Rhodium phosphine-π-arene intermediates in the hydroamination of alkenes

A detailed mechanistic study of the intramolecular hydroamination of alkenes with amines catalyzed by rhodium complexes of a biaryldialkylphosphine is reported. The active catalyst is shown to contain the phosphine ligand bound in a κ(1), η(6) form in which the arene is π-bound to rhodium. Addition of deuterated amine to an internal olefin showed that the reaction occurs by trans addition of the N-H bond across the C═C bond, and this stereochemistry implies that the reaction occurs by nucleophilic attack of the amine on a coordinated alkene. Indeed, the cationic rhodium fragment binds the alkene over the secondary amine, and the olefin complex was shown to be the catalyst resting state. The reaction was zero-order in substrate, when the concentration of olefin was high, and a primary isotope effect was observed. The primary isotope effect, in combination with the observation of the alkene complex as the resting state, implies that nucleophilic attack of the amine on the alkene is reversible and is followed by turnover-limiting protonation. This mechanism constitutes an unusual pathway for rhodium-catalyzed additions to alkenes and is more closely related to the mechanism for palladium-catalyzed addition of amide N-H bonds to alkenes.     

pi-Acidic alkene ligand effects in Pd-catalysed cross-coupling processes: exploiting the interaction of dibenzylidene acetone (dba) and related ligands with Pd(0) and Pd(II)

pi-Acidic alkene (olefin) ligands positively influence Pd-catalysed cross-coupling processes, interacting with both palladium(0) and palladium(ii) species, in some cases stabilising key catalytic intermediates. Rates of oxidative addition and reductive elimination are both affected. In certain cases, beta-hydrogen elimination can be slowed down by pi-acidic alkenes, which opens up new reaction pathways (e.g. interception of sigma-alkylpalladium(ii) species by appropriate nucleophiles). pi-Acidic alkene ligands can act independently or in a synergistic fashion with another two-electron donor ligand (e.g. amine, phosphine or N-heterocyclic carbene). The purpose of this perspective article is to highlight the impressive results that can be obtained using pi-acidic alkene ligands, with a particular focus on dibenzylidene acetone (dba) derivatives. Other types of alkene ligands, e.g. macrocyclic alkenes, are also discussed.     

Iodine atom transfer [3 + 2] cycloaddition reaction with electron-rich alkenes using N-tosyliodoaziridine derivatives as novel azahomoallyl radical precursors

Treatment of N-tosyliodoaziridine derivatives with Et(3)B efficiently produces various azahomoallyl radical (2-akenylamidyl radical) species which give oxygen-functionalized pyrrolidine derivatives through iodine atom transfer [3 + 2] cycloaddition with electron-rich alkenes such as enol ethers and ketene acetal. The present cycloaddition reaction proceeds regioselectively via C-N bond cleavage of an aziridinylalkyl radical intermediate and addition of the resulting azahomoallyl radicals to the terminal carbon of an alkene. The reaction of alkenes with the cyclohexenylamidyl radical generated from an optically active bicyclic iodoaziridine [(1S,2S,6S)-2-iodo-7-(p-toluenesulfonyl)-7-azabicyclo[4.1.0]heptane, 94% ee] also proceeds to give optically active octahydroindole derivatives (84-93% ee).     

Migration of Double Bond of Alkene in the Reaction of Iron-Catalyzed Olefination from Sulfonylcarbanion

Sulfonylcarbanion can be converted into alkene by elimination⁽¹⁾(eq 1),mix-coupling⁽²⁾ (eq 2) or homocoupling⁽³⁾ (eq 3) under iron catalysis. Besides desired alkenes, isomerized alkenes were detected in the reaction of elimination or mix-coupling. However, no migration of double bond in alkene was found in homocoupling reaction. Some experimental results were obtained to discuss the alkene isomerism in the iron-catalyzed sulfonylcarbanion olefination and some evidences were given to support the proposed mechanism of iron-catalyzed sulfonylcarbanion olefination.     

