Unsymmetrical Alkynes Research Articles

Regioselective Reductive Cross-Coupling Reactions of Unsymmetrical Alkynes.

The present microreview summarizes our progress over the last few years in defining regioselective reductive cross-coupling reactions of unsymmetrical alkynes with terminal- and internal alkynes, aldehydes, and imines. We begin with a brief historical perspective of metal-mediated reductive dimerization reactions of aromatic alkynes and discuss the challenges associated with "crossed" versions of this mode of reactivity. Next, a collection of available methods that allow for regioselective reductive cross-coupling of internal alkynes with terminal and internal alkynes, aldehydes, and imines is summarized. After an examination of the requirements for regioselectivity in these cases, the logic behind our design of alkoxide-directed titanium-mediated reductive cross-coupling reactions is presented. A nomenclature is introduced to delineate the presumed mechanistic origin of regioselection associated with each reaction design, and a presentation of alkoxide-directed regioselective reductive cross-coupling reactions of alkynes follows. Throughout, principal issues related to reactivity and selectivity are discussed to assess scope and limitations of available methods and to describe the broad challenges that exist for defining complex fragment union reactions based on reductive cross-coupling chemistry.

ChemInform Abstract: Predictable and Regioselective Insertion of Internal Unsymmetrical Alkynes in Rhodium‐Catalyzed Cycloadditions with Alkenyl Isocyanates.

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Rh-Catalyzed Cycloadditions of Unsymmetrical Alkynes with Isocyanates

Rh-Catalyzed Cycloadditions of Unsymmetrical Alkynes with Isocyanates

Ni-Catalyzed Synthesis of 4-Quinolones from Alkynes and Isatoic Anhydrides

The synthesis of 4-quinolones 3 is reported via the Ni-catalyzed decarboxylative carboamination of alkynes 2 with isatoic anhydrides 1. The reaction is tolerant of a range of R¹ substituents, including alkyl, acyl and both electron-rich and electron-poor aryl groups. A variety of alkynes may also be used. Unsymmetrical alkynes gave mixtures of the two possible regioisomers (ratio from 2:1 to 6:1), where the larger group is placed preferentially at C-3, most likely due to steric repulsion between the larger alkyne substituent (R4) and the Ni ligand (see scheme).

Predictable and Regioselective Insertion of Internal Unsymmetrical Alkynes in Rhodium-Catalyzed Cycloadditions with Alkenyl Isocyanates

A regioselective, rhodium-catalyzed cycloaddition between a variety of internal, unsymmetrical alkynes is described. We document the impact of both steric and electronic properties of the alkyne on reaction course, efficiency, and enantioselectivity. The substituent that better stabilizes a positive charge or the larger group, all else being equal, inserts distal to the carbonyl moiety in a predictable and controllable fashion. The reaction scope is broad and the enantioselectivities are high, providing an "instruction manual" for substrate choice when utilizing this reaction as a synthetic tool.

Copper-catalyzed alkyne–aryne coupling reaction under microwave conditions: preparation of unsymmetric and symmetric di-substituted alkynes

Several unsymmetric and symmetric alkynes were prepared excellent to modest yields by generating benzyne from the reaction of 2-(trimethylsilyl)phenyl triflate with CsF in the presence of CuI and terminal alkyne under microwave heating for 30 min at 150 °C. Using conventional heating, the reactions required 24 h reaction time.

Indole Synthesis via Rhodium Catalyzed Oxidative Coupling of Acetanilides and Internal Alkynes

The oxidative synthesis of highly functionalized indoles from simple anilines and internal alkynes mediated by a rhodium(III) catalyst is described. Good yields are obtained for a variety of aniline substrates, and good regioselectivity is obtained for the more sterically accessible position when meta-substituted anilines are used. Symmetrical and unsymmetrical alkynes react efficiently with high (>40:1) C2/C3 regioselectivity when aryl/alkyl substituted alkynes are used.

