Novel ring transformations of pyrazines by intramolecular diels-alder reactions

Pyrazines carrying the ω-alkyne side-chain -XCH₂CH₂C≡CH, (X = 0,N,S,SO,SO₂, C(CN)₂) undergo on heating an intramolecular Diels-Alder reaction. Pyrazines with the electron donating atom (0,N or S) in the side-chain afford [c]-fused pyridines as main products, whereas (3-buty-nylsulfinyl)pyrazine and (3-butynylsulfonyl)pyrazine are exclusively converted into [b]-fused pyridines. A [b]-fused pyridine is also the major product in the reaction of 2,5-bis(1,1-dicyano-4-pentynyl)pyrazine.     

Stereoselectivity of the Diels-Alder Reaction of (E)-γ-Oxo-α,β-Unsaturated Thioesters with Cyclopentadiene

The stereoselectivity of the Diels-Alder reaction of (E)-γ-oxo-α,β-unsaturated thioesters 3a-3d with cyclopentadiene is greatly enhanced in the presence of Lewis acids favoring the endo acyl isomers 4a-4d . In the absence of Lewis acid, Diels-Alder reaction of 3a-3d with cyclopentadiene at 25 °C gave two adducts 4a-4d and 5a-5d in a ratio of 1:1 respectively. In the presence of Lewis acids, Diels-Alder reaction of 3a-3d with cyclopentadiene gave 4a-4d and 5a-5d in ratios of 75-94:25-6 respectively. The stereoelectivity was enhanced to ratios of 95-98:5-2 with lowering the reaction temperature. The stereochemistry of the cycloadducts 4 and 5 was confirmed by iodocyclization. Reaction of the endo-thioester 5c with I₂ in aqueous THF at 0 °C gave the novel methylthio group rearranged product 6c in 80% yield, the first example of iodo-lactonization of endo-thioesters. Reaction of the endo-acyl isomer 4b with I₂ under the same reaction conditions gave an isomeric mixture of 7b and 8b in 1:2 ratio. The stereochemistry of the thioester group in 8b was proved by X-ray single-crystal analysis. The solvent effect on the endo selectivity of (Z)-γ-oxo-α,β-unsaturated thioester 2b was also examined.     

Homo‐ and copolymerization of norbornene derivatives with ethene by ansa‐fluorenylamidodimethyltitanium activated with methylaluminoxane

(t‐BuNSiMe₂Flu)TiMe₂ ( 1 ) activated with Me₃Al‐free methylaluminoxane (dried MAO) which conducts vinyl addition polymerization of norbornene (N) with very high activity was applied for homopolymerization of N derivatives (i.e., 5‐vinyl‐2‐norbornene (5V2N), 5‐ethylidene‐2‐norbornene (5E2N), dicyclopentadiene (DCPD)) at 40 °C. The activities for the N derivatives were about two orders of magnitude lower than that for N and decreased in the following order: 5E2N ? 5V2N ? DCPD. Copolymerization of ethene (E) and 5E2N under an atmospheric pressure of E was then conducted by 1 ‐dried MAO. The copolymerization proceeded with better activity than the homopolymerization of 5E2N and gave poly(E‐co‐5E2N) with narrow molecular weight distribution. The content of the ethylidene group in poly(E‐co‐5E2N) was controlled by the feed ratio of 5E2N/E. The T_g value of the copolymer changed from 70 °C to 155 °C according to the 5E2N content from 27 mol % to 68 mol %. The addition of N as a third monomer to the E‐5E2N copolymerization improved the activity and raised the T_g values of the terpolymer above 200 °C. The content of 5E2N was controlled by the 5E2N/N ratio with keeping the high T_g values. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4581–4587, 2007     

Synthesis and Reactions of a Stable Precursor to Dienes Containing Both Silicon and Sulfur Substituents

3-(Phenylthio)-4-(trimethylsilyl)-3-sulfolene (2) was readily prepared by chlorosulphenylation-dehydrochlorination of 3-(trimethylsilyl)-3-sulfolene (4b) . Treatment of 2 with n-butyllithium at ?105 °C followed by an alkylating agent gave only C5 alkylation products 6 demonstrating the stronger carbanion stabilizing ability of phenylthio group than that of trimethylsilyl group. 2-(Phenylthio)-3(trimethylsilyl)-1,3-butadiene (3a) was readily prepared by thermal extrusion of sulfur dioxide from 2 . Selective oxidation of 3a with MCPBA gave the sulfinyl (3b) and sulfonyl (3c) derivatives. The Diels-Alder reactions of 3a-c were studied, and the regiocontrolling power of the substituents follows the order PhS >PhSO ～ PhSO₂ >Me₃Si.     

