Olefins asymmetric cyclopropanations

Other types of new AT-containing ligands have been described as effective chiral inductors for copper-catalyzed asymmetric cyclopropanation. Hence, Fu and Lo [42] prepared a new planar-chiral hgand, namely the C2-symmetric bisazaferrocene (structure 34 in Scheme 18), which was fbimd to be efficient for the cyclopropanation of various olefins with large diastereomeric excesses and ee values up to 95%. 

In 1966, Nozaki et al. reported that the decomposition of o-diazo-esters by a copper chiral Schiff base complex in the presence of olefins gave optically active cyclopropanes (Scheme 58).220 221 Following this seminal discovery, Aratani et al. commenced an extensive study of the chiral salicylaldimine ligand and developed highly enantioselective and industrially useful cyclopropanation.222-224 Since then, various complexes have been prepared and applied to asymmetric cyclo-propanation. In this section, however, only selected examples of cyclopropanations using diazo compounds are discussed. For a more detailed discussion of asymmetric cyclopropanation and related reactions, see reviews and books.17-21,225... 

Intermolecular cyclopropanation of olefins poses two stereochemical problems enantioface selection and diastereoselection (trans-cis selection). In general, for stereochemical reasons, the formation of /ra ,v-cyclopropane is kinetically more favored than that of cis-cyclopropane, and the asymmetric cyclopropanation so far developed is mostly /ram-selective, except for a few examples. Copper, rhodium, ruthenium, and cobalt complexes have mainly been used as the catalysts for asymmetric intermolecular cyclopropanation. 

Two strategies have been adopted for asymmetric cyclopropanation. First, there are auxiliary-based methods, involving a covalently attached adjacent chiral moiety on either the olefin or the cyclopropylating agent. The second process, on the other hand, employs a chiral ligand on a metal catalyst. This method is more applicable to route b or c, and this is an issue that warrants further discussion. 

The next major contribution in asymmetric cyclopropanation was the introduction of chiral semicorrin ligands 184 by Fritschi et al.95 This ligand has been used for coordinating with copper and has been found to provide improved enantiocontrol in the cyclopropanation of monosubstituted olefins. Copper(I), coordinated by only one semicorrin ligand, is believed96 to be the catalytically active oxidation state. The copper(I) oxidation state can be reached directly... 

Lo and Fu112 have reported a new type of planar-chiral ligand 203 for the enantioselective cyclopropanation of olefins. As shown in Scheme 5-62, asymmetric cyclopropanation in the presence of chiral ligand 203 proceeds smoothly, giving the cyclopropanation product with high diastereoselectivity and enantioselectivity. 

SCHEME 90. Schiff base-Cu-catalyzed asymmetric cyclopropanation of olefins. 

Asymmetric cyclopropanation of olefins can also be achieved by the Simmons-Smith reaction (231). Reaction of ( )-cinnamyl alcohol and the diiodomethane-diethylzinc mixed reagent in the presence of a small amount of a chiral sulfonamide gives the cyclopropylcarbinol in up to 75% ee (Scheme 97) (232a). ( )-Cinnamyl alcohol can be cyclopro-... 

The glyoxime-Co(II)-catalyzed asymmetric cyclopropanation shown in Scheme 94 is noteworthy (226). The results of the detailed kinetic study are consistent with the mechanism of Scheme 92, however, the intermediary Co carbenoid species has substantial radicaloid properties, and only styrene and other conjugated olefins can be used as substrates. Simple alkenes are not cyclopropanated by diazo compounds. The reaction of deuterated styrene proceeds in non-stereospecific manner without retention of geometrical integrity. 

Zhang s group developed highly active chiral (porphyrin)cobalt(II) complexes 327b-d, which catalyzed the cyclopropanation reactions of styrenes [356-358] and even of ot,(3-unsaturated esters or nitriles [358, 359] by diazoacetates. Nitrodi-azoacetates [360] or sulfonyldiazomethane [361] also proved to be useful in asymmetric cyclopropanation reactions of styrenes, acrylic derivatives, and in some cases even simple olefins with good to high de and moderate to excellent ee (highlight [362]). 

This collection begins with a series of three procedures illustrating important new methods for preparation of enantiomerically pure substances via asymmetric catalysis. The preparation of 3-[(1S)-1,2-DIHYDROXYETHYL]-1,5-DIHYDRO-3H-2.4-BENZODIOXEPINE describes, in detail, the use of dihydroquinidine 9-0-(9 -phenanthryl) ether as a chiral ligand in the asymmetric dihydroxylation reaction which is broadly applicable for the preparation of chiral dlols from monosubstituted olefins. The product, an acetal of (S)-glyceralcfehyde, is itself a potentially valuable synthetic intermediate. The assembly of a chiral rhodium catalyst from methyl 2-pyrrolidone 5(R)-carboxylate and its use in the intramolecular asymmetric cyclopropanation of an allyl diazoacetate is illustrated in the preparation of (1R.5S)-()-6,6-DIMETHYL-3-OXABICYCLO[3.1. OJHEXAN-2-ONE. Another important general method for asymmetric synthesis involves the desymmetrization of bifunctional meso compounds as is described for the enantioselective enzymatic hydrolysis of cis-3,5-diacetoxycyclopentene to (1R,4S)-(+)-4-HYDROXY-2-CYCLOPENTENYL ACETATE. This intermediate is especially valuable as a precursor of both antipodes (4R) (+)- and (4S)-(-)-tert-BUTYLDIMETHYLSILOXY-2-CYCLOPENTEN-1-ONE, important intermediates in the synthesis of enantiomerically pure prostanoid derivatives and other classes of natural substances, whose preparation is detailed in accompanying procedures. 

