Cyclotrimerization complex catalysis

Since Reppe s discovery of the cyclotrimerization of acetylene to benzene in the presence of nickel carbonyl-phosphine complexes, the use of nickel catalysts in many organic transformations has become popular. Transition metal complex catalysis provides many elegant entries to carbon-carbon bond-forming reactions in organic synthesis. One notable example is carbocyclic ring expansion mediated by nickel(O) complexes. ... 

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. 

The complexes 222-226 exhibited a high catalytic activity on the cyclotrimeriza-tion of aromatic isocyanates to produce triaryl isocyanurates (Scheme 83). They are the first reported Cp-free rare-earth metal complexes showing high activity and selectivity on the cyclotrimerization of aryl isocyanates. For comparison, the starting trisamide complex [Yb N(SiMe3)2 3( T-Cl)Li(THF)3] was studied for the catalysis and showed a catalytic activity comparable with those of the new complexes 222-226. All the complexes showed no catalytic activity on the cyclotrimerization of 4-nitrophenylisocyanate and exhibited a relatively low catalytic activity on the cyclotrimerization of aliphatic isocyanates [167]. 

The first application of phosphinines in catalysis was reported by Zenneck et al. in 1996 in the case of t 6-Fe complexes [46, 98], It was shown that complex 73 could catalyse the cyclotrimerization of dimethyl acetylenedicarboxylate as well as the formation of pyridines from alkynes and nitriles. Importantly, the catalytic activity of this complex was found to be superior to that of the corresponding benzene [Fe(n6-C6FI6)(COD)] complex. Reactions of methylpropargyl ether with butyronitrile in the presence of complex 73 as catalyst in a ratio 620 2,720 1 afforded a mixture of functional benzenes and pyridines. Turn over numbers (TON) for the conversion into pyridines reached 160 and those for the formation of functional benzenes reached 326, thus corresponding to a chemoselectivity of 0.49 (Scheme 26). 

The catalysis of the cyclotrimerization of acetylenes by transition metal complexes has been extensively studied (reviews Bird, 1967 Hoogzand and HUbel, 1968 Maitlis, 1971, 1973 Heck, 1974). Various mechanistic studies have been done on this type of reaction (Blomquist and Maitlis, 1962 Meriwether et al., 1962 Schrauzer, 1964 Collman et al., 1968 Whitesides and Ehmann, 1969 Yamazaki and Hagihara, 1970 Miiller and Beissner, 1973 Gardner et a/., 1973). The iridium-promoted trimerization of dimethyl acetylenedicarboxylate has been demonstrated to occur through an iridocycle as shown in Scheme 2 (Collman and Kang, 1967 Baddley and Tupper, 1974). 

Intermediates containing metallapentadiene rings are frequently involved in metal-catalysed cyclotrimerization of acetylenes to benzenes, a fact illustrated by the facile catalysis of the cyclotrimerization of PhC2Ph by the nickelole complex (35). ... 

Metallocyclopentadiene complexes are often involved in cyclotrimerization reactions of alkynes. Studies on the complex (36) reveal that these alkenyl-metallocyclo-pentadiene complexes are more efficient catalysts for cyclotrimerization reaction than (37). The significant enhancement of catalysis by the alkenyl ligand may be a... 

In 2008, Yu and coworkers [31 ] reported the Au nanoparticle-supported Pd(II) microwave-assisted tilkyne cyclotrimerization reaction in ionic liquids. The Pd complexes were immobilized onto Au nanoparticles via chelation to the surface-bound dipyridyls. The catalysis was performed in bmimPF under microwave irradiation and the recovered catalysts could be recycled many times. The conversions were excellent, the regioselectivity was good, and the scope was broad. 

Despite the observation that phenyl- and 1-adamantylisocyanate may be used as substrates for hydroamination catalysis without polymerization or oligomerization of the unsaturated molecule (see Sect. 2.1.4), in the presence of catalytic quantities of a calcium carbene complex [ (ArN=PPh2)2C)Ca(THF)2] phenyl- and cyclohexylisocyanate have been reported to undergo a selective cyclotrimerization to form the corresponding isocyanurate (Scheme 24) [106, 146], Reactirms proceed at low temperature and with phenylisocyanate requiring a lower catalyst loading and shorter reaction time to reach higher conversion than cyclohexylisocyanate. 

Cyclopentadienylcobalt complexes are also good for co-cyclotrimerization of alkynes with other unsaturated compounds containing the carbon-heteroatom double bonds, especially when they are part of the cumulene system such as isocyanates, diimides, and carbon dioxide. The reaction conditions are essentially the same as in the previously mentioned processes. However, the biggest problem remains the selectivity for the formation of heterocycles, because of the strong competition for the formation of benzene derivatives. Whereas co-cyclotrimerization of diimides and isocyanates results in the formation of reasonable yields of the corresponding heterocycles 170 and 171 (Scheme 75), in the case of carbon dioxide the yields are generally low [108, 109]. Recently, it has been shown that the ruthenium complex 106 is capable of efficient catalysis of co-cyclotrimerization of diynes and isocyanates [110] and isothiocyanates [111] under mild reaction conditions. 

The first application of a phosphinine-based catalyst in homogeneous catalysis was reported in 1996 by Zenneck et al. The f/ -phosphinine-iron complex 33 was used in the catalytic [2 -h 2 -h 2] cyclotrimerization of the electron-poor alkyne dimethyl acetylenedicarboxylate to give C6(C02Me)6 and in the co-cyclotrimer-ization of methyl propargylether with butyronitrile affording functionalized pyridine derivatives (Scheme 6.8) [60]. Turnover-numbers of up to 160 for the pyridine derivatives and chemoselectivities of up to 1.4 1 (pyridinesiarenes) were observed. Nevertheless, the efficiency of 33 is significantly lower compared to classical CpCo catalysts [61, 62]. 

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