Acetylene catalysts, nickel complexes

Nickel plays a role in the Reppe polymeriza tion of acetylene where nickel salts act as catalysts to form cyclooctatetraene (62) the reduction of nickel haUdes by sodium cyclopentadienide to form nickelocene [1271 -28-9] (63) the synthesis of cyclododecatrienenickel [39330-67-1] (64) and formation from elemental nickel powder and other reagents of nickel(0) complexes that serve as catalysts for oligomerization and hydrocyanation reactions (65). 

One of the most interesting alternatives to the Shirakawa catalyst has been the systems disclosed by Luttinger 22-23) and later elaborated by Lieser et al. 24). The tris(2-cyanoethyl)phosphine complex of nickel chloride reacts with sodium boro-hydride to produce a catalyst system capable of polymerizing acetylene in solutions in either alcohol or, quite remarkably, water. A more efficient catalyst is obtained by replacing the nickel complex with cobalt nitrate. Interest in Luttinger polyacetylene seems to have waned in the last few years. 

Acetylene hydrogenation. Selective hydrogenation of acetylene to ethylene is performed at 200°C over sulfided nickel catalysts or carbon-monoxide-poisoned palladium on alumina catalyst. Without the correct amount of poisoning, ethane would be the product. Continuous feed of sulfur or carbon monoxide must occur or too much hydrogen is chemisorbed on the catalyst surface. Complex control systems analyze the amount of acetylene in an ethylene cracker effluent and automatically adjust the poisoning level to prepare the catalyst surface for removing various quantities of acetylene with maximum selectivity. 

Vinyl sulfides have been prepared by the catalytic addition of the S—H bond of thiols (85) to terminal alkynes (86) under solvent-free conditions using the nickel complex Ni(acac)2 (47). High alkyne conversions (up to 99%) were achieved after 30 min at 40 °C in favor of the corresponding Markovnikov products (87) (equation 23). Other metal acetylacetonate complexes were examined for this reaction, but none showed any improvement over the nickel catalyst. Mechanistic details suggest that alkyne insertion into the Ni—S bond is important to the catalytic cycle and that nanosized structural units comprised of [Ni(SAr)2] represent the active form of the catalyst. Isothiocyanates and vinyl sulfides have been produced in related Rh(acac)(H2C=CH2)2 (6) and VO(acac)2 (35) catalyzed sulfenylation reactions of aryl cyanides and aryl acetylenes, respectively. 

The tendency of acetylene to undergo thermal oligo- and polymerization to give (actually in very low yields ) linear polyenes or benzene is a reaction known for almost a century. Formally, the same process is implied in the formation of 137 from four acetylene molecules. Yet this deceptively simple cyclotetramer-ization scheme turned to be a viable reaction only after Reppe s discovery that a simple catalyst, nickel(n) cyanide, could serve as a highly efficient device to control the course of acetylene oligomerization. It is the ability of this catalyst to form a complex, 401, with four molecules of acetylene that ensured the required selectivity of cyclotetramer formation (Scheme 2.135). 

The most efficient catalysts for the homo Diels-Alder reactions of norbornadiene were found to be cobalt and nickel complexes. The general mechanistic pathway that has been proposed for these reactions has been depicted in equation 161. According to this mechanism, co-ordination of norbornadiene and the olefin or acetylene to the metal center gives 557, which is in equihbrium with metallocyclopentane complex 558. Then, insertion of the olefin or acetylene in the metal-carbon bond takes place to form 559. Reductive elimination finally liberates the deltacyclane species. 

This was the appearance of publications by W. Reppe and co-workers in followed by Badische Anilin und Soda Fabrik patents/ They showed that various triphenylphosphine complexes of nickel, especially [Ni(CO)2(PPh5)2](Ph = QHs), were more effective than other nickel complex catalysts for the polymerization of olefinic and acetylenic substances and that others, especially [NiBr2(PPh3)2], catalyzed the formation of acrylic acid esters from alcohols (ROH), acetylene, and carbon monoxide ... 

Giacomelli and his co-workers have described a further application of the bis-(N-methylsalicilaldimine)nickel, [Ni(mesal)2], complex as a catalyst. This complex effects the head-to-tail dimerization of terminal acetylenes in the presence of stoicheiometric amounts of di-isobutylzinc to give conjugated enynes (102). The conversion and yield are both reduced in the presence of bulky substituents on the acetylene, but two equivalents of PhaP added to the nickel complex can improve the yield in these cases. 

