Synthesis of internal alkynes

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Cyanophosphates (CPs) can be easily prepared from carbonyl compounds, and their reaction with azides provides tetrazoles. A successive termal fragmentation of the tetrazoles generates alkylidene carbenes that undergo [1,2]-rearrangement and are transformed into homologous internal and terminal alkynes in good yields under neutral conditions.
H. Yoneyama, M. Numata, K. Uemura, Y. Usami, S. Harusawa, J. Org. Chem., 2017, 82, 5538-5556.

A mild catalytic synthesis of alkynes via a tandem Pd-catalyzed decarboxylation/elimination of enol triflates as readily available starting materials. The method offers broad functional group tolerance, and the ability to access terminal, internal, and halogenated alkynes.
C. Munteanu, D. E. Frantz, Org. Lett., 2016, 18, 3937-3939.

Tetra-n-butylammonium is a mild and efficient base for the elimination of bromoalkenes. Treatment of 1,1-dibromoalkenes, (Z)-1-bromoalkenes, and internal bromoalkenes with 5 equiv of TBAF • 3 H₂O in DMF yielded terminal and internal alkynes in high yields without problems regarding the presence of water.
M. Okutani, Y. Mori, J. Org. Chem., 2009, 74, 442-444.

A convenient copper-catalyzed radical reaction of readily available N-tosylhydrazones provides vinyl sulfones with excellent E stereoselectivity and broad substrate scope. An efficient one-pot synthesis of alkynes from N-tosylhydrazones has also been achieved.
S. Mao, Y.-R. Gao, X.-Q. Zhu, D.-D. Guo, Y.-Q. Wang, Org. Lett., 2015, 17, 1692-1695.

Aromatic aldehyde p-tosylhydrazones can directly be converted into 1-arylprop-1-ynes in the presence of calcium carbide as an acetylene source. This simple protocol offers use of a readily available source of acetylene, operational simplicity, high yield, and broad substrate scope.
L. Gao, Z. Li, Synlett, 2019, 30, 1580-1584.

Terminal and internal acetylenes were obtained in good to excellent isolated yields from carbonyl compounds by converting the carbonyl functionality into the enol nonaflate intermediate followed by elimination to give the C-C triple bond. The one-pot transformations were uniformly induced by phosphazene bases combined with mildly electrophilic nonafluorobutane-1-sulfonyl fluoride.
I. M. Lyapkalo, M. A. K. Vogel, E. V. Boltukhina, J. Vavřík, Synlett, 2009, 558-561.

Coupling of benzaldehydes and benzyl chloride derivatives under basic conditions with an organocatalyst gives good yields of alkynes. The catalyst, a highly reactive sulfenate anion, is readily generated in situ from air-stable precursors. This method represents an attractive organocatalytic alternative to well-established stoichiometric approaches to alkynes and to transition-metal-based alkyne functionalization methods.
M. Zhang, T. Jia, C. Y. Wang, P. J. Walsh, J. Am. Chem. Soc., 2015, 137, 10346-10350.

A palladium-catalyzed C-C cross-coupling reaction of gem-dibromoalkenes and halobenzenes provides terminal alkynes in one pot through a tandem elimination-hydrodebromination process. This convenient reaction proceeded under copper-free Sonogashira coupling reaction conditions in good to excellent yields.
Y. Ji, N. Zhong, Z. Kang, G. Yan, M. Zhao, Synlett, 2018, 29, 209-214.

A palladium-catalyzed domino coupling reaction of 1,1-dibromo-1-alkenes with triarylbismuth nucleophiles furnishes disubstituted alkynes directly. The couplings are very fast, affording high yields of alkynes in a short reaction time.
M. L. N. Rao, D. N. Jadhav, P. Dasgupta, Org. Lett., 2010, 12, 2048-2051.

The ionic liquid 1-methyl-3-pentylimidazolium fluoroborate, [pmIm]BF₄, is an efficient catalyst as well as solvent for the stereoselective debromination of various vicinal-dibromides to the corresponding (E)-alkenes in high yields under microwave irradiation. This reaction does not require any conventional reducing agent, and the ionic liquid can be recycled.
B. C. Ranu, R. Jana, J. Org. Chem., 2005, 70, 8621-8624.

Dichloroindium hydride (Cl₂InH) generated in situ from the combination of a catalytic amount of indium(III) chloride and sodium borohydride in acetonitrile reduces activated vic-dibromides to the corresponding (E)-alkenes in excellent yields.
B. C. Ranu, A. Das, A. Hajira, Synthesis, 2003, 1012-1014.

d² Re(V) alkylidyne complexes bearing two decorated phosphino-phenolates (POs) and a labile pyridine ligand can efficiently promote homometathesis as well as ring-closing alkyne metathesis (RCAM) of methyl-capped diynes. A wide range of functional groups including ethers, esters, ketones, aldehydes, alcohols, phenols, amines, amides, and heterocycles are tolerated.
M. Cui, H. H. Y. Sung, I. D. Williams, G. Jia, J. Am. Chem. Soc., 2022, 144, 6349-6360.