Intermolecular turnover numbers

Du Bois originally used rhodium(n) acetate and rhodium triphenylacetate (tpa) as catalysts and found that regio-and diastereocontrol was influenced by the catalysts, but neither was particularly effective when low catalyst loadings were used. Inspired by the bridged dirhodium catalysts which have been developed for carbenoid chemistry,40,273,274 a second generation catalyst Rh2(esp)2 116 (esp = a,a,a, o -tetramethyl-l,3-benzenedipropionate) was designed which was capable of much higher turnover numbers (Scheme ll).275 Furthermore, this catalyst was effective in intermolecular reactions. 

In 1995 Herrmann discovered highly efficient palladacyde catalysts in Heck and related reactions of aryl halides with catalyst turnover numbers (TONs) up to 500,000 [ 117]. Later, TONs of the intermolecular Heck reaction reached up to 8,900,000 [ 118]. On the other hand, few syntheses of chiral palladacyde catalysts were envisioned and most of these attempts failed. Recently, the first AHR using a chiral phosphapalladacycle catalyst was reported by Buono et al. [ 119]. The chiral phosphapalladacycle catalyst 129, which was prepared from Pd(OAc)2... 

In 1980, Miller et al. [76] reported the first example of an intermolecular hydroacylation of an aldehyde with an olefin to give a ketone, during their studies of the mechanism of the rhodium-catalyzed intramolecular cyclization of 4-pentenal using ethylene-saturated chloroform as the solvent. Later James and Young [77] reported that the reaction of propionaldehyde with ethylene can be conducted in the presence of RuCl2(PPh3)3 as the catalyst without any solvent at 210 °C, resulting in the formation of 3-pentanone in 2-4% yield (turnover number of 230) (Eq. 49). 

The first report of a catalytic intermolecular cyclization was made by Pauson and Khand in 1974 [22], but the scope was limited to gaseous acetylene as the alkyne partner, strained olefins such as norbornene and norbornadiene as the alkene component, and TON s (turnover numbers) were modest (8-11). Several subsequent reports detailed the production of cyclopentenones from a substoi-chiometric amount of Co2(CO)g, but none were as efficient as Pauson s initial work [23,24]. Using ethylene as the alkene component, Rautenstrauch demonstrated the first efficient catalytic Pauson-Khand cyclization with a TON of 220, Eq. (5) [25]. A more general catalyst system employing (indenyl)Co(cod) was recently reported by Chung and Jeong, Eq. (6) [26]. The reaction was quite effec-... 

In general, however, the activity of the triazolium salts in this asymmetric Stetter protocol is quite low, i.e. the total turnover numbers obtained ranged from 0.5 to 8. The development of more active catalysts which are also suitable for the intermolecular Stetter reaction is desirable. 

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