Many of these nickel carbonyl-base compounds have been prepared primarily for use in infrared studies, some of the conclusions of which are summarized briefly in Section II, C 30,41,46,47,48,50,51,127,349). The phosphine-nickel complexes have catalytic activity in the polymerization of acetylenes, and the mechanisms of these polymerizations have been studied 350, 351). Interest in these catalysts has led to an investigation of their phosphorus-31 NMR spectra, which may be qualitatively correlated with the accepted ideas on metal-ligand bonding (72). 

In the presence of the nickel complex bis-(N-methylsalicylaldimine)nickel, [NiCmesaOj], 1-bromo-alk-l-ynes react rapidly with trialkylalanes to give the corresponding alkylated acetylenes (43) in 80% yield, with the exception of 1 -bromo-1 -phenylacetylene which only reacts in low yield. In the absence of the nickel catalyst the reaction is very slow. One drawback of this method is that as in related reactions only one of the alkyl groups of the trialkylalane is used. 

The reaction of [2+2+2] cycloaddition of acetylenes to form benzene has been known since the mid-nineteenth century. The first transition metal (nickel) complex used as an intermediate in the [2+2+2] cycloaddition reaction of alkynes was published by Reppe [1]. Pioneering work by Yamazaki considered the use of cobalt complexes to initiate the trimer-ization of diphenylacetylene to produce hexasubstituted benzenes [54]. Vollhardt used cobalt complexes to catalyze the reactions of [2+2+2] cycloaddition for obtaining natural products [55]. Since then, a variety of transition complexes of 8-10 elements like rhodium, nickel, and palladium have been found to be efficient catalysts for this reaction. However, enantioselective cycloaddition is restricted to a few examples. Mori has published data on the use of a chiral nickel catalyst for the intermolecular reaction of triynes with acetylene leading to the generation of an asymmetric carbon atom [56]. Star has published data on a chiral cobalt complex catalyzing the intramolecular cycloaddition of triynes to generate a product with helical chirality [57]. 

A more complex reaction is involved in the cooligomerization of acetylenes and tert-butyl isocyanide using nickel acetate as the catalyst (Scheme 20)43 the nature of intermediate complexes leading to the formation of 2-cyano-5-terf-butylaminopyrroles has not been established. Cocyclization of tert-butyl isocyanide with coordinated hexafluoro-2-butyne gives rise to coordinated cyclopentadienone anils for molybdenum systems,44 hence the nature of acetylene substitutents and of the organometallic catalyst play crucial roles in these processes. The pyrrole products from the former reaction can be decomposed by sulfuric acid and the overall sequence provides a simple synthesis of 5-amino-2-cyanopyrroles (Scheme 20). 

Cyclotetramerization to form cyclooctatetraene occurs only with nickel.46,63 68 The best catalysts are octahedral Ni(II) complexes, such as bis(cyclooctatetraene) dinickel.46 Internal alkynes do not form cyclooctatetraene derivatives but participate in cooligomerization with acetylene. Of the possible mechanistic pathways, results with [l-13C]-acetylene81 favor a stepwise insertion process or a concerted reaction, and exclude any symmetric intermediate (cyclobutadiene, benzene). The involvement of dinuclear species are in agreement with most observations.46,82-84... 

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethylbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages. 

When an aqueous organic solution of acetylene is treated with CO at tSO C and 30 atm, in the presence of a catalytic amount of NifCOK. acrylic acid is formed with a selectivity of about 90%. In the presence of alcohols, the corresponding acrylic ester is formed with a selectivity of about 85%. The interesting thing with methyl acetylene, is that the major product ( 80%) is methyl methacrylate. The preferred catalyst t undoubtedly based on nickel, althou other Group Vlll metal-carbonyl complexes (e.g. Fe(CO)s) will catalyze these reactions. 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

The trimerization reaction with benzene derivatives could sometimes follow a similar path but, since it also occurs with disubstituted acetylenes such as HOCH2C = CCH2OH, some other mechanism must also be operating. Schrauzer explained the cyclization of tolane with a catalyst based on bisacrylonitrilenickel and triphenylphosphine by the so-called V-complex, multicenter processes. With this process, three alkyne molecules would successively coordinate with nickel and then the ring closure would take place (See Figure 6). 

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