Cyclooctatetraene from acetylene

Nickel catalysts for the syntheses of cyclic compounds were first successfully utilized by Reppe, who was able to prepare cyclooctatetraene from acetylene (65). This eight-membered ring synthesis, and also the preparation of cyclic products from strained olefins (e.g., bicycloheptene and norbornadiene) and acrylonitrile, have been adequately reviewed elsewhere (7) and will therefore not be considered further. A short account of the cyclization reactions of butadiene using nickel-containing catalysts has appeared previously in this series (/). The discovery of new synthetic possibilities and a deeper understanding of the mechanism of these reactions justify a more extensive treatment. 

The first breakthrough came m 1911 when Richard Willstatter prepared cyclooc tatetraene by a lengthy degradation of pseudopelletienne a natural product obtained from the bark of the pomegranate tree Today cyclooctatetraene is prepared from acetylene m a reaction catalyzed by nickel cyanide... 

The photoreaction of dimethyl acetylenedicarboxylate at room temperature was also reported by Grovenstein and co-workers [57,58]. From acetylene and benzene, only traces of cyclooctatetraene could be detected [59,60],... 

One of the first and perhaps most interesting examples of a metal-assisted, symmetry-forbidden reaction was Reppe s synthesis of cyclo-octatetraene from acetylene 34). in a careful study of this system, Schrauzer proposed a concerted mechanism in which the four a bonds of the cyclo-octatetraene are essentially formed simultaneously 35). He proposed an octahedral complex (54) with four acetylene hgands fitted to adjacent ligand coordination positions, spatially defining the incipient cyclooctatetraene. 

Metal, TT complexes of cyclooctatetraene were first described by Reppe ei al. (211) in 1948. Silver and copper complexes were obtained during the synthesis of this hydrocarbon from acetylene. These complexes of cyclooctatetraene have since been investigated in considerable detail by several groups of investigators (50, 114, l j l )- An x-ray analysis of the complex [(CgHg)Ag]+N03 (XXIV) indicates that the silver ion is coordinated to two nonadjacent bonds of cyclooctatetraene but is situated at differences from each. [(CgHg)Ag]+ units are, furthermore, joined together by weak bonds. 

Thermal cyclooligomerizations of olefins and alkynes require severe and often dangerous reaction conditions and the yields of cyclic products are usually very low. Acetylene ean be trimerized to benzene at 500 °C [1] and butadiene (BD) dimerizes at 270 °C and under high pressure to give small amounts of 1,5-cyclo-octadiene [2]. Reppe s discovery in 1940 that acetylene can be cyclotetramerized to cyclooctatetraene (COT) using a nickel catalyst [3] shows that transition metals can act as templates for the synthesis of cyclic hydrocarbons from acetylenic or olefinic building blocks (Scheme 1). 

A number of metal complexes catalyses specific alkyne polymerizations, giving rise to four-, six- or eight-membered carbocyclic rings. The first work in this area was the nickel-catalysed formation of cyclooctatetraene (40) from acetylene by the group of Reppe ", but since then formation of cyclic systems from acetylenes has been found to be also catalysed by molybdenum, cobalt, iridium and tantalum. ... 

An early synthesis of nylon 8 used cyclooctatetraene, which was formed from acetylene and then converted to nylon 8 as follows ... 

Another classical example is the synthesis of cyclooctatetraene in one step from acetylene when using a nickel-based catalyst (a Reppe process), whereas the first classical synthesis required a painful, multistep approach. 

Cuprous salts catalyze the oligomerization of acetylene to vinylacetylene and divinylacetylene (38). The former compound is the raw material for the production of chloroprene monomer and polymers derived from it. Nickel catalysts with the appropriate ligands smoothly convert acetylene to benzene (39) or 1,3,5,7-cyclooctatetraene (40—42). Polymer formation accompanies these transition-metal catalyzed syntheses. 

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). 

