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.     

Stereocontrolled synthesis of bicyclic ureas and sulfamides via Pd-catalyzed alkene carboamination reactions

The synthesis of bicyclic ureas and sulfamides via palladium-catalyzed alkene carboamination reactions between aryl/alkenyl halides/triflates and alkenes bearing pendant cyclic sulfamides and ureas is described. The substrates for these reactions are generated in 3–5 steps from commercially available materials, and products are obtained in good yield with up to >20:1 diastereoselectivity. The stereochemical outcome of the sulfamide alkene addition is consistent with a mechanism involving anti-aminopalladation of the alkene, whereas the stereochemical outcome of the urea alkene addition is consistent with a syn-aminopalladation mechanism.     

Sc(OTf)3-catalyzed synthesis of indoles and SnCl4-mediated regioselective hydrochlorination of 5-(arylamino)pent-3-yn-2-ones

Highly substituted indole derivatives bearing alkyl and aryl moieties can be prepared by Sc(OTf)(3)-catalyzed Friedel-Crafts alkenylation of 5-(arylamino)pent-3-yn-2-ones. In addition, a method for regioselective hydrochlorination of 5-(arylamino)pent-3-yn-2-ones mediated by SnCl(4) in moderate to good yields (up to 84%) has been developed. The resulting exclusive Z-selectivity of the C-Cl bond can be further exploited using cross C-N coupling reactions.     

Stereoselective synthesis of imidazolidin-2-ones via Pd-catalyzed alkene carboamination. Scope and limitations

A method for the synthesis of imidazolidin-2-ones from N-allylureas and aryl or alkenyl bromides via Pd-catalyzed carboamination reactions is described. The N-allylurea precursors are prepared in one step from readily available allylic amines and isocyanates, and the Pd-catalyzed reactions effect the formation of a C-C bond, a C-N bond, and up to two stereocenters in a single step. Good diastereoselectivities are obtained for the conversion of substrates bearing allylic substituents to 4,5-disubstituted imidazolidin-2-ones, and excellent selectivity for the generation of products resulting from syn-addition across the alkene is observed when substrates derived from cyclic alkenes or E-1,2-disubstituted alkenes are employed. A brief discussion of reaction mechanism and product stereochemistry is presented.     

Alkene-directed, nickel-catalyzed alkyne coupling reactions

In alkene-directed, nickel-catalyzed coupling reactions of 1,3-enynes with aldehydes and epoxides, the conjugated alkene dramatically enhances reactivity and uniformly directs regioselectivity, independent of the nature of the other alkyne substituent (aryl, alkyl (1 degrees , 2 degrees , 3 degrees )) or the degree of alkene substitution (mono-, di-, tri-, and tetrasubstituted). These observations are best explained by a temporary interaction between the alkene and the transition metal center during the regioselectivity-determining step. The highly substituted 1,3-diene products are useful in organic synthesis and, in conjunction with a Rh-catalyzed, site-selective hydrogenation, afford allylic and homoallylic alcohols that previously could not be prepared in high regioselectivity (or at all) with related Ni-catalyzed alkyne coupling reactions.     

Transition-Metal-Free Coupling of Polyfluorinated Arenes and Functionalized,Masked Aryl Nucleophiles

A chemoselective C(sp²)−C(sp²) coupling of sufficiently electron-deficient fluorinated arenes and functionalized N-aryl-N’-silyldiazenes as masked aryl nucleophiles is reported. The fluoride-promoted transformation involves the in situ generation of the aryl nucleophile decorated with various sensitive functional groups followed by a stepwise nucleophilic aromatic substitution (S_NAr). These reactions typically proceed at room temperature within minutes. This catalytic process allows for the functionalization of both coupling partners, furnishing highly fluorinated biaryls in good yields.     