Information on the location of carbon-carbon double bonds in C6−C23 linear alkenes from carbon addition reactions in a quadrupole ion trap equipped with a pulsed sample-inlet system

Ion-molecule reactions of a number of alkene molecular ions with different neutral alkenes were studied in a quadrupole ion trap equipped with a pulsed sample-inlet system. The molecules studied include several isomeric unbranched hexenes, heptenes, octenes, and nonenes, as well as representative alkenes with ten, twelve, fourteen, and twenty-three carbon atoms. Transfer of structurally characteristic number of methylene units between the ionic and neutral reactants dominates the product distributions for all the alkenes studied, with the exception of 1-alkenes. Isomeric alkenes can be readily distinguished on the basis of their products from reactions with neutral alkenes. It is suggested that distonic intermediates are generated in these reactions, and that they fragment by alkene elimination after 1,2- and 1,5-hydride shifts. The ability to vary the reaction time, pressure of the neutral reagents, and the type of ions and neutral molecules present in the reaction chamber during each stage of the experiment sequence makes it possible to maximize the amount of structural information obtained for alkenes in these experiments. Use of CS ₂ ^+? to generate the alkene molecular ion by charge exchange yields the same information without the need to carry out a mass-selection step for the ionized alkene.     

Intermolecular hydroaminations via strained (E)-cycloalkenes

A photoinduced procedure for the intermolecular hydroamination of alkenes using azoles is described. This reaction occurs in modest to good yield for 6- and 7-membered cyclic alkenes. Upon irradiation at 254 nm in the presence of methyl benzoate and a small amount of triflic acid as an additive (20 mol %), imidazoles, pyrazoles, triazoles, and tetrazole can react with the alkene to afford complex Markovnikov adducts. The proposed mechanism involves photoisomerization to generate highly strained (E)-cycloalkene intermediates and (E)-cycloalkene protonation followed by reaction with the azole nucleophile. Alkene isomerization was found to be a competing side reaction under these conditions.     

Formal and standard ruthenium-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,6-diynes to alkenes: a mechanistic density functional study

"Formal" and standard Ru(II)-catalyzed [2 + 2 + 2] cycloaddition of 1,6-diynes 1 to alkenes gave bicyclic 1,3-cyclohexadienes in relatively good yields. The neutral Ru(II) catalyst was formed in situ by mixing equimolecular amounts of [Cp*Ru(CH3CN)3]PF6 and Et4NCl. Two isomeric bicyclic 1,3-cyclohexadienes 3 and 8 were obtained depending on the cyclic or acyclic nature of the alkene partner. Mechanistic studies on the Ru catalytic cycle revealed a clue for this difference: (a) when acyclic alkenes were used, linear coupling of 1,6-diynes with alkenes was observed giving 1,3,5-trienes 6 as the only initial reaction products, which after a thermal disrotatory 6e-pi electrocyclization led to the final 1,3-cyclohexadienes 3 as probed by NMR studies. This cascade process behaved as a formal Ru-catalyzed [2 + 2 + 2] cycloaddition. (b) With cyclic alkenes, the standard Ru-catalyzed [2 + 2 + 2] cycloaddition occurred, giving the bicyclic 1,3-cyclohexadienes 8 as reaction products. A complete catalytic cycle for the formal and standard Ru-catalyzed [2 + 2 + 2] cycloaddition of acetylene and cyclic and acyclic alkenes with the Cp*RuCl fragment has been proposed and discussed based on DFT/B3LYP calculations. The most likely mechanism for these processes would involve the formation of ruthenacycloheptadiene intermediates XXIII or XXVII depending on the alkene nature. From these complexes, two alternatives could be envisioned: (a) a reductive elimination in the case of cyclic alkenes 7 and (b) a beta-elimination followed by reductive elimination to give 1,3,5-hexatrienes 6 in the case of acyclic alkenes. Final 6e-pi electrocyclization of 6 gave 1,3-cyclohexadienes 3.     

