Showing papers on "Double bond published in 1991"

The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules

Abstract: Alkanes. Halocompounds. Alcohols and Phenols. Ethers and Peroxides. Alkenes. Acetylenes. The -C=N and -N=C Groups. Compounds Containing the Carbonyl Group. Compounds Containing -NH2, -NHR, and -NR2 Groups. The Nitro Group. Double Bonds Containing Nitrogen Atoms. Cumulated Double Bonds. Organic Sulfur Compounds. Organosilicon Compounds. Organophosphorous Compounds. Aromatic and Heteroaromatic Rings. Selected Infrared and Raman Spectra. Appendices. Index.

Improved methodology for photocyclization reactions

Abstract: To photocyclize stilbenes, a stoichiometric amount of iodine plus propylene oxide in the absence of air is superior to a catalytic amount of iodine in the air. The propylene oxide prevents HI from photoreducing double bonds. The absence of air prevents photooxidative side reactions, shown in one case to be caused by a photogenerated oxidant, possibly hydrogen peroxide

Strained‐Ring and Double‐Bond Systems Consisting of the Group 14 Elements Si, Ge, and Sn

Abstract: The organometallic chemistry of the Group 14 elements E = Si, Ge, Sn in the 1980's is highlighted by the successful construction and characterization of three systems previously thought to be too reactive to exists: (1) three-membered ring compounds including cyclotrisilane, cyclotrigermane, and cyclotristannane, (2) molecules containing EE double bonds including disilene, digermene, and distannene, and (3) strained polycycles containing a skeleton of Group 14 elements, such as bicyclo[1.1.0]tetrasilane, hexagemaprismane, and octasilacubane. The majority of these numerous compounds now available are fully substituted with bulky ligands to suppress the reactivity intrinsic to the systems. These compounds permit examinations of (1) the variation of physical and chemical properties of a system with these elements and also with the ligands and (2) how two systems are interrelated thermally and photochemically with the intermediacy of the divalent (carbene-like) species. Theoretical calculations on virtually all of the parent compounds discussed in this review are evaluated alongside the experimental results. Some polycycles may constitute a stepping-stone on the way to compounds with a triple bond.

Tris(trimethylsilyl)silane as a radical-based reducing agent in synthesis

Abstract: Tris(trimethylsily1)silane is an effective reducing agent for organic halides, selenides, xanthatee, and isocyanide, as well as an effective hydrosilylating agent for dialkyl ketones and alkenes. The silane functions as a mediator in the formation of intermolecular carbon-carbon bonds via radicals and allows a variety of organic substrates to be used as alkyl radical percursors. Absolute rate constants for the reaction of (MeSSi),@' radicals with a variety of organic compounds have been measured in solution by laser flash photolysis. At 294 K rate constants are >5 X 10' M-' s-l for C=C double bonds that are activated by neighboring ?r-electron systems or by electron-withdrawing groups. For other substrates, reactivities decreased in the order xanthate > selenide > isocyanide > nitro > sulfide.

Kinetics of the reactions of allylsilanes, allylgermanes, and allylstannanes with carbenium ions

Abstract: AbsbPct Second-order rate constants for the reactions of para-substituted diarylmrbenium ions (ArAr‘CH’ = 1) with allylsilanes 2, allylgermanes 3, and allylstannanes 4 have been determined in CH2C12 solution at -70 to -30 OC. Generally, the attack of ArAr’CH+ at the CC double bond of the allylelement compounds 2-4 is rate-determining and leads to the formation of the j3-element-stabilized carbenium ions 5, which subsequently react with the negative counterions to give the substitution products 6 or the addition products 7. For compounds H2C=CHCH2MPh3, the relative reactivities are 1 (M = Si), 5.6 (M = Ge), and 1600 (M = Sn). From the relative reactivities of compounds H2C==CHCH2X (X .= H, SiBu3, SnBu3), the activating effect of an allylic trialkylsilyl (5 X IOs) and trialkylstannyl group (3 X lo9) is derived. This effect is strongly reduced, when the alkyl groups at Si or Sn are replaced by inductively withdrawing substituents, and an allylic SiCI, group deactivates by a factor of 300 (comparison isobutene/2k). A close analogy between the reactions of alkenes and allylelement compounds with carbenium ions is manifested, and the different reaction series are connected by well-behaved linear free energy relationships. The relative reactivities of terminal alkenes and allylelement compounds are almost independent of the electrophilicities of the reference carbenium ions (constant selectivity relationship), thus allowing the construction of a general nucleophilicity scale for these compounds. Allylsilanesl and allylstannanes2 have extensively been used as allyl anion equivalents during the last two decades. Their regioselective reactions with electrophiles have been explained by the intermediate formation of carbenium ions, which are hyperconjugatively stabilized by the carbonsilicon or carbon-tin bond in the 8-position (Scheme I).3 Competition experiments have shown that allyltrimethylsilane is 5 orders of magnitude more reactive toward in situ generated diarylcarbenium ions than propene; Le., the allylic trimethylsilyl group reduces the activation energy for electrophilic attack at the CC double bond by 18 kJ m01-I.~ Analogous investigations on the effect of allylic germy1 or stannyl groups on the nucleophilicity of CC double bonds have, to our best knowledge, not been performed. Indirect evidence for the magnitude of these effects can be obtained from the solvolysis rates of 8-element-substituted alkyl halidesS or trifluoroacetates (Chart 1): rates of acid-catalyzed R3MOH eliminations from 8-metal-substituted alcohols,6 or the rates of hydride abstraction in alkyl-substituted silanes, germane, and stannanes (Scheme II),’

