO–CO₂Me][X]. The first NHO‐SO₂ adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol.     

Synthesis of tri-substituted vinyl boronates via ruthenium-catalyzed olefin cross-metathesis

Tri-substituted vinyl pinacol boronates, which are key reactive intermediates in a variety of transformations, are synthesized using ruthenium-catalyzed olefin cross-metathesis of α-substituted vinyl boronates and various alkenes. Cross-metathesis of 2-isopropenyl pinacol boronate proceeds with moderate yield and high Z-selectivity when sterically unhindered cross partners are used. Cross-metathesis of vinyl boronates that possess α-substitution larger than a methyl group is also achieved. Yield and Z-selectivity are lower in these cases, and the success of a cross-metathesis reaction is highly dependent on the steric bulk surrounding the double bond of the α-substituted vinyl boronate.     

Regiospecific insertion into the Co–Co bond of the mixed-metal cluster Co2Rh2(CO)12 by an electron-poor alkyne: X-ray diffraction structure of the butterfly cluster Co2Rh2(CO)10(μ-HCCCO2Me)

The reaction between the mixed-metal tetrahedral cluster Co₂Rh₂(CO)₁₂ (1) and the electron-poor alkyne methyl propiolate in hexane at room temperature furnishes a mixture of products consisting of Co₃Rh(CO)₁₂ (2), Co₃Rh(CO)₁₀(μ-HCCCO₂Me) (3), Co₂Rh₂(CO)₁₀(μ-HCCCO₂Me) (4), and CoRh₃(CO)₉(μ-HCCCO₂Me)₃ (5). The isolation and solution spectroscopic data of these compounds are described, and the solid-state structure of Co₂Rh₂(CO)₁₀(μ-HCCCO₂Me) determined by X-ray diffraction analysis. The title cluster crystallizes in the triclinic space group. The solid-state structure of Co₂Rh₂(CO)₁₀(μ-HCCCO₂Me) provides proof for the regiospecific insertion of the methyl propiolate ligand into the Co–Co bond of the starting cluster Co₂Rh₂(CO)₁₂. The stability of clusters 3 and 4 in the presence of added methyl propiolate is discussed.     

Oxidative Methoxycarbonylation of Propyne and Allene with Carbon Monoxide and Methanol in the Presence of Copper-palladium Catalyst

A direct oxidative methoxycarbonylation of propyne with carbon monoxide in methanol medium in the presence of copper-palladium catalytic system results in methyl 2-butynoate in 18-31% yield. Depending on reaction conditions allene provides either a mixture of methyl 2-(chloromethyl)acrylate and methyl 2-(methoxymethyl)acrylate (3-4:1) in overall yield 16-23%, or methyl 2-(methoxymethyl)acrylate in 19% yield.     

Shedding Light on the Diverse Reactivity of NacNacAl with N-Heterocycles

The aluminum(I) compound NacNacAl (NacNac=[ArNC(Me)CHC(Me)NAr]⁻, Ar=2,6-iPr₂C₆H₃, 1 ) shows diverse and substrate-controlled reactivity in reactions with N-heterocycles. 4-Dimethylaminopyridine (DMAP), a basic substrate in which the 4-position is blocked, induces rearrangement of NacNacAl by shifting a hydrogen atom from the methyl group of the NacNac backbone to the aluminum center. In contrast, C−H activation of the methyl group of 4-picoline takes place to produce a species with a reactive terminal methylene. Reaction of 1 with 3,5-lutidine results in the first example of an uncatalyzed, room-temperature cleavage of an sp² C−H bond (in the 4-position) by an Al^I species. Another reactivity mode was observed for quinoline, which undergoes 2,2′-coupling. Finally, the reaction of 1 with phthalazine produces the product of N−N bond cleavage.     

Electronic and Steric Structure of the Methanofullerene C61(CO2Me)[P(O)(OMe)2]. A Density Functional Study

The asymmetric addend in the methanofullerene C₆₁(CO₂Me)[P(O)(OMe)₂] polarizes and divides the fullerene shell into four nonequivalent fragments. According to DFT/PBE calculations, the most stable conformers of the methanofullerene C₆₁(CO₂Me)[P(O)(OMe)₂] involve Coulomb interactions of the phosphoryl oxygen with one of the fullerene carbon atoms, which produces polarization of the corresponding fragment and asymmetry in bond lengths and atomic charges in the fullerene shell. Alternation and attenuation of changes in bond lengths along the conjugation branches was revealed.     