Mo‐Catalyzed Cross‐Metathesis Reaction of Propynylferrocene (Eur. J. Inorg. Chem. 25/2008)

AbstractThe cover picture shows the reaction scheme for the cross‐metathesis of (prop‐1‐yn‐1‐yl)ferrocene with substituted propynes effected by catalysts generated from [Mo(CO)6] and halophenols, situated on a background full of typical ferrocene, autumn‐like colors. The metathesis reaction affords good yields of unsymmetric ferrocenyl alkynes, thus offering a new alternative approach toward their preparation. Also shown are the crystal structure of methyl 4‐[(ferrocenyl)ethynyl]benzoate and the orbital diagram of its solid‐state π···π stacked dimer as calculated by DFT methods. A survey of various substrates and catalysts, results of X‐ray structure analysis and DFT calculations as well as electrochemical data for a series of (phenylethynyl)ferrocenes substituted at the phenyl ring are discussed in the article by M. Kotora, P. Štěpnička et al. on p. 3911 ff.

Synthetic, Mechanistic, and Computational Investigations of Nitrile-Alkyne Cross-Metathesis

The terminal nitride complexes NW(OC(CF 3) 2Me) 3(DME) ( 1-DME), [Li(DME) 2][NW(OC(CF 3) 2Me) 4] ( 2), and [NW(OCMe 2CF 3) 3] 3 ( 3) were prepared in good yield by salt elimination from [NWCl 3] 4. X-ray structures revealed that 1-DME and 2 are monomeric in the solid state. All three complexes catalyze the cross-metathesis of 3-hexyne with assorted nitriles to form propionitrile and the corresponding alkyne. Propylidyne and substituted benzylidyne complexes RCW(OC(CF 3) 2Me) 3 were isolated in good yield upon reaction of 1-DME with 3-hexyne or 1-aryl-1-butyne. The corresponding reactions failed for 3. Instead, EtCW(OC(CF 3)Me 2) 3 ( 6) was prepared via the reaction of W 2(OC(CF 3)Me 2) 6 with 3-hexyne at 95 degrees C. Benzylidyne complexes of the form ArCW(OC(CF 3)Me 2) 3 (Ar = aryl) then were prepared by treatment of 6 with the appropriate symmetrical alkyne ArCCAr. Three coupled cycles for the interconversion of 1-DME with the corresponding propylidyne and benzylidyne complexes via [2 + 2] cycloaddition-cycloreversion were examined for reversibility. Stoichiometric reactions revealed that both nitrile-alkyne cross-metathesis (NACM) cycles as well as the alkyne cross-metathesis (ACM) cycle operated reversibly in this system. With catalyst 3, depending on the aryl group used, at least one step in one of the NACM cycles was irreversible. In general, catalyst 1-DME afforded more rapid reaction than did 3 under comparable conditions. However, 3 displayed a slightly improved tolerance of polar functional groups than did 1-DME. For both 1-DME and 3, ACM is more rapid than NACM under typical conditions. Alkyne polymerization (AP) is a competing reaction with both 1-DME and 3. It can be suppressed but not entirely eliminated via manipulation of the catalyst concentration. As AP selectively removes 3-hexyne from the system, tandem NACM-ACM-AP can be used to prepare symmetrically substituted alkynes with good selectivity, including an arylene-ethynylene macrocycle. Alternatively, unsymmetrical alkynes of the form EtCCR (R variable) can be prepared with good selectivity via the reaction of RCN with excess 3-hexyne under conditions that suppress AP. DFT calculations support a [2 + 2] cycloaddition-cycloreversion mechanism analogous to that of alkyne metathesis. The barrier to azametalacyclobutadiene ring formation/breakup is greater than that for the corresponding metalacyclobutadiene. Two distinct high-energy azametalacyclobutadiene intermediates were found. These adopted a distorted square pyramidal geometry with significant bond localization.

Synthesis of Naphthalenes through Three-Component Coupling of Alkynes, Fischer Carbene Complexes, and Benzaldehyde Hydrazones via Isoindole Intermediates

The synthesis of naphthalene derivatives through three-component coupling of 2-alkynylbenzaldehyde hydrazones with carbene complexes and electron-deficient alkynes has been examined. The reaction involves formation of an isoindole derivative, followed by intramolecular Diels-Alder reaction, followed by nitrene extrusion. The reaction was highly regioselective using unsymmetrical alkynes.