Ortho photocycloaddition was first reported in a U.S. patent [1] dated September 3, 1957. Irradiation of benzonitrile in the presence of various alkenes resulted in the formation of derivatives of l-cyanobicyclo[4.2.0]octa-2,4-diene. The first ortho photocycloaddition to benzene was reported in 1959 by Angus and Bryce-Smith [2], who discovered that benzene and maleic anhydride react to form a stable adduct at 60°C under the influence of ultraviolet radiation. This 1 2 adduct was formed from one molecule of benzene and two molecules of maleic anhydride. Two years later, Bryce-Smith and Lodge [3] found that acetylenes could also be photoadded to benzene. The isolated products were cyclooctatetraenes, formed by ring opening of the primarily formed bicyclo[4.2.0]octa-2,4,7-trienes. Since those early years, hundreds of examples of ortho photocycloadditions of alkenes to the benzene ring and many mechanistic investigations have been reported and they will be discussed in this chapter. 

The primary ortho adducts formed from benzene derivatives and acetylenes are derivatives of bicyclo[4.2.0]octa-2,4,7-triene. These products usually are not isolated but isomerize during the irradiation to cyclooctatetraenes [58,59,68,72], From combinations of hexafluorobenzene and pentafluoroalkoxybenzenes with various disubstituted acetylenes, however, the isolation of relative stable primary ortho adducts has been reported [65-67], In Scheme 46, the products of the photochemical reaction of hexafluorobenzene with but-2-yne are shown [67],... 

Benzene and cyclooctatetraene (COT) derivatives are formed by [2+2+2] and [2+2+2+2] cycloadditions of alkynes. At first the metallacyclopropene 107 and metallacyclopentadiene 108 are formed. Benzene and COT (106) are formed by reductive elimination of the metallacycloheptatriene 109 and the metallacyclononate-traene 110. Formation of benzene by the [2+2+2] cycloaddition of acetylene is catalysed by several transition metals. Synthesis of benzene derivatives from... 

The mass spectrum of cyclooctatetraeneiron tricarbonyl shows stepwise loss of the three carbonyl groups from the molecular ion, followed by elimination of acetylene giving C6H6Fe+, and further breakdown gives Fe+ (119). The mass spectra of two substituted cyclooctatetraene complexes (see Table VIII) show the molecular ion and stepwise loss of the carbonyl groups (83). 

Acrylonitrile and related compounds displace all the carbonyl groups from nickel carbonyl to form [(RCH CHCN)2Ni], in which the nitrile bonds through the olefinic double bond 222, 418). The bis(acrylonitrile) complex catalyzes many reactions, including the conversion of acrylonitrile and acetylene to heptatrienenitrile and the polymerization of acetylene to cyclooctatetraene 418). Cobalt carbonyl gave a brown-red amorphous material with acrylonitrile, which had i cn absorptions typical of uncoordinated nitrile groups, but interestingly, the presence of C=N groups was also indicated 419). In acidic methanol, cobalt carbonyl converts a,j8-unsaturated nitriles to saturated aldehydes 459). 

Reppe process. Any of several processes involving reaction of acetylene (1) with formaldehyde to produce 2-butyne-l,4-diol which can be converted to butadiene (2) with formaldehyde under different conditions to produce propargyl alcohol and, from this, allyl alcohol (3) with hydrogen cyanide to yield acrylonitrile (4) with alcohols to give vinyl ethers (5) with amines or phenols to give vinyl derivatives (6) with carbon monoxide and alcohols to give esters of acrylic acid (7) by polymerization to produce cyclooctatetraene and (8) with phenols to make resins. The use of catalysts, pressures up to 30 atm, and special techniques to avoid or contain explosions are important factors in these processes. 

The mechanism of the Reppe reaction has been studied by Colborn and Vollhardt. They used C-labelled acetylene and studied the labelling pattern of the cyclooctatetraene. Their results were not consistent with reaction through cyclobutadiene or by recombination of carbyne units resulting from triple-bond cleavage, but were consistent with either the concerted or stepwise mechanisms outlined below. 

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