Unsymmetrical Alkenes Research Articles

Does crystal or gel matter to stereochemistry of a reaction? Silver complexation-promoted solid-state [2+2] reaction of an unsymmetrical olefin

Head-to-tail and head-to-head [2+2] photodimerization of an unsymmetrical olefin containing benzimidazole and pyridyl groups was achieved by irradiating Ag(I) complexed olefin in crystalline state and gel state, respectively, in stereoselective manner. The [2+2] reaction indicates that the molecular arrangement in a gel is different from that of a xerogel.

Ligand Control of the Cobalt-Catalysed 1,4-Hydrovinylation Reaction

The application of cobalt catalysts to the hydrovinylation of unsymmetrical alkenes and 1,3-dienes was investigated using modular phosphine-phosphite ligands to evaluate the influence of the ligands on the regioselectivity of the reaction. While a ligand with a TADDOL subunit (SchmalzPhos) gave the best results for the β-4 regioisomer, the corresponding BINOL-derived ligand gave predominantly the β-1 regioisomer. The application of (E)-2-methylpenta-1,3-diene in combination with the (nonracemic) BINOL-derived ligand generated the β-4 regioisomer in excellent yields and very high regioselectivities for a number of terminal alkenes bearing various functional groups. In one case, the enantiomeric excess of a chiral product, i.e. a 3,5-dimethylhexa-1,4-diene derivative, was determined by chiral RP-HPLC to be 78% ee.

Cu(I)/Cu(II)-catalyzed allylic amination of alkenes

Allylic hydrocarbons are selectively converted to the corresponding allyl amines in good to excellent yield by reaction with aryl hydroxylamines catalyzed by a 1:1 mixture of CuCl and CuCl 2 (10 mol %). Under these conditions unsymmetrical olefins react highly regioselectively with N-functionalization at the less substituted vinylic carbon. Trapping experiments indicate that a free nitrosoarene is not an intermediate in these reactions.

Regiochemistry of the Cycloaddition of Unsymmetrical Alkenes with an Imino-1,2,4-Dithiazole

Two cycloaddition reactions were conducted with 5-(diphenylamino)-3-phenylimino-3H-1,2,4-dithiazole and the unsymmetrical alkenes, methyl acrylate and acrylonitrile, to produce thiazolidines 6 and 7, respectively. X-ray crystal structures of both compounds were obtained. The crystal of 6 is orthorhombic and space group Pbca with crystallographic parameters: a = 10.7090(14) A, b = 19.612(4) A, c = 21.353(3) A, α = 90°, β = 90°, γ = 90°, V = 4484.7 (14) A3, Z = 8, Dc = 1.326 Mg/m3, F(000) = 1872. The crystal of 7 is monoclinic and space group P2(1)/c with crystallographic parameters: a = 13.4143(5) A, b = 16.1088(6) A, c = 10.1644(6) A, α = 90°, β = 107.282(5)°, γ = 90°, V = 2097.25(17) A3, Z = 4, Dc = 1.313 Mg/m3, F(000) = 864. The crystal structures confirmed the thiazolidine structure and the predicted regiochemistry of the cycloaddition based on the electronics of the imino-1,2,4-dithiazole and the alkenes. Cycloaddition reactions were conducted between 5-(diphenylamino)-3-phenylimino-3H-1,2,4-dithiazole and methyl acrylate or acrylonitrile producing thiazolidine products in which the predicted regiochemistry was established by single-crystal X-ray diffraction.

Reaction of O,O-diethyl phosphorodithioic acid with hexadec-1-ene. Catalysis with zinc chloride

As shown earlier [1–6] the regiochemistry of addition of O,O-diethyl phosphorodithioic acid (I) to unsymmetrical alkenes, i.e. formation of Markovnikov or anti-Markovnikov products, is defined by the nature of substituents at the double bond and, in some cases, on the presence or absence of catalysts in the reaction mixture. Alkenes with electron-acceptor groups at the double bond take up dithio acid I under base catalysis or without it occurs against Markovnikov’s rule [2]. The reactions of unactivated aliphatic alkenes (oct-1and oct-2-enes, cyclohexene, diisobutylene) at 100°C give rise to Markovnikov products [4–6]. Phos-phorodithioc acids not been subjected to special purification with aqueous sodium carbonate [4–6] contain an admixture of phosphorus sulfide (P4S10) which can destroy peroxide admixtures in commercial alkenes and thus favors formation of Markovnikov products. At the same time, cumene hydroperoxide additive in a mixture of dithio acid I and oct-1-ene favors an antiMarkovnikov reaction pathway. The reaction of commercial oct-1-ene (with a natural content of peroxides) with purified dithio acid I provides a mixture of two isomeric Markovnikov adducts [6].

