Ethers, allyl propargyl Pauson-Khand reaction

In terms of functional group compatibility, ethers, alcohols, tertiary amines, acetals, esters, amides and heterocycles are compatible with the Pauson-Khand reaction. In the intramolecular version, relatively few carbon skeletons undergo the cyclization. Most intramolecular PKRs use systems derived from hept-l-en-6-yne (6) or propargyl allyl ethers (7) or amines (8). Other interesting and more recent substrates are enynes connected through aromatic rings like 9-11, which have allowed us and other groups to obtain aromatic polycycles (Fig. 1) [28-31]. 

We prefer Pauson-Khand reactions for the other two compounds as they have both been mace that way. The first needs a simple allyl propargyl ether readily formed from the adduct of aceuier. i and acetone and allyl bromide. The cobalt carbonyl complex gives a good yield of ere cyclopentenone. 

The catalytic [2 + 2 + 1]-cycloaddition reaction of two carbon—carbon multiple bonds with carbon monoxide has become a general synthetic method for five-membered cyclic carbonyl compounds. In particular, the Pauson-Khand reaction has been widely investigated and established as a powerful tool to synthesize cyclopentenone derivatives.110 Various kinds of transition metals, such as cobalt, titanium, ruthenium, rhodium, and iridium, are used as a catalyst for the Pauson-Khand reaction. The intramolecular Pauson-Khand reaction of the allyl propargyl ether and amine 91 produces the bicyclic ketones 93, which bear a heterocyclic ring as shown in Scheme 31. The reaction proceeds through formation of the bicyclic metallacyclopentene intermediate 92, which subsequently undergoes insertion of CO to give 93. 

Because of their ease of preparation, allyl propargyl ethers and amines have been extensively studied. Although their thermal Pauson-Khand reactions often fare poorly [127], a great many proceed well using both dry-state absorption [113] and amine oxide [101,110, 119, 128] methodologies [e.g., Eq. (59)]. 

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