Alkynes, terminal

Terminal alkyne anions are popular reagents for the acyl anion synthons (RCHjCO"). If this nucleophile is added to aldehydes or ketones, the triple bond remains. This can be con verted to an alkynemercury(II) complex with mercuric salts and is hydrated with water or acids to form ketones (M.M.T. Khan, 1974). The more substituted carbon atom of the al-kynes is converted preferentially into a carbonyl group. Highly substituted a-hydroxyketones are available by this method (J.A. Katzenellenbogen, 1973). Acetylene itself can react with two molecules of an aldehyde or a ketone (V. jager, 1977). Hydration then leads to 1,4-dihydroxy-2-butanones. The 1,4-diols tend to condense to tetrahydrofuran derivatives in the presence of acids. 

Terminal alkynes are only reduced in the presence of proton donors, e.g. ammonium sulfate, because the acetylide anion does not take up further electrons. If, however, an internal C—C triple bond is to be hydrogenated without any reduction of terminal, it is advisable to add sodium amide to the alkyne solution Hrst. On catalytic hydrogenation the less hindered triple bonds are reduced first (N.A. Dobson, 1955, 1961). 

Terminal alkynes undergo the above-mentioned substitution reaction with aryl and alkenyl groups to form arylalkynes and enynes in the presence of Cul as described in Section 1.1.2.1. In addition, the insertion of terminal alkynes also takes place in the absence of Cul, and the alkenylpalladium complex 362 is formed as an intermediate, which cannot terminate by itself and must undergo further reactions such as alkene insertion or anion capture. These reactions of terminal alkynes are also treated in this section. 

Insertion of alkyne Tandem insertion polycyclic Terminal alkyne Coupling 

The thioboration of terminal alkynes with 9-(alkylthio)-9-borabicyclo[3.3.1]-nonanes (9-RS-9-BBN) proceeds regio- and stereoselectively by catalysis of Pd(Ph,P)4 to produce the 9-[(Z)-2-(alkylthio)-l-alkeny)]-9-BBN derivative 667 in high yields. The protonation of the product 667 with MeOH affords the Markownikov adduct 668 of thiol to 1-alkyne. One-pot synthesis of alkenyl sulfide derivatives 669 via the Pd-catalyzed thioboration-cross-coupling sequence is also possible. Another preparative method for alkenyl sulfides is the Pd-catalyzed cross-coupling of 9-alkyl-9-BBN with l-bromo-l-phe-nylthioethene or 2-bromo-l-phenylthio-l-alkene[534]. 

Metal derivatives of terminal alkynes, RC2H. Transition metals form complex acetylides (e.g. (M(C = CR) ]- ) often containing the metal in low oxidation states. 

The alkynyl iodide 359 undergoes cross-coupling with a terminal alkyne to give the 1,3-diyne 360[264]. No homocoupling product is formed. This reaction offers a good synthetic method for unsymmetrical 1,3-diynes. 

The alkenylzirconium 685, prepared by hydrozirconation of a terminal alkyne with hydrozirconocene chloride, reacts with alkenyl halide to afford the conjugated diene 686(545]. The Zr reagent can be used even in the presence of the carbonyl group in 687, which is sensitive to Al and Mg reagents. 

The novel intramolecular reaction of the alkenyl bromide with the terminal alkyne in 328, followed by intramolecular Diels-Alder reaction, afforded the highly strained dynemicin A structure 329 in one stepf237]. 

The formation of disubstituted alkynes by coupling of terminal alkynes, followed by intramolecular attack of an alcohol or amine, is used for the preparation of benzofurans and indoles. The benzo[il)]furan 356 can be prepared easily by the reaction of o-iodophenol with a terminal alkyne[262]. The 2-substituted indole 358 is prepared by the coupling of 2-ethynylaniline (357) with aryl and alkenyl halides or triflates, followed by Pd(ll)-catalyzed cycliza-tion[263]. 

The 2,3-alkadienyl acetate 851 reacts with terminal alkynes to give the 2-alkynyl-1,3-diene derivative 852 without using Cul and a base. In the absence of other reactants, the terminal alkyne 853 is formed by an unusual elimination as an intermediate, which reacts further with 851 to give the dimer 854. Hydrogenolysis of 851 with formic acid affords the 2, 4-diene 855[524]. 

The carbonylation of aryl iodides in the presence of terminal alkynes affords the acyl alkynes 565. Bidentate ligands such as dppf give good results. When PhjP is used, phenylacetylene is converted into diphenylacetylene as a main product[4l5]. Triflates react similarly to give the alkynyl ketones 566[4I6], In 

Chlorobenzenes activated by coordination of Cr(CO)3 react with terminal alkynes[253). The 1-bromo-1,2-alkadiene 346 reacts with a terminal alkyne to afford the alka-l,2-dien-4-yne 347[254], Enol tritlates are used for the coupling with terminal alkynes. Formation of 348 in the syntheses of ginkgolide[255] and of vitamin D are examples[256] Aryl and alkenyl fluorides are inert. Only bromide or iodide is attacked when the fluoroiodoalkene 349 or fluoroiodoar-ene is subjected to the Pd-catalyzed coupling with alkynes[257-259]. 

Both chlorines of 1,1-dichloroethylene (340) react stepwise with different terminal alkynes to form the unsymmetrical enediyne 341 [250]. The coupling of the dichloroimine 342 with tin acetylide followed by hydrolysis affords the dialkynyl ketone 343[2511. The phenylthioimidoyl chloride 344 undergoes stepwise reactions with two different tin acetylides to give the dialkynylimine 345[252], 

The alkynyl ketones 840 can be prepared by the reaction of acyi chlorides with terminal alkynes, Cul in the presence of Et3N is the cocatalyst[719]. (1-Alkynyl)tributylstannanes are also used for the alkynyl ketone synthesis[720]. The a,. 3-alkynic dithio and thiono esters 842 can be prepared by the reaction of the corresponding acid chloride 841 with terminal alkynes[721,722]. 

Reactions of Propargylic Compounds Catalyzed by Pd(Q) 5.2 Reactions with Alkenes and Terminal Alkynes 

Many examples of insertions of internal alkynes are known. Internal alkynes react with aryl halides in the presence of formate to afford the trisubstituted alkenes[271,272]. In the reaction of the terminal alkyne 388 with two molecules of iodobenzene. the first step is the formation of the phenylacetylene 389. Then the internal alkyne bond, thus produced, inserts into the phenyl-Pd bond to give 390. Finally, hydrogenolysis with formic acid yields the trisubstituted alkene 391(273,274], This sequence of reactions is a good preparative method for trisubstituted alkenes from terminal alkynes. 

Organoboranes undergo transmetallation. 1-Hexenylboronic acid (438) reacts with methyl acrylate via the transmetallation with Pd(OAc)2, giving methyl 2,4-nonadienoate (439)[399], The ( )-alkenylboranes 440, prepared by the hydroboration of terminal alkynes, are converted into the alkylated ( )-alkenes 441 by treatment with an equivalent amount of Pd(OAc)2 and triethylamine[400]. The ( )-octenylborane 442 reacts with CO in MeOH in the 

Using a catalyst system of PdCl2, CuCH, HCl, and O2, the internal alkyne 20 is carbonylated at room temperature and 1 atm to give unsaturated esters[19]. This apparently oxidizing system leads to non-oxidative cu-hydroesterilica-tion. With terminal alkynes, however, oxidative carbonylation is observed. 

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