Enantioselective synthesis of β-amino acids based on BINAP—ruthenium(II) catalyzed hydrogenation

Abstract: BINAP—Ru(II) catalyzed hydrogenation of β-substituted ( E )-β-(acylamino)acrylic acids allows efficient enantioselective synthesis of β-amino acids. The Z double bond isomers which possess an intramolecular hydrogen bond between amide and ester groups are more reactive but are hydrogenated with poor enantioselectivity. BINAP—Rh(I) complexes afford only moderate stereoselectivity with the opposite sense of enantioselection.

Methanol copolymerization of ethylene

Abstract: An improvement in the process for the continuous copolymerization of ethylene with certain polar comonomers having reactive double bonds, wherein a single phase is maintained in the reactor by means of methanol introduced concurrently with the reactants, whereby reactor fouling is significantly reduced or eliminated.

Effect of alkyl chain unsaturation and cholesterol intercalation on oxygen transport in membranes: a pulse ESR spin labeling study

Abstract: Transport and diffusion of molecular oxygen in phosphatidylcholine (PC)-cholesterol membranes and their molecular mechanism were investigated. A special attention was paid to the molecular interaction involving unsaturated alkyl chains and cholesterol. Oxygen transport was evaluated by monitoring the bimolecular collision rate of molecular oxygen and the lipid-type spin labels, tempocholine phosphatidic acid ester, 5-doxylstearic acid, and 16-doxylstearic acid. The collision rate was determined by measuring the spin-lattice relaxation times (T1's) in the presence and absence of molecular oxygen with long-pulse saturation-recovery ESR techniques. In the absence of cholesterol, incorporation of either a cis or trans double bond at the C9-C10 position of the alkyl chain decreases oxygen transport at all locations in the membrane. The activation energy for the translational diffusion of molecular oxygen in the absence of cholesterol is 3.7-6.5 kcal/mol, which is comparable to the activation energy theoretically estimated for kink migration or C-C bond rotation of alkyl chains [Trauble, H. (1971) J. Membr. Biol. 4, 193-208; Pace, R. J., & Chan, S. I. (1982) J. Chem. Phys. 76, 4241-4247]. Intercalation of cholesterol in saturated PC membranes reduces oxygen transport in the headgroup region and the hydrophobic region near the membrane surface but little affects the transport in the central part of the bilayer. In unsaturated PC membranes, intercalation of cholesterol also reduces oxygen transport in and near the headgroup regions. In contrast, it increases oxygen transport in the middle of the bilayer. On the basis of these observations, a model for the mechanism of oxygen transport in the membrane is proposed in which oxygen molecules reside in vacant pockets created by gauche-trans isomerization of alkyl chains and the structural nonconformability of neighboring lipids, unsaturated PC and cholesterol in particular, and oxygen molecules jump from one pocket to the adjacent one or move along with the movement of the pocket itself. The presence of cholesterol decreases oxygen permeability across the membrane in all membranes used in this work in spite of the increase in oxygen transport in the central part of unsaturated PC-cholesterol membranes because cholesterol decreases oxygen transport in and near the headgroup regions, where the major barriers for oxygen permeability are located. Oxygen gradients across the membranes of the cells and the mitochondria are evaluated. Arguments are advanced that oxygen permeation across the protein-rich mitochondrial membranes can be a rate-limiting step for oxygen consumption under hypoxic conditions in vivo.

