Cobalt complex, intramolecular activation

It should be noted that there is also a report of intramolecular C-H activation by a relevant cobalt complex, the identity of which was inferred to be the cobalt imido compound Tp Co = NSiMe3. However, this imido species has not been discretely observed. 

The proposed reaction mechanism is shown in Figure 16. The catalytic activity is due to the superoxocobalt(in) complex L2C0-O-O which binds a p-keto ester or aldehyde (left and right end of the horizontal row, respectively). Intramolecular H-atom transfer affords an enolato-cobalt complex (left) or a coordinated acyl radical (right). Both of these release an... 

Half-sandwich complexes of cobalt with intramolecular phosphorous coordination have been known for the cobalt atoms in oxidation state - -l and - -ll (see Sections 7.01.3.3 and 7.01.3.4). Butenschon has published a comprehensive review recently on cyclopentadienyl metal complexes bearing pendant phosphorous arsenic and sulphur ligands. Intramolecular coordination to the cobalt center via a substituted Cp ring also oecurred in a rare reaction which combined C-H and C-F activation at the same time, thus leading to the coupling between a pentafluorophenyl ring and a pentamethylcyclopentadienyl ring (see Scheme 10 above). 

Tracer studies have shown that the isomerization of /ra/zj-[Co(en)2(NH3)-(SCN)] to the A-thiocyanato-linkage isomer in basic aqueous solution is intramolecular. The detailed mechanism may involve an intimate ion pair which does not exchange its thiocyanate with the thiocyanate in solution, a species in which the thiocyanate is Ji-bonded to the cobalt, or intramolecular attack of a deprotonated amine nitrogen at the thiocyanate carbon (Scheme 5). Aquation of [Co(NH3)5(sulphamate)] - is preceded by isomerization of the starting complex to an equilibrium mixture of N- and 0-bonded isomers. The specific rate constant for approach to equilibrium is l.lxlO s at 25 °C composite activation parameters are AH = 24.7 kcal mol and AS = 10.9 cal deg mol . Inability to estimate the equilibrium constant satisfactorily precludes the separation of these kinetic parameters into their forward and backward components. ... 

Cycloaddition of aUcynes catalysed by transition metals is one of the most efficient and valuable ways to prepare benzene and pyridine systems [12], Among the possible catalytic systems able to catalyse this reaction, cobalt and iron complexes containing NHCs as ligands have shown high catalytic activity in the intramolecular cyclotrimerisation of triynes 36 (Scheme 5.10) [13]. The reaction was catalysed with low loading of a combination of zinc powder and CoC or FeClj with two or three equivalents of IPr carbene, respectively. 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

The data in Table XXXV show that common features for these ammonia and amine complexes are very fast isomerization between the cis and trans isomers of the diaqua species and the fact that the trans diaqua isomers are generally more stable than the cis isomers. In the ammine system the activation parameters for k2 and k 2 are consistent with an isomerization process at cobalt(III), but it is at present not clear how this occurs. It need not be a simple cis-trans isomerization occurring at one of the Co(III) centers, but might involve the participation of both metal centers. The isomerization reaction may proceed via intramolecular proton transfer between a water ligand and one of the two hydroxo bridges with simultaneous bridge cleavage and formation... 

Both intramolecular and intermolecular attack by M—OHn+ species are well established for cobalt(III) and other kinetically inert metal centres (Section 61.4.2.2.3). However, reactions of this type are not as well defined with labile metal ions. In copper(II) complexes, the pKa values for coordinated water ligands usually fall within the range pKa 6-8. If coordinated hydroxide ion is an important nucleophile in copper(II)-promoted reactions, the reactions would be expected to become independent of [OH-] at pH 8 when the bulk of the complex was converted to the active hydroxo species. Studies of the pH dependence of a number of copper(II)-promoted reactions to such pH levels have been carried out and no evidence obtained for the production of catalytically active hydroxo complexes however, some reactions do proceed by this pathway. 

Among the carbonylative cycloaddition reactions, the Pauson-Khand (P-K) reaction, in which an alkyne, an alkene, and carbon monoxide are condensed in a formal [2+2+1] cycloaddition to form cyclopentenones, has attracted considerable attention [3]. Significant progress in this reaction has been made in this decade. In the past, a stoichiometric amount of Co2(CO)8 was used as the source of CO. Various additive promoters, such as amines, amine N-oxides, phosphanes, ethers, and sulfides, have been developed thus far for a stoichiometric P-K reaction to proceed under milder reaction conditions. Other transition-metal carbonyl complexes, such as Fe(CO)4(acetone), W(CO)5(tetrahydrofuran), W(CO)5F, Cp2Mo2(CO)4, where Cp is cyclopentadienyl, and Mo(CO)6, are also used as the source of CO in place of Co2(CO)8. There has been significant interest in developing catalytic variants of the P-K reaction. Rautenstrauch et al. [4] reported the first catalytic P-K reaction in which alkenes are limited to reactive alkenes, such as ethylene and norbornene. Since 1994 when Jeong et al. [5] reported the first catalytic intramolecular P-K reaction, most attention has been focused on the modification of the cobalt catalytic system [3]. Recently, other transition-metal complexes, such as Ti [6], Rh [7], and Ir complexes [8], have been found to be active for intramolecular P-K reactions. 

Two mechanisms of cobalt(III)-mediated peptide-bond cleavage have been investigated. The first one involves hydrolysis of a directly activated amino acid ester, or peptide (equation 4). The other mechanism involves the intramolecular attack of an amino acid ester or peptide by a cis coordinated hydroxide or water molecule (equation 5). In both cases, the cobalt(III) complex must have two open coordination sites cis to each other. For the directly activated mechanism, these sites are needed to bind the amino acid ester or peptide. The intramolecular reaction requires one site for coordination of the ester or peptide, and one site for the coordination of the hydroxy or water molecnle. One of the initial cobalt(III) complexes to be investigated was... 

Any detailed description of the mechanism of an octahedral substitution must also account for the stereochemical changes that accompany reaction. Werner recognized this and made use of it in his discussions of the stereochemistry of reactions of cobalt(III) complexes. The available experimental results can be explained on the basis of possible molecular rearrangements and some cautious predictions can even be made. The base hydrolysis of cobalt III)ammines appears to be unique in that it often occurs with rearrangement it also affords the few known examples of optical inversion. These results can be explained by formation of a 5-coordinated species with a trigonal bipyramidal structure. Optically active metal complexes racemize by either an intramolecular or an in-termolecular process. Substitution reactions of platinum metal complexes often occur with retention of configuration. 

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