Cycloaddition de derives organiques insatures du silicium,du germanium et de l'etain

The effect of atom M and of its substituents Σ upon dienophilic activity of Σ₃MCHCH₂ and Σ₃MCCH (MC, Si, Ge, Sn) is interpreted in terms of a perturbation scheme involving the lowest unoccupied orbital of the dienophile and the highest occupied orbital of the diene. This scheme also explains the stereochemistry of Diels-Alder reactions and the regioselectivity of the ene reaction with organometallic compounds.     

Reaktionen an Isocyanidverbindungen

By the reaction of FSO₂N?PCl₃ with perfluorpropionic acid FSO₂NHC(O)C₂F₅ is formed, which yields FSO₂N?C(Cl)? C₂F₅ (I) with PCl₅. The chlorine atom in (I) could be replaced by the substituents NH₂ (II) and N(C₂H₅)₂ (III). FSO₂N?C(Cl)? CF₃ reacts with AgOCN, AgSCN, unhydrous HF and 2,3-dimethylbutadiene. FSO₂N(CH₃)? C(O)F reacts with elemental fluorine under exchange of a proton against a fluorine atom to give FSO₂N(CH₂F)? C(O)F, which liberates at room temperature COF₂ and trimerises to form 1,3,5-Tris-fluorosulfonyl-s-triazine (VIII). The amides FSO₂N?C(CH₃)NH₂ and FSO₂N?C(CF₃)NH₂ react with SF₄ in the presence of NaF to yield the iminosulfur difluorides FSO₂N?C(CH₃)? NSF₂ (IX) and FSO₂N?C(CF₃)? NSF₂ (X)     

Prostaglandin synthesis from a fulvene with the ω-side chain equivalent

The acid-catalyzed hydrolysis of the Diels-Alder adduct of 6-(2,2-ethylene-dioxyheptyl) fulvene with 2-chloroacrylonitrile gave 7-anti-(3-oxo-1-trans-octenyl)-2-chloro-2-cyanobicyclo[2.2.1]hept-5-ene, which is an intermediate of 12-epi-prostaglandine. On the other hand, the hydrolysis of 7-(3,3-ethylenedioxyoctylidene)bicyclo-[2.2.1]hept-5-en-2-one, obtained from the Diels-Alder adduct of the fulvene with 2-chloroacryloyl chloride, gave a mixture (3:2) of 7-anti- and 7-syn-(3-oxo-1-trans-octenyl)bicyclo[2.2.1]hept-5-en-2-one. The latter compound was oonverted into prostaglandin F_2α.     

Diels-Alder adducts of 4-chloro-1,6-dihydrophosphinine derivatives: A new precursor of 2-phosphapropene

The 4-chloro-1,6-dihydrophosphinine derivatives, prepared as a mixture of 3- and 5-methyl isomers from the thermolysis of dichlorocarbene adducts with 1-R-3-methylphospholene oxide (R  Me, MeO, EtO, n-PrO, i-PrO), participate in Diels-Alder reactions with dimethyl acetylenedicarboxylate and N-phenylmaleimide. The former reactant is of special value since the 2-phosphabicyclo[2.2.2]octa-5,7-diene framework is thermally labile and undergoes retrocycloaddition by a different path, eliminating the C P bridging unit as a low-coordinate species. Thus, in the case of R  Me, the initial phosphine oxides (a mixture of isomers) have been reduced to the phosphines and these undergo straightforward fragmentations on heating at 50°C to produce 2-phosphapropene. The reactions of this transient species with water and alcohols have been investigated. Unlike the case of stabilized phosphaalkenes, hydration occurs smoothly without catalysis to produce Me₂P(O)H; addition of alcohols gives phosphinites, Me₂POR. The structure of the Diels-Alder adducts was confirmed by ³¹P, ¹H, and ¹³C NMR spectroscopy. Each of the isomeric 1,6-dihydrophosphinine oxides gives two diastereomeric adducts; partial separation of some of the resulting four-component mixtures was achieved with silica gel chromatography.     

