Metal-alkyne complexes cobalt

In fact, this approach constituted one of the earliest trials. One is introduction of a chiral ligand like glyphos on cobalt by replacing one of the carbon monoxides to obtain an enantiomerically enriched new metal-alkyne complex. The second approach is use of the propargyl alcohol bearing a chiral auxiliary. Replacement of only one carbon monoxide would lead to a mixture of diastereomers. In both cases, a I I mixture of products was obtained, and each diastereomer was separated before the... 

Transition-metal-promoted cycloaddition is of much interest as a powerful tool for synthesis of carbocyclic stmcture in a single step. Utilization of carbon monoxide as a component of the cycloaddition reaction is now widely known as the Pauson-Khand reaction, which results in cyclopentenone formation starting from an alkyne, an alkene, and carbon monoxide mediated by cobalt catalyst. Although mechanistic understanding is limited, a commonly accepted mechanism is shown in Scheme 4.16. Formation of dicobalt-alkyne complex followed by alkene... 

Strained cycloalkynes can be stabilized by coordination to one or more transition metal centers (198). The unusual vicinal defluorination reaction of perfluoro-l,3-cyclohexadiene with [Co2(CO)8] to give the /i-alkyne complex 45 (see Section III,E) prompted a study of the reactions of OFCOT with cobalt carbonyl precursors. 

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. 

Although heterobinuclear alkyne complexes have been less intensively studied, a number of routes involving cobalt as one of the metals have been developed. Compounds with two different metals are expected to have reactivity patterns different from those of the respective homonuclear species and also have applications in the preparation of chiral complexes. In a manner analogous to Eq. (1) the mixed-metal complexes [MCo (CO)7Cp] (M = Mo, W) and [CoMn(CO)9] react with a range of alkynes to afford complexes of the type [MCo(ju-alkyne)(CO)5Cp]1314 and [CoMn (jU-alkyne)(CO)7],15 as illustrated in Eq. (3). 

The heterobinuclear complexes [MCo(/u,-FiCC2CF3)2(CO)3Cp] have been prepared by addition of octacarbonyldicobalt to the mononuclear alkyne complexes [M(F3CC2CF3)2(Cl)Cp] (M = Mo or W)24 demonstrating that cobalt-carbonyl fragments can coordinate to an alkyne already bound to another metal-ligand fragment see Eq. (5). 

X-ray crystallography amino(l-alkynyl) carbenes, 170 chalcogen-bridged metal-carbonyl complexes, 244-248, 250-253, 255-256, 258-264, 266-272, 274-279, 281, 283-284, 287, 289, 292-293, 295-310 cobalt-alkyne complexes, 76-77, 82-83, 89-90, 94-96... 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

These metal-alkynyl complexes can be protonated to afford the free alkynes and parent cobalt hydroxo complex (comparable reactivity to their alkyl and aryl congeners), but have proven inert toward oxygenation and carbonylation. They are also thermally stable up to 100 °C. Attempts to explore the reactions of these compounds with unsaturated hydrocarbons were typically fruitless. The one exception is the reaction between 53 and its parent alkyne (HC = C02Me, Scheme 6), which under benzene reflux effects catalytic, stereospecific, linear trimerisation of the alkyne to afford ( , )-buta-l,3-dien-5-yne. The reaction was, however, slow (4.5 turnovers in 20 h) and suffered from catalytic deactivation due to hydrolysis of 53, which subsequently reacted with adventitious CO2 to irreversibly form an inert /x-carbonato complex. The catalytic cycle was concluded to involve initially a double coordination-insertion of the C = C bond of methylpropiolate into the Co-Caikyne linkage. Subsequent hydrolysis of the Co-C bond by a third equivalent of HC = CC02Me would then afford the observed product and regenerate 53. However, a definitive explanation for the stereospecificity of the process was not established. 

