Cobalt complex, intramolecular

A second, less used, strategy encompasses the Lewis acid catalyzed intramolecular reaction of a silyl enol ether with a propargyl cation. The latter can be conveniently generated by a cobalt complexed propargyl ether. This complexation strongly helps the carbocation formation. By using cobalt complexation, intramolecular aldol type reactions (for R = OR ) have been accomplished. ... 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

Intramolecular Electronic Communication in Polynuclear Cobalt Complexes... 

The novel cobalt complex came about as a result of the intramolecular coordination of a double bond, present in one of the R groups on the acetylene, to one of the cobalt atoms - taking the place of a CO ligand (67 - 68). They found that the new pentacarbonyl complex could be readily formed in CDC13 at room temperature from the hexacarbonyl dicobalt precursor. Attempts to use the pentacarbonyl complex as a substrate in the PK reaction led to no formation of cyclopentenone product. It was proposed that this is due to the alkene occupying a pseudo-equatorial site -alkene insertion is thought to occur from the axial position (see section 2.4). 

A complexation-induced intramolecular Diels-Alder cycloaddition of furan is depicted in Scheme 34. Upon exposure to silica gel, the alkyne-Co2(CO)6 complex 61 was transformed to the cycloadduct that contained a seven-membered ring <20000L871>. This facile process was supposed to be arisen from the bending of the linear triple bond to a structure with a 140° angle between the two carbon substituents in the cobalt complex 61. 

Vitamin B12 (1) was discovered some 55 years ago as the (extrinsic) antipemicious anemia factor, important for human and animal metabolism The cobalt complex (1) was structurally characterized by X-ray analysis in the laboratory of D.C. Hodgkin. In these studies, the unique buildup of the tetrapyrrolic corrin ligand and the intramolecularly coordinating pseudo-nucleotide function of the vitamin were discovered. ... 

ABCDE-ring analog of fredericamycin A was prepared using an intramolecular [4 - - 2] cycloaddition of a cobalt-complexed alkyne (Scheme 242). 

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... 

An additional advantage of the intramolecular protocol stems from the opportunity to prepare easily the required polyfunctional precursors via cobalt carbonyl stabilized propargyl cations. The approach based on the tandem utilization of Co-mediated alkylation and Pauson-Khand annulation was developed in Schreiber s studies to elaborate short pathways for the synthesis of polycyclic compounds. An example of the efficiency of this protocol is the two-step transformation of the acyclic precursor 409 into the tricyclic derivative 410. The cobalt-complexed acetal 409 was first transformed into the cyclooctyne derivative 411 via intramolecular reaction of the in situ generated propargyl cation 409a with the allylsilane moiety. Cyclooctyne 411 underwent smooth cycloaddition in the presence of carbon monoxide to give the target compound 410 with excellent stereoselectivity. 

Fig. 43 Possible intramolecular electron transfer processes present in a cobalt complex with two semiquinone anion ligands...

Upon extended heating with stoichiometric CpCo(CO)2 all-intramolecular cycloadditions of 1-ene-6,12-diynes to form cobalt-complexed cyclohexadienes take place in good yields (Scheme 23). - High diastereoselectivity in the formation of the complex is sometimes observed in these processes, as is considerable tolerance for steric hindrance. - ... 

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