Pauson-Khand reaction origin

The Pauson-Khand reaction was originally developed using strained cyclic alkenes, and gives good yields with such substrates. Alkenes with sterically demanding substituents and acyclic as well as unstrained cyclic alkenes often are less suitable substrates. An exception to this is ethylene, which reacts well. Acetylene as well as simple terminal alkynes and aryl acetylenes can be used as triple-bond component. 

As dibromocyclopropanes can easily be synthesized by reacting a cycloalkene with bromoform in the presence of a base [16], this method affords an alternative procedure for cyclopentenone annelation onto cyclic alkenes. It should be noted that in the Pauson-Khand reaction, which is probably the most direct cyclopentenone annelation reaction, the reaction using cyclohexene gives the product only in very low yield [11,17]. Also, the position of the original alkynyl substituent on the product double bond is opposite to that in the present reaction. Thus the two reactions are complementary. 

Increasing reactivity in the Pauson-Khand reaction. The PK reaction originally suffered from a lack of substrate scope and low reaction yields which prevented it from being widely employed. The discovery of new reaction conditions (additives and modified methods) led to an improvement in yields and reaction times, allowing the scope of the reaction to be expanded. 

Pauson-Khand Cycloaddition. Pauson Khand cycloaddition (see Pauson-Khand Reaction) is a cobalt-mediated method to prepare cyclopentenone from the cyclization of an alkyne with an alkene and CO (equation 14). This method is widely used to produce cychc ketones. Originally, stoichiometric amounts of Co2(CO)g were used in these reactions with the cobalt carbonyl being the CO source. However, it was shown that a strict temperature profile and high-purity reagents allowed the use of catalytic amounts of Co2(CO)g for reactions with 1 atm of CO. Currently, there is intense interest in developing catalytic cobalt starting materials for use in Pauson-Khand reactions. 

As follow-ups of the original reaction procedure, numerous improvements have been made, including the lowering of the reaction temperature, use of other transition metals, and the catalytic use of various metal species. In the 1990s, cobalt, titanium, and ruthenium complexes were found to serve as catalysts for the Pauson-Khand reaction, but these catalytic systems often need to employ medium to high pressures of... 

In 1997, Murai s [18] group developed the first rathenium-catalyzed Pauson-Khand reaction, which originally was carried out by a cobalt catalyst. They showed that good yields of cyclopentenones could be achieved in an intramolecular, ratheniumsubstituted alkynes could be employed, showing the tolerance of the reaction. Notably, when a 2-PyMe2Si-substituted alkyne was... 

The Pauson-Khand reaction has been studied extensively in academic laboratories and has been used to prepare a number of natiual products. The PKR is included in this text because of its extensive use in synthesis, but the presentation is brief because it has not experienced the same development into industrial scale synthesis as the other carbonyla-tion processes described in this chapter. This section presents the original reaction, the modifications that increase the rate, allenic PKRs, asymmetric variants, and the application of the PKR in modem organic synthesis. 

The Pauson-Khand reaction forms cyclopentenones from three groups, a C=C, a C=C, and a CO molecule (Eq. 14.28). Originally, stoichiometric and based on Co, the Rh-catalyzed version is now widely adopted. Equation 14.29 shows the formation of the carbon skeleton of guanacastepene A, a novel antibiotic candidate." ... 

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]. 

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