Acetylene complexes with carbon monoxide

Titanium-acetylene complexes 29 generated in situ from acetylenes, Ti(0-i-Pr)4 and /-PrMgX react with imines to form azatitanacyclopentenes 30 which then react with carbon monoxide under atmospheric pressure to provide pyrroles 31 <96TL7787>. This reaction, which utilizes commercially available reagents is an improvement over a related procedure via the corresponding zirconium complexes under 1500 psi CO <89JA776>. 

When dicobalt octacarbonyl is treated with acetylenes (ac) and carbon monoxide under pressure, stable crystalline complexes of the type [Co2(CO)s(ac)] are formed. They can also be prepared from [Co2(CO)6(ac)] and carbon monoxide (199) ... 

In 2001, Van den Hoven and Alper reported the unexpected 2(Z)-6(ft)-47/-[l,4]-thiazepin-5-one 215, as the major product, from the reaction of acetylenic thiazole 214 with carbon monoxide and hydrogen in presence of a zwitterionic rhodium complex and triphenyl phosphite. After optimization of the reaction condition, the pressure, and the temperature, up to 90% yield is achieved with good selectivity for thiazepine 215 over thiazole side products 216-218 (Scheme 38) <2001 JA1017>. 

The polymer of methyl methacrylate (MMA) is known as Perspex. It is a clear transparent glasslike material with high hardness, resistance to fracture, and chemical stability. The conventional route, as shown by reaction 4.10, involves the reaction between acetone and hydrocyanic acid, followed by sequential hydrolysis, dehydration, and esterification. This process generates large quantities of solid wastes. An alternative route based on a homogeneous palladium catalyst has recently been developed by Shell. In this process a palladium complex catalyzes the reaction between propyne (methyl acetylene), methanol, and carbon monoxide. This is shown by reaction 4.11. The desired product is formed with a regioselectivity that could be as high as 99.95%. 

The conditions for the synthesis must differ, as the electronic configuration of each metal changes, but the intermediate in each case probably is a complex in which acetylene and carbon monoxide are each linked to two metal atoms. Cobalt and iron compounds having both acetylene and carbonyl bridges have already been synthesized 27). The report of the preparation of a dimeric nickel hydrocarbonyl, [NiH(CO)a]2 by Behrens 28) may well lead to the isolation of a siipilar acetylene complex with nickel. 

In some cases the insertion reactions are reversible. This is the case with the palladium complexes for carbon monoxide and sulfur dioxide. With acetylenes, preliminary studies indicate that the insertion products can be made to eliminate the acetylene on photolysis. The remaining A-frames shown in Figure 5.16 are formed irreversibly. 

In inert organic solvents nickel carbonyl reacts with diphenylacetylene to form tetraphenylcyclopentadienone and bis-tetraphenylc clopentadienone nickel (cf Fe(CO)5 p 229). In the presence of aqueous adds (e.g. acetic, hydrochloric) acetylenes are converted into a/3-unsaturated adds, e.g. acetylene itself yields acrylic add. With carbon monoxide under pressure, nickel carbonyl is continuously regenerated, so that the reaction becomes catalytic. The mechanism of this process is not yet understood, but it is found that water is essential. It is possible that formation of an intermediate <7-alkenyl nickel complex is involved, which affords the unsaturated add after carbonylation and hydrolysis of the acyl derivative ... 

An alternative route based on a homogeneous palladium catalyst was developed by Shell. In this process, a palladium complex catalyzes the reaction between propyne (methyl acetylene), methanol, and carbon monoxide. This is shown by reaction 4.5.2, and the product is formed with a regioselectivity that could be as high as 99.95%. 

Figure 1.6 Representative TEM image (a) and particle size distribution (b) obtained for a Au/Ti02 catalyst prepared by grafting of a [Au6(PPh3)6](BF4)2 complex onto Ti02 particles followed by appropriate reduction and oxidation treatments [42], The gold particles exhibit approximately spherical shapes and an average particle size of 4.7 nm.The measured Au particle sizes could be well correlated with the activity of the catalyst for carbon monoxide oxidation and acetylene hydrogenation. (Reproduced with permission from Springer.)...