Synthesis of 1,1-disubstituted alkenes via a Ru-catalyzed addition.

The synthesis of 1,1-disubstituted alkenes typically involves reactions that lack atom economy such as olefination protocols. The use of various ruthenium complexes to effect the addition of terminal alkynes to alkenes is explored as an atom economical strategy. Two new ruthenium complexes have been discovered that effect this reaction at ambient temperature, cyclopentadienylruthenium (triphenylphosphine) camphorsulfonate and cyclopentadienylruthenium tris(acetonitrile) hexafluorophosphate. Using these complexes as catalysts, reactions proceed at ambient temperature in acetone or DMF, respectively. Regioselectivity favoring the formation of a 1,1-disubstituted over a 1,2-disubstituted alkene typically ranges from 9:1 to >25:1. The reaction demonstrates extraordinary chemoselectivity-even di- and trisubstituted alkenes such as present in the products do not compete with the starting monosubstituted alkene. Free hydroxyl groups as well as silyl and PMB ethers are tolerated as are ketones, esters, and amides. The mechanism of the reaction is believed to invoke formation of a metallacyclopentene. To account for the chemo- and regioselectivity, the initial formation of the metallacycle is believed to be reversible. While formation of the 2,5-disubstituted ruthenacyclopentene, which produces the linear product, is believed to be kinetically preferred, the rate of beta-hydrogen elimination from the 2,4-disubstituted ruthenacyclopentene, which produces the branched product, is believed to be faster. Thus, the competition between the rate of beta-hydrogen elimination and cycloreversion rationalizes the results.     

Polyoxotungstate photoinduced alkylation of electrophilic alkenes by cycloalkanes

Alkyl radical obtained by irradiation of tetrabutylammonium decatungstate in acetonitrile in the presence of cycloalkanes (C5H10, C6H12, C7H14) are efficiently trapped by electrophilic alkenes (acrylonitrile, isopropylydenmalonitrile, isopropylydencyanoacetate) to give the corresponding alkylated aliphatic nitriles. The reaction can be carried out up to complete conversion of the alkene with reasonable (in most cases 60-65 %) yields. Addition of the radicals to the alkene is followed by electron transfer from reduced decatungstate regenerating the sensitizer (turn over number up to 60). Steady-state measurements, EPR evidence, deuteration experiments and attempted intramolecular trapping of the adduct radical support the mechanistic proposal.     

Mechanisms for Manganese(III) Oxidations with Alkenes

In the reaction of manganese(III) acetate with carboxylic acids and alkenes, three distinct processes have been identified which involve the alkene and two processes which are independent of alkene. A combination of product studies, rearrangements, dilution experiments and literature kinetic data allow the proposal of a unified mechanistic picture to describe these processes. Specifically, the role of α-H acidity of the carboxylic acid component, electron deficient radical additions, metal complexed organic radicals, and the importance of an oxo-centered manganese(III) triangle are discussed as they relate to the lactone annulation reaction. Single electron transfer oxidation of alkenes is described as a route toward 1,2-diacetates of alkenes within the 8.1-7.5 eV I.P. range. Three less common modes of Mn(III) reaction are discussed and compared with the two primary processes of lactone annulation and 1,2-diacetate formation.     

Synthesis of functionalized hydroquinones via [Cp*RuCl2]2-catalyzed cocyclization of alkynes, alpha,beta-unsaturated carbonyl compounds, and carbon monoxide

[reaction: see text] Catalytic [2 + 2 + 1 + 1] cocyclization reaction of an alkyne, an alkene, and two molecules of carbon monoxide, leading to functionalized hydroquinones, was studied. Using [Cp*RuCl2]2 as a catalyst, we found that a variety of electron-deficient alkenes, such as alpha,beta-unsaturated ketones, esters, amides, and nitriles, can be employed as an alkene coupling partner to give the corresponding hydroquinones.     

