Cobalt complexes cyclization

Upon treatment with suitable cobalt complexes, methylbutynol cyclizes to a 1,2,4-substituted benzene. Nickel complexes give the 1,3,5-isomer (196), sometimes accompanied by linear polymer (25) or a mixture of tetrasubstituted cyclooctatetraenes (26). 

RCM of 132 to the medium-sized enyne 135, for example, appears to be highly unlikely. This transformation was achieved by conversion of 132 to the cobalt complex 133, which is cyclized to the protected cycloenyne 134. Deprotection yields 135, and a subsequent Pauson-Khand reaction yields the interesting tricyclic structure 136 (Scheme 27) [125c]. 

In a more recent and improved approach to cyclopropa-radicicol (228) [ 110], also outlined in Scheme 48, the synthesis was achieved via ynolide 231 which was transformed to the stable cobalt complex 232. RCM of 232 mediated by catalyst C led to cyclization product 233 as a 2 1 mixture of isomers in 57% yield. Oxidative removal of cobalt from this mixture followed by cycloaddition of the resulting cycloalkyne 234 with the cyclic diene 235 led to the benzofused macrolactone 236, which was converted to cyclopropa-radicicol (228). 

Reaction (62) reports the cyclization of a thermally instable propargyl bromide cobalt complex mediated by Ph2SiH2 at room temperature and Et3B/02 as the... 

Under the conditions of the cobalt-mediated carbonylative A-oxide-promoted cocyclization (Pauson-Khand reaction) at room temperature, compound 547 provides exocyclic 1,3-diene 548 as the major product (>98%) together with only traces of the corresponding carbonylative product 549. Owing to the relative instability of the diene, it is more efficient to perform a one-pot cobalt cyclization/Diels-Alder process after A-oxide-promoted cyclization of the cobalt complexes. Compound 550 is obtained as a single diastereomer in 39% overall yield if MTAD is used as a dienophile (Scheme 90) <2003JOC2975>. 

A mechanism was proposed in which entry into the catalytic cycle is achieved via Et2AlCl-mediated cobalt hydride generation. Diene hydrometallation affords the cobalt-complexed -jr-allyl A-5, which inserts the tethered alkene to furnish intermediate B-4. Elimination of LnCoOBn provides the cyclization product. Reduction of LnCoOBn by Et2AlCl regenerates cobalt hydride to complete the catalytic cycle (Scheme 17). 

Similar cyclizations can be performed using cobalt complexes (vitamin B12 and Bi2 analogs) as mediators [97-100] (Scheme 68). 

In Reaction (7.20) is reported the cyclization of a thermally unstable propar-gyl bromide cobalt complex mediated by Ph2SiH2 at room temperature and Et3B/02 as the radical initiator. However, a mixture of reduced and bromine atom-transfer products (1 1.8 ratio) are isolated due to the low hydrogen donation of the employed silane [31]. 

In many of the above cyclization reactions it is not clear to what extent the metal ion is important. In most cases a geometrical effect is significant but electronic activation is probably not always of importance. The corphin synthesis (see Scheme 68) is a good example of precise geometrical requirements. This cyclization succeeds for the palladium complex but not for nickel or cobalt complexes. The imidate condensation seems to require steric acceleration and more is provided by the desire of palladium to attain planarity than the urge for nickel or cobalt to do so. 

Cobalt complexes of a-alkynylpyranosides undergo TfOH-catalyzed ring cleavage (Scheme 45) to afford the ring-opened products, which cyclize to dehydrooxepanes <1994T12883>. 

Cyclization of cobalt complex 112 followed by decomplexation with hydrogen over 5% Rh/C in EtOH (Equation 45) affords the dihydrooxepine 113 <1998T2509>. 

Indoline, in trinuclear Ru and Os clusters, 6, 725 Indolizidine, via cyclization-hydrosilylation, 11, 389-390 Industrial processes, in Ziegler-Natta polymerizations, 4,1040 Inflammation, drugs for, 12, 460 Infrared spectroscopy applications, 1, 501 cobalt complexes, 1, 488... 

The same radical cyclization can be performed using a more simplified vitamin B12 model, such as Z w(dimethylglyoximato)(pyridine)Cobalt chloride Co3+ Cl -Py complex (B), as shown in eq. 11.3. Treatment of propargyl P-bromoethyl ether (3) with NaBH4 and a catalytic amount of the cobalt complex (B) provides fi-exo-methylene tetrahydrofuran via 5-exo-dig manner [2-7]. 

Fig. 58 Cyclizations of aryl iodides catalyzed by low-valent cobalt complexes...

Pattenden and coworkers studied the cyclization of allyl ort/io-iodophenyl ether 293a (R=H) catalyzed by (salen)cobalt complex 294 (Fig. 70) [317], Dihydro-benzofuran 295a (R=H) was obtained in a moderate 45% yield. 

As commented above, Young has reported one example of a Nicholas/PKR starting from cobalt complex 74, prepared by means of a RCM reaction [111]. The reaction of 74 with allyl alcohol in the presence of BF3Et20 gave enyne 75 which cyclized with f-BuSMe promotion to give tricycle 76 (Scheme 22). 

Indole N-substituted diyne tetracobalt complexes 1467 undergo a Lewis acid-mediated dimerization-cyclization reaction through the indole 3-position to afford indolophanetetrayne cobalt complexes 1468 (Scheme 280) <20030L1003>. 

Intramolecular allene alkyne cyclizations using cobalt complexes are nonspecific and both double bonds of the allene participate in the reaction affording [4.3.0] and [3.3.0]ring systems (Scheme 260). Both inter- and intramolecnlar Pauson - Khand-type reactions have been reported using... 

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