Cobalt complexes vinylidenes

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

Strong bases (pKa > 11) also convert alkyl cobaloximes and alkyl cobalamins into -complexes such as 73. This is usually followed by further decomposition to olefins and alkanes. The stability of complexes such as 73 depends very much upon X and the nature of the axial ligand in the cobalt chelate.98-218-227 230 Strong nucleophiles such as RS or CN can cause decomposition of LCo—R as well.98-231 Under the normal conditions of radical polymerization, Markovnikov organocobaloxime should form whenever the hydride, LCoH, appears in the polymerization mixture. If 1,2-vinylidene monomers are being polymerized, then thermally unstable tert-alkyl-cobaloximes are obtained. These species are expected to undergo homolytic Co—C cleavage to yield tertiary radicals. 

Cobalt tetraphenylporphyrin complex promotes a chain-transfer reaction in the radical polymerization of MMA to give an MMA oligomer with vinylidene unsaturation at the chain end.124 An alternative method of introducing the terminal unsaturation was disclosed by Meijs et al,125 Substituted allylic sulphides are used as chain transfer agents in which sulphide groups act as leaving group as follows ... 

These derivatives (Type B) are at least formally prepared by the insertion of a fragment into a M-N bond to yield a new M-X-N unit. Such species have been suggested as possible intermediates in the insertion of an oxygen atom into a G-H bond by cytochrome P-450. Several of these derivatives have formally had a carbene fragment inserted into a M-N bond. Such derivatives include a Ni derivative and a cobalt(III) species that has undergone two such insertion reactions. Other species represent the formal reaction of a vinylidene with iron(III) (two different crystalline forms). This iron derivative has an intermediate-spin state. Other complexes result from the insertion of a nitrene or an oxene This last derivative can also be considered to be a porphyrin N-oxide derivative and the structure of a free base species of a porphyrin N-oxide has also been reported". Appropriate stereochemical parameters for the members of this class are found in Table IX. 

The use of a cyclopentadienyl ligand containing a pendant phosphine arm allowed the isolation of the cobalt alkyne complexes [<5o(ti5 i -C5H4CH2CH2l Bu 2)(Tl -PhCCR)] (R = H, Hi). In the case of ethyne, the alkyne complex could not be isedated and the vinylidene complex [ 50(115 I-C5H4CH2CH2PBu 2)(=C=CH2)1 was observed. 

Some of the vinylidene complexes include cobalt, rhodium and rhenium in halfsandwich complexes, which are synthesized from acetylene complexes". This reaction involves an intermediate alkinyl(hydrido) complex, which can sometimes be isolated. The bonding between the metal and the a-carbon atom in vinylidene rhodium complexes is shorter than in carbene rhodium complexes, which indicates a high electron density on the center atom. 

Nishibayashi and coworkers [10] have used a variant of the same reaction that does not require the cobalt carbonyl group. Here, a diruthenium catalyst is employed, which reacts with a terminal alkyne, reportedly going through a vinylidene complex and opening the cyclopropane ring in situ. Reaction with the aldehyde followed by ring closure provides the product, regenerating the catalyst (Scheme 10.8). 

---