Cyclization of acetylene

Reversible interaction of the carbonyl group with an azine lone-pair (cf. 245) should facilitate substitution adjacent to the heteroatom by the anion of a )3-hydroxyethyl ketone. A similar cyclic intermediate (246) is presumably responsible for the cyclization of acetylene dicarboxylic esters with azines. Similar cyclic intermediates... 

Scheme 12 Reductive cyclization of acetylenic aldehydes via Rh-catalyzed asymmetric hydrogenation...

In summary, the Zn-Al and Zn-Ga based-hydrotalcites were found as very effective supports for docking a Rh(TPPTS)3Cl complex. The process was carried out via ionic exchange and occurred without any damage of die support structure or of the complex. The resulted catalysts were found to catalyze the cyclization of acetylenic carboxylic acids to the corresponding 5-membered ring heterocycles in good to excellent yields. The basic properties of the support also allow a clean and selective reaction of unsubstituted acetylenic carboxylic acids. 

The cyclization of acetylenic homolog 124 leads to the selective formation of cyclopentylideneindane 125, and in this case, the nucleophile size does not affect the reaction. 

Recently, addition of organorhodium species to nitriles has been reported.420 4203 4201 Intermolecular reaction of benzonitrile with phenylborate (accompanied with r//w-aryiation) (Equation (65)), arylative cyclization of acetylenic nitriles (Equation (66)), and cyclization of 2-cyanophenylboronic acid with alkynes or strained alkenes (Equation (67)) are proposed to proceed via this process. 

Reductive Cyclization of Acetylenic and Allenic Carbonyl Compounds 524... 

The transition metal-catalyzed domino reactions will undoubtedly have a splendid future which is underlined by the increasing number of publications in this area Steglich et al. presented an approach to arcyriacyanin A via a domino Heck reaction between a bromo(indolyl)maleiimide and 4-bromoindole.1811 The synthesis of 33 -bifurans by Ling et al. was achieved using a novel palladium-catalyzed domino dimerization and subsequent cyclization of acetylenic ketones.1821 Other applications of a combination of Heck reactions for domino processes are the syntheses of aza-heterocydes developed by our group.1831... 

Palladium(0)-catalysed coupling of non-conjugated dienes, aryl iodides and stabilized carbon nucleophiles has been developed468. An incredibly high yield (86%) of pentacycle 343 has been obtained from a Pd(0)-catalysed zipper reaction of acetylenic pentaene 342. The reaction is triggered off by a Pd-catalysed cyclization of acetylenic bond and the first olefinic bond469. 

Very recently, new ruthenium catalysts, for example RuCl2(triazol-5-ylidene) (p-cymene) [11] and the catalytic system generated in situ from [RuCl2(p-cym-ene)]2, tris(p-chlorophenyl)phosphine, and 4-dimethylaminopyridine [12], have provided efficient catalysts for synthesis of the same type of enol ester. The regio-selective cyclization of acetylenic acids containing a terminal triple bond to give unsaturated lactones was performed in the presence of catalytic amounts of Ru(tris(pyrazolyl)borate)(PhC=C(Ph)C CPh)(PMe2iPr2) [13],... 

For the corresponding cyclization of acetylenic amides 418, Nagasaka etal.347,347a used a mixture of silver(i) triflate and lithium hexamethyldisilazide as the precatalyst. The process is probably initiated by the coordination of AgN(SiMe3)2 (generated in situ from AgOTf and LHMDS) to the triple bond, followed by nucleophilic attack of the lithium amide at the activated alkyne which affords (ZVy-alkylidene-y-butyrolactams 419 in high yields (Scheme 122). 

The indium-mediated cyclization of acetylenic aldehyde 59 affords the corresponding ethenylidenecyclohexanol derivative 60 in 44% yield (Equation (37)).217 A similar intramolecular allenylation of 61 prompted by indium in aqueous THF produces chromane 62 in good yield (Equation (38)). 

Shono and coworkers [91] reported an interesting reductive cyclization of acetylenic ketones (XII) [Eq. (12)]. The cyclized products (XIII) were formed in good yields with some stereoselectivity. Perichon and coworkers [92] also reported that the reductive crosscoupling of 1,3-diynes with carbon dioxide gave exclusively the corresponding cis addition products in the presence of a Ni(II) complex, as shown in Eq. (13). 

Isomerization of acetylenic Grignard reagents affords the corresponding cyclic alkenyl Grignards [2,7]. Many cyclizations of acetylenic Grignard reagents are found in the literature [9]. Equations (37) [118,119] and (38) [119] represent recent, synthetically useful examples, in which the silyl group plays a dual role to facilitate the cyclization and to increase the stereoselectivity. 

