Acetylene trimerization

Philipsborn and coworkers [83] have successfully used the Co signals in the substituted Co(i)Cp complexes 72, in connection with understanding the mechanism of pyridine/acetylene trimerization reactions. The metal resonance was found to vary strongly with the catalyst structure and a correlation of d Co with reactivity was observed. 

In many cases the reaction of osmium carbonyls and acetylenes does not stop at the first stages as in [56], [57], or [57]. Instead, two or more acetylene molecules are incorporated, and in some cases acetylene trimerization to benzenes takes place (182, 371, 379). Incorporation of two acetylene molecules can lead to metallacyclo-pentadiene clusters like [55] (126,168,171,182,184, 223, 371), or to metallacyclo-hexadienone clusters hke [59] (126, 223). And the complex [90], another intermediate, is related to [55] by an intramolecular oxidative addition reaction (168,169). 

According to Saito(Ref 7), the acetylene trimers obtained as byproducts in the prepn of CHa CH. C CH by condensation of acetylene are expl. They can be stabilized by hydrochlorination in the presence of a complex salt of CuCl and NH,C1 to yield additive compds contg 1 or 2 mols of HC1, from which they are separated by distn. Nakagawa(Ref 8) reviews the chemistry of poly acetylenes and gives 25 references. Polymerization of acetyl-... 

The RCCo3(CO)9 catalysts in these reactions, however, only appear to be sources of simpler cobalt carbonyl species through their thermolysis, and it is the latter which cause the acetylene trimerization through well-established routes (73). Indeed, in some cases acetylene-derived cobalt carbonyl complexes are obtained (72) ... 

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. 

A number of transition metal ion-exchange zeolites are active for acetylene trimerization (159, 160), and the criterion for activity appears to be an even, partially filled d-orbital, i.e., d8 (Ni2 +, Co+), d( (Fe2+), d4 (Cr2 + ). This has led to the suggestion that the mechanism must involve a complex in which there is simultaneous coordination of two acetylene molecules to the transition metal ion. The active oxidation state for CuNaY butadiene cyclodimerization catalysts has been unambiguously defined as monovalent copper (172-180). The d10 electronic configuration of Cu+ is consistent with the fact that isoelectronic complexes of Ni° and Pd° are active homogeneous catalysts for this reaction. The almost quantitative cyclodimerization selec-... 

Fig. 10. MINDO/3 D3h surface for acetylene trimerization ( ). The open circles are obtained by STO-3G calculations...

The possibility to catalyze the acetylene trimerization depends critically on the ability of the metal center to coordinate and activate two C2H2 molecules and then to bind the C4H4 intermediate according to reaction ... 

Catalytic activity of cation-exchanged zeolites containing transition metal ions was reported by Rouchaud et al. (82), who found that the oxidation of n-hexane to acetic acid is catalyzed by Mn -exchanged Y at 160° and 25 atm. As another example of the effect of transition metal ions, Kruerke (52) found that acetylene trimerized to benzene on transition metal-exchanged Y at near room temperature. The activity changes with the transition metal Ca and Na = O < Mn " < < Co " = Ni " > > Cu " = = O. This activity pattern is consistent with a model accord-... 

In this chapter, an overview is presented of studies that deal with the electronic and chemical properties of Pd in bimetallic systems. We will focus on palladium for three main reasons. First, bimetallic catalysts that contain Pd or other Group-10 metals have many uses isomerization of hydrocarbons, olefin hydrogenation, CO oxidation, alcohol synthesis, acetylene trimerization, etc. [8,10,19-21]. Second, palladium is very sensitive to the formation of bimetallic bonds [22-24]. And third, there is a vast number of experimental and theoretical articles in the literature that examine the properties of Pd in bimetallic systems [14,15,19-23,25-44]. From this large volume of work, one can get a general idea of how deep is our knowledge about the basic nature of bimetallic bonding and how it affects the properties of a metal. 

Judai K, Worz AS, Abbet S, Antonietti JM, Heiz U, Del Vitto A, Giordano L, Pacchioni G (2005) Acetylene trimerization on Ag, Pd and Rh atoms deposited on MgO thin films. Phys Chem Chem Phys 7 955... 

Some Ziegler-type catalysts have been used to polymerize terminal acetylenes (56, 57). Highly dispersed or soluble catalysts based on TiCl, Ti(OR), metal chelates (Co, Ni, V, Fe) plus AlEt have been most successful. Acetylenes, unlike a-olefins, polymerize with Group VIII transition metal compounds. With some acetylenes, trimerization reactions leading to the corresponding substituted benzene derivatives take place. 

Similar studies of the regioselectivity in the HDA reaction between 2-substituted norbomadienes and unactivated terminal acetylenes catalyzed by a cobalt catalyst were also undertaken. Unfortunately, 2-sub-stituted norbomadienes are much less reactive with unactivated acetylenes in the cobalt-catalyzed HDA reaction than with electron-deficient olefins in the Ni-catalyzed HDA cycloadditions. When an electron-withdrawing group (Y = COOMe) or an electron-donating group (Y = OMe) is attached to the 2-position of the NBD, no desired [2 + 2 + 2] cycloadduct is observed with 1-hexyne, even in refluxing toluene for three days under the usual cobalt-catalyzed conditions (Scheme 12). Instead, some NBD dimers and acetylene trimers are detected. With a... 

The actual mechanism of the thermally induced acetylene reaction has not been determined and was first proposed as a simple acetylenic trimerization. It is our opinion, however, that the terminal acetylene groups can simultaneously react via a number of alternate routes in addition to trimerization. Reaction of phenylacetylene and the model com-... 

Rh(CaH4)2L] (L = indenyl or 1-methylindenyl) and but-2-yne give [Rha-(CH=CH2)(MeC=CHMe)L2l and [Rh2(C4Me4)La]. The former has the divinyl structure (43) and is a catalyst for acetylene trimerization the latter contains a rhodiacyclopentadiene group bonded to the second RhL unit by a metal-metal bond and the pentadiene double bonds. ... 

In these reactions it seems likely that the RCCo3(CO)9 complexes are undergoing thermal degradation to give simpler cobalt carbonyl species. The latter then would induce acetylene trimerization through well-established routes. One may note that in some cases acetylene-derived cobalt carbonyl complexes were isolated (Dickson and Tailby, 1970) ... 

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