The time dependence of the competing processes leading to the loss of C2H4 from tetralin

Field ionization kinetic measurements indicate that loss of C₂H₄ from the molecular ion of tetralin occurs via competing processes, viz. (a) at short times the retro Diels-Alder decomposition which is a high energy process, (b) loss from positions 1+2 (=3+4) and (c) loss after complete C and H scrambling which gains in importance for long ion lifetimes.     

Synthesis of the carbocyclic skeleton of abyssomicins C and D

[reaction: see text] Intramolecular Diels-Alder substrate trienyl methylenebutenolide 5 was prepared in six steps by coupling 3-methoxy-4-methylenebutenolide (6) with trienone keto aldehyde 7. Heating 5 in CHCl(3) for 2 d at 70 degrees C afforded 80% of a single Diels-Alder adduct 4 with the complete carbon skeleton of abyssomicin C. Addition of thiophenoxide to the enone double bond of 4 followed by an intramolecular Michael addition afforded 15 with the abyssomicin D carbon skeleton.     

Cycloaddition reactions of β-trifluoroacetylvinyl ethers

β-Trifluoroacetyl vinyl ethers ROCHCHCOCF₃ react smoothly as dienophiles with α,β-unsaturated carbonyl compounds to give the unexpected 2-alkoxyl-5-trifluoroacetyl-3,4-dihydro-2H pyrans. These products are formed by elimination and addition of the alcohol to the products of the normal hetero Diels-Alder reaction (2-alkoxyl-3-trifluoroacetyl-2,3-dihydro-2H pyrans). In contrast, 1,3-dipolar cycloaddition of ROCHCHCOCF₃ with ArCHN(O)Me proceeds via a Z-endo transition state to give regio- and stereospecific 4-trifluoroacetyl substituted isoxazolidines and their derivatives.     

Acyl- und Alkylidenphosphane. XXIV. (N,N-Dimethylthiocarbamoyl)trimethylsilylphosphane und 1,2-Di(tert-butyl)-3-dimethylamino-1-thio-4-trimethylsilylsulfano-1λ5,2λ3-diphosphet-3-en

Acyl- and Alkylidenephosphines. XXIV. (N,N-Dimethylthiocarbamoyl)trimethylsilyl-phosphines and 1.2-Di(tert-butyl)-3-dimethylamino-1-thio-4-trimethylsilylsulfano-1λ⁵, 2λ³-diphosphet-3-ene In contrast to bis(trimethylsilyl)phosphines R? P[? Si(CH₃)₃]₂ 1 {R ? H₃C a ; (H₃C)₃C b ; H₅H₆ c ; H₁₁C₉ d ; (H₃C)₃Si e }, the more nucleophilic lithium trimethylsilylphosphides 4 react with N,N-dimethylthiocarbamoyl chloride already at ?78°C to give (N,N-dimethylthiocarbamoyl)trimethylsilylphosphines 2 . Working up the reaction, a dismutation of the mesityl derivative 2d is observed, whereas the tert-butyl compound 2b dissolved in toluene, eliminates dimethyl(trimethylsilyl)amine to form 1,2-di(tert-butyl)-3-dimethylamino-1-thio-4-trimethylsilyl-sulfano- 1λ⁵, 2λ³-diphosphet-3-ene 6b , nearly quantitatively within several days at +20°C.     

