Rhodium porphyrin chiral

The chiral rhodium porphyrin catalyst (90) shows a high turnover rate, though enantioselec-tivity is modest (less than 60% ee). It is, however, noteworthy that cw-selective cyclopropanation of simple olefins (cisjtrans = 2—14.2/1) was realized for the first time with (90).249 250... 

Replacement of the 2-naphthyl groups by 2-dimethylaminomethylphenyl groups in H2(npOEP) also led to a rhodium porphyrin being able to extract leucine from water, however, the situation is complicated by dimerization of the rhodium porphyrin due to intermolecular amine-rhodium bonding [286]. A rhodium complex of a trifunctional chiral bis(2-hydroxynaphthyl)porphyrin related to the above-mentioned RhCl(npOEP) system was used to separate diastereomers formed via two-point fixation of amino acids [287],... 

So far, while there is a relative abundance of synthetically useful cyclopropana-tion catalysts, all of them provide a mixture of diastereomers with the anti product predominating. Thus, a catalyst able to provide optically active syn cyclopropyl esters would constitute a useful complement to existing methodology. Rhodium complexes of bulky porphyrins ( chiral fortress porphyrins) have been developed for this purpose [27]. The porphyrin ligands bear chiralbinaphthyl groups appended directly to the meso positions. Their rhodium(III) complexes provide predominantly the syn cyclopropane with diazoesters, with very good stereoselectivity in some cases. However, the enantioselectivities observed are modest. 

A similar type of chiral rhodium porphyrin was found to be effective for the carbene-insertion reaction to olefins, where formation of the carbene complex takes place. Chiral rhodium complexes for catalytic stereoselective-carbene addition to olefins were prepared by condensation of a chiral aldehyde and pyrrole. Formation of the metal-carbene complex and substrate access to the catalytic center are crucial to the production of optically active cyclopropane derivatives. Optically active a-methoxy-a-(trifluoro-methyOphenylacetyl groups are linked witfi the amino groups of a,p,0L,p isomers of tetrakis-(2-aminophenyI)por-phyrin through amide bonds. Oxidation reactions of the... 

Asymmetric C-H amination has progressed through the apphcation of rathenium(II) porphyrin catalysts. Che has employed fluorinated ruthenium porphyrin complexes with added AI2O3 (in place of MgO) to catalyze suifamate ester insertion (Scheme 17.31) [98]. These systems show exceptional catalyst activity (>300 turnovers) and afford product yields that are comparable to rhodium tetracarboxylate-promoted reactions. Of perhaps greater significance is that the use of the chiral rathenium complex... 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

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. 

As for other catalysts, chiral Rh(III) porphyrin catalysts 23a were also foiuid to catalyze asymmetric cyclopropanation of alkenes with EDA (84). Although moderate enantiocontrol (<60% ee) was observed, the reaction was cis-selective tic = 1 2.5) with >1800 turnover number. Other newly developed Rh(III) porphyrin catalysts 23b gave slightly improved enantioselectivity (68% ee) with moderate diastereoselectivity (85). Similar to their achiral covmterparts, a perpendicular approach of alkene to rhodium-carbene was proposed as a mechanism. Apart from porphyrins, phosphine catalysts also gave interesting results. Catalyst 24, which contains backbone chirality, catalyzed the cyclopropanation of styrene with EDA to give 91% ee for the cis-isomer and 90 10 of the cis trans ratio (86). 

C-H alkylation and amination reactions involving metal-carbenoid and metal-nitrenoid species have been developed for many years, most extensively with (chiral) dirhodium(ll) carboxylate and carboxamidate complexes as catalysts [45]. When performed in intramolecular settings, such reactions offer versatile methods for the (enantioselective) synthesis of hetero- and carbocy-cles. In the past decade, Zhang and coworkers had explored the catalysis of cobalt(II)-porphyrin complexes for carbene- and nitrene-transfer reactions [46] and revealed a radical nature of such processes as a distinct mechanistic feature compared with typical metal (e.g., rhodium)-catalyzed carbenoid and nitrenoid reactions [47]. Described below are examples of heterocycle synthesis via cobalt(II)-porphyrin-catalyzed intramolecular C-H amination or C-H alkylation. 

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