Hydrogen-Atom-Transfer-Initiated Radical/Polar Crossover Annulation Cascade for Expedient Access to Complex Tetralins

A radical/polar crossover annulation between allyl-substituted arenes and electron-deficient alkenes is described. Cobalt-catalyzed hydrogen atom transfer (HAT) facilitates tandem radical C−C bond formation that generates functionalized tetralin products in the face of potentially problematic hydrofluorination, hydroalkoxylation, hydrogenation, alkene isomerization, and radical polymerization reactions. The reactions proceed under mild conditions that tolerate many functional groups, leading to a broad substrate scope. This powerful ring-forming reaction very quickly assembles complex tetralins that are the formal products of the largely infeasible Diels–Alder cycloadditions of styrenes.     

Selectivity in the addition reactions of organometallic reagents to aziridine-2-carboxaldehydes: the effects of protecting groups and substitution patterns

Good to excellent stereoselectivity has been found in the addition reactions of Grignard and organozinc reagents to N-protected aziridine-2-carboxaldehydes. Specifically, high syn selectivity was obtained with benzyl-protected cis, tert-butyloxycarbonyl-protected trans, and tosyl-protected 2,3-disubstituted aziridine-2-carboxaldehydes. Furthermore, rate and selectivity effects of ring substituents, temperature, solvent, and Lewis acid and base modifiers were studied. The diastereomeric preference of addition is dominated by the substrate aziridines' substitution pattern and especially the electronic character and conformational preferences of the nitrogen protecting groups. To help rationalize the observed stereochemical outcomes, conformational and electronic structural analyses of a series of model systems representing the various substitution patterns have been explored by density functional calculations at the B3LYP/6-31G* level of theory with the SM8 solvation model to account for solvent effects.     

Photochemistry of 4-(2'-aminoethyl)quinolones: enantioselective synthesis of tetracyclic tetrahydro-1aH-pyrido[4',3':2,3]- cyclobuta[1,2-c] quinoline-2,11(3H,8H)-diones by intra- and intermolecular [2 + 2]-photocycloaddition reactions in solution

Enantioselective [2 + 2]-photocycloaddition reactions on 4-(2'-aminoethyl)quinolones in solution were studied using the enantiomerically pure complexing agent 1 as source of chirality. The intermolecular reactions of fully N-protected substrates 5a-5c with different 2-alkyl-substituted acrylates 12-15 represent the first systematic study on the diastereoselectivity of their intermolecular [2 + 2]-photocycloadditions to unsymmetrically 1,1-disubstituted olefins (75-91% yield, d.r. = 58/42-95/5). N-Benzylic-protected photoproducts exo-16a/b-19a/b could easily be converted into lactams 20a/b-23a/b by a sequence of Boc deprotection and thermal lactamization (74-98% yield). Identical products 20a-22a were directly accessible by the intramolecular [2 + 2]-photocycloaddition of acrylic acid amides 2-4 (41-61% yield). The suitability of both pathways for an enantioselective reaction variant was proven (70-92% ee). Thus, tetracyclic lactams possessing the carbon framework C were obtained with good yields and enantioselectivities of up to 92% ee in intramolecular reactions. Comparative investigation of both routes showed that quinolone dimerization was the single most decisive factor preventing a complete chirality transfer. Functional group manipulations were successfully conducted with the primary photoproduct exo-17a. Finally, a new and unexpected type of benzylic hydrogen abstraction-radical cyclization reaction was discovered for substrate 5a, which explains the photochemical instability of substrates 2-5 under short wavelength irradiation (lambda = 300 nm).     

Organometallic dithiolene complexes of the group 8-10 metals: reactivities, structures and electrochemical behavior