Synthesis of Multisubstituted Pyridines from Ketoximes and Alkynes

A rhodium-catalyzed coupling of α,β-unsaturated ketoximes with alkynes to afford substituted pyridines is reported. Two unsymmetrical alkynes (R4 = Ph, R5 = H, SiMe3) were tested and their regioselective coupling was demonstrated. The substrate scope was fairly well studied. Based on known metal-catalyzed reactions, a chelation assisted CH-activation mechanism was proposed and supported by isolation of an intermediate, which was shown to be converted into the final product under refluxing toluene conditions. This appears to be the first report of this type of reaction, that is, a catalytic addition of an ­alkenyl C-H bond to alkynes followed by cyclization and elimination.

Reactivity of triruthenium thiophyne and furyne clusters: competitive S–C and P–C bond cleavage reactions and the generation of highly unsymmetrical alkyne ligands

The synthesis and reactivity of the thiophyne and furyne clusters [Ru3(CO)7(mu-dppm)(mu3-eta2-C4H2E)(mu-P(C4H3E)2)(mu-H)] (E = S, O) is reported. Addition of P(C4H3E)3 to [Ru3(CO)10(mu-dppm)] (1) at room temperature in the presence of Me3NO gives simple substitution products [Ru3(CO)9(mu-dppm)(P(C4H3E)3)] (E = S, 2; E = O, 3). Mild thermolysis in the presence of further Me3NO affords the thiophyne and furyne complexes [Ru3(CO)7(mu-dppm)(mu3-eta2-C4H2E)(mu-P(C4H3E)2)(mu-H)] (E = S, 4; E = O, 6) resulting from both carbon-hydrogen and carbon-phosphorus bond activation. In each the C4H2E (E = S, O) ligand donates 4-electrons to the cluster and the rings are tilted with respect to the mu-dppm and the phosphido-bridged open triruthenium unit. Heating 4 at 80 degrees C leads to the formation of the ring-opened cluster [Ru3(CO)5(mu-CO)(mu-dppm)(mu3-eta3-SC4H3)(mu-P(C4H3S)2)] (5) resulting from carbon-sulfur bond scission and carbon-hydrogen bond formation and containing a ring-opened mu3-eta3-1-thia-1,3-butadiene ligand. In contrast, a similar thermolysis of 3 affords the phosphinidene cluster [Ru3(CO)7(mu-dppm)(mu3-eta2-C4H2O)(mu3-P(C4H3O))] (7) resulting from a second phosphorus-carbon bond cleavage and (presumably) elimination of furan. Treatment of 4 and 6 with PPh3 affords the simple phosphine-substituted products [Ru3(CO)6(PPh3)(mu-dppm)(mu3-eta2-C4H2E)(mu-P(C4H3E)2)(mu-H)] (E = S, 8; E = O, 9). Both thiophyne and furyne clusters 4 and 6 readily react with hydrogen bromide to give [Ru3(CO)6Br(mu-Br)(mu-dppm)(mu3-eta2-eta1-C4H2E)(mu-P(C4H3E)2)(mu-H)] (E = S, 10; E = O, 11) containing both terminal and bridging bromides. Here the alkynes bind in a highly unsymmetrical manner with one carbon acting as a bridging alkylidene and the second as a terminally bonded Fisher carbene. As far as we are aware, this binding mode has only previously been noted in ynamine complexes or those with metals in different oxidation states. The crystal structures of seven of these new triruthenium clusters have been carried out, allowing a detailed analysis of the relative orientations of coordinated ligands.

Platinum-Catalyzed Regio- and Stereoselective Arylthiolation of Internal Alkynes

Unsymmetrical internal alkynes such as ethyl phenylpropiolate (2b) successfully underwent Pt-catalyzed decarbonylative arylthiolation by thioesters. The regio- and stereoselective insertion of 2b into an S-Pt bond was confirmed by reaction with a platinum complex with an S-Pt-Cl framework.