Targeted drug delivery via phosphonate derivatives

Abstract: The invention provides compounds of formula (I) or a pharmaceutically acceptable salt thereof, wherein [D] is the residue of a drug having a reactive functional group, said functional group being attached, directly or through a bridging group, via an oxygen-phosphorus bond to the phosphorus atom of the moiety (I'); R1 is C1-C8 alkyl, C6-C10 aryl or C7-C12 aralkyl; R2 is hydrogen, C1-C8 alkyl, C6-C10 aryl C4-C9 heteroaryl, C3-C7 cycloalkyl, C3-C7 cycloheteroalkyl or C7-C12 aralkyl; and R3 is selected from the group consisting of C1-C8 alkyl; C2-C8 alkenyl having one or two double bonds; (C3-C7 cycloalkyl)-CrH2r- wherein r is zero, one, two or three, the cycloalkyl portion being unsubstituted or bearing 1 or 2 C1-C4 alkyl substituents on the ring portion; (C6-C10 aryloxy)C1-C8 alkyl; 2-, 3- or 4- pyridyl; and phenyl-CrH2r- wherein r is zero, one, two or three and phenyl is unsubstituted, or is substituted by 1 to 3 alkyl each having 1 to 4 carbon atoms, alkoxy having 1 to 4 carbon atoms, halo, trifluoromethyl, dialkylamino having 2 to 8 carbon atoms or alkanoylamino having 2 to 6 carbon atoms. The compounds are adapted for targered drug delivery, especially to the brain.

4.5 – Additions to N-Acyliminium Ions

Abstract: The addition of strongly polarized carbon-carbon double bonds (enols) to simple iminium ions, known as the Mannich reaction (equation 1), is a fundamentally important route to amines, especially in biosynthetic processes.1,2 This reaction type is less synthetically useful if weakly polarized or unpolarized carbon-carbon π-bonds are used as nucleophiles (equation 2).3 Because of the low electrophilic reactivity of the iminium moiety, the reverse reaction, known as the Grob fragmentation (equation 2), is often the more important process.4 However, if the iminium moiety has a carbonyl substituent on nitrogen, its electrophilicity is strongly enhanced, and reactions with CC bonds are irreversible and synthetically useful (equation 3).

4.3 – Vinyl Substitutions with Organopalladium Intermediates

Abstract: Organopalladium halides, acetates, triflates and similar derivatives lacking β sp3-bonded hydrogens generally react easily with unhindered alkenes to form new alkenes in which an original, vinyl hydrogen is replaced by the organic group of the palladium reactant. The reactions occur in solution. Other Group VIII metals are ineffective in bringing about this transformation in acceptable yields. In the palladium reaction the organopalladium(II) reactant is reduced to the zero valent state while the halide, acetate, triflate or other, initial anionic ligand is converted into the corresponding acid (equation 1). If alkyl groups are attached directly to the double-bond carbons, double-bond rearrangement may occur and allylically substituted products can be produced. Normally, substitution occurs before any double-bond rearrangement occurs in the reactant alkene and the substituent is placed on one of the initial double-bond carbons (equation 2). Rearrangement of the double bond along a hydrocarbon chain to positions more distant from the substituent than the allylic position is occasionally observed, most often when X is a halogen and the halogen acid produced by the reaction is not neutralized as it is formed by inclusion of a base.

Carbon-carbon double-bond formation in the intermolecular acetonitrile reductive coupling promoted by a mononuclear titanium(II) compound. Preparation and characterization of two titanium(IV) imido derivatives

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Intramolecular bis-silylation of carbon-carbon double bonds leading to stereoselective synthesis of 1,2,4-triols

Abstract: Intramolecular Bis-Silylation of Carbon -Carbon Double Bonds Leading to Stereoselective Synthesis of 1,2,4-Triols

Chemistry of substituted (2-butene-1,4-diyl)magnesium : a facile approach to complex carbocycles, functionalized ketones and alcohols, and silicon-containing heterocycles

