Cobalt catalyzed cyclotrimerization

Scheme 9. Cobalt-catalyzed cyclotrimerization of propargylated glycosides.90...

Nevertheless, nitrile functionalized ILs have been used for cobalt-catalyzed cyclotrimerization reactions of monosubstituted aromatic alkynes also in this case, the nitrile functionality was able to stabilize the transient cobalt(i) catalytic species, ensuring good conversions. ... 

Chang and co-workers developed an intramolecular cobalt-catalyzed cyclotrimerization of both the symmetric and the asymmetric nitrile-diyne... 

Yong et al. developed a cobalt-catalyzed [2+2+2] cyclotrimerization of terminal alkynes in good yields in aqueous media (80/20 mixture of water and ethanol) at room temperature. A cyclopentadienyl cobalt complex bearing a pendant phosphine ligand was used as a catalyst (Eq. 4.59). The cyclotrimerization of internal alkynes resulted in lower yields and required an elevated temperature, most likely due to steric interactions. For example, cyclotrimerization of 2,5-dimethyl-3-hexyne gave hexaisopropylbenzene in 51% yield and the reaction of diphenylethyne resulted in a 47% yield of hexaphenylbenzene.112... 

Novel transition metal-mediated strategies were also well represented this past year. Takahashi and co-workers reported a s nickel-catalyzed reaction between azaziconacyclopentadienes (9) and alkynes to form pyridines (10) of varying substitution patterns <00JA4994>. This methodology, a formal cyclotrimerization, is also noteworthy since two different alkynes can be used. In similar fashion, Eaton reported an aqueous, cobalt(II) catalyzed cyclotrimerization between two identical acetylenes and one nitrile to afford substituted pyridines . 

The cobalt catalyzed cocyclization of alkynes with heterofunctional substrates is not limited to nitriles. cpCo-core complexes are capable of co-oligomerizing alkynes with a number of C,C, C,N or C,0 double bonds in a Diels-Alder-type reaction. Chen, in our laboratories, has observed that these cycloadditions are best performed with the help of stabilizers such as ketones or acetic esters that are weakly coordinated to the cobalt and prevent the alkynes from being cyclotrimerized at the metal center... 

Vollhardf s group adopted the cobalt-catalyzed photochemical cyclotrimerization of alkynes for the rapid construction of a novel class of helicenes, helical [n]phe-... 

In contrast to carbocyclic alkyne cyclotrimerizations, the catalytic pyridine synthesis from alkynes and nitriles relies exclusively on cobalt catalysts with a few exceptions where rhodium [16] and iron complexes [17] could be applied. The cobalt-catalyzed pyridine synthesis can even be carried out in a one-potreac-tion generating the catalyst from C0CI2 6 H20/NaBH4 -1- nitrile/alkyne in situ [18]. 

The regiochemical product distribution of the co-cyclization of two or three different alkynes occurs statistically. In some cases carefully controlled reaction conditions allow isolation of a main product from mixed cyclotrimerizations. For example, l,2,3,4-tetraphenyl-5,6-diethylbenzene can be obtained from cobalt-catalyzed reaction of tolane and 3-hexyne in good yield [62]. The first example of an intermolecular, regiospecific cross-benzannulation reaction catalyzed by Pd(PPh3)4 was reported by Yamamoto [63]. The reaction of 2-alky 1-but-l-ene-3-yne with disubstituted diynes leads exclusively in high yields to... 

Jonas and Tadic [71] have investigated the homogeneous cobalt-catalyzed co-cyclotrimerization of acetylene and olefins. The reaction with -Ind-cobalt bis(ethylene) as the catalyst was carried out with ethylene, a-olefins and 2-butene as well as cyclohexene and cyclooctene (eq. (25)). 

Compared with the Diels-Alder reaction, the [2+2+2]-cycloaddition is potentially more powerful since the number of new bonds as well as chirality centers that are formed is higher. Unfortunately, the reaction seems to be entropically or kinetically unfavorable. This disadvantage can, however, be overcome by the use of transition metal catalysts (templates). Among the most successful examples of this reaction type, the nickel(II) catalyzed Reppe reactions 96), the cobalt(I) catalyzed cocyclizations of a,to-diynes with alkynes 97), the cobalt(I) catalyzed pyridine synthesis 985 and last but not least the palladium(0) catalyzed cyclotrimerizations of 3,3-dialkylcyclopropenes to frans-cr-tris-homobenzenes must be mentioned. The latter has been known for ten years 99>. 

Scheme 28. Cobalt catalyzed [2 + 2 + 2] cyclotrimerization of the diaryl alkyne (63) tothehexa-phenylbenzene (64) and modern Scholl-type reaction in the synthesis of hexabenzocoronene (62,[112])...

When a monosubstituted acetylene RCsCH is heated with nickel or cobalt carbonyls, it gives the 1,2,4-trisubstituted benzene as major product, the 1,3,5-benzene as minor product, and little if any of the 1,2,3-isomer. Coordination of the metal with the triple bond has been considered to be involved [52]. Thus, in the transition-metal-catalyzed cyclotrimerization, the... 

The l,2-di-(D-flrabt o-tetritol-l-yl)pyridin-4-one C-nucleoside 842 was reported to be formed, among other products, when 1-amino-l-deoxy-D-fructose (239) reacted with malondialdehyde (92MI6) (Scheme 236), Formation of the pyridine nucleus of the nucleoside 844 was effected by cobalt-catalyzed co-cyclotrimerization of the L-threononitrile derivative 843 with acetylene (94TA299) (Scheme 237). 

Kotora et al. extended the scope of the reaction and showed that cobalt-catalyzed [2+2+2] cyclotrimerization of diynes with benzonitrile is a convenient method to prepare bis-ort/zo-substituted arylpyridines (06T968). Pyridine 267 was oxidized to give 268. 

The first [4]circulene has been obtained only very recently. King and coworkers have prepared 158 in a three-step synthesis starting from the naphthoquinone photodimer 157, with a cobalt-catalyzed alkyne cyclotrimerization as the key step... 

A concise synthesis of the cytotoxic alkaloids dehydrotylophorine (127) and (+)-tylophorine (128) has been accomplished by a cascade consisting of a cobalt-catalyzed crossed [2 - - 2 - - 2] cyclotrimerization that is followed by an intramolecular ring annulation reaction (Scheme 7.27) [37],... 

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