Alkyne reactions with cobalt carbonyl complexes

Use of Co2(CO)8 in reactions involving 1,2-propadienes remains for the most part unexplored. It has been reported that terminal 1,2-propadienes react with Co2(CO)8 to form unidentified complexes, and that excess 1,2-propadiene is polymerized concurrently [30]. It has also been reported by Nakamura that a novel dimeric complex 54, in which a carbonyl ligand is connected to the central carbon of 1,2-propadiene, is produced by the reaction of 1,2-propadiene itself with Co2(CO)8 (Scheme 23) [31]. However, unlike the well-known chemistry of alkyne-Co2(CO)6 complexes, these 1,2-propadiene-cobalt carbonyl complexes have rarely been applied in synthetic reactions, probably due to their high activity in catalyzing the polymerization of 1,2-propadienes [32]. 

Although the oxidative promoters have been quite effective, the reaction with less reactive olefins is still troublesome. Under the conditions with the oxidative promoter, the PKR is inevitably competing with the demetallation of the alkyne-cobalt carbonyl complex to give the metal-free alkynes. This competition is insignificant when the reactive olefins are employed, but it is prone to give more demetallated alkynes when the less reactive olefins are used (Scheme 3). This is reasoned by the fact that it is hard to expect from those promoters to oxidize only one of the COs on metal, and, in other words, the decarbonylation is not discriminative. As a result, the finding of the optimum condition is critical to favor the desired PKR product. 

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. 

In chapter 6 we described the use of the remarkable Pauson-Khand reaction for the synthesis of cyclopentenones. If the components (CO, alkene and alkyne) are tethered by a nitrogen atom, a heterocycle will also be formed. The first stage in this process is to couple the cobalt carbonyl complex, e.g. 236, of ahalo-alkyne with an amine containing the alkene in the side chain. The best way to do this is to react 234 with Co2(CO)9 to give 235 and then 236 and to capture this complex with the amine without isolation of intermediates.34... 

Combination of three unsaturated compounds, i.e., alkyne, alkene, and CO provides a convenient means of catalytically synthesizing useful products such as cyclic unsaturated ketones in a one-pot process. On the basis of fundamental studies of the reactions of alkyne-coordinated cobalt carbonyl complex with olefins, a catalytic process to synthesize cyclic ketones has been developed (Eq. 1.20) [134],... 

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. 

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]. 

A cobalt carbonyl complex can be available for the synthesis of pyridinophanes, 2-oxopyridinophanes and cyclophanes in the reaction of diynes with nitriles, isocyanates, alkynes, respectively. Meta/para selectivity was varied by the tether length of diynes and the substituents of alkynes. 

While platinum and rhodium are predominantly used as efficient catalysts in the hydrosilylation and cobalt group complexes are used in the reactions of silicon compounds with carbon monooxide, in the last couple of years the chemistry of ruthenium complexes has progressed significantly and plays a crucial role in catalysis of these types of processes (e.g., dehydrogenative silylation, hydrosilylation and silylformylation of alkynes, carbonylation and carbocyclisation of silicon substrates). 

The Pauson-Khand reaction starts with the replacement of two CO molecules, one from each Co atom, with the alkyne to form a double a complex with two C-Co a bonds, again one to each Co atom. One CO molecule is then replaced by the alkene and this n complex in its turn gives a a complex with one C-Co a bond and one new C-C a bond, and a C-Co bond is sacrificed in a ligand coupling reaction. Then a carbonyl insertion follows and reductive elimination gives the product, initially as a cobalt complex. 

Addition of a catalytic amount of BPK to a 14/dppm/THF solution at 288 K affords a quantitative yield of 15, in which the dppm is monodentate, within 1 min (Scheme 6). The thermal reaction of 14 with dppm in hexane affords the dppm-bridged complex, 16. The substitution of equatorial carbonyls to form pt-r -dppm products has been confirmed in a number of cases by X-ray crystallography, for example, [Co2(/a-PhC2Ph) (ja-dppm)(CO)4j (Fig. 7).60 Solution dynamics studies of [Co2( t-aIkyne) (ju.-dppm)(CO)4] complexes show that an effective mirror plane that contains the cobalt-cobalt bond and the two phosphorus atoms is generated on the NMR time scale.61 This can be interpreted as a rocking motion of the alkyne about the cobalt-cobalt bond. Complex 16 is also produced... 

The use of Cp rings with pendant phosphines in CpCoL2 complexes has also been reviewed. Carbonyl complexes of this type (Cp Co(CO)2) lose CO at room temperature to afford pendant phosphane adducts (equation 46). The chelated phosphane can then be uncoordinated with ligands such as cod substitution of cod (Section 5.1.4) with alkynes allows the cobalt complex to participate in cychzation reactions (Scheme 26). 

An important question in light of the ease of chelation in the synthesis of the carbonyl complexes is whether it is possible to decoordinate the phos-phane arm, possibly to create a vacant coordination site for further chemistry. The question was addressed by treatment of 327 with 1,5-cyclooctadiene under photochemical reaction conditions, using the diene as the solvent, and resulted in a 41% yield of nonchelated cyclooctadiene complex 336 (Scheme 61). Treatment of this complex with diphenylethyne under reaction conditions normally allowing alkyne di- or trimerization reactions gave tetraphenylcyclo-butadiene complex 337 in 64% yield, showing that chemistry at the cobalt atom is possible without inhibition by a chelating phosphane arm. ... 

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