Research progress in the organometallic dithiolene complexes such as [Cp(or Cp*)M(dithiolene)] (M = Co, Rh, Ir, Ni), [(C(6)R(6))Ru(dithiolene)] and [(C(4)R(4))Pt(dithiolene)] complexes during the past decade is described and the reactivities, structures and electrochemical behavior are summarized in this paper. The five-membered metalladithiolene ring (MS(2)C(2)) undergoes addition reactions to the M[double bond, length as m-dash]S bond to form 18-electron adducts by an imido, alkylidene, alkene or norbornene group and also undergoes dimerizations on the basis of the unsaturation in the ring. The aromaticity of the ring causes substitution reactions on the dithiolene carbon by a C-centered radical, S-centered radical or succinimide group when the ring has a C-H bond. Furthermore a dithiolene-dithiolene homo-coupling reaction by an acid or dithiolene-aryl cross-coupling occurs based on the aromaticity in the ring. Dissociations of the 18-electron adducts are observed by those thermolyses, photolyses, electrochemical redox reactions and other chemical reactions with tertiary phosphorus compounds. One representative example of them is the imido adduct dissociation with PR(3) under heating toward the intramolecular imido migration to a Cp ligand. Since all products are rearomatized by those adduct dissociations, it is concluded that the 'coexistence of aromaticity and unsaturation' in the metallacycle mediates the diverse chemical reactions.     

Synthesis and anti-HIV activity of new homo acyclic nucleosides, 1-(pent-4-enyl)quinoxalin-2-ones and 2-(pent-4-enyloxy)quinoxalines

A series of acyclonucleosides 6,7-disubstituted 1-(pent-4-enyl)quinoxalin-2-one derivatives and the O-analogs were synthesized by a one-step condensation of the corresponding quinoxaline bases with 5-bromo-1-pentene.The acyclonucleosides prepared were assayed against HIV-1 and HIV-2 in MT-4 cells. 6,7-Dimethyl-2-(pent-4-enyloxy)quinoxaline showed inhibition of HIV-1 with EC₅₀ value of 0.22 ± 0.08 μg/ml and a therapeutic index of 13. This means that it was cytotoxic to MT-4 cells at CC₅₀ of 2.6 ± 0.1 μg/ml. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1243–1250, August, 2007.     

Nickel-catalyzed Heck-type reactions of benzyl chlorides and simple olefins

Nickel-catalyzed intermolecular benzylation and heterobenzylation of unactivated alkenes to provide functionalized allylbenzene derivatives are described. A wide range of both the benzyl chloride and alkene coupling partners are tolerated. In contrast to analogous palladium-catalyzed variants of this process, all reactions described herein employ electronically unbiased aliphatic olefins (including ethylene), proceed at room temperature, and provide 1,1-disubstituted olefins over the more commonly observed 1,2-disubstituted olefins with very high selectivity.     

Enantiomerically pure alpha-amino acid synthesis via hydroboration-suzuki cross-coupling

The Garner aldehyde-derived methylene alkene 5 and the corresponding benzyloxycarbonyl compound 25 undergo hydroboration with 9-BBN-H followed by palladium-catalyzed Suzuki coupling reactions with aryl and vinyl halides. After one-pot hydrolysis-oxidation, a range of known and novel nonproteinogenic amino acids were isolated as their N-protected derivatives. These novel organoborane homoalanine anion equivalents are generated and transformed under mild conditions and with wide functional group tolerance: electron-rich and -poor aromatic iodides and bromides (and a vinyl bromide) all undergo efficient Suzuki coupling. The extension of this methodology to prepare meso-DAP, R,R-DAP, and R,R-DAS is also described.     

Synthesis of unsaturated amino alcohols through unexpectedly selective Ru-catalyzed cross-metathesis reactions

[reaction: see text]. A two-step synthesis of N-protected unsaturated amino alcohols is disclosed that relies on an unexpectedly selective cross-metathesis (CM) involving allyl cyanide and pent-4-en-1-ol. The solution concentration and the identity of the Ru complex used are critical to the selectivity and efficiency of CM reactions. The intermediate obtained by CM is converted efficiently to the final desired products through a one-pot nitrile reduction/amine protection procedure.     

