Cobalt catalyzed cyclization

Organometallic reactions leading to condensed pyrans containing three or more rings have been achieved by cyclooligomerization processes and by intramolecular coupling. The former type of reaction is based on intramolecular trapping of o-xylylenes produced in cobalt-catalyzed cyclization... 

Wilfried Hess was born in Wiesbaden in 1979. He studied chemistry at the Philipps-University in Marburg and is currently a Ph.D. student working in the group of Prof. Hilt in the field of cobalt-catalyzed cyclization reactions. 

Fig. 99 Catalytic cycle of the cobalt-catalyzed cyclization to tetrahydrofurans...

A Heck-type cobalt-catalyzed cyclization of 6-halo-1-hexenes into 3-... 

Silicon analogues 124 of the powerful musk odorant Versalide 123 were synthesized by a cobalt-catalyzed cyclization of l,2-bis(ethynyldi-methylsilyl)ethane (101) and alkynes (07OM1295) (Scheme 24). 

Cobalt-catalyzed cyclization of 6-halo-l-hexenes into methylene cyclopentanes has also been reported [175]. Iodo acetal 172a was selected as model substrate. Trimethyl-silylmethylmagnesium chloride (1.0 m THF solution, 1.5 mmol) was added to a mixture of cobalt(II) chloride (0.05 mmol) and l,4-bis(diphcriylphosphino)butane (0.06 mmol) in THF. The resulting mixture was stirred for 5 min and iodo acetal 172 a (0.50 mmol) was then added to the mixture at 0°C. The whole mixture was heated under reflux for 5 min. Aqueous work-up then sihca gel column purification afforded the desired product 173 a in 84% yield in addition to its saturated analog 174 a (Scheme 3.161). 

Paolesse R, Tassoni E, Licoccia S, Paci M, Boschi T (1996) One-pot synthesis of corrolates by cobalt catalyzed cyclization of formylpyrroles. Inorg Chim Acta 241 55-60... 

Liccocia S, Tassoni E, Paolesse R, Boschi T (1995) The effect of steric hindrance in the synthesis of corrolates via the cobalt catalyzed cyclization of 2-(a-hydroxyalkyl)pyrroles. Inorg Chim Acta 235 15-20... 

In contrast, cyanoynamines react with phenyl isocyanate via an initial [2+2] cycloaddition reaction across the CM3 bond of the isocyanate to give a quinoline derivative Intramolecular cobalt catalyzed cyclization of alkynes and isocyanates was employed in the total synthesis of camptothein, an antitumor alkaloid. The key step of the synthesis involves the reaction of the acetylene group-containing isocyanate 185 with the acetylene derivative 186 to give 187. ... 

To summarize this section, to date 19 phenylenes have yielded to synthesis. They can be divided broadly into 5 families ( of examples) linear (3), angular/ helical (7), zigzag (2), bent (3) and branched (4). Their topologies have been accessed through 26 different routes, 15 of which involved in the crucial step an all-intramolecular cobalt-catalyzed cyclization, 9 used intermolecular variants and 2 a combination of the two strategies. 

The cobalt-catalyzed reaction was studied by isolation of the lactones formed by hydrogenation and lactonization at higher temperatures (73). The hydroformylation was conducted at 140°C and 300 atm, followed by hydrogenation and cyclization at 200°-240°C, Eq. (33). 

In a related cobalt-catalyzed transformation, 1,3-dienes tethered to ally lie ethers engage in Et2AlCl-mediated reductive cyclization.463 Exposure of benzylic ether 22a to Co(acac)3-PPh3 in the presence of Et2AlCl results in formation of divinylcyclopentane 22b with excellent /raar-diastereoselectivity. As demonstrated by the conversion of 23a to 23b, this method is also applicable to the stereocontrolled formation of six-membered rings (Scheme 16). 

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

For lead references pertaining to iron- and cobalt-catalyzed [4+2 + 2] intermolecular cyclizations see (a) Greco, A. ... 

A [2+2+2] cyclization reaction of acetylenes and nitriles leads to the formation of furo[3,4-r]pyridines under cobalt-catalyzed conditions (Scheme 11) <2005SL1188>. 

Oshima provided a number of examples of cobalt-catalyzed tandem radical 5-exo cyclization/cross-coupling reactions (Fig. 59, Table 4). Allyl a-haloaldehyde... 

Fig. 59 Low-valent cobalt-catalyzed radical cyclization/Kumada coupling reactions...

Fig. 60 Cobalt-catalyzed tandem reactions involving radical cyclizations...

A tandem radical 5-exo cyclization/radical addition/allylic substitution reaction was subsequently described [292]. Allylic ot-bromo acetal 242b cyclized cobalt-catalyzed. Addition to diene 245 and subsequent coupling with coformed organocobalt(I) species generates an allylcobalt complex, which undergoes reductive elimination to cyclic product 246 in 93% yield (cf. Fig. 56). 

Fig. 66 Cobalt-catalyzed radical cyclizations of using other radical acceptors and termination reagents...

Fig. 70 Electrochemically mediated cobalt-catalyzed aryl radical 5-exo cyclizations...

Fig. 98 Cobalt-catalyzed alkoxyl radical cyclization reactions...

In a rather comprehensive work aimed at synthesizing diverse carbapentofuranoses, Desire and Prandi [8] exploited a cobalt-catalyzed radical cyclization/oxygenation of a wide number of 6-iodohex-l-enitols to assemble the carbocyclic frame of the targets. As an example, Scheme 4 displays how 4a-carba-a-D-arabinofuranose 27 was prepared by starting with readily available glucopyranoside 24. 

Estrones. An A —> BCD approach to estrones involves a cobalt catalyzed intramolecular 2 + 2 -y 2] cycloaddition of the enediyne 1 to form the B, C. and D rings in one step. The high stereoselectivity suggests that the cyclization involves a Dicls-Alder-type reaction of the vinyl group with a cobaltacyclopentadiene formed by coupling of the two alkyne units. The homoannular diene obtained on demetalation is isomcrized easily to the diene 3. 

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