Abstract: Highly reactive magnesium reacts with a wide variety of substituted 1,3-dienes to give the corresponding substituted (2-butene-1,4-diyl) magnesium complexes. Reactions of symmetrical (2-butene-1,4-diyl) magnesium with α,ω-alkylene dihalides form three-, four-, five-, and six-membered carbocycles. Significantly, the cyclizations are always stereospecific and completely regioselective. Depending on the initial 1,3-diene and specific electrophiles, uncyclized products can be obtained. Stepwise reactions of (2,3-dimethyl-2-butene-1,4-diyl) magnesium with two different electrophiles afford polyfunctionalized ketones with the generation of a quaternary center. Formal 1,2-additions can be effected in this manner. Substituted five- and six-membered cyclic ketones can also be synthesized in one step by this approach. Treatment of unsymmetrical (2-butene-1,4-diyl) magnesium complexes with triorganosilyl chlorides followed by cyclohexanone results in additions across a terminal double bond with high regioselectivity. Silicon-containing heterocycles or spiro compounds can be readily synthesized by using the bis-Grignard reagents

Thermally irreversible photochromic systems. reversible photocyclization of non-symmetric diarylethene derivatives

Abstract: Non-symmetric diarylethenes with an indole ring on one end and a thiophene, a benzo[b]thiophene, or a pyrrole ring on the other end of the double bond were synthesized in an attempt to get thermally irreversible photochromic compounds having absorption bands at longer wavelengths 2-(1,2-Dimethyl-3-indolyl)-3-(2,4,5-trimethyl-3-thienyl)maleic anhydride(8a) underwent photoinduced cyclization/ring-opening reactions with relatively high quantum yields (cyclization quantum yield: 015; ring-opening quantum yield: 040), and the both isomers were thermally stable The closed-ring form had the absorption maximum at 595 nm with the edge extending to 760 nm Cyclization/ring-opening reactions of 8a and 2-(1,2-dimethyl-3-indolyl)-3-(2-methyl-3-benzo[b]thienyl)maleic anhydride (9a) were induced by Ar ion (488 nm) and He–Ne (633 nm) lasers

Photoelectron spectroscopy of free radicals with cm−1 resolution: The benzyl cation

Abstract: We report four vibrational bands from 0–650 cm−1 for benzyl+−h7, benzyl+−αd2, and benzyl+−d7. The exocyclic C—C bond of benzyl cation has substantial double bond character. Band assignments from ab initio frequencies illuminate the mechanism of vibronic coupling in the 1 2A2–2 2B2 system of neutral benzyl. The adiabatic ionization potential of benzyl−h7 is 7.2484±0.0006 eV.

Catalytic epoxidation of styrene–butadiene triblock copolymer with hydrogen peroxide

Abstract: Styrene-butadiene-styrene linear block copolymer (SBS) can be epoxidized with hydrogen peroxide in the presence of methyltrioctylammonium tetrakis (diperoxotungsto) phosphate(3-) as the catalyst in a biphasic system. The effects of reaction time, temperature, solvent, the ratio of the organic phase to the aqueous phase, the concentration of the catalyst, hydrogen peroxide and polymer, respectively, are studied on the conversion of double bonds to oxirane groups. 1H-NMR analysis confirms the absence of ring opening side reaction in this epoxidation reaction system up to at least 70% conversion of the double bonds. When the conversion of double bonds is over about 70% the resultant polymer is insoluble in chloroform even at reflux but soluble in polar solvents such as DMSO when heated. Toluene is a better solvent for the reaction than dichloroethane. The reaction rate constants are measured at four temperatures and the activation energy for the reaction with toluene as solvent is determined as 49.9 kJ/mol.

Additionsreaktionen an intramolekular basenstabilisierte GeN- und GeS-Doppelbindungen

Abstract: Addition Reactions of Intramolecularly Base-Stabilized GeN and GeS Double Bonds. The GeN bond of 1 adds hydrogen chloride, ethanol, tert-butyl alcohol, methyllithium, and methyl iodide to form 3–6 and 8. The GeS bond in 2 analogously adds methyl iodide to yield 10. In the resulting products the electropositive part of the polar reactand is bound to nitrogen or sulfur, while the electronegative one is on the germanium atom. In the lithium derivative 6 and in the iodine derivative 8 Li and I can be replaced by hydrogen to form two isomers of the same composition 7 and 9, which differ only in the positions of the hydrogen and methyl groups with respect to Ge or N. The X-ray structure analyses on 4, 9, and 10 reveal that the germanium atom is fourfold coordinated by two nitrogen atoms and two other ligands. The intramolecular nitrogen base of the silazyl moiety in 1 and 2 is, thus, expelled from the coordination sphere of germanium by insertion of the double bonds into polar σ bonds, and the tricyclic molecule is changed to a bicyclic one. Thus these “base stabilized π bonds” in 1 and 2 behave as normal unsaturated systems.