Olefin Cross-Metathesis Reactions at Room Temperature Using the Nonionic Amphiphile “PTS”: Just Add Water

The first examples of unsymmetrical olefin cross-metathesis reactions in water, involving water-insoluble substrates, at room temperature and using commercially available catalysts are reported. The key to success is to include small percentages of the nonionic, vitamin E-based amphiphile "PTS". The nanometer micelles formed accommodate water-insoluble substrates, along with a readily available Ru-based metathesis catalyst. Reactions proceed at ambient temperatures with high efficiency and very high E-selectivity, and products are easily isolated.

Construction of Isochromenes via a Ruthenium-Catalyzed Reaction of Oxabenzonorbornenes with Propargylic Alcohols

The ruthenium-catalyzed cyclization of a propargylic alcohol with an oxabenzonorbornene in methanol leads to the creation of an isochromene framework. The proposed mechanism herein discussed for the formation of the product involves six major steps, the first four being oxidative cyclization, β-hydride elimination, hydroruthenation, followed by [2+2] cycloreversion. The ruthenium carbene formed at this stage undergoes a [1,3]-alkoxide shift that provides the observed product after reductive elimination. This process, believed to occur via a cationic ruthenium species, is in competition with two other pathways, ruthenium-catalyzed [2+2] cycloaddition and cyclopropanation. Although both [Cp*Ru(CH3CN)3]PF6 and Cp*Ru(COD)Cl are effective catalysts, the latter gives better yield and product ratio. The reaction was also found to proceed with high regioselectivity and product selectivity when unsymmetrical alkenes bearing a coordinating functional group at the bridge junction were used.

Nonchelated d0 Zirconium−Alkoxide−Alkene Complexes

The reaction of Cp'2Zr(O(t)Bu)Me (Cp' = C5H4Me) and [Ph3C][B(C6F5)4] yields the base-free complex [Cp'2Zr(O(t)Bu)][B(C6F5)4] (6), which exists as Cp'2Zr(O(t)Bu)(ClR)+ halocarbon adducts in CD2Cl2 or C6D5Cl solution. Addition of alkenes to 6 in CD2Cl2 solution at low temperature gives equilibrium mixtures of Cp'2Zr(O(t)Bu)(alkene)+ (12a-l), 6, and free alkene. The NMR data for 12a-l are consistent with unsymmetrical alkene bonding and polarization of the alkene C=C bond with positive charge buildup at C(int) and negative charge buildup at C(term). These features arise due to the lack of d-pi* back-bonding. Equilibrium constants for alkene coordination to 6 in CD2Cl2 at -89 degrees C, K(eq) = [12][6](-1)[alkene](-1), vary in the order: vinylferrocene (4800 M(-1)) >> ethylene (7.0) approximately alpha-olefins > cis-2-butene (2.2) > trans-2-butene (<0.1). Alkene coordination is inhibited by sterically bulky substituents on the alkene but is greatly enhanced by electron-donating groups and the beta-Si effect. Compounds 12a-l undergo two dynamic processes: reversible alkene decomplexation via associative substitution of a CD2Cl2 molecule, and rapid rotation of the alkene around the metal-(alkene centroid) axis.

A simple, highly regioselective and efficient reaction of indole with epoxides under solvent-free conditions

Lithium perchlorate has been found to be an inexpensive and efficient catalyst for the ring opening of epoxides by indole and N-methylindole, which provides an environmentally friendly method for the synthesis of substituted indoles. A complete regioselectivity in favor of nucleophilic attack at the benzylic carbon atom of aromatic epoxides, such as styrene oxide, is observed. However, aliphatic unsymmetrical alkene oxides undergo selective nucleophilic attack at the sterically less-hindered carbon atom. This catalyst offers several advantages, such as short reaction time, high yields, lower catalytic loading, simple experimental procedure, and easy isolation of the products, under solvent-free conditions.

