Alkyl-substituted Acetylenes Research Articles

C(sp)-H, S-H, and Sn-H Bond Activation with a Cobalt(I) Pincer Complex.

This study focuses on the stoichiometric reactions of {2,6-(iPr2PO)2C6H3}Co(PMe3)2 with terminal alkynes, thiols, and tin hydrides as part of an effort to develop catalytic, two-electron processes with cobalt. This specific Co(I) pincer complex proves to be effective for cleaving the C(sp)-H, S-H, and Sn-H bonds to give oxidative addition products with the general formula {2,6-(iPr2PO)2C6H3}CoHX(PMe3) (X = alkynyl, thiolate, and stannyl groups) along with the free PMe3. These reactions typically reach completion when the substituents on acetylene, sulfur, and tin are electron-withdrawing groups (e.g., phenyl, pyridyl, and alkenyl groups). In contrast, alkyl-substituted acetylenes, 1-pentanethiol, and tributyltin hydride are partially converted due to the equilibria with the corresponding oxidative addition products. The Co(I) pincer complex is not a hydrothiolation catalyst but capable of catalyzing the hydrostannation of terminal alkynes with Ph3SnH to produce β-(Z)-alkenylstannanes selectively.

Anti-Diradical Formation in 1,3-Dipolar Cycloadditions of Nitrile Oxides to Acetylenes.

By means of high level quantum chemical calculations (B2PLYPD and CCSD(T)), the mechanisms of the reaction of nitrile oxides with alkenes and alkynes were investigated. We were able to show that in the case of alkenes, regardless of the chosen substituents, the concerted mechanism is always energetically favored as compared to a two-step process, which runs through an anti-diradical species. In the case of alkynes, the concerted mechanism is favored only for the reaction of alkyl-substituted acetylenes. For aryl-substituted acetylenes, the activation barrier toward the anti-diradical is equal to or lower than the activation barrier of the concerted reaction. This reversal of the reaction paths is not only limited to nitrile oxides as dipolarophiles. Conditions favoring the anti-diradical path are the presence of a triple bond in both the 1,3-dipole and the dipolarophile and additionally an aryl substituent attached to the alkyne. The featured energy relationships between the reaction paths are able to explain the experimentally observed byproducts of the reaction of nitrile oxides with arylacetylenes. The discovered differences for the preferred reaction path of 1,3-dipolar cycloadditions to acetylenes should be of considerable interest to a broader field of chemists.

Investigation of alkyne regioselectivity in the ni-catalyzed benzannulation of cyclobutenones.

A Ni-catalyzed benzannulation reaction of cyclobutenones and alkynes provides a rapid synthesis of heavily substituted phenols. The regioselectivity of this reaction can be modulated by variation of substituents on the alkyne. Though the incorporation of Lewis basic donors provides modest selectivities, the use of aryl substituents can provide high levels of regiocontrol. Finally, alkynylboronates derived from alkyl-substituted acetylenes provide both high yields and regioselectivities. This study suggests that alkynes bearing one sp(2) - and one sp(3) -based substituent can undergo benzannulation with high levels of regiocontrol whereby the sp(3) -based group is incorporated ortho-to the phenolic OH.

Synthesis and Reactions of Ferrocenyl-capped Long-chain Alkynes

Alkyl-substituted acetylenes and diacetylenes with terminal ferrocenyl groups were synthesised starting from ferrocene. The reactivity of the triple bonds in the resulting compounds was used to prepare cobalt-alkyne complexes. Cyclotrimerisation of the monoacetylene resulted in a hexa-substituted benzene derivative with six pending ferrocenyl groups.

Synthesis of isoquinolines and pyridines by the palladium/copper-catalyzed coupling and cyclization of terminal acetylenes and unsaturated imines: the total synthesis of decumbenine B.

Monosubstituted isoquinolines and pyridines have been prepared in good to excellent yields via coupling of terminal acetylenes with the tert-butylimines of o-iodobenzaldehydes and 3-halo-2-alkenals in the presence of a palladium catalyst and subsequent copper-catalyzed cyclization of the intermediate iminoalkynes. In addition, isoquinoline heterocycles have been prepared in excellent yields via copper-catalyzed cyclization of iminoalkynes. The choice of cyclization conditions is dependent upon the nature of the terminal acetylene that is employed, as only aryl and alkenyl acetylenes cyclize under the palladium-catalyzed reaction conditions that have been developed. However, aryl-, vinylic-, and alkyl-substituted acetylenes undergo palladium-catalyzed coupling and subsequent copper-catalyzed cyclization in excellent yields. The total synthesis of the isoquinoline natural product decumbenine B has been accomplished in seven steps and 20% overall yield by employing this palladium-catalyzed coupling and cyclization methodology.

Synthesis of Isoquinolines and Pyridines by the Palladium- and Copper-Catalyzed Coupling and Cyclization of Terminal Acetylenes

3-Substituted isoquinolines have been synthesized by the coupling of aryl- and alkenyl-substituted terminal acetylenes with the tert-butylimine of o-iodobenzaldehyde in the presence of a palladium catalyst. Alkyl-substituted acetylenes fail to undergo this annulation process. However, the intermediate iminoalkynes containing aryl, alkenyl, and alkyl substituents produced from these palladium-catalyzed reactions undergo copper-catalyzed cyclization in excellent yields and short reaction times. Isoquinolines and pyridines have thus been prepared in a one-pot synthesis via coupling of aryl-, alkenyl-, and alkyl-substituted terminal acetylenes with the tert-butylimines of o-iodobenzaldehydes or 3-halo-2-alkenals in the presence of a palladium catalyst and subsequent copper-catalyzed cyclization of the resulting iminoalkynes. The total synthesis of the isoquinoline natural product decumbenine B has been accomplished in seven steps and 20% overall yield by employing this palladium-catalyzed coupling/cyclization...

