Cobalt tethering

Synthesis of Tetrakis(polyethyleneoxy)phthalocyanine cobalt (II) (9). The polyethylene-tethered phthalonitrile (0.827 g, 1.0 mmol), 1,5-diazabicyclo [4.3.0] non-5-ene (0.062 g, 0.50 mmol) and cobalt acetate (0.044 g, 0.25 mmol) were added to a reaction vial equipped with a magnetic stir bar. The reaction was then heated to 175°C for 2 hours before reducing the temperature to 95°C and adding toluene. The reaction mixture was then poured into methanol, filtered, and the sohd washed further with methanol. The collected product was then dried under vacuum to yield... 

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). 

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). 

In the presence of nickel(0), tethered diene-VCPs react to produce eight- and five-membered ring products (Scheme 2). Palladium(O) and cobalt(m) were also tried but produced only decomposition products. However, in the presence of Wilkinson s catalyst (RhCl(PPh3)3), tethered diene-VCP 1 was cleanly converted to triene 4 in 91% yield. Although the desired cycloaddition reaction was not obtained, the cleavage of the cyclopropane ring was encouraging.22... 

The separation of dioxygen from air has been carried out using a cobalt porphyrin complex tethered to the surface of a Vycor glass mem-... 

Liu has reported recently a new Co2(CO)8-mediated tandem [5+ 1]/ [2 + 2+ 1]-cycloaddition reaction that gives tricyclic 5-lactones from cis-epoxy enynes 186. This process possibly involves an initial opening of the epoxide in the cobalt hexacarbonyl complex 187 to from the complexed al-lene 189 via 188. Further coordination of the tethered olefin leads to 190 and further oxidative cyclization gives cobaltacycle 191 which inserts CO leading to the final compound 192. When performing the reaction under N2,191 suffers a reductive elimination to give cyclobutane 193 (Scheme 55) [172],... 

The structure of 19 was unambiguously confirmed by an X-ray diffraction study. A mechanistic rationale is depicted in Scheme 11. After Co2(CO)6-complexed alkyne A is obtained, a Sn2 attack of the Co2(CO)6 fragment opens the epoxide moiety to afford intermediate B, which subsequently incorporates CO to give C. The latter rearranges into cobalt-stabilized cyclic allene species D. The net result is a [5 + 1] cyclization that creates the lactone group. Coordination of the tethered olefin leads to oxidative cyclization to give E. Finally, insertion of CO followed by reductive elimination affords the desired product 19. 

It is noteworthy that the methylenecyclopentane 54 was generated in situ via a cobalt-catalyzed Conia-ene type reaction. By a subtle change in the position of the /J-kclocslcr substituent relative to the tether, the sequence could be reversed and allow the formation of the basic skeleton of the kaurane family [50]. 

Cobalt-cycloheptyne complexes tethered with propargylic ethers were found to undergo also intermolecular as well as intramolecular [2+2+2] reactions to provide polycyclic benzocycloheptanes <03CC2936>. 

Ogawa and coworkers have examined peptide y -strand mimics of the general type 24 (R = CH(CH3)2, n = 1-3), consisting of a ruthenium(II) polypyridyl electron donor tethered to a cobalt(III) pentammine electron acceptor by an polyvaline peptide chain [107]. A related parallel ) -sheet mimic has also been studied [108]. These compounds adopt the conformational properties found within the individual strands of a y -pleated sheet in both aqueous and methanol solutions. Emission lifetime measurements and HPLC product analyses suggest that the binuclear donor-acceptor compounds undergo photoinduced electron transfer. The values of et decrease with increasing donor acceptor distance according to y = 1.1 A, which is observed for electron transfers both in water at 298 K and in ethanol-... 

Opening an epoxide by an alkoxide moiety can be done intramolecularly, and a new cyclic ether is generated. Ethers of various ring sizes can be produced depending on the length of the tether between the alkoxide unit and the epoxide. Specialized conditions are common, as in the conversion of 116 to 117. Another variant of this transformation used a cobalt-salen catalyst. A specialized version has the alkoxide moiety on the carbon adjacent to the epoxide, leading to the Payne rearrangement, where a 2,3-epoxy alcohol is converted to an isomeric one, by treatment... 

In chapter 6 we described the use of the remarkable Pauson-Khand reaction for the synthesis of cyclopentenones. If the components (CO, alkene and alkyne) are tethered by a nitrogen atom, a heterocycle will also be formed. The first stage in this process is to couple the cobalt carbonyl complex, e.g. 236, of ahalo-alkyne with an amine containing the alkene in the side chain. The best way to do this is to react 234 with Co2(CO)9 to give 235 and then 236 and to capture this complex with the amine without isolation of intermediates.34... 

The indole 2-3 double bond can participate in a cobalt-mediated combination with two alkyne units. In particular, when one of the alkynes is tethered to the indole nitrogen atom by an acyl group, cyclization occurs to give an unusual dihydrocarbazole such as compound (286), which can be demetallated either to the dihydrocarbazole (287) or to the carbazole (288) (Scheme 84) <86JA2091>. 

Reactions in which the third cyclization partner is tethered to the indole nucleus either at the 3-position 257 or as a dialkyne imit off the nitrogen atom 260 have also been reported (Scheme 69). It should be noted that the cobalt complexes resulting from these [2+2+2] cycloadditions can be easily demetalated by using CuCl2 or Fe (N03)3 to obtain the cobalt-free products in excellent yields. Alternatively, Ce" may be used to produce the demetalated aromatized carbazole analogs. 

Table 5.2 Sites reported for tethering iron and cobalt polymerization catalysts.

Unlike tethered diynes that are commonly seen in [2 + 2 + 2]-pyridine syntheses, nitrile-diyne substrates are less explored. Recently, Snyder and coworkers reported microwave promoted cobalt-catalyzed [2 + 2 + 2]-reaction of nitrile-diynes 275 to afford tetrahydronaphthyridines 276 in moderate to excellent yields. ... 

In her review, Laschat summarizes the strategies that have been employed to control stereochemistry. These strategies include diastereo-selectivity from enantiopure starting materials and enantioselectivity with chiral additives. The use of enantiopure starting materials falls into three catagories The controlling stereocenter is in the tether for an intramolecular PKR, the controlling stereocenter is in a chiral auxiliary on the alkyne or alkene, or a chiral cobalt complex controls stereochemistry. 

Spicer has reported that dimethylacetylene dicarboxylate reacted with norbomene at room temperature in the presence of TMANO to give the cyclopentenone in 73% yield. However, the cobalt complex is thermally unstable and mild PKR conditions were required for success. The intramolecular and inter-molecular PKR of alkynones has been reported by Hoye in 30-90% yields. In the intramolecular examples, improved yields were observed when a geminal dimethyl was incorporated into the tether. In the intermolecular examples, the ketone was found in the 3-position of the cyclopentenone. The vinylogous 48 ynone cyclised in 62% yield to provide the tricyclic product 49 as a 2.7 1 mixture of cis/trans isomers. An interesting intramolecular example was disclosed by de Meijere in which ketone 50 produced the spirocyclopropane 51 in 63% yield. ... 

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