The Catalytic Option

Only catalytic versions fulfill the economic demands of industry. The handling of huge amounts of cobalt carbonyls in the stoichiometric versions is cumbersome, expensive, and environmentally unpleasant. A first breakthrough was made by Rautenstrauch at Firmenich S. A., Geneva [7, 8]. He showed that the dihydrojas- 

Subsequent work of the Korean group of Jeong and Chung [9] led to further improvements via catalyst modification - e. g. addition of phosphites - or the application of supercritical (sc) media [10]. Supercritical carbon dioxide at 20 MPa and 90 °C at a CO partial pressure of 3 MPa gave an 82 % product yield after 24 hours according to eq. (6) (cf. Section 3.1.13). 

Good yields also resulted from the photoactivation of Co2(CO)g at only low CO pressure [11]. However, from experience with the photochemistry of this particular class of metal carbonyl compounds there is not the slightest chance of an upscaled application of this procedure. 

Two Japanese research groups reported also on an efficient Ru-based catalytic PKR 2 mol% of the commercial Ru3(CO)i2 effected intramolecular PKRs in good yields at 140-160 °C under a CO pressure of 1-1.5 MPa [12, 13]. 

A major step was made when titanocenes were employed for the cyclization a number of functional groups are tolerated, the C-C coupling works even with dis-ubstituted olefins, only low CO pressure is necessary, and the yields are beyond 85 %. Stereoselectivity is induced by ani a-titanocenes such as [(S,S)-(ebthi)-Ti(CO)2] as chiral catalyst. Ees of up to 96 % are thus achieved (see eq. (8)). However, the catalyst is still required in amounts of 5-20 mol% [18]. 

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