Diphenylacetylene complexes of tungsten containing π-acceptor ligands

Reaction of PhC2Ph with [WCl2(PhC2Ph)(PMe3)3] gave [WCl2(PhC2Ph)2(PMe3)2]1 for which IR and NMR spectral data indicated cis-chloro ligands. trans phosphines and mutually-cis diphenylacetylene ligands. This geometry was confirmed by X-ray crystallography. The average W–C bond lengths are 2.071 (3)A. The 13C NMR spectrum showed the acetylenic carbon resonance at δ 185.45. The alkynes HC2H and PhC2H reacted with [WCl2(PhC2Ph)(PMe3)3] giving [WCl2(PhC2Ph)(HC2H)(PMe3)2]2 and [WCl2(PhC2Ph)(PhC2H)(PMe3)2]3. The phenylacetylene ligand gave rise to asymmetry in complex 3 leading to an AB system 31P-{1H} NMR spectrum. Different values of 1J(PW) indicated small differences in the W–P bonding. Alkyl-substituted acetylenes did not react cleanly with [WCl2(PhC2Ph)(PMe3)3]. The complexes [WCl2(PhC2Ph)(PrnC2Me)(PMe3)2]4 and [WCl2(PhC2Ph)(MeC2Me)(PMe3)2]5 were prepared by sodium–mercury amalgam reduction of [WCl3(PhC2Ph)(PMe3)2] with hex-2-yne or but-2-yne present but were not particularly stable in solution. Sodium-mercury amalgam reduction of [WCl3(PhC2Ph)(PMe3)2] under ethylene or propene gave [WCl2(PhC2Ph)(CH2CH2)(PMe3)2]6 or [WCl2(PhC2Ph)(MeCHCH2)(PMe3)2]9. Complex 9 was unstable in solution and was characterised by NMR spectroscopy. The 13C NMR acetylenic carbon resonances in complexes 6 and 9 are at δ 218.11 and 218.82. Reduction of [WCl3(PhC2Ph)(PMe3)2] under cis- or trans-but-2-ene or 2-methylpropene afforded [WCl2(PhC2Ph)(PMe3)3] and [WCl2(PhC2Ph)(PMe3)2]x10 as characterised by NMR spectroscopy. Reduction of [WCl3(PhC2Ph)L2](L = PMe3or PMePh2) under CO gave [WCl2(PhC2Ph)(CO)(PMe3)2]11 and [WCl2(PhC2Ph)(CO)(PMePh2)2]12 for which X-ray crystal structure determinations showed cis-chloro ligands, trans phosphines and mutually-cis PhC2Ph and CO ligands. The W–C and C–C bond lengths for complex 11 are 2.009(5), 2.024(5) and 1.341 (6)A, respectively. Overall the reactions showed that the diphenylacetylene ligand in these complexes acts like an organoimido or oxo ligand.

Synthesis of fluorine-containing arylacetic acids via the DielsAlder reaction of polyfluorinated cyclohexa-2,4-dienones with acetylenes

2,3,4,5,6-Pentafluoro-6-chloro-2,4-cyclohexadien-1-one ( I) and 3-(pentafluorophenoxy)-6-chloro-2,4,5,6-tetrafluoro-2,4-cyclohexadien-1-one( II) readily undergo the DielsAlderreaction with aryl- and alkyl-substituted acetylenes giving [2+4]-cycloadducts in goodyield. The latter undergo aromatization by nucleophilic agents under mild conditions toα-chlorophenylacetic acid derivatives containing two or three fluorine atoms in the aromaticring.

ChemInform Abstract: Regio‐ and Stereochemistry of Bromochlorinations of Alkynes with Molecular Bromine Chloride and Dichlorobromate(1‐) Ion.

AbstractThe bromochlorination of the acetylenes (I) using the reagents (IV) or (VIII) can occur in a regiospecific or non‐regiospecific as well as in a stereospecific or non‐stereospecific manner.

Regio- and Stereochemistry of Bromochlorinations of Alkynes with Molecular Bromine Chloride and Dichlorobromate(1−) Ion

Abstract The bromochlorination of phenyl- and alkyl-substituted acetylenes with tetrabutylammonium dichlorobromate(1−) (1) in dichloromethane was found to be anti-stereospecific and nonregiospecific (regiospecific in the case of phenylacetylene). Whereas the addition of molecular bromine chloride (2) to phenyl-substituted acetylenes was found to give nonstereospecific and regiospecific adducts, the reaction of alkyl-substituted acetylenes gave anti-stereospecific and nonregiospecific adducts. These results suggest that the addition of 1 involves an attack of chloride ion to a three-centered π-complex in the product-forming stage, and that the addition of 2 to phenyl-substituted acetylenes involves a vinyl cation intermediate (but a bridged bromonium ion intermediate in the case of alkyl-substituted acetylenes).

Polymerization of terminal alkynes by rare earth compound/trialkylaluminum catalysts

Rare earth naphthenates and phosphonates with trialkylaluminum compounds were investigated as catalysts for the polymerization of 1-alkynes. The neodymium naphthenate/ triisobutylaluminum system was found to promote high conversions to polymers of relatively high molecular weights in the case of alkyl-substituted acetylenes. The polymers obtained showed, by infrared spectroscopy, high cis content in the polyconjugated chains.

One-step synthesis of tertiary alkyl-substituted acetylenes from silylacetylenes

The reaction of silylacetylenes with tertiary alkylyhalides under Lewis acid catalysis gives tertiary alkyl-substituted alkunes in high yields.