A brand-new methodology for synthesizing glycals from noncarbohydrate precursors, one based on cyclization of acetylenic alcohols, has emerged from the field of metalorganics. Molybdenum pentacarbonyl-trialkylamine complexes have been found to efficiently catalyze cyclization of l-alkyn-4-ols to substituted dihydrofurans [233,234]. This same transformation has been successfully carried out on asymmetrically substituted alcohols the furanoid glycals 132, 134, and 135 (O Scheme 45) so obtained have in turn been used as intermediates in the synthesis of nucleosides [235]. 

The growth of Pd on Au(lll) using atomic beam deposition has been studied by Koel et al. [67]. Initial stages of Pd on Au deposited by electrochemistry have also been investigated on Au(lll) and Au(lOO) [68, 69]. The analysis of the reactivity of Pd deposits with respect to CO chemisorption [70] and to the cyclization of acetylene to benzene [71] has been the subject of a few experimental works. A theoretical work has been devoted to the study of electronic factors governing ethylene hydrogenation and dehydrogenation activity of pseudomorphic Pd on Au(l 11) [21]. 

Badderley CJ, Ormerod RM, Stephenson AW, Lambert RM (1995) Surface-structure and reactivity in the cyclization of acetylene to benzene with Pd overlayers and Pd/Au surface alloys on Au(lll). J Phys Chem 99 5146... 

Pacchioni G, Lambert RM (1994) Cyclization of acetylene over Pd(lll) a theoretical study of reaction mechanisms and surface intermediates. Surf Sci 304 208... 

The preferred catalysts for the one-step co-cyclization of acetylene and acetonitrile (or alkyl cyanides in general) to give a-picoline (or 2-alkylpyridines) are i/ -Cp-cobalt cod or / -trimethylsilyl-Cp-cobalt cod (eq. (2)). The a-picoline synthesis is best performed in pure nitrile without any additional solvent [5 d]. 

The catalytic reaction may also be ctirried out using two different alkynes. For example, the co-cyclization of acetylene and propyne with acetonitrile yields a mixture of dimethylpyridines (lutidines) in addition to a-picoline and the isomeric collidines. The co-cyclization, however, turned out to be nonselective. For experimental details see [5 g]. The cobalt-catalyzed co-cyclization of benzonitrile and acetylene at 77 -Cp-cobalt cod gives 2-phenylpyridine in high yield. 

Acetylenic Compounds.— The reductive cyclization of acetylenic ketones with naphthalene radical-anions has been studied for a variety of 4,5-secocholest-3-yn-5-one analogues (139). Products vary with structural type three examples are illustrated in Scheme 5. Results are interpreted in terms of initial electron transfer from the reagent to form a ketyl radical-anion, followed by intramolecular radical attack on the triple bond. ... 

Syntheses of heterocycles by catalytic intramolecular cyclization of acetylenic compounds 03MI31. 

DICARBOXYLATE. A rhodium-catalyzed cyclization of acetylenes with a-diazocarbonyls provides a nice route to ETHYL 2-METHYL-5-PHENYL-... 

Cyclization of acetylenic ketones to allyl alcohols by one electron reduction with Li/NH3 also electrochemically (Shono) or by Sml2 (Kagan-Molander) (see 1st edition). 

Several mechanisms have been proposed for the cyclization of acetylene to benzene derivatives. Whitesides and Ehrmann proved the concerted cyclotri-merization mechanism by carrying out the cyclization reaction with CD3C=CCH3 catalyzed by Cr, Co, Ni, and Ti complexes and analyzing the distribution of D in the reaction products 141>. 

It is well known that palladium chloride is an active catalyst for the cyclization of acetylene to form cyclobutadiene as well as benzene derivatives. In this reaction an intermediate complex was isolated which has a palladium carbon a-bond, the formation of which was explained by an insertion mechanism, not by concerted cyclotrimerization. When this complex obtained from butyne and palladium chloride was decomposed by various means, 5-vinyl-l,2,3,4,5-penta-methylcyclopentadiene and 5-(l-chlorovinyl)-l,2,3,4,5-pentamethylcyclopenta-diene were obtained in addition to hexamethylbenzene... 

An extension of this intramolecular carbolithiation was recently disclosed for the preparation of bis-exocyclic 1,3-dienes by the 5-exo-dig cyclization of -acetylenic vinyllithiums. Through subsequent Diels-Alder chemistry, they serve as useful precursors of polycyclic ring systems [14a] (Scheme 7-13). 

---