Silylative Pauson-Khand reactions

Modern developments have included allyl groups functionalised at the ends, particularly those derived from silylated enals such as 72. The application shown here creates a cyclopentenone 74 by combination of the allyl nickel 73 with an alkyne and CO rather in the style of the Pauson-Khand reaction (chapter 6).18... 

Allenamides are functionalized at the a-position by virtue of the regioselective lithiation. Thus, on introduction of an alkynyl chain, valuable precursors of the Pauson-Khand reaction can be prepared. Lithiation of the silyl ethers of propargyl or allenyl alcohols engenders O —>C transsilylation. ... 

Another cobalt-mediated cyclization reaction is the Pauson Khand reaction (PKR). In a [2 -H 2 - - 1] fashion, the reaction of TMSA with alkenes delivers regioselectively silylated cyclopen-tenones. Following the trend typically observed in the PKR, the bulky TMS-group is always positioned a to the carbonyl group (eqs 37 and 38)7 ... 

Since the intramolecular reactions are so much the best, others have linked the alkene and the alkyne by a weak bond that can later be sacrificed. This is the tether strategy you will meet in chapter 36. An N-0 bond is ideal and with the alkyne additionally blocked with a silyl group 124, good yields of Pauson-Khand product 125 and of the amino alcohol 126 could be achieved even with an amine A-oxide as promoter. Samarium(II) iodide was used as the reducing agent.35... 

Bicyclic cyclopentenones. Cyclization of the Pauson-Khand type from enynes is achievable with Ni(cod)j (12 examples, 38-85%). A bulky bis-ketimine ligand and a CO equivalent are present in the reaction medium. For the latter (i-Pr)3SiCN is adequate. The silyl cyanide is in equilibrium with the isocyanide. The primary products are the cyclopentenone imines, which undergo hydrolysis on workup. 

---