Abnormal Beckmann rearrangement of steroidal α-chlorocyclobutanone oximes: the fragmentation–substitution reaction

α,α-Dichlorocyclobutanones of various substitution patterns, readily available by the thermal cycloaddition of dichloroketene to steroid olefins, were selectively dehalogenated and further transformed to α-chlorocyclobutanone oximes. In contrast to the Beckmann rearrangement of the α-unsubstituted oximes, α-chlorocyclobutanone oximes, on treatment with thionyl chloride in benzene, gave normal and abnormal reaction products. In all reactions studied, the Beckmann fragmentation–substitution was the major process. The rearrangement of α-chlorocyclobutanone oximes is the key step of the novel method for geminal functionalization of carbonyl carbon atom in ketones and regioselective vicinal functionalization of unsymmetrical olefins. The regiochemistry observed in the rearrangements is supported by semiempirical calculations (AM1).

Synthesis of novel sulfonyl-stabilized phosphorus ylides, and the kinetics and mechanism of their conventional and flash vacuum pyrolysis reactions

Nine substituted sulfonyl-stabilized phosphorus ylides were prepared by treating their intermediate ylide analogues with phenylmethanesulfonyl fluoride. The stoichiometric ratio of the reactants for each preparation needed to be adjusted according to the basicity of each ylide intermediate. The nine ylide compounds were then subjected to conventional (sealed-tube) gas-phase pyrolysis at 470–545 K. The pyrolytic reactions were homogeneous and obeyed a first-order rate equation. The values of the Arrhenius log A (s–1) and Ea (kJ mol–1) obtained for these reactions averaged 11.12 ± 2.00 and 131.8 ± 24.4, respectively. Analysis of the pyrolysates from conventional pyrolysis and from flash vacuum pyrolysis at 600 K showed the products to be complex mixtures of triphenylphosphine, triphenylphosphine oxide, triphenylphosphine sulfide, and symmetric and unsymmetric alkenes. Conventional pyrolysis also gave novel mixed sulfones and, for the p-methoxyaryl substituent, p-anisaldehyde. The products of the reactions under study are explained on the basis of a mechanism involving a sulfonyl carbene intermediate, and the reaction mechanism is used to rationalize the kinetic results and molecular reactivities.Key words: ylides, synthesis, pyrolysis, kinetics, mechanism.

Kinetics of the Allylic Amination of Olefins by Nitroarenes Catalyzed by [CpFe(CO)2]2

The kinetics of the [CpFe(CO)2]2-catalyzed allylic amination of α-methylstyrene derivatives by nitroarenes is reported. The reaction is found to be first-order in both alkene and nitroarene, zero-order in CO, and half-order in catalyst. Relative rate studies of para-substituted alkenes and nitroarenes show that the rate is increased for electron-deficient nitro compounds and for electron-rich alkenes. On the basis of these results, together with the regioselectivity of the reactions with unsymmetrical alkenes and the outcome of trapping experiments for free nitrosoarenes and nitrenes, a mechanistic scheme is proposed involving C−N bond formation from a CpFe(CO)(RNO)(alkene) intermediate.

Effect of cyclodextrin complexation in bromine addition to unsymmetrical olefins: evidence for participation of cyclodextrin hydroxyl groups

Multiple recognition by cyclodextrin in a bimolecular reaction, namely bromination of styrene, methyl cinnamate, phenylacetylene and allylbenzene, has been studied. Bromohydrin is obtained as a major product along with dibromide in the bromination of styrene and methyl cinnamate. The percentage of bromohydrin decreases as the cavity size increases. With phenylacetylene, bromophenylacetylene and phenacyl bromide are obtained in addition to the dibromides. In the bromination of cyclodextrin complexes of allylbenzene, the product distribution is the same as in solution bromination. The observed results demonstrate the efficiency of cyclodextrin in stabilizing the open carbocationic intermediate and thus provide chemical evidence for the participation of cyclodextrin hydroxyl groups.