1,3,4‐Triphenyl‐7‐trifluoromethyl‐1H‐pyrazolo[3,4‐b]quinoline at 293 and 100 K

In the structure of the title compound, C₂₉H₁₈F₃N₃, belonging to the space group P6₅ (or P6₁), three symmetry‐independent molecules are arranged in two chains, with two molecules alternating along the 3₂ axes, whereas the remaining molecule forms a chain along [0001] due to the 6₅ screw axis. The conformation of each of the molecules is stabilized by an intramolecular C—H...N hydrogen bond, with C...N distances in the range 2.964 (6)–3.069 (5) Å at room temperature (293 K) and 2.943 (4)–3.084 (4) Å at low temperature (100 K). One molecule has its –CF₃ group ordered even at 293 K, which can be explained only by considering its involvement in two weak intermolecular C—H...F interactions, with C...F distances in the range 3.084 (6)–3.302 (5) Å at 293 K and 3.070 (3)–3.196 (3) Å at 100 K, and also a C—F...N interaction, with a C...N distance of 3.823 (5) Å at 293 K and 3.722 (4) Å at 100 K. The trifluoromethyl groups in the two remaining molecules are disordered at 293 K, whereas at 100 K the continuous (dynamic) positional disorder of one of the –CF₃ groups (of the molecule forming the chain along [0001]) is totally eliminated while the –CF₃ group disorder remains for the third molecule.     

Planarity of benzoylhydrazine–dithiocarbazoate tuberculostatics. I. N′‐Methyl‐substituted 3,4‐dichlorobenzoyl monoesters

Methyl 2‐(3,4‐dichlorobenzoyl)‐1‐methylhydrazinecarbodithioate, C₁₀H₁₀Cl₂N₂OS₂, (F1), butyl 2‐(3,4‐dichlorobenzoyl)‐1‐methylhydrazinecarbodithioate, C₁₃H₁₆Cl₂N₂OS₂, (F2), and 3,4‐dichloro‐N‐(2‐sulfanylidene‐1,3‐thiazinan‐3‐yl)benzamide, C₁₁H₁₀Cl₂N₂OS₂, (F3), were studied by X‐ray diffraction to test our hypothesis that planarity of aryloylhydrazinedithiocarbazic acid esters is a prerequisite for tuberculostatic activity. All compounds examined in this study are inactive and nonplanar due to twists along two specific bonds in the central frame of the molecules. The significant twist at the N—N bond, with an C—N—N—C(S) torsion angle of about 85°, results from repulsion caused by a methyl substituent at the N′ atom of the hydrazide group. The other twist is that within the benzoyl group at the C(O)—Ph bond, i.e. the N—C(=O)—C(phenyl)—C torsion angle: the values found in the studied structures (25–30°) are in agreement with those observed in other compounds containing a similar fragment. As some nonplanar benzoyl derivatives are active, it seems that planarity of the hydrazinedithioate fragment is more important for tuberculostatic activity than planarity of the aryloyl group.     

S‐Doping of an Fe/N/C ORR Catalyst for Polymer Electrolyte Membrane Fuel Cells with High Power Density

Fe/N/C is a promising non‐Pt electrocatalyst for the oxygen reduction reaction (ORR), but its catalytic activity is considerably inferior to that of Pt in acidic medium, the environment of polymer electrolyte membrane fuel cells (PEMFCs). An improved Fe/N/C catalyst (denoted as Fe/N/C‐SCN) derived from Fe(SCN)₃, poly‐m‐phenylenediamine, and carbon black is presented. The advantage of using Fe(SCN)₃ as iron source is that the obtained catalyst has a high level of S doping and high surface area, and thus exhibits excellent ORR activity (23 A g^?1 at 0.80 V) in 0.1 M H₂SO₄ solution. When the Fe/N/C‐SCN was applied in a PEMFC as cathode catalyst, the maximal power density could exceed 1 W cm^?2.     

Titanium and Zirconium Complexes with Non‐Salicylaldimine‐Type Imine–Phenoxy Chelate Ligands: Syntheses,Structures, and Ethylene‐Polymerization Behavior

