Alkynes reaction with dicobalt octacarbonyl

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

An interesting cyclization reaction was reported that involved the reaction of dienes, diynes, or ene-ynes with transition metals to form cyclopentenone derivatives in the presence of carbon monoxide.363 in a simple example, ene-yne 444 was heated with dicobalt octacarbonyl and CO to give a 68% yield of 445.364 jjjj transformation has become an important synthetic tool known as the Pauson-Khand reaction.365 jhe mechanism probably involves insertion of the alkene (or alkyne) into the transition metal bond, which is why it is presented in this section. Formally, it is a [2+2+l]-cycloaddition, but the accepted mechanism is the one proposed by Magnus,364 and shown in Figure 13.8.366 n has been stated that further study is required to... 

This reaction has found its way into organic synthesis fi and is attractive because of its generality and selectivity. In Drummond s synthesis of hydroxymethylacylfulvene,3 0 for example, a Pauson-Khand reaction of the allene-alkyne 446 gave a 69% yield of 447. In this case, molybdenum hexacarbonyl was used. In Takano s synthesis of dendrobine, 448 was treated with dicobalt octacarbonyl and then NMO to give an 89%... 

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). 

In 1990, Rautenstrauch and co-workers reported the catalytic reaction of 1-heptyne (43) with ethene (Scheme 17).51 They were able to use just 0.22 mol % of dicobalt octacarbonyl (with respect to the alkyne) in the reaction and obtained a moderate yield of the desired cyclopentenone (44). 

The expected hexacarbonyl dicobalt complex was formed when the al-kyne and dicobalt octacarbonyl were stirred in CH2C12 (73 —> 74). If the same reaction was performed in the presence of ten equivalents of NMO, the novel pentacarbonyl complex could be isolated (73->75). On dissociation of the CO ligand, accelerated by the addition of NMO, the sulfur atom is able to coordinate to the vacant site creating a stable complex. Treatment of the complex with CO resulted in regeneration of the hexacarbonyl complex and, unlike the isolated alkene-pentacarbonyldicobalt-alkyne species 68, heating of the pentacarbonyl complex 75 led to the formation of the desired cyclopentenone. A similar sulfur stabilised intermediate was isolated by Pericas and co-workers while investigating tethered chiral auxiliaries in the PK reaction.87,88... 

Cyclopropene can also be used as the aUcene component and affords bicyclo[3.1.0]hexen-2-ones upon reaction with alkyne dicobalt octacarbonyl complexes in the presence of NMO (Scheme 250). Vinyl ethers and vinyl esters serve as ethene equivalents in Pauson-Khand reactions. For example, reaction of vinyl benzoate with complex (169) furnished cyclopentenone (170) (Scheme 251). This reaction was used in a synthesis of (-l-)-taylorine and nortaylorine. Allenes participate in intermolecular Pauson-Khand reactions affording alkylidene-substituted cyclopentenones (Scheme 252). ... 

A particularly useful synthesis of cyclopentanones involves the coupling of an alkene, an alkyne and carbon monoxide in the presence of dicobalt octacarbonyl (equation 17). The reaction proceeds via an al-kyne-cobalt complex (7) and with relatively unreactive alkenes such as cyclopentene it is preferable to synthesize the complex in a separate step. With highly strained alkenes such as norbomadiene, how-... 

A significant advance in the Pauson-Khand reaction was made by the discovery that various additives, such as tertiary amine A-oxides, promote the cycloaddition reaction. For example, treatment of the dicobalt complexed alkyne 187 with trimethylamine A-oxide at only 0 °C provides the cyclopentenone 188 in good yield (1.192). More recent advances have been made in catalytic Pauson-Khand reactions. " Only 3 mol% of dicobalt octacarbonyl [Co2(CO)8] under one atmosphere of CO effects the formation of the cyclopentenone 188 from the alkyne 189 in benzene at 70 °C (an improvement in the yield to 90% was achieved in the presence of the additive Bu3P=S) (1.193). " ... 

The Pauson-Khand reaction provides another new approach to the metal-catalyzed synthesis of heterocycles. This reaction involves the interaction of the multiple bonds of an alkyne with an alkene and carbon monoxide in the presence of dicobalt octacarbonyl (Co2(CO)g), or with just this reagent as a source of CO. The overall process has been described as a [2 -h 2 -h 1] cycloaddition. Only a few applications to heterocyclic synthesis have been reported so far. A 2008 paper that is illustrative of the process describes the use of this reaction for the construction of a heterocyclic ring that is part of an azabicy-clo[3.3.1]nonane derivative. This ring system is present in the alkaloid (-)-alstonerine (4.37), which prompted this study. 

Apart from cobalt carbonyl catylyzed hydroformylation, Pauson-Khand (PK) reaction is another type of reaction catalyzed with bimetallic carbonyl complex. Formally Pauson-Khand (PK) is a [2 -i- 2 -i- 1] cycloaddition of an alkyne, an alkene, and a CO group into cyclopentenone [128-130]. This process was initially discovered in 1973 [131], and early studies focused on using dicobalt octacarbonyl as both reaction mediator and the source of the carbonyl functional group. Since several variants of the original thermal protocol were introduced, PK reaction has received more and more fundamental and organic synthesis interests [132, 133]. 

The reaction of l,r-dialkynylferrocene [(Ti5-C5H4CCR)2Fe] (R = Ph, SiMe3, Me, ferrocenyl) with excess dicobalt octacarbonyl afforded [ (T 5-C5H4CCR)2Fe Co2(CO)6)2l. structurally characterised for R = Ph, in which a Co2(CO)6) group coordinates to each of the alkyne moieties. 

The Nicholas reaction enables efficient substitution reactions of propargyl alcohols, ethers, and acetates. Prior to the substitution step, dicobalt octacarbonyl reacts with the alkyne to yield cobalt-alkyne complex 1. The resulting organometallic complex reacts with inter- or intramolecular nucleophiles in the presence of a Lewis or protic acid to furnish desired substitution products 2. The cobalt-complexed alkyne can be oxidatively removed after this step or used to further fiinctionalize the Nicholas reaction products. The stereoselective synthesis of chiral products using the title reaction is also possible. ... 

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