Metal Hydrides. Metal hydrides generally react readily with acetylenes, often by an insertion mechanism. Cobalt hydrocarbonyl gives complicated mixtures of compounds with acetylenes. The only products which have been identified so far are dicobalt hexacarbonyl acetylene complexes (34). Greenfield reports that, under conditions of the hydroformy lation reaction, acetylenes give only small yields of saturated monoaldehydes (30), probably formed by first hydrogenating the acetylene and then reacting with the olefin. Other workers have identified a variety of products from acetylene, carbon monoxide, and an alcohol with a cobalt catalyst, probably cobalt hydrocarbonyl. The major products observed were succinate esters (74,19) and succinate half ester acetals (19). 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

Substituted cyclopentadienones react with iron carbonyls to form stable, diamagnetic 7r-co triplexes of the type [Fe(CO)3(cyclopentadienone)] (215). The proposed structure is shown in (XX). These complexes undergo reactions typical of metal carbonyls, e.g., displacement of carbon monoxide by tertiary phosphines, but the carbonyl group of the ligand does not show reactions characteristic of a keto-group. These complexes are also formed by interaction of acetylenes with iron carbonyls (see Section VI,C). Interaction of tetracyclone and Fe3(CO)i2 gives unstable complexes which contain the sandwich anion [Fe(tetracyclone)2]2 analogous to the anion (XXV) (215). 

Only seven of the nine carbonyl groups can be liberated as carbon monoxide, indicating that two carbonyl groups have combined with the acetylene to form a new ligand. X-ray studies on the complex derived from acetylene show that the complex contains an unsaturated lactone ring as shown in (LXIII) (160). An important structural feature of (LXIII) is that the... 

In a similar manner, UV irradiation of (triphenylsilyl)phenylacetylene with cyclopentadienylmanganese tricarbonyl 121) and cyclopentadienyl-niobium tetracarbonyl 122) results in elimination of carbon monoxide and formation of monodentate acetylene complexes (R = Ph) ... 

The production of two moles of carbon monoxide and the 18-electron rule lead us to predict that the acetylene molecule is acting as a four-electron donor. In fact this is just one of many complexes in which alkynes bind in this fashion.81 For example, the structure of the diphenylacetylene complex in Fig. 15.26 shows that the positions of the two rhodium atoms are such as to allow overlap with both tr orbitals in the carbon-carbon triple bond.82 The extent of back donation into the antibondirg orbitals determines the lengthening of the C—C bond and the extent to which the C—H bonds are bent away from the complex. Bond length values vary greatly from system to... 

Organogold(I) complexes undergo a number of insertion reactions with unsaturated molecules, such as olefins, acetylenes, and sulfur dioxide. Insertion of carbon monoxide or carbon dioxide has not been achieved, although the reverse reaction has been observed with C02 (7/). 

The proposed course of reaction is shown below. The acetylene molecule displaces the 7r-allyl group from one coordination site. Presumably thiourea plays a similar role to carbon monoxide in the reaction mechanism. Such donor-stabilized 77-allyl complexes are known to react with acetylene and... 

A further subtlety is the cyclization of the initial 2,5-hexadienoyl complex to form a six-membered ring, as an alternative to the cyclopentene system. In the reaction of allyl chloride with acetylene and carbon monoxide in the presence of Ni(CO)4, only traces of six-membered ring systems... 

Cocyclization of acetylene with isocyanides gives interesting new cyclic compounds 103, 116). The reaction patterns are generally similar to the cocyclization wdth carbon monoxide which is already known 103, 117). Low-valent nickel, palladium, or cobalt complexes are active in the following reactions 102, 103) for which intervention of acetylene complexes has been suggested ... 

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. 

Munakata and coworkers (1991) have recently characterised copper complexes with ethylene, acetylene, carbon monoxide and cod and studied the structures by X-ray analysis to investigate the bonding involved. Ethylene and acetylene are sideways bonded to copper (Fig. 5-18). Sigma donation from ethylene to copper predominates and ti-back donation is very weak. 

They noted that in dimethoxyethane or in iso-octane (path a), the major product was dicarbonylcyclopentadienylcobalt (2) which must arise as a result of a retro Diels-Alder reaction of the norbornadiene (which would lead to the formation of acetylene and cyclopentadiene). When the solvent was changed to an aromatic hydrocarbon such as benzene or toluene (path b), the major cobalt-containing product was shown to be a complex derived from Co4(CO)i2, with three CO ligands on an apical cobalt being replaced by a molecule of the aromatic solvent (3). The group noted that they were also obtaining hydrocarbon and ketonic products derived from norbornadiene, acetylene and carbon monoxide .1,2... 

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