This Chapter is subdivided into five Sections dealing with alkylation and related reactions activation of aryl and vinyl halides by transition metals to manufacture fine chemicals butadiene chemistry olefin oligomerization car-bene chemistry and asymmetric cyclopropanation. 

Carbene species can be stabilized by complexation to metals and transferred to olefinic substrates in catalytic reactions. Although the main industrial application of carbenes is in metathesis (Chapter 6), an important application in the area of fine chemicals is to asymmetric cyclopropanation. 

Asymmetric Copper-Catalyzed Cyclopropanation. Since the pioneering work of Nozaki and his co-workers, several chiral ligands have been designed to achieve high enantio and diastereoselectivity in copper-catalyzed asymmetric cyclopropanation of olefins. Masamune introduced C2-symmetric bisoxa-... 

Satisfactory asymmetric cyclopropanations of olefins bearing chiral substituents have been achieved in several cases A variety of cyclopropyl ketones as well as... 

Charette, A B, Cote, B, Marcoux, J E, Carbohydrates as chiral auxiliaries asymmetric cyclopropanation reaction of acyclic olefins, J. Am. Chem. Soc., 113, 8166-8167, 1991. 

Pyrethroids occupy a central position among insecticides because of their high selectivity and low toxicity [34]. Chrysanthemic esters (33), the carboxylic acid components of this important class of compounds, can be synthesized by asymmetric cyclopropanation of olefins (cf Section 3.1.7) by diazoacetates in the presence of a chiral Schiff base-Cu complex (Scheme 9 and Structures 34 and 35) [35-37]. 

Jacobsen reported in 1990 that Mnm complexes of chiral salen ligands (41) were the most efficient catalysts available for the enantioselective epoxidation of alkyl- and aryl-substituted olefins.118 This stimulated a rapid development in the chemistry and applications of chiral SB complexes, which offer promising catalytic applications to several organic reactions, such as enantioselective cyclopropanation of styrenes, asymmetric aziridination of olefins, asymmetric Diels-Alder cycloaddition, and enantioselective ring opening of epoxides.4,119... 

Table 12. Asymmetric cyclopropanation by 19Sa-catalyzed decomposition of /-methyl diazoacetate in olefins - ...

Cu(ll) complexes ofbisoxazohnes 42a, 42b and 43 (Figure 7.12) have been reported to catalyze the asymmetric cyclopropanation of olefins with diazo-compounds in ionic liquids (Table 7.11). The catalytic activities increased in ionic liquids compared with that in organic solvent (compare entry 1 with 9 entry 2 with entries 3 and 6) [63]. One important finding here was that catalytically less-active but cheaper and moisture-stable CuCfi could be activated in ionic liquids, in which the anion of the ionic liquid may exchange with chloride to generate the more reactive... 

Recently, another cobalt(II)/camphor-derived complex was developed for performing the asymmetric cyclopropanation of olefins [38]. The complex 18 was prepared by reacting the ligand 17, synthesized by condensation of (lR)-3-hydroxymethylenebornane-2-thione and the corresponding diamine, with co-balt(II) dichloride hexahydrate in degassed ethanol (Scheme 11). The cyclopropane derivatives were obtained in 50-60% yield using 3 mol % of the catalyst 18 and ethyl diazoacetate in styrene or 1-octene as solvent. The diastereomeric ratios were low for both styrene and 1-octene. 

The cyclopropanation of styrene with f-butyl diazoacetate in the presence of 5 mol % of (salen)Co(III) bromide 23 produced the corresponding trans-cyclo-propane-carboxylate, with high diastereomeric ratio and enantiomeric excess (Eq. (5)). The asymmetric cyclopropanations of other styrene derivatives also showed high enantioselectivities as well as high transxis selectivities. However, the reaction of disubstituted olefins, such as indene, was sluggish. The use of Co(III) instead of Co(II) seemed to be critical, since Nakamura reported that much lower enantioselectivities were observed with optically active (salen)co-balt(II) complexes. 

Nishiyama and coworkers reported that chiral ruthenium(II) bisfoxazoli-nyl)pyridine complexes were efficient catalysts for the asymmetric cyclopropanation reaction of terminal olefins [42,43]. The fra s-RuCl2(pybox-i-Pr)(ethyl-ene) complex 26 was produced from a mixture of optically active bis(2-oxazolin-... 

Recently, Denmark and coworkers have confirmed that asymmetric cyclopropanation with chiral palladium complexes was not efficient [55]. They reported a systematic study toward the development of an enantioselective diazomethane-based cyclopropanation reagent derived from bis(oxazoline)palladium(II) complexes. In addition to the novel carbon-bound complexes, several simple palladium chelates were synthesized and evaluated in the cyclopropanation of various electron-deficient olefins (Fig. 2). Although all catalysts were effective at inducing the cyclopropanation, the products obtained were racemic in all the... 

Scheme 6.34. Aratani s copper-catalyzed asymmetric cyclopropanation of olefins, (a) trans-, 2-disubstituted [127]. (b) 1,1, -disubstituted [128]. (c) monosubstituted, trans favored [127]. (d) trisubstituted, cis favored [127]. (e) dienes, trans favored [126,128]. Inset chiral auxiliary and coordinatively unsaturated chiral catalyst.

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