Exploration of synthetic strategies for the stereoselective preparation of novel tetrahydrofuran-containing biaryls: A high-pressure promoted Diels-Alder approach

Three stereoselective synthetic approaches to tetrahydrofuran-containing biaryl scaffolds are described. All approaches involve a high-pressure promoted Diels-Alder reaction of substituted diene with methyl propiolate to give, after aromatization, the corresponding biaryl. The tetrahydrofuran moiety can be created starting from aryl-Br or aryl-CO₂Me functional groups through a γ-phenylseleno ketone intermediate.     

Electronic control of the regiochemistry in palladium-phosphine catalyzed intermolecular Heck reactions

Density functional theory calculations of the transition-state structures and reaction barriers for the C-C coupling between monosubstituted eta(2)-olefins and eta(1)-vinyl for neutral [PdI(PH(3))(vinyl)(RCHCH(2))] and cationic [Pd(H(2)PCH(2)PH(2))(vinyl)(RCHCH(2))](+) (R = OMe, Me, and CN) depend mostly on the regiochemistry and not on the starting position of the olefin substituent. The regiochemistry is thus implicit in the electronic structure of the precursor complex. A selectivity index, Omega, based on electrostatic and frontier orbital interactions gives a good correlation with experiment for vinylations or arylations. The model correctly predicts that the regiochemistry for R = OMe, Me, and CN is the same for both neutral and cationic Pd complexes while for R = CH(2)OH the regiochemistry reverses. The latter is confirmed by explicit calculations of the transition-state energies. Selectivity indices are computed for 13 substituents: CO(2)Me, CN, CF(3), Ph, H, Me, CH(2)OH, CH(2)NMe(2), 2-pyrolidone, CH(2)SiMe(3), OAc, OMe, and F. Cationic conditions systematically give larger Omega values and thus tend to favor coupling at the alpha carbon on the olefin. The Omega values are approximately additive and can be used to predict the regiochemistry for disubstituted olefins.     

Synthesis of δ-cyclohexyl- and δ,δ-alkylene-α,α-dicarbonyl-substituted dienes and study of their valence isomerization

β-Cyclohexylacrolein, β-cyclohexylmethacrolein, or α-cycloalkylidenalkanals were condensed with methyl acetoacetate or dimethyl malonate to give the δ-cyclohexyl- and δ,δ-alkylene-substituted α,α-dicarbonyl-containing α,β∶γ,δ-dienes. The structures of the reaction products were studied using¹H NMR,¹³C NMR, and UV spectroscopy. The diene keto esters bearing no substituents at the γ-position were shown to be in fact three-component equilibrium mixtures comprised ofE- andZ-isomers of the diene (at the α,β bond) and a corresponding 2H-pyran. On the other hand, for keto esters with a Me group at the γ-position the equilibrium is shifted entirely to the 2H-pyrans. In contrast with the keto esters, dienic diesters exist only in the open form.     

The concerted addition of HBr to aryl alkynes; orthogonal pi bond selectivity

In a weakly acidic solution, the addition of HBr to 1-phenylprop-1-yne produces predominantly the anti-Markovnikov product. In this paper, we consider five possible explanations for this behavior and conclude that the concerted addition is occurring on the acetylenic pi bond orthogonal to the extended aromatic pi system. The electronic effect of the distal methyl group and the steric hindrance of the coplanar phenyl ring combine to promote bromide attack at the beta carbon. Attack on this pi bond is insensitive to the electronic effect of meta and para substituents on the ring but is very (sterically) sensitive toward all ortho substituents.     

Electrophilic cleavage of the iron-carbon sigma bond: Mechanism and the stereochemistry at iron

Electrophilic cleavage of the iron-carbon sigma bond in the resolved chiral complexes CpFE(CO)(PPh₃)R (where R = Me, Et, CH₂CO₂ (menthyl), and CH₂O (menthyl)) by I₂, ICl, and HgI₂ reveals that the reaction proceeds with net retention of configuration at iron, but stereospecifity is not high and is insensitive to the nature of the alkyl group. In every case, recovered starting material is partially racemized, but the source of this is not yet certain. Formation of CpFe(CO)(PPh₃)I from cleavages using ICl indicates that initial electrophilic attack is at the metal. A mechanism is discussed which accounts for net retention at metal in terms of steric and ion pairing rather than electronic effects. Cleavage using HO₂CCF₃ forms CpFe(CO)(PPh₃)O₂CCF₃ of ca. 50% enantiomeric excess as shown by the use of an optically active Eu NMR shift reagent, but net retention or inversion at iron cannot be inferred.     