The use of both LIU and HIU has been shown to increase the efficiency of the P-K reaction, which involves the formation of cyclopentenone from the annulation of a cobalt alkynyl carbonyl complex and an alkene. The use of low-power ultrasound, as for example, from a cleaning bath, although capable of producing intramolecular P-K reactions, generated relatively low cyclization yields. The motivation for the use of high intensity came from its ability, as previously described, to effectively decarbonylate metal carbonyl and substituted metal carbonyl complexes. Indeed, HIU produced by a classic horn-type sonicator has been shown to be capable of facile annulation of norbornene and norbornadiene in under 10 min in the presence of a trimethylamine or trimethylamine N-oxidc dihydrate (TMANO) promoter, with the latter promoter producing cleaner product mixtures. This methodology also proved effective in the enhancement of the P-K reaction with less strained alkenes such as 2,5-dihydrofuran and cyclopentene, as well as the less reactive alkenes -fluorostyrene and cycloheptene. The mechanism has been postulated to involve decarbo-nylation of the cobalt carbonyl alkyne, followed by coordination by the amine to the vacant coordination sites on the cobalt. 

Dicobalt-hexacarbonyl-alkyne complexes are another class of organometallic compounds with good stability imder physiological conditions. Complexation of the alkyne proceeds smoothly under mild conditions by reaction with Co2(CO)g imder loss of two molecules of CO [79]. The applicability of this reaction to peptides was shown by Jaouen and coworkers by the reaction of Co2(CO)g with protected 2-amino-4-hexynoic acid (Aha) and dipeptides thereof (Boc-Phe-Aha-OMe and Ac-Aha-Phe-OMe) [80]. Similarly, Cp2Mo2(CO)4 complexes of these alkynes were obtained. It has been shown that the C-terminal Met" in SP can be replaced by isostere analogs without appreciable loss of physiological activity. The same is true for the C-terminal Met in neurokinin A (NKA), another tachykinin peptide hormone (Scheme 5.16). Alkyne analogs of SP and NKA were obtained by replacement of these methionines with norleucine acetylene residues. Alternatively, Lys in NKA may be replaced by an alkyne derivative which can also be complexed to Co2(CO)g as shown in Scheme 5.16. Complexation with Co2(CO)g proceeds smoothly in about 50% yield for all derivatives [81]. After HPLC purification, these cobalt alkyne peptides were comprehensively characterized spectroscopically. Most notably, they exhibit typical IR absorptions for the metal carbonyl moieties between 2000-2100 cm [3]. Recently, there is renewed interest in Co2(CO)5(alkyne) complexes because of their cytotoxicity [82-84]. 

While there are T -alkyne complexes of many metals, the most important for organic synthesis are the dicobalt complexes 7.2 formed simply by stirring an alkyne 7.1 with dicobalt octacarbonyl in an inert solvent (Scheme 7.1). The complexes are tetrahedral with carbon atoms at two vertices and cobalt atoms at the other two (Figure 7.1). They are, however, often drawn as simple alkyne complexes 73. Although this is a very convenient shortcut, drawing them in this way may hide mueh of the chemistry. It is easy to release the alkyne from the hold of the cobalts decomplexation can be achieved by mild oxidation using a reagent such as FeCls or h. 

Reaction of [S(CCPh)2] with [Co2(CO)g] afforded 2 jhe crystallographically characterised adduct [(PhCC)S Co2(CO)6(Ti2-CCPh) ] which, on treatment with further [Co2(CO)g] afforded [S Co2(CO)6(Tl2-CCPh) 2l. The reaction of pentacarbonyl allyloxy((4-methylphenyl)ethynyl)carbene) chromium and tungsten complexes with dicobalt octacarbonyl afforded 63 tpe alkyne complexes (62) (M = Cr, W), which failed to undergo intramolecular Pauson-Khand reactions instead, at room temperature, a 1,2-elimination occurs, regenerating the metal hexacarbonyl with predominant formation of dinuclear cobalt complexes (63) - (65). 

---