Lithium Bromide, an Inexpensive and Efficient Catalyst for Opening of Epoxide Rings by Amines at Room Temperature under Solvent‐Free Condition.

Lithium bromide has been found to be an inexpensive and efficient catalyst for the opening of epoxide rings by amines, and this provides an environmentally friendly method for the synthesis of β-amino alcohols. Aromatic and aliphatic amines react with cycloalkene oxides to exclusively form trans-2-(aryl/alkylamino)cycloalkanols in high yields. A 98−100% selectivity in favour of nucleophilic attack at the benzylic carbon atom of styrene oxide is observed with aromatic amines. However, aliphatic amines exhibit a marginal preference for the reaction at the terminal carbon atom of the epoxide ring in styrene oxide. Non-styrenoidal, unsymmetrical alkene oxides undergo selective nucleophilic attack at the sterically less hindered carbon atom by aniline. The chelation effect of the Li+ ion enables selective opening of the epoxide ring in 3-phenoxypropylene oxide in the presence of styrene oxide. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Lithium Bromide, an Inexpensive and Efficient Catalyst for Opening of Epoxide Rings by Amines at Room Temperature under Solvent‐Free Condition

AbstractLithium bromide has been found to be an inexpensive and efficient catalyst for the opening of epoxide rings by amines, and this provides an environmentally friendly method for the synthesis of β‐amino alcohols. Aromatic and aliphatic amines react with cycloalkene oxides to exclusively form trans‐2‐(aryl/alkylamino)cycloalkanols in high yields. A 98−100% selectivity in favour of nucleophilic attack at the benzylic carbon atom of styrene oxide is observed with aromatic amines. However, aliphatic amines exhibit a marginal preference for the reaction at the terminal carbon atom of the epoxide ring in styrene oxide. Non‐styrenoidal, unsymmetrical alkene oxides undergo selective nucleophilic attack at the sterically less hindered carbon atom by aniline. The chelation effect of the Li+ ion enables selective opening of the epoxide ring in 3‐phenoxypropylene oxide in the presence of styrene oxide. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2004)

Synthesis of Substituted Stilbenes via the Knoevenagel Condensation

Knoevenagel condensations between aldehydes and substrates containing active methylene groups were carried out in ethanol at room temperature, in the presence of potassium phosphate, to afford unsymmetrical olefins. These condensations have been shown to afford only the E-isomers in greater than 80% yields. Salicylaldehyde first produces the Knoevenagel condensation products, which undergo a subsequent heterocyclization to give coumarin derivatives. The structures of the synthesized compounds were established on the basis of UV, IR, MS and NMR spectroscopy.

An efficient synthesis of 2-amino alcohols by silica gel catalysed opening of epoxide rings by amines.

Silica gel (60-120 mesh) efficiently catalyses the opening of epoxide rings by amines at rt under solvent-free conditions providing an easy method for the synthesis of 2-amino alcohols. Aromatic and aliphatic amines react with cyclohexene oxide with exclusive formation of the trans-2-aryl/alkylaminocyclohexanols in high yields. A complementary regioselectivity is exhibited by aromatic and aliphatic amines during the reaction with styrene oxide. The epoxide ring of non-styrenoidal unsymmetrical alkene oxide undergoes selective nucleophilic attack at the sterically less hindered carbon by aniline.

Copper‐Catalyzed Allylic Amination of Olefins with Nitrosoarenes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Copper-catalyzed allylic amination of olefins with nitrosoarenes

The transition metal catalyzed allylic amination of olefins are studied. A screening of catalysts known for the intermediacy of PhNO in the amination of alkene with phenylhydroxylamine reveals that the hydrated copper salt in conjugation with copper powder as reductant has the best catalytic properties using nitrosobenzene as nitrogen-fragment donor. The catalytic system has been studied for a variety of alkenes. Unsymmetrical alkenes react with high regioselectivity with N-functionalization at the less substituted vinylic carbon.

Cu(I)‐Catalyzed Allylic Amination of Olefins.

AbstractFor Abstract see ChemInform Abstract in Full Text.