Palladium-catalyzed synthesis of 2,1'-disubstituted tetrahydrofurans from gamma-hydroxy internal alkenes. Evidence for alkene insertion into a Pd-O bond and stereochemical scrambling via beta-hydride elimination

Palladium-catalyzed reactions of gamma-hydroxy internal acyclic alkenes with aryl bromides afford 2,1'-disubstituted tetrahydrofurans in good yields with diastereoselectivities of 3-5:1. The analogous transformations of substrates bearing internal cyclic alkenes afford fused bicyclic and spirocyclic tetrahydrofuran derivatives in good yields with excellent diastereoselectivities (>20:1). A series of deuterium labeling experiments indicate that the origin of the modest diastereoselectivity in reactions of acyclic internal alkene substrates likely derives from a series of reversible beta-hydride elimination and sigma-bond rotation processes that occur following a rare intramolecular alkene syn-insertion into an intermediate Pd(Ar)(OR) complex. In addition, these studies shed light on the chemoselectivity of insertion, suggesting that the alkene inserts into the Pd-O bond in preference to the Pd-C bond.     

Cationic carbohydroxylation of alkenes and alkynes using the cation pool method

The reactions of an N-acyliminium ion pool with alkenes and alkynes gave gamma-amino alcohols and beta-amino carbonyl compounds, respectively, after treatment with H(2)O/Et(3)N. The present reaction serves as an efficient method for cationic carbohydroxylation of alkenes and alkynes. When vinyltrimethylsilane was used as an alkene, the reaction was highly diastereoselective and served as an access to an enantiomerically pure alpha-silyl-gamma-amino alcohol. [reaction: see text]     

Theoretical prediction on the reactivity of the Co-mediated intramolecular Pauson-Khand reaction for constructing bicyclo-skeletons in natural products

This work performed a theoretical investigation to explore the mechanism and reactivity of the Co-mediated intramolecular Pauson-Khand reaction for constructing bicyclo-skeletons.     

Mechanism of the intramolecular hydroamination of alkenes catalyzed by neutral indenyltitanium complexes: a DFT study

Three mechanistic pathways for the [Ind(2)TiMe(2)]-catalyzed intramolecular hydroamination of alkenes have been investigated by employing density functional theory calculations on the possible intermediates and transition states. The results indicate that the reaction cycle proceeds via a Ti-imido-amido complex as the catalytically active species. However, at the moment, the question as to whether this imido-amido complex is involved in a [2+2]-cycloaddition with the alkene or a newly proposed insertion of the alkene into a Ti--N single bond cannot be answered; the calculated barriers of both the insertion mechanism and the [2+2]-cycloaddition mechanism are similar (143 vs. 136 kJ mol(-1)), and both pathways are in accordance with the experimentally observed rate law (first-order dependence on the aminoalkene concentration). Interestingly, the newly proposed insertion mechanism that takes place by an insertion of the alkene moiety into the Ti--N single bond of an imido-amido complex seems to be much more likely than a mechanism that involves an alkene insertion into a Ti--N single bond of a corresponding trisamide. The latter mechanism, which has been proposed in analogy to rare-earth-metal-catalyzed hydroamination reactions, can be ruled out for two reasons: a surprisingly high activation barrier (164 kJ mol(-1)) and the fact that the rate-limiting insertion step is independent of the aminoalkene concentration. This is in sharp contrast to the experimental findings for indenyltitanium catalysts.     

Cationic palladium-catalyzed hydrosilylative cross-coupling of alkynes with alkenes

A cationic palladium complex-catalyzed cross-coupling of alkynes with alkenes is presented, which occurs selectively under the hydrosilylation conditions using trichlorosilane. The unique reaction might be well understood in terms of an initial hydropalladation of a given 1-alkyne to form regioselectively a 1-alkenylpalladium species, which, in turn, undergoes easily and specifically an alkene insertion. The resulting homoallylic organopalladium species terminates one catalytic cycle by substituting the palladium center with a trichlorosilyl group to give product(s).     

