Enantioselective Pauson-Khand-type Reaction

As previously mentioned, this might be attributed to the diminished Lewis acidity of the phosphine-bound catalysL and implies that a catalyst with strongly bound phosphine ligands is required to suppress the reaction occurring with a phosphine-free catalyst. 

Asymmetric catalytic reactions under solvent-free conditions have also been reported. Contrary to the previous result, a neutral rhodium(I) complex provided comparable enantioselectivity with high chemical yield [24c], Eor certain cases, benzaldehyde gave improved enantioselectivity over cirmamaldehyde (Tab. 11.6), although the rationale behind choosing this particular CO source is not entirely clear. Additionally it should be noted that when the reaction was carried out using a stoichiometric amount of an aldehyde as the CO source in xylene, the reaction takes much longer and the enantioselectivity decreases substantially. 

Domino reactions attract significant attention due to their ability to allow for multi-step operations in one pot. Two classes of domino transformations are known for the PK reaction. The first involves the use of a dual-catalyst system, while the second uses a single catalyst with modification of the reaction parameters to control reactivity. 

The combination of Pd2(dba)3 CHCI3 and dppb with [RhCl(CO)dppp]2 provides the optimal balance of reactivity for the one-pot two-step allylic alkylation followed by PK 

Disappointingly, the trimethylphosphite-modified Wilkinson catalyst, which had proven effective for the allylic substitution reaction [30], furnished only a trace amount of the PK product. By screening various rhodium catalysts for both reactions, it was determined that [RhCl(CO)dppp]2 was the optimum complex for the sequential pro- 

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By contrast, in 2000 Shibata reported the Ir-catalyzed enantioselective Pauson-Khand-type reaction of enynes [30aj. The chiral Ir catalyst was readily prepared in situ from [lrCl(cod)]2 and tolBINAP (2,2 -bis(di-p-tolylphosphino)-l,T-binaphthyl), both of which are commercially available and air-stable, and the reaction proceeded under an atmospheric pressure of carbon monoxide. The Ir-catalyzed carbonylative coupling had a wide generality in enynes with various tethers (Z), substituents on the alkyne terminus (R ) and the olefinic moiety (R ). In the case of less-reactive enynes, a lower partial pressure of carbon monoxide achieved a higher yield and ee-value (Table 11.1) [30b]. 

Subsequently, it was found that aldehydes could be used as a CO source rather than the toxic CO gas. However, the choice of aldehydes proved to be very important for example, when Shibata used cinnamaldehyde and Chan used decanal, highly enantioselective Ir-catalyzed Pauson-Khand-type reactions were achieved independently [30b, 32] (Scheme 11.20). Whilst Shibata realized at an early stage that the Rh-tolBlNAP complex-catalyzed enantioselective Pauson-Khand-type reaction served as a CO source [33], it was apparent that the Ir-catalysts could induce a greater enantioselectivity. 

Table 11.1 Ir-catalyzed enantioselective Pauson-Khand-type reaction ofenynes.

Tab. n.5 Enantioselective Pauson-Khand-type reaction with a cationic rhodium(l) complex. 

For the synthesis of heterocycles, an efficient strategy has been introduced utilizing the dual transition metal sequences (Scheme 6).11,lla The key issue is the compatibility of the two catalyst systems. Jeong et al. studied the one-pot preparation of bicyclopentenone 35 from propargylsulfonamide 33 and allylic acetate.11 This transformation includes two reactions the first palladium-catalyzed allylation of 33 generates an enyne 34 and the following Pauson-Khand type reaction (PKR) of 34 yields a bicyclopentenone 35. The success of this transformation reflects the right combination of catalysts which are compatible with each other because the allylic amination can be facilitated by the electron-rich palladium(O) catalyst and the PKR needs a Lewis-acidic catalyst. Trost et al. reported the one-pot enantioselective... 

Co complexes, Buchwald reported the Ti-catalyzed carbonylative coupling of enynes-the so-called Pauson-Khand-type reaction [28]-and realized the first such catalytic and enantioselective reaction using a chiral Ti complex [29]. Here, a variety of enynes were transformed into bicyclic cyclopentenones with good to high ee-values however, several steps were required to prepare the chiral Ti catalyst, while the low-valent complex proved to be so unstable that it had to be treated under oxygen-free conditions in a glove box. 

The Ir-tolBINAP catalyst also functions well in the desymmetrization of dienynes, where a highly enantioselective and diastereoselective Pauson-Khand-type reaction proceeded to give vinyl-substituted bicyclic cyclopentenones with two chiral centers (Scheme 11.19) [31]. 

Under pressure of CO, cyclopentenones can be obtained in good yields in the presence of a catalytic amount of the titanocene Cp2Ti(CO)2 (Eq. 42) [41]. By using an enantiomerically pure analogue, Buchwald was able to perform a highly enantioselective catalytic Pauson-Khand type reaction (Eq. 42) [42]. 

Considerable efforts have been made to develop asymmetrical variants of the classical Pauson-Khand reaction. Initial investigations have shown that compounds derived from cobalt complexes of type 1, in which a carbonyl ligand is replaced by a chiral phosphane (glyphos), react with high enantioselectivity [22], However, the procedure is too complex to be of preparative value. The concept of Kerr et al., who achieved significant enantioselectivities (max. 44 % ee) in intermolecular Pauson-Khand reactions by... 

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