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Organocobalt porphyrins

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... [Pg.290]

On the basis of H NMR studies, the controlled polymerization of 20 with organocobalt porphyrins (7a) is considered to take place as shown in Scheme 11. Homolysis of the cobalt-alkyl bond in 7a produces 54 and a carbon-centered radical (R-) (Scheme 11 A) which initiates polymerization by reacting with 20 to form 55 (Scheme 1 IB). Then, 55 either combines reversibly with 54 to give... [Pg.154]

Although the previously described polymerization with organocobalt porphyrins (7) is the first example of controlled radical polymerization, the applicability of this system is only limited to acrylic esters (20). Use of 7 for free-radical polymerization of methacrylic esters (21) results in a chain-transfer reaction with respect to the a-methyl group, to give oligomers with terminal unsaturation. [Pg.154]

Cobalt. Alkyl exchange reactions of organocobalt porphyrins have been followed by NMR spectroscopy.The peroxidation of (EtO)2CHCH2-Co porphycenes has been investigated by NMR spectroscopy. NMR... [Pg.74]

Gridnev, A.A. Ittel, S.D. Fryd, M. Wayland. B. B. Formation of Organocobalt Porphyrin Complexes from Reactions of Cobalt(II) Porphyrins and Dialkylcyanomethyl Radicals with Organic Substrates Chemical Trapping of a Transient Cobalt Porphyrin Hydride, Organometallics 1993,12 (12), 4871-80. [Pg.141]

Organocobalt porphyrin complexes have also yielded useful mechanistic information. The reactions of (tetrakis(p-methoxyphenyl)porphyrinato)cobalt(II) ((TAP)Co(II)) with radicals derived from dialkylazo thermal initiators with acrylic monomers provide evidence for the intermediacy of Co(III)-H species in CCTP. Reaction of (TAP)Co (II) with tertiary alkyl radicals, for example, as derived from AIBN in the presence of monomers that form stable Co-alkyl complexes, such as methyl acrylate, results in quantitative formation of Co(III)-alkyl. Whereas with monomers leading to tertiary C-Co bonds, such as MMA, the Co (II) is very much a spectator as normal polymerization ensues. Thermodynamic and activation parameters have been measured for the dissociation of (TAP)Co(III)-C(CH3)2CN to Co(II) and organic radical in solution as a probe into CCTP mechanism by low-spin Co(II). ... [Pg.255]

Since Wayland et al. first used organocobalt porphyrins to mediate the OMRP of MA in 1994, Co porphyrin complexes (Co-1) have been used for the polymerization of acrylates, acrylic acid, and VAc, in which the DT process dominated the controlled propagation for VAc (Figure 11). Cobalt(II)... [Pg.446]

The migration reactivity of cr-bonded ligands of organocobalt porphyrin is also depending upon the oxidation states of the cobalt center (Scheme 16). The migration of the (R) group onto the macrocycle of [(TTP)Co (R)]+... [Pg.29]

In some cases where a reaction involving a radical species occurred within cobalt porphyrin complexes, it has been possible to trap transient cobalt porphyrin hydride species. This was indeed observed during the synthesis of organocobalt porphyrin that resulted from the reaction of cobalt(n) porphyrin and dialkylcyanomethylradicals with alkenes, alkynes, alkyl halides, and epoxide. A transient hydride porphyrin complex was also involved in the cobalt porphyrin-catalyzed chain transfer in the free-radical polymerization of methacrylate. The catalytic chain transfer in free-radical polymerizations using cobalt porphyrin systems has been extensively investigated and will not be treated in this section. Gridnev and Ittel have published a comprehensive overview of the catalytic chain transfer in free-radical polymerizations. ... [Pg.30]

Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. [Pg.250]

In this section, we present material dealing with the direct oxidation and reduction of a variety of organocobalt species, including complexes with more than one cobalt center, electrodes functionalized with cobalt complexes, cobalt-containing SchifF-base complexes, cobalt porphyrins and corroles, and macrocyclic tetraamines. [Pg.536]

Uyemura, M. Aida. T. Steric control of organic transformation by a dendrimer cage Organocobalt dendrimer porphyrins as novel coenzyme 8,2 mimics. J. Am. Chem. Soc. 2002. 124 (38). 11392-11103. [Pg.1575]

Direct aerial oxidation of cobalt(ii) in the presence of acetone, acetonitrile, and malononitrile has been used for the first time in the porphjrrin series to generate organocobalt(m) porphyrin derivatives. The acetonyl species [Co-(TPP)CHaCOCH3] was characterized by an JlT-ray crystal structure and found to have (Co—C)=2.028 A. The complexes are stable in solution and apparently fairly stable to light. A further novel alkylation of prophyrins involves electro-... [Pg.438]

The area started to boom after 1994, by which time awareness of controlled radical polymerization methods was rising, when the well-controlled polymerization of acrylates was reported almost simultaneously by the groups of Harwood and Wayland using organocobalt(III) compounds based on glyoxime (cobaloximes) and porphyrins as initiators (Figure While II is a thermal initiator, I needs photo-... [Pg.352]

The stability constants for the coordination of pyridine or substituted pyridines to various alkylcobalt porphyrin systems have been reported. This study has been done in order to observe the m-influence of hydroporphyrin macrocycles on axial position and alkyl exchange reactions. The alkyl exchange reactions of organocobalt(lll) porphyrins with a cobalt(ll) complex of a distinguishable porphyrin or tetrapyrrolc have been studied. The equilibrium constants for the alkyl transfer have been reported. The exchange of the axial ligand is reversible and it follows a bimolecular mechanism. Thermodynamic and activation parameters for homolytic Co-C bond dissociation have been obtained on the (TAP)CoC(CH3)2CN complex (TAP = tetraanisylporphirinato). ... [Pg.29]


See other pages where Organocobalt porphyrins is mentioned: [Pg.245]    [Pg.256]    [Pg.285]    [Pg.838]    [Pg.1484]    [Pg.153]    [Pg.274]    [Pg.924]    [Pg.98]    [Pg.245]    [Pg.256]    [Pg.285]    [Pg.838]    [Pg.1484]    [Pg.153]    [Pg.274]    [Pg.924]    [Pg.98]    [Pg.280]    [Pg.286]    [Pg.288]    [Pg.332]    [Pg.387]    [Pg.447]    [Pg.856]    [Pg.167]    [Pg.855]    [Pg.682]    [Pg.28]   
See also in sourсe #XX -- [ Pg.285 ]

See also in sourсe #XX -- [ Pg.153 , Pg.154 ]




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Organocobalt

Oxidation organocobalt porphyrins

Porphyrins, organocobalt derivatives

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