Pauson-Khand reactions sulfoxide

Alkynylallenes have proved to be viable substrates for Pauson-Khand reactions (PKR), providing a-methylenecyclopentenones (Scheme 16.44) [44, 45]. It has been found that a combination of Mo(CO)6 and dimethyl sulfoxide (DMSO) is an effective catalytic system. The most commonly used Co2(CO)g catalyst for PKR is not effective since it causes polymerization. 

In a similar manner, Brummond et al. demonstrated the first total synthesis of 15-deoxy-A12,14-prostaglandin J2 (162) that was completed using a silicon-tethered allenic Pauson-Khand reaction to obtain the highly unsaturated cyclopentenone substructure [36]. Treatment of alkynylallene 160 with molybdenum hexacarbonyl and dimethyl sulfoxide affords the desired cycloadduct 161 in 43% yield (Scheme 19.30). Trienone 161 was obtained as a 2 1 Z E mixture of isomers in which the Z-isomer could be isomerized to the desired E-isomer. The silicon tether was cleaved and the resulting product converted to 15-deoxy-A12,14-prostaglandin J2 (162). 

Several reports have appeared on the effect of additives on the Pauson-Khand reaction employing an alkyne-Co2(CO)6 complex. For example, addition of phosphine oxide improves the yields of cyclopentenones 119], while addition of dimethyl sulfoxide accelerates the reaction considerably [20]. Furthermore, it has been reported that the Pauson-Khand reaction proceeds even at room temperature when a tertiary amine M-oxide, such as trimethylamine M-oxide or N-methylmorpholine M-oxide, is added to the alkyne-Co2(CO)6 complex in the presence of alkenes [21]. These results suggest that in the Pauson-Khand reaction generation of coordinatively unsaturated cobalt species by the attack of oxides on the carbonyl ligand of the alkyne-Co2(CO)6 complex [22] is the key step. With this knowledge in mind, we examined further the effect of various other additives on the reaction to obtain information on the mechanism of this rearrangement. 

The idea that alkenes possessing electron-withdrawing groups are not adequate substrates for Pauson-Khand reactions has in recent years turned out not to be precise. Carretero has reported several examples involving electron-deficient alkenes 3), such as os/Tunsaturalcd ketones, esters, nitriles, sulfoxides and sulfones. In these reactions they reach good yields of PK products (4) and isolate small amounts of dienes 5, that come from a /J-climinalion competitive reaction (Scheme 5) [26,27]. 

In many cases, yields have been modest. Various tricks have been introduced to boost yields. The Pauson-Khand reaction can be promoted by a wide range of additives, including amine oxides, phosphine oxides, sulfoxides, amines and sulfides. Water can have an effect. " Special reaction conditions, including ultrasonication, ultraviolet irradiation and dry-state absorption on silica gel, have been employed. 

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