Studies in organic mass spectrometry—V: N-arylsulphonyliminopyridinium betaines

The mass spectral fragmentation of N-arylsulphonyliminopyridinium betaines has been rationalized using high resolution mass spectrometry as well as deuterium labelling. The most characteristic features are a very facile N S bond cleavage and skeletal rearrangements accompanied by the expulsion of SO₂ from the molecular ion and the [M 1] ion to the corresponding ionized N-aryliminopyridinium betaines and azacarbazoles, respectively. The presence of methyl substituents at the α-position of the pyridine ring has a significant effect on the mode of the fragmentation.     

Structure effect of benzofuranone on the antioxidant activity in polypropylene

This work deals with the antioxidant activity of benzofuranone compounds in polypropylene (PP). The antioxidant activities of ten benzofuranone compounds in PP were compared using melt flow index (MFI) values of PP stabilized by benzofuranone compounds primarily. The results show: firstly, that the increase of electron donating ability of substituent in 3-substituted benzene ring is beneficial to the improvement of antioxidant activity. Secondly, it has been verified that the steric hindrance of 2′-position substituent can weaken the antioxidant activity of benzofuranone. But when 2′-position substituent forms a hydrogen bond with 3-position reactive hydrogen, the steric hindrance is offset efficiently. Finally, the methyl and tert-butyl groups in the 5 and 7-position of parent benzene ring do not affect the antioxidant activity of benzofuranone compounds in PP obviously.     

Metal‐Free Reduction of CO2 with Hydroboranes: Two Efficient Pathways at Play for the Reduction of CO2 to Methanol

Guanidines and amidines prove to be highly efficient metal‐free catalysts for the reduction of CO₂ to methanol with hydroboranes such as 9‐borabicyclo[3.3.1]nonane (9‐BBN) and catecholborane (catBH). Nitrogen bases, such as 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD), 7‐methyl‐1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (Me‐TBD), and 1,8‐diazabicycloundec‐7‐ene (DBU), are active catalysts for this transformation and Me‐TBD can catalyze the reduction of CO₂ to methoxyborane at room temperature with TONs and TOFs of up to 648 and 33 h^?1 (25 °C), respectively. Formate HCOOBR₂ and acetal H₂C(OBR₂)₂ derivatives have been identified as reaction intermediates in the reduction of CO₂ with R₂BH, and the first C?H‐bond formation is rate determining. Experimental and computational investigations show that TBD and Me‐TBD follow distinct mechanisms. The N?H bond of TBD is reactive toward dehydrocoupling with 9‐BBN and affords a novel frustrated Lewis pair (FLP) that can activate a CO₂ molecule and form the stable adduct 2 , which is the catalytically active species and can facilitate the hydride transfer from the boron to the carbon atoms. In contrast, Me‐TBD promotes the reduction of CO₂ through the activation of the hydroborane reagent. Detailed DFT calculations have shown that the computed energy barriers for the two mechanisms are consistent with the experimental findings and account for the reactivity of the different boron reductants.     

Synthesis of Ψ[CH(RF)NH]Gly-peptides: The dramatic effect of a single fluorine atom on the diastereocontrol of the key aza-Michael reaction

We describe in full-detail the synthesis of new ψ[CH(R_F)NH]-peptidomimetics, having different fluoroalkyl groups R_F, as peptide bond surrogates. A key step in the synthesis is a stereoselective aza-Michael addition of chiral α-amino acid esters to β-fluoroalkyl-α-nitroethenes. The diastereoselection of the process was influenced by the electronegativity, rather than by the steric bulk, of the fluorinated residue R_F in the β-position of the nitroalkene acceptors. Replacement of a single F atom of R_F by a hydrogen or methyl group brings about a dramatic drop of stereocontrol, whereas Br, Cl and CF₃, albeit bulkier than F, provide inferior results in terms of stereocontrol. A mechanistic hypothesis is provided.     

Regiochemical switching in Diels-Alder cycloadditions by change in oxidation state of removable diene sulfur substituents. Synthesis of carbazoles by sequential heteroannulation and Diels-Alder cycloaddition

The palladium-catalyzed heteroannulation of N-carbobenzyloxy-o-iodoanilines with 1-phenylthio-1,3-butadiene afforded indolines 7, which were oxidized with DDQ to produce vinylogous 2-(phenylthio)indoles 8. The latter compounds underwent highly regioselective Diels-Alder cycloadditions with methyl propiolate in the presence of MeAlCl(2) or AlCl(3), with simultaneous elimination of benzenethiol, to afford methyl N-(carbobenzyloxy)carbazole-3-carboxylates 9 and, in some cases, the N-deprotected derivatives 11. This is the opposite regiochemistry of that observed previously with the corresponding sulfone analogues of 8. Thus, the regiochemistry of the cycloaddition can be effectively controlled by appropriate choice of oxidation state of the diene sulfur substituent.