Pauson-Khand-Type Reactions

TRANSITION-METAL CATALYZED CYCLOADDITIONS 4.3.1 Pauson-Khand-Type Reactions... 

Currently, these reactions are typically conducted with Rh(l) or Ir catalysts. The Pauson-Khand-type reaction of allenynes has also witnessed important developments, especially in its applications to natural products synthesis.388 Brummond s group has been very productive in both areas. Duality in the reaction of allenynes is shown below. In the context of diversity-oriented synthesis, simply changing the reaction conditions gives versatile heterocycles in high yields (Scheme 116).389... 

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. 

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]. 

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]. 

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. 

Allenyne represents an interesting substrate for the intramolecular Pauson-Khand(-type) reaction, where an allene moiety acts as an ene component. Here, there are two possible reaction pathways (Scheme 11.21) (i) the reaction of an external tr-bond of allene moiety gives a bicyclic dienone (type A) or (ii) the reaction of an internal 7i-bond gives a bicyclic cyclopentenone with an alkylidene substituent (type B). 

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

Recent reviews of Pauson-Khand-type reaction (a) Hanson, B.E. (2002)... 

Jeong and coworkers have executed a desymmetrization of a dienyne by asymmetric Pauson-Khand-type reaction. Intriguingly, the use of a Rh catalyst resulted in preferential formation of one diastereomer, while a switch to the analogous Ir system provided the other diastereomer in excellent selectivity [Eq. (10.62)] the system has been shown to be tolerant of oxygen in the linker as well as modest substitution on the alkyne (Ph) ... 

Scheme 11.3 A proposed catalytic cycle for the Pauson-Khand-type reaction.

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

Pauson-Khand-type reactions of enynes are mediated by other metal complexes, such as Fe(CO)5 [100], [W(CO)5THF] and Mo(CO)6 [101]. Cyclization of the allene-yne system 227 to the a-methylenecyclopentenone 228 is promoted by Mo(CO)6. The products depend on the substituents of the allene, and the cyclization of 229 afforded 230 as the main product [102]. 

The cyclopentenone 277 and a small amount of the cyclopentanone 278 are obtained by the carbonylation (1 atm) of titanacycle 276, generated from 1,6-enyne 275 and 273 [120], However, this Pauson-Khand type reaction of the 1,6-enyne proceeds with a catalytic amount of Cp2Ti(CO)2. Furthermore, asymmetric... 

Buchwald and coworkers studied Cp2Ti complexes and developed a titanium(II)-catalyzed Pauson-Khand-type reaction (Fig. 14) [133]. When a number of ortho-allylacetophenones 42 was subjected to 5-20 mol% of Cp2Ti(PMe3)2 in the... 

Fig. 14 Titanium-catalyzed Pauson-Khand-type reactions...

Cycloadditions of an Alkyne, a Carbonyl Functionality, and CO Hetero-Pauson-Khand-Type Reaction... 

There has been significant interest in substituting cobalt for other transition metals giving rise to what are known as Pauson-Khand type reactions.60 In 1996, Buchwald and co-workers showed that a titanium -cyclopentadienyl complex (45) could be used sub-substoichiometrically in the reaction (46->47 - Scheme 18).61,62... 

Pauson-Khand Type Reactions with Metallocenes... 

Similarly, intramolecular Pauson-Khand reactions can be utilized for tetrahydrofuran synthesis, when the tethers are oxa substituted. Ligand effects on stereoselectivity in Rh(i)-catalyzed asymmetric Pauson-Khand-type reactions have been investigated and ee s of up to 92% have been achieved with the 2,2-bis(diphenyl-phosphanyl)-l,l-binaphthyl (BINAP) ligand and a Rh(i) precatalyst (Equation 91). However, it has to be noted that the ee is highly substrate dependent, and considerably lower in most other cases <2006S4053>. 

A stereoisomer library of 16 murisoUn analogues was recently reported and early in 2005, an FMS strategy for the production of cyclopentenone scaffolds with Pauson-Khand-type reactions was published. ... 

A Pauson-Khand type reaction of enynes, where the CO source is an aldehyde, has been reported by Morimoto and coworkers. This CO-transfer carbonylation system was carried ont with monomeric or dimeric rhodium complexes supported by monodentate or chelating phosphine ligands (e g. [RhCl(cod)]2/dppe or dppp [RhCl(CO)PPh3]). This reaction is snccessfiil for a series of enynes and aldehydes (Scheme 28). 

Intramolecular allene alkyne cyclizations using cobalt complexes are nonspecific and both double bonds of the allene participate in the reaction affording [4.3.0] and [3.3.0]ring systems (Scheme 260). Both inter- and intramolecnlar Pauson - Khand-type reactions have been reported using... 

Bonaga LVR, Krafft ME. When the Pauson-Khand and Pauson-Khand type reactions go awry a plethora of unexpected results. Tetrahedron 2004 60 9795-9833. 

Shibata T. Recent advances in the catalytic Pauson-Khand-type reaction. Adv. Synth. Catal. 2006 348 2328-2336. 

Buchwald, S. L., Hicks, F. A. Pauson-Khand type reactions. Comprehensive Asymmetric Catalysis /-///1999, 2, 491-510. 

Kellogg, R. M. Rhodium(l)-catalyzed asymmetric intramolecular Pauson-Khand-type reaction. Chemtracts 2000, 13, 708-710. 

Cyclopentenones. Allenynes undergo a Pauson-Khand-type reaction in the presence of Mo(CO)g in DMSO. The substitution pattern of the allenyl moiety has profound... 

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