Rhodium Cope rearrangement catalyst

A method for highly efficient asymmetric cyclopropanation with control of both relative and absolute stereochemistry uses vinyldiazomethanes and inexpensive a-hydroxy esters as chiral auxiliaries263. This method was also applied for stereoselective preparation of dihydroazulenes. A further improvement of this approach involves an enantioselective construction of seven-membered carbocycles (540) by incorporating an initial asymmetric cyclopropanation step into the tandem cyclopropanation-Cope rearrangement process using rhodium(II)-(5 )-N-[p-(tert-butyl)phenylsulfonyl]prolinate [RhjtS — TBSP)4] 539 as a chiral catalyst (equation 212)264. 

In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [1418], and other natural products [1419]. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal [3 + 4] cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee [1420]) can be attained using enantiomerically pure rhodium(II) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. 

Rhodium(II) (iV-dodecylbenzenesulfonyl)prolinate has been found to act as an effective catalyst for the enantioselective decomposition of vinyldiazoacetates to c -divinylcyclopropanes. Combination of this process with a subsequent Cope rearrangement has resulted in a highly enantioselective synthesis of a variety of cycloheptadienes containing multiple stereogenic centres (see Scheme 40). The tandem... 

Davies and co-workers [12, 35] have exploited one particular aspect of the asymmetric cyclopropanation of alkenes with vinyl diazoacetates, namely, application to substrates suitable for subsequent Cope rearrangement. Cyclopropanation of dienes with predominant cfs-1,2-divinyl diastereoselection makes possible subsequent facile [3,3]-sigmatropic rearrangement with entry to 1,4-cycloheptadienes or bicyclic dienes. Two such examples employing cyclopenta-diene and penta-l,3-diene as substrates and the rhodium(II) prolinate catalyst, Rh2(2S-TBSP)4 in Fig. 1, are shown in Eq. (6) and Eq. (7),respectively cfs-l,2-di-vinylcyclopropanes are presumed to be intermediates in these annulation reactions. In contrast, ethyl diazoacetate and styrene with the prolinate catalyst (Fig. 

Independently, Austin and co-workers also adopted the isomunchnone generation and trapping protocol to the solid-phase synthesis of furans. Model studies revealed that rhodium perfluorobutyroamidate (Rh2(pfm)4) afforded none of the by-product 495, which forms via a tandem cyclopropanation-Cope rearrangement. Moreover, this catalyst is more soluble in organic solvents than is rhodium trilluoroacetate (Scheme 4.21). The cycloadducts 494 and 497 readily fragment to the corresponding furans (e.g., 498) on heating in benzene, which was found to be superior as a solvent to methanol or chloroform. 

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