Porphyrin, chiral wall

Callot and co-workers established in 1982 that iodorhodium(III) porphyrin complexes could be used as cyclopropanation catalysts with diazo esters and alkenes with c/.s-disubstituted alkenes these catalysts provide preferential production of cis(syn) disubsdtutcd cyclopropancs (syn/anti up to 3.3 with 1,4-cyclohexadiene) [72], More recently, chiral porphyrins have been designed and prepared by Kodadek and co-workers [73], and their iodorhodium(lll) complexes have been examined for asymmetric induction in catalytic cyclopropanation reactions [74,751. The intent here has been to affix chiral attachments onto the four porphyrin positions that are occupied in tetraphenylporphyrin by a phenyl group. Iodorhodium(III) catalysts with chiral binaphthyl (27, called chiral wall porphyrin [74]) and the structurally analogous chiral pyrenyl-naphthyl (28,... 

Thus, O Malley and Kodadek [224] prepared the chiral wall porphyrin ligand (61) by condensation of (/ )-binaphthaldehyde with pyrrole (Figure 12). The corresponding manganese complex associated with NaOCl produced epoxidation of various styrene derivatives in 20-40% ee with extremely high catalytic efficiency. Furthermore, this complex is quite robust under oxidation conditions. 

Figure 12 Structure of the chiral wall porphyrin. Reproduced from J. Am. Chem. Soc., Ill, 9116 (1989) by permission of the American Chemical Society...

Among other examples of catalysed asymmetric cyclopropanation using rhodium (II) complexes are those involving Kodadek s chiral wall and chiral fortress porphyrins [26, 27], e.g., IRh(l -BNPP)4 in Fig. 8. These unique designs provide high turnover numbers (>1,800) and relatively high diastereoselectivi-ties (>70 30, cis trans),hut enantiocontrol in cyclopropanation with EDA was at best moderate (<60% ee). The Rh2(4S-IBAZ)4 catalyst (Fig. 4) exerts comparable diastereocontrol and significantly better enantioselectivity. 

Iodorhodium(IIl) porphyrins also efficiently catalyze the reaction of ethyl diazoacetate with simple alkenes. generally providing the cw-isomers as the major product77 79110. The cis( tram ratio increases when bulkier porphyrins, such as tetramesitylporphyrin (TMP), are employed. The mechanism of this rhodium-catalyzed cyclopropanation with diazoacetate is interpreted as proceeding via carbene complexes79 80 111,112. Based on these results, asymmetric cyclopropanation of alkenes with ethyl diazoacetate is achieved if catalyzed by a chiral wall porphyrin81. An earlier described binaphthyl-system of this type82113114, introduced as an iodorhodium(lll) complex, 6, forms an extremely active catalyst and leads to m-cyclopropanes (preferred over the rran.v-products) with moderate to poor enantioselectivities if styrene, 1- and 3-phenylpropene are used as substrates (10-60% ee)81. 

Optically active chiral wall porphyrin iodorhodium(III) complexes promote the cyclopropa-nation of several olefins with ethyl diazoacetate with good cis selectivity, but with modest enantioselectivities (10-60% ee)120. 

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