Synthesis of 1,4‐Dithiins from Pentathiepins.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Hydroheteroarylation of Alkynes under Mild Nickel Catalysis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

A Study of the Scope and Regioselectivity of the Ruthenium-Catalyzed [3 + 2]-Cycloaddition of Azides with Internal Alkynes

[3 + 2]-Cycloadditions of alkyl azides with various unsymmetrical internal alkynes in the presence of CpRuCl(PPh3)2 as catalyst in refluxing benzene have been examined, leading to 1,4,5-trisubstituted-1,2,3-triazoles. Whereas alkyl phenyl and dialkyl acetylenes undergo cycloadditions to afford mixtures of regioisomeric 1,2,3-triazoles, acyl-substituted internal alkynes react with complete regioselectivity. In addition, propargyl alcohols and propargyl amines were found to react with azides to afford single regioisomeric products.

Direct Coupling Reactions of Alkynylsilanes Catalyzed by Palladium(II) Chloride and a Di(2‐pyridyl)methylamine‐Derived Palladium(II) Chloride Complex in Water and in NMP

AbstractSymmetrical internal alkynes can be prepared either by diarylation of mono‐ and bis(trimethylsilyl)acetylene (TMSA and BTMSA) catalyzed by ligand‐less palladium(II) chloride or by a di(2‐pyridyl)methylamine‐derived palladium(II) chloride complex 1 (typical 0.1–1 mol % of Pd loading) in water using pyrrolidine as base and tetra‐n‐butylammonium bromide as additive. Alternatively, this same process is performed in NMP in the presence of tetra‐n‐butylammonium acetate (TBAA) as base with even lower Pd loadings (0.001–1 mol % Pd). The same reaction conditions are applied to the synthesis of unsymmetrical internal alkynes by monoarylation of silylated terminal alkynes. Aryl iodides can be coupled with TMSA, BTMSA and silylated terminal alkynes under heating or at room temperature, whereas for aryl bromides couplings are performed under water reflux or at 110 °C in the case of NMP. Complex 1 can be reused during several cycles either in water or in NMP without loss of catalytic activity. These simple reaction conditions allow the preparation of internal alkynes without secondary products, most probably by succesive protiodesilylation‐Sonogashira coupling.

Synthesis of 1,4-Dithiins from Pentathiepins

Fused aromatic and heterocyclic 1,2,3,4,5-pentathiepins react with triphenylphosphine and alkynes bearing electron-withdrawing groups to give the corresponding 1,4-dithiins in high yields. Unsymmetrical alkynes add regioselectively to afford products in agreement with the electron distribution in a proposed reaction intermediate. A mechanism for these reactions is proposed.

Hydroheteroarylation of Alkynes under Mild Nickel Catalysis

Nickel complexes having a bulky tri(sec-alkyl)phosphine ligand catalyze hydroheteroarylation of alkynes at 35 degrees C. Selective activation of an Ar-H bond over an Ar-CN bond of N-protected 3-cyanoindoles is achieved by a proper choice of ligand and/or an N-protecting group. The catalysis is applicable to a diverse range of heteroarenes to afford cis-hydroheteroarylation products in highly chemo- and stereoselective manners. Excellent regioselectivity is observed with unsymmetrical alkynes to give the corresponding heteroaryl-substituted ethenes having a larger substituent trans to an aryl group.

Nickel-Mediated Regioselective [2 + 2 + 2] Cycloaddition of Carboryne with Alkynes

Transition metal-benzyne complexes have found many applications in organic synthesis, mechanistic studies, and the synthesis of functional materials. In sharp contrast, the chemistry of transition metal-carboryne complexes, especially late transition metal complexes, is virtually unknown. This communication reports a novel nickel-mediated regioselective [2 + 2 + 2] cycloaddition reaction of carboryne with alkynes via the Ni-carboryne intermediate (eta2-C2B10H10)Ni(PPh3)2. Because of the bulkiness of the carborane moiety, a high regioselectivity was achieved in the reactions involving unsymmetrical alkynes. This work furnishes a novel method for the preparation of highly substituted benzocarboranes which are difficult to obtain by other methods.