Cobalt triple bonds

Various other reducing methods are employed for the conversion of (3-nitro alcohols to amino alcohols, namely, electrochemical reduction.107 The selective electrohydrogenation of ni-troaliphatic and nitroaromatic groups in molecules containing other groups that are easy to hydrogenate (triple bond, nitrile, C-I) are carried out in methanol-water solutions at Devarda copper and Raney cobalt electrodes (Eq. 6.55).107... 

In 1996, Malacria et al. [139] reported on cobalt-mediated reactions of the related allenynes. Heating the allenyne 222 in the presence of cpCo(CO)2 accompanied by a photochemical activation of this organometallic compound delivered the cross-conjugated trienes 223 (Scheme 15.71). The second triple bond present in the substrate did not participate in the reaction, underlining the higer reactivity of the allene unit. 

Cobalt, as its CpCo(CO)2 complex, has proven to be especially suited to catalyze [2 + 2 + 2] cycloadditions of two alkyne units with an alkyne or alkene. These cobalt-mediated [2 + 2 + 2] cycloaddition reactions have been studied in great detail by Vollhardt337. The generally accepted mechanism for these cobalt mediated cycloadditions, and similar transition metal mediated cycloadditions in general, has been depicted in equation 166. Consecutive co-ordination of two triple bonds to CpCo(CO)2 with concomitant extrusion of two molecules of carbon monoxide leads to intermediates 578 and 579 via monoalkyne complex 577. These react with another multiple bond to form intermediate 580. The conversion of 578 to 580 is said to be kinetically favored over that of 579 to 580. Because intermediates like 580 have never been isolated, it is still unclear whether the next step is a Diels-Alder reaction to form the final product or an insertion to form 581. The exact circumstances might determine which pathway is followed. 

Formation of 772-complexes is known for both mono- and bis-phospho-nio-benzophospholides and has been observed (Scheme 18) in the reactions of the cation 23 with Jonas reagent to give the cobalt complex 49 [49], addition of the zwitterion 25 to a Mo-Mo triple bond to afford the dinuclear complex 50 [47], and finally, upon treatment of 26 with copper iodide to yield the complex 51 [46] which is peculiar because of the presence of the same ligand in two different coordination modes. Whereas it is clear that the metal atoms in all complexes supply inappropriate templates for the formation of 77 -complexes, the preference of rf-(,n)- over a possible a-coordina-tion is less well understood [49]. 

Again several alkyls add—molybdenum, chromium, iron, cobalt, nickel, the alkali metal alkyls and aluminum alkyls react. A tin alkoxide has recently been studied by Russian workers and found to add to acetylenes. Mercury chloride, of course, adds and two cobalt—cobalt bonded compounds add to acetylene. The second is questionable because it dissociates in solution and the reaction may be a radical reaction, one cobalt adding to each end of the triple bond. 

Dicyclopentadienyldinickeldiphenylbutadiyne-dicobalt hexacarbonyl has been prepared from diphenyldiacetylene in which one triple bond acts as a bridging group between two nickel atoms and the other between two cobalt atoms (203). Reduction of the diphenylacetylene complex (R = R = Ph) with sodium and alcohol in liquid ammonia yields dibenzyl, showing that the diphenylacetylene grouping is bonded only to the nickel atoms. The corresponding complex of acetylene (R = R = H) has also been prepared from nickeloccne and acetylene (69) ... 

Recently, cobalt(II) chloride has been found to catalyze the allylzincation of a variety of 1-aryl-1-alkynes111. As illustrated for 154, a syn addition to the triple bond occurred with high regioselectivity and afforded an intermediate a-arylzinc species that could be further functionalized (equation 76). Several tri- or tetrasubstituted styrenes have been prepared by this method. 

The triple bond of the nitrile group can be cotrimerized with two alkynes to produce pyridines. The cobalt-catalysed cocyclization of alkyne and nitrile in a ratio of 2 1 is a good synthetic route to pyridine derivatives [70], Two regioisomers, 174 and 175, are obtained by the reaction of propyne with MeCN. The reaction is carried out in a large excess of MeCN, and potentially useful for commerical production of pyridine derivatives [71]. The reaction of acetylene itself with various nitriles produces the a-substituted pyridine 176 in the presence of water under irradiation [72], HCN cannot be used for this cocyclization. The reaction has been applied to alkaloid synthesis. 

Acetylenic molecules are versatile ligands in organometallic chemistry [152— 154]. Examples of acetylene-metal reactions include facile complexations of one triple bond with Co2(CO)s [155-157] and two triple bonds with CpCo(CO)2 (Scheme 31) [158,159]. The hb-PYs contain numerous triple bonds and should be readily metallized through their complexations with the cobalt carbonyls. Upon admixing hb-P20 and the cobalt carbonyls in THF at room temperature, the solution color changed from yellow to brown, accompanied by CO gas evolution. The mixtures remained homogenous towards... 

It has become clear that the carbon-rich hb-PYs are readily curable (from ca. 150 °C), thermally stable (up to ca. 550 °C), and pyrolytically carboniz-able (yield up to 80% at 900 °C). Furthermore, their triple bonds are easily metallizable by the complexations with cobalt carbonyls. Since the polymer complexes contain a large number of metal atoms, we tried to utilize them as precursors for fabrication of metalloceramics. The pyrolyses of the polyyne-cobalt complexes at 1000 °C for 1 h under nitrogen furnished ceramic products 83 and 84 in 50%-65% yields (cf., Scheme 31). All the ceramics were magnetizable and could be readily attracted to a bar magnet. 

In previous works this group had observed a competition between the PKR and a [2 + 2 + 2] cyclization in the second reaction step of three triple bonds. Thus, when reacting linear triynes 174 under catalytic, high CO pressure, cobalt mediated PKR conditions, they obtained mixtures of products 175 coming from two [2 + 2 + 1] cycloadditions, and 176 from a [2 + 2 + 1]/ [2 + 2 + 2] tandem reaction. When the triple bonds were ether linked, the latter was the favored reaction, while with substrates lacking oxygen atoms, the iterative PKRs was the major pathway (Scheme 51) [166]. When the reaction was performed intramolecularly between a diyne and an alkyne, the only reaction products were the result of a [2 + 2 + 1 ]/[2 + 2 + 2] tandem cycloaddition [167,168]. 

The study of polynuclear cobalt-alkyne clusters has provided insight into the coordination, protection, and activation of the carbon-carbon triple bond. It is possible to draw analogies between alkynes coordinated to molecular species and those to surfaces. 

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. 

Highly strained cyclic compounds containing an alkyne group can be prepared upon complexation to Co2(CO)6. The success of these reactions relies on the severely bent nature of the alkyne triple bond when complexed to cobalt. Cyclization of (137) to form bicyclo[7.3.0]tridecane (138), a calichiamycinone derivative has been reported (Scheme 204). The nine-membered enyne complex (140) was prepared by ring-closing metathesis of (139) (Scheme 205). A related reaction was used to give complex (141) (Scheme 206). It should be noted that no cyclization occurs without prior complexation to cobalt in the latter case. A complexation-initiated... 

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