New Ti and Zr complexes that bear imine–phenoxy chelate ligands, [{2,4‐di‐tBu‐6‐(RCH=N)‐C₆H₄O}₂MCl₂] ( 1 : M=Ti, R=Ph; 2 : M=Ti, R=C₆F₅; 3 : M=Zr, R=Ph; 4 : M=Zr, R=C₆F₅), were synthesized and investigated as precatalysts for ethylene polymerization. ¹H NMR spectroscopy suggests that these complexes exist as mixtures of structural isomers. X‐ray crystallographic analysis of the adduct 1 ?HCl reveals that it exists as a zwitterionic complex in which H and Cl are situated in close proximity to one of the imine nitrogen atoms and the central metal, respectively. The X‐ray molecular structure also indicates that one imine phenoxy group with the syn C?N configuration functions as a bidentate ligand, whereas the other, of the anti C?N form, acts as a monodentate phenoxy ligand. Although Zr complexes 3 and 4 with methylaluminoxane (MAO) or [Ph₃C]⁺[B(C₆F₅)₄]^?/AliBu₃ displayed moderate activity, the Ti congeners 1 and 2 , in association with an appropriate activator, catalyzed ethylene polymerization with high efficiency. Upon activation with MAO at 25 °C, 2 displayed a very high activity of 19900 (kg PE) (mol Ti)^?1 h^?1, which is comparable to that for [Cp₂TiCl₂] and [Cp₂ZrCl₂], although increasing the polymerization temperature did result in a marked decrease in activity. Complex 2 contains a C₆F₅ group on the imine nitrogen atom and mediated nonliving‐type polymerization, unlike the corresponding salicylaldimine‐type complex. Conversely, with [Ph₃C]⁺[B(C₆F₅)₄]^?/AliBu₃ activation, 1 exhibited enhanced activity as the temperature was increased (25–75 °C) and maintained very high activity for 60 min at 75 °C (18740 (kg PE) (mol Ti)^?1 h^?1). ¹H NMR spectroscopic studies of the reaction suggest that this thermally robust catalyst system generates an amine–phenoxy complex as the catalytically active species. The combinations 1 /[Ph₃C]⁺[B(C₆F₅)₄]^?/AliBu₃ and 2 /MAO also worked as high‐activity catalysts for the copolymerization of ethylene and propylene.     

Controlling silica surfaces using responsive coupling agents

The surface chemistry of silica was modified using coupling agents capable of participating in oxidation or in the Diels-Alder (and retro Diels-Alder) reactions. The synthesis of the latter coupling agents, using trialkoxysilane groups linked to a cyclopentadiene structure, was achieved by the condensation of sodium cyclopentadienolide with 1-chlorodimethylsilyl-3-triethoxysilylpropane to give 1-cyclopentadienyldimethylsilyl-3-trialkoxysilylpropane. The cyclopentadiene ring in this structure was shown to undergo normal Diels-Alder chemistry with maleimides or maleic anhydride to give 7-(dimethylsilylpropyltrialkoxysilane)-5-norbornene-2,3-dicarboxylic acid anhydride. The retro Diels-Alder reaction of 7-(dimethylsilylpropyltrialkoxysilane)-5-norbornene-2,3-dicarboxylic acid anhydride in solution was not very efficient: the adduct is very stable and only undergoes the retro Diels-Alder reaction at temperatures in excess of 200 °C. Once grafted to the surface, however, the retro Diels-Alder reaction was achieved at a level greater than 90% by use of thermolysis in the presence of free cyclopentadiene. In addition, a polyene-based coupling agent derived from squalene was prepared by hydrosilylation using HMe₂SiOSiMe₂CH₂CH₂Si(OEt)₃. Once grafted to the surface, oxidation by ozone led to ozonides that could be reduced to ketone/aldehyde groups. These in turn could be trapped by functional groups such as hydrazines to make surface-bound hydrazones. With both types of coupling agents, the surface energy and nature of the functional groups bound to the surface could be changed on demand in response to an external stimulus.     

Asymmetric Diels-Alder and Ficini reactions with alkylidene β-ketoesters catalyzed by chiral ruthenium PNNP complexes: mechanistic insight

