Cyclotetramerization, alkynes

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

Cyclooctatetraene, CgHg, has a D2g tublike structure with four rather isolated double bonds. Cyclooctatetraenes can be the products of the catalytic cyclotetramerization of alkynes, and cyclobutadienes may be the intermediates. The BN homologues of cyclooctatetraenes have been known since 1962 (67). Like cyclooctatetraene, molecules of [(SCN)BNtBu]4 were shown to have a tublike ring structure of S4 symmetry with alternating bond lengths of 140 and 146 pm, the shorter ones perpendicular to the direction of the S4 axis (68). 

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... 

Reppe and co-workers reported in 1948 the Ni-catalyzed cyclotetramerization of acetylene to give cyclooctatetraene (106) [58]. After this discovery, the reaction was expanded to monosubstituted alkynes. Monosubstituted alkyne 138 is cyclized smoothly to give tetrasubstituted cyclooctatetraene 139 [59]. Internal alkynes are... 

Cyclotetramerization of a series of phosphaalkynes at a vanadium(v) center, metal-to-phosphoms transfer of a BuN-fragment, and subsequent elimination of alkyne are the main steps in the formation of azatetraphosphaqua-dricyclanes 50 with their lA3,3A3-diphosphetane basic ring <1998AGE1233, 2000CEJ4558, 2001ZNB951>. 

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). 

Monosubstituted alkynes may be included in this cyclization giving 1,2,4,7-, 1,2,4,6-, and 1,3,5,7-tetrasubstituted cyclooctatetraene derivatives [86]. A special case is the cyclotetramerization of 1 -phenylpropyne giving the octasubstituted C product besides the hexasubstituted benzene derivative [87] (eq. (30)). 

Whereas the transition metal catalyzed cyclotrimerization and cyclotetramerization of alkynes leading to benzene or cyclooctatetraene and their derivatives is a rather common reaction, there exist only a few examples of cooligomerizations between alkynes and alkenes or 1,3-butadienes leading to 1,3- or 1,4-cyclohexadiene derivatives20S). It is therefore surprising that the [3+2]-cycloaddition between methylenecyclopropanes and alkynes, catalyzed by triarylphosphite modified Ni(0) compound, is a rather convenient method to synthesize 4-methylenecyclopentenes 206). A wide range of methylenecyclopropanes and alkynes, in the latter case mainly 1,2-disubstituted ones, can be used for these reactions (Eqs. 98-100, see p. 127-128). 

Similar to cyclodimerizations, transition metal catalyzed cyclooligomerizations and cocyclooligomerizations involve dicarborative addition steps. The most important examples of this type are represented by cyclotrimerization and cyclotetramerization of alkenes. dienes, alkynes and systems with hetero multiple bonds, leading to carbocyclic and heterocyclic six-, eight- or twelve-membered rings1 ... 

Cyclotrimerization and cyclotetramerization of alkynes to form benzene and cyclooctatetraene derivatives was first achieved with various nickel catalysts26. Several other metal systems are also active and numerous applications have been reported3 5. Various concerted or stepwise reaction paths have been suggested3,5,2 -29. 

With alkynes alone, typical products are those of cyclotrimerization, i.e., arenes, although cyclotetramerization to cyclooctatetraenes has been directed selectively by Ni catalysts [1], providing an early commercial process for cyclooctatetraene. Numerous catalysts are known for the former process and, depending on the catalyst and substrate, moderate to high regioselec-tivity (1 4- or 1,3,5-) can be obtained with unsymmetrical alkynes [95, 96]. Selective mixed... 

Treatment of propiolic esters with catalysts of the type Ni(PX3)4 affords small amounts of tetrasubstituted cyclooctatetraenes in addition to benzene derivatives. This is the only other reported instance of cyclotetramerization of an alkyne using a Ni(0) catalyst 86). The synthesis of heptatrienenitrile was discovered in 1952 by Cairns et al. 87). These authors treated acetylene with acrylonitrile in the presence of phosphine-substituted nickel carbonyls. Under these conditions the reaction requires an induction period during which bis(acrylonitrile)-nickel and/or its phosphine adducts are formed. Bis(acrylonitrile)-nickel in the presence of triphenylphosphine is now known to be immediately reactive under similar conditions 42). Formation of (XL) must involve a hydrogen shift, and may proceed according to Eq. (22)... 

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