Cyclooligomerization trimerization

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). 

The cyclooligomerization of isoprene (eq. (7)) has received attention, since the trimerization products are of interest in the perfume industry [16]. 

Allene cyclooligomerization.1 Allene is catalytically converted into a mixture of isomeric trimers, a tetramer, a pentamer, isomeric hexamers, and polymers by this complex. The reaction involves the initial formation of [(C6H3)3P]2NiC3H4, which can be isolated. 

The catalytic behavior of Pd in 1,3-diolefin oligomerizations is quite different from that of Ni. Cyclooligomerization, as in the case of Ni catalysis, is normally not observed. Pd-catalyzed reactions of 1,3-diolefins are characterized by linear oligomerizations with H-migrations yielding, in the absence of nucleophiles, linear dimers or trimers . Another typical feature of Pd catalysis is the facile formation of dimeric telomers with incorporation of various nucleophiles (alcohols, carboxylic acids, amines) ". A variety of Pd(0) complexes and also of Pd(II) complexes, which are more easily accessible and may be reduced in situ to Pd(0), have been examined . Some selected examples are given in Table 1. 

In the presence of suitable cocatalysts such as alcohols, phenols, or secondary amines, 1,3-diolefins are oligomerized to linear dimers or trimers by the same nickel-ligand systems, which are effective for cyclooligomerization (see 14.5.2.5.1). Reactions may be accompanied by telomerization. Typical examples are given in Table 1. 

Oligomerization reactions catalyzed by complexes other than those described also give quite different cyclooligomerization and oligomerization products see equations (13.109) and (13.110). The same products (the tetramer and trimers) are formed if [Ni(CO) P(OPh)3 4 ,] (x= 1-3) is utilized as a catalyst. 

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