Cycloisomerization of enynes catalyzed by iron(0)-ate complexes

The 18-electron half-sandwich iron(0) complex [CpFe(C2H4)2] [Li(tmeda)] (1a), which is readily available in multigram quantities from inexpensive starting materials (ferrocene, ethylene, Li sand), is shown to be an efficient catalyst for the Alder-ene reaction of various 1,6(7)-enynes. Thereby, the presence of the labile alkene ligands in the ferrate catalyst is essential since the analogous complex [CpFe(CO)2]Na is catalytically incompetent. The cycloisomerizations catalyzed by 1a are compatible with various functional groups and turned out to be highly diastereoselective with regard to the configuration of the newly formed alkenes as well as relative stereochemistry at the ring junction. The alkyne moiety in the substrates may be terminal, silylated, or substituted with various groups, including cyclopropane rings. Likewise, the alkene substructure can be varied to a large extent, with cycloalkenes of ring sizes >/=7 being particularly suitable.     

Recent advances in "formal" ruthenium-catalyzed [2+2+2] cycloaddition reactions of diynes to alkenes

"Formal" and standard RuII-catalyzed [2+2+2] cycloaddition of 1,6-diynes to alkenes gave bicyclic 1,3-cyclohexadienes in relatively good yields. When terminal 1,6-diynes 1 were used, two isomeric bicyclic 1,3-cyclohexadienes 4 or 6 were obtained, depending on the acyclic or cyclic nature of the alkene partner. When unsymmetrical substituted 1,6-diynes 7 were used, the reaction with acyclic alkenes took place regio- and stereoselectively to afford bicyclic 1,3-cyclohexadienes 8. A cascade process that behaves as a "formal" RuII-catalyzed [2+2+2] cycloaddition explained these results. Initially, a Ru-catalyzed linear coupling of 1,6-diynes 1 and 7 with acyclic alkenes occurs to give open 1,3,5-trienes of type 3, which after a thermal disrotatory 6e(-) pi-electrocyclization led to the final 1,3-cyclohexadienes 4 and 8. When disubstituted 1,6-diyne 10 was used with electron-deficient alkenes, new exo-methylene cyclohexadienes 12 arose from a competitive reaction pathway.     

Rh-catalyzed enantioselective diboration of simple alkenes: reaction development and substrate scope

[Reaction: see text]. The rhodium-catalyzed reaction between bis(catecholato)diboron and simple alkenes results in the syn addition of the diboron across the alkene. The resulting 1,2-bis(boronate) is subsequently oxidized to provide the 1,2-diol. In the presence of enantiomerically enriched Quinap ligand, high enantioselection in the diboration can be achieved. The reaction is highly selective for trans- and trisubstituted alkenes and can be selective for some monosubstituted alkenes as well. The development of this reaction is described as is the substrate scope and experiments that are informative about the reaction mechanism and competing pathways.     

Revealing carbocations in highly asynchronous concerted reactions: The ene-type reaction between dithiocarboxylic acids and alkenes

The ene-type reaction between (dithio)carboxylic acids and alkenes has been studied computationally by DFT and topological (analysis of the electron localization function, ELF) methods. The reaction proceeds under kinetic control and the observed differences in regioselectivity are well-explained by the relative stability of the different transition structures. In agreement with experimental observations, electron-rich alkenes lead to Markownikoff adducts while electron-poor alkenes lead to Michael adducts. In all cases the reaction proceeds through an only transition structure (one kinetic step) although a different synchronicity was observed depending on the alkene electronics. The ELF analysis of the reactions corroborates the existence of a transient carbocation (hidden intermediate) in the reactions with electron-rich alkenes. On the other hand, electron-poor alkenes proceed through a more synchronous concerted mechanism. It can be predicted that with electron-rich alkenes bearing highly donating the transient carbocations might be captured by a nucleophile.