Pauson-Khand reactions diynes

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

It is not quite clear which step takes place first - the Co-catalyzed [2+2+1] cycloaddition of the outer alkyne moiety, or the Diels-Alder reaction of the diene with the inner alkyne to form a 1,4-cyclohexadiene, which then undergoes a Pauson-Khand reaction with the remaining alkyne. Recently, it has been shown that a domino reaction can also be performed using 1 mol of a 1,7-diphenyl-1,6-diyne 6/4-20 and a 1,3-diene 6/4-21 in the presence of Co/C at 150 °C under 30 atm CO, to give the polycyclic compounds 6/4-22 as sole product (Scheme 6/4.7) [282]. 

In sharp contrast to the unique pattern for the incorporation of carbon monoxide into the 1,6-diyne 63, aldehyde 77 was obtained as the sole product in the rhodium-catalyzed reaction of 1,6-enyne 76 with a molar equivalent of Me2PhSiH under CO (Scheme 6.15, mode 1) [22]. This result can be explained by the stepwise insertion of the acetylenic and vinylic moieties into the Rh-Si bond, the formyl group being generated by the reductive elimination to afford 77. The fact that a formyl group can be introduced to the ole-finic moiety of 76 under mild conditions should be stressed, since enoxysilanes are isolated in the rhodium-catalyzed silylformylation of simple alkenes under forcing conditions. The 1,6-enyne 76 is used as a typical model for Pauson-Khand reactions (Scheme 6.15, mode 2) [23], whereas formation of the corresponding product was completely suppressed in the presence of a hydrosilane. The selective formation of 79 in the absence of CO (Scheme 6.15, mode 3) supports the stepwise insertion of the acetylenic and olefmic moieties in the same molecules into the Rh-Si bond. 

When this reaction is carried out under 1 atm of nitrogen or GO atmosphere, a cyclopentane 276 is formed selectively in a minute at 25 °G (Scheme 13, mode 2). Although the Pauson-Khand reaction of 1,6-enyne 273 (Scheme 13, mode 3) gives 21H, this transformation is completely suppressed under the conditions of mode 1. Even simple alkyne silylformylation product 277 is not detected at all. This contrasts sharply to the silylformylation of l-penten-4-yne 48 carried out under similar conditions (Equation (12)). These results can be explained by a pathway similar to the reaction of 1,6-diynes (i) stepwise insertion of the acetylenic and olefmic moieties into the Rh-Si bond in this order, and (ii) subsequent interaction of GO and Mc2PhSiH with the resultant intermediate to give 275. The... 

This tandem reaction is believed to proceed via Pauson-Khand reaction between the diyne and carbon monoxide to give an intermediate bicyclopen-tadienone. 

Fenestranes are compounds of theoretical interest in which the central carbon atom undergoes severe planarization distortion. Reactions sequences involving double intramolecular Pauson-Khand reactions of ene-diynes, or intramolecular Pauson-Khand of dienynes followed by photochemical [2 + 2] cycloaddition, successfully lead to [5.5.5.5]- or [4.5.5.5]fenestrane, respectively [42], For instance, compound 37 was obtained from ene-diyne 36 in moderate yield as a single all-czs stereoisomer [43] (Scheme 18). 

Son et al. reported that the cobalt-catalyzed [2 + 2 + 1]-cycloaddition of the diyne 102 with CO, followed by Diels—Alder reaction of the resulting product with 2,3-dimethyl-1,3-butadiene, gave the tricyclic heterocycle 107 in a good yield (Scheme 37).115 The Pauson-Khand reaction of 69, followed by the [2 + 2 + 2]-cycloaddition of the resulting bi-cyclopentadienone with the cobaltacyclopentadiene... 

Cyclization of enynes, dienynes, and diynes with aryl isocyanides. This combination of reagents (1 2) generates a Ni(0) catalyst (1) which is easier to handle than the air-sensitive bis(l,5-cyclooctadiene)nickel(0), Ni(COD)2. Reaction of 1,6-cnyncs (2) with an aryl isocyanidc in the presence of this Ni(0) catalyst combined with Bu3P (2 cquiv.) results in bicyclic iminocyclopentenes (3) which can be hydrolyzed to the corresponding ketone (4). The overall reaction is an alternative to the Pauson-Khand reaction. 

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... 

Terminal alkynes undergo oxidative coupling in the presence of the GuGl-TMEDA catalytic system in [G4GiIm]PF6 under aerobic conditions to produce 1,3-diynes. " Intermolecular Pauson-Khand reactions of strained alkenes with alkynes and Go2(GO)g were performed in [G4GiIm]PF6 either thermally or in the presence of... 

Whereas both Pauson-Khand based reactions presented above are intramolecular processes, a bimolecular route to [5,5,6]-tricyclic compounds is also available [37]. Indeed, it was shown that diynes could be reacted with monoalkynes using a catalytic amount of Co2(CO)s. In this way, compound 27 and phenylacetylene were converted into tricyclic product 28 in 68% yield (Scheme 13). Although high temperature and pressure of CO were required, compound 28 was obtained as a single regio- and diastereomer. 

It is also worthy of note that the construction of tetracyclic compounds 44 by cobalt-catalyzed tandem Pauson-Khand/[2 + 2 + 2] cycloaddition reactions of 1,6-diynes 43 was also reported by Chung et al. [45,46]. It was experimentally shown that the [2 + 2 + 2] cycloaddition reaction occurs after the [2 + 2 + 1] cycloaddition between CO and the substrate (Scheme 22). 

Other Pauson-Khand (PK) Type Reactions 15.5.1 PK-Type Reactions of Diynes... 

---