Hydride abstraction from the β-position of the enolato ligand of the previously reported complex [Ru(3a-H)(PNNP)]PF(6) (5a; 3a-H is the enolate of 2-tert-butoxycarbonylcyclopentanone) with (Ph(3)C)PF(6) gives the dicationic complex [Ru(6a)(PNNP)](2+) (7a) as a single diastereoisomer, which contains the unsaturated β-ketoester 2-tert-butoxycarbonyl-2-cyclopenten-1-one (6a) as a chelating ligand. The methyl analogue 2-methoxycarbonylcyclopentanone (3b) gives [Ru(3b-H)(PNNP)]PF(6) as a mixture of noninterconverting diastereoisomers (ester group of 3b trans to P, 5b; or to N, 5c), which were separated by column chromatography. Hydride abstraction from 5b (or 5c) yields diastereomerically pure [Ru(6b)(PNNP)](2+) (7b or 7c). Complexes 7b and 7c do not interconvert at room temperature in CD(2)Cl(2) and form opposite enantiomers of the Diels-Alder adduct upon reaction with Dane's diene (1 equiv). X-ray studies of 7a, 5b, and 5c give insight into the origin of enantioselection and the sense of asymmetric induction in the previously reported asymmetric Diels-Alder and Ficini cycloaddition reactions with 2,3-disubstituted butadienes and ynamides, respectively. Stoichiometric reactions (substrate coordination, cycloaddition, and product displacement) between [Ru(OEt(2))(2)(PNNP)](2+) (2), 6b (or 6a), and Dane's diene (15, to give estrone derivatives) or N-benzyl-N-(cyclohexylethynyl)-4-methylbenzenesulfonamide (17, to give cyclobutenamides) suggest that product displacement from the catalyst is turnover limiting.     

How the oxazole fragment influences the conformation of the tetraoxazocane ring in a cyclohexanespiro‐3′‐(1,2,4,5,7‐tetraoxazocane): single‐crystal X‐ray and theoretical study

Single crystals of (2S,5R)‐2‐isopropyl‐5‐methyl‐7‐(5‐methylisoxazol‐3‐yl)cyclohexanespiro‐3′‐(1,2,4,5,7‐tetraoxazocane), C₁₆H₂₆N₂O₅, have been studied via X‐ray diffraction. The tetraoxazocane ring adopts a boat–chair conformation in the crystalline state, which is due to intramolecular interactions. Conformational analysis of the tetraoxazocane fragment performed at the B3LYP/6‐31G(d,2p) level of theory showed that there are three minima on the potential energy surface, one of which corresponds to the conformation realized in the solid state, but not to a global minimum. Analysis of the geometry and the topological parameters of the electron density at the (3,?1) bond critical points (BCPs), and the charge transfer in the tetraoxazocane ring indicated that there are stereoelectronic effects in the O—C—O and N—C—O fragments. There is a two‐cross hyperconjugation in the N—C—O fragment between the lone electron pair of the N atom (lpN) and the antibonding orbital of a C—O bond (σ*_C—O) and vice versa between lpO and σ*_C—N. The oxazole substituent has a considerable effect on the geometry and the topological parameters of the electron density at the (3,?1) BCPs of the tetraoxazocane ring. The crystal structure is stabilized via intermolecular C—H…N and C—H…O hydrogen bonds, which is unambiguously confirmed with PIXEL calculations, a quantum theory of atoms in molecules (QTAIM) topological analysis of the electron density at the (3,?1) BCPs and a Hirshfeld analysis of the electrostatic potential. The molecules form zigzag chains in the crystal due to intermolecular C—H…N interactions being electrostatic in origin. The molecules are further stacked due to C—H…O hydrogen bonds. The dispersion component in the total stabilization energy of the crystal lattice is 68.09%.     

Wolframkomplexe von Diphosphanylacetylenen

Tungsten Complexes of Diphosphanylacetylenes Diphosphanylacetylenes, R₂P? C?C? PR₂ [R?N(C₂H₅)₂ ( 1 ), N[(CH₂)₂]₂O ( 2 ), OCH₃ ( 3 )] and W(CO)₅ · tetrahydrofurane form the mononuclear complexes R₂P? C?C? PR₂ · W(CO)₅ [R?N(C₂H₅)₂ ( 1a ), N[(CH₂)₂]₂O ( 2a )], the dinuclear complexes (CO)₅W? PR₂? C?C? PR₂? W(CO)₅ [R?N(C₂H₅)₂ ( 1b ), N[(CH₂)₂]₂O ( 2b ), OCH₃ ( 3b )], and the trinuclear complex (CO)₅W? PR₂? C?C? PR₂? W(CO)₄? PR₂? C?C? PR₂? W(CO)₅ [R?N(C₂H₅)₂ ( 4 )]. The new compounds are characterized by their NMR, mass, and IR spectra. The results of an X-ray structural analysis of 4 are reported.