Photoinduced three-component reactions of tetracyanobenzene with alkenes in the presence of 1,3-dicarbonyl compounds as nucleophiles

Photoinduced three-component reactions between tetracyanobenzene (TCNB), an aromatic olefin, and a beta-dicarbonyl compound afford products composed of the three components via formal elimination of hydrogen cyanide, leading to the vicinal dialkylation of the olefin and the alpha-alkylation of the beta-dicarbonyl compounds. It is shown that these reactions are initiated by photoinduced electron transfer (PET) from the olefin to the singlet excited TCNB and proceed by a nucleophile-olefin combination, aromatic substitution (NOCAS) reaction sequence with the enolized beta-dicarbonyl compound as a nucleophile. Therefore, aromatic olefins are suitable substrates in photo-NOCAS reactions when TCNB is used as the electron acceptor. In addition, these results show that the enol of beta-dicarbonyl compound serves as a carbon nucleophile to trap the alkene cation radical in PET reactions to lead to C-C bond formation.     

Intramolecular alkene carboamination reactions for the synthesis of enantiomerically enriched tropane derivatives

The synthesis of tropane derivatives via intramolecular Pd-catalyzed alkene difunctionalization reactions is described. Enantiopure N-aryl-γ-aminoalkenes bearing an aryl or alkenyl halide adjacent to the amino group were converted to benzo- or cycloalkenyl-fused tropane products in good yield and with no loss of enantiopurity.     

Copper-catalyzed intramolecular alkene carboetherification: synthesis of fused-ring and bridged-ring tetrahydrofurans

Fused-ring and bridged-ring tetrahydrofuran scaffolds are found in a number of natural products and biologically active compounds. A new copper-catalyzed intramolecular carboetherification of alkenes for the synthesis of bicyclic tetrahydrofurans is reported herein. The reaction involves Cu-catalyzed intramolecular addition of alcohols to unactivated alkenes and subsequent aryl C-H functionalization provides the C-C bond. Mechanistic studies indicate a primary carbon radical intermediate is involved and radical addition to the aryl ring is the likely C-C bond-forming mechanism. Preliminary catalytic enantioselective reactions are promising (up to 75% ee) and provide evidence that copper is involved in the alkene addition step, likely through a cis-oxycupration mechanism. Catalytic enantioselective alkene carboetherification reactions are rare and future development of this new method into a highly enantioselective process is promising. During the course of the mechanistic studies a protocol for alkene hydroetherification was also developed.     

Dealkenylative Alkenylation: Formal σ-Bond Metathesis of Olefins

The dealkenylative alkenylation of alkene C(sp³)−C(sp²) bonds has been an unexplored area for C−C bond formation. Herein 64 examples of β-alkylated styrene derivatives, synthesized through the reactions of readily accessible feedstock olefins with β-nitrostyrenes by ozone/Fe^II-mediated radical substitutions, are reported. These reactions proceed with good efficiencies and high stereoselectivities under mild reaction conditions and tolerate an array of functional groups. Also demonstrated is the applicability of the strategy through several synthetic transformations of the products, as well as the syntheses of the natural product iso-moracin and the drug (E)-metanicotine.     

Allenyl azide cycloaddition chemistry. synthesis of pyrrolidine-containing bicycles and tricycles via the possible intermediacy of azatrimethylenemethane species

Thermolysis of 5-azidoallenes bearing a C(1) methyl group and either an aryl ring or an alkene on C(1) furnishes tricyclic (from the aryl substrates) or bicyclic (from the alkenyl substrates) pyrrolidine products following formal H-CN addition across an intermediate imine. High levels of diastereoselectivity are observed in all cases studied. This reaction cascade presumably passes through unobserved triazoline and azatrimethylenemethane diyl intermediates en route to product.     

Efficient synthesis of 1,3,4-oxadiazoles promoted by NH4Cl

An efficient and inexpensive approach to the synthesis of 2-substituted and 2,5-disubstituted 1,3,4-oxadiazoles from arylhydrazides and orthoesters is reported using catalytic NH₄Cl. The conditions are mild, and thus, compatible with a variety of functional groups. The optimized reaction is performed using 30 mol % of NH₄Cl in 100% EtOH and is generally complete within 1 h for non-aromatic orthoesters and 2–10 h for aromatic orthoesters. The reaction permits both electron-releasing and electron-withdrawing groups on the arylhydrazide substrate. Most products are formed in high yields and require only minimal purification. Compared with earlier reports, the current reactions proceed in shorter time and require less of the orthoester.