Catalytic cyclopropanation

Cyclopropane ring scission occurs readily either under reducing conditions or upon the action of electrophilic or nucleophilic agents. These possibilities offer multiple options for the synthetic utilization of the cyclopropane moiety in organic synthesis. One of the most important applications is based upon the use of the cyclopropanation-catalytic hydrogenation sequence as a method for the creation of the gem-dimethyl moiety, a fragment frequently encountered in many naturally occurring compounds. A typical example is shown in Scheme 2.161. 

Other Catalysts. Other catalysts with metals of rhenium and platinum have shown catalytic reactivities for cyclopropanation. Methylrhenium trioxide (MTO) was the first rhenium catalyst for catalytic cyclopropanation, with yields of 57-87% obtained for the cyclopropanation of alkyl or aryl alkenes with EDA (45). As for platinum, a number of complexes have been screened for cyclopropanation catalytic activity (46). PtCl4 was the most active, giving good yield (79%) of cyclopropane from styrene and EDA. However, all reactions had to proceed at elevated temperature. Nonmetal catalysts such as tris(4-bromophenyl)-aminium hexachloroantimonate have been utilized as catalysts for mechanistic studies of cyclopropanation of a series of raras-stilbenes with EDA (47). A cation radical mechanism for this catalysis has been proposed. 

Cyclopropanes from unsaturaled carbonyl compounds via pyraaolines by catalytic pyrolysis... 

Catalytic, enantioselective cyclopropanation enjoys the unique distinction of being the first example of asymmetric catalysis with a transition metal complex. The landmark 1966 report by Nozaki et al. [1] of decomposition of ethyl diazoacetate 3 with a chiral copper (II) salicylamine complex 1 (Scheme 3.1) in the presence of styrene gave birth to a field of endeavor which still today represents one of the major enterprises in chemistry. In view of the enormous growth in the field of asymmetric catalysis over the past four decades, it is somewhat ironic that significant advances in cyclopropanation have only emerged in the past ten years. 

From a historical perspective it is interesting to note that the Nozaki experiment was, in fact, a mechanistic probe to establish the intermediacy of a copper carbe-noid complex rather than an attempt to make enantiopure compounds for synthetic purposes. To achieve synthetically useful selectivities would require an extensive exploration of metals, ligands and reaction conditions along with a deeper understanding of the reaction mechanism. Modern methods for asymmetric cyclopropanation now encompass the use of countless metal complexes [2], but for the most part, the importance of diazoacetates as the carbenoid precursors still dominates the design of new catalytic systems. Highly effective catalysts developed in... 

The discussion of the catalytic, asymmetric variants will incorporate a significant emphasis on the interplay of mechanistic investigations and synthetic optimization studies to provide a unified picture of the cyclopropanation methods. Finally, recent insights provided by computational analysis of the transition structures for cyclopropanation will be discussed. 

These early studies on zinc carbenoids provide an excellent foundation for the development of an asymmetric process. The subsequent appearance of chiral auxiliary and reagent-based methods for the selective formation of cyclopropanes was an outgrowth of a clear understanding of the achiral process. However, the next important stage in the development of catalytic enantioselective cyclopropanations was elucidation of the structure of the Simmons-Smith reagent. 

The landmark report by Winstein et al. (Scheme 3.6) on the powerful accelerating and directing effect of a proximal hydroxyl group would become one of the most critical in the development of the Simmons-Smith cyclopropanation reactions [11]. A clear syw directing effect is observed, implying coordination of the reagent to the alcohol before methylene transfer. This characteristic served as the basis of subsequent developments for stereocontrolled reactions with many classes of chiral allylic cycloalkenols and indirectly for chiral auxiliaries and catalysts. A full understanding of this phenomenon would not only be informative, but it would have practical applications in the rationalization of asymmetric catalytic reactions. 

The discovery of viable substrate-direction represents a major turning point in the development of the Simmons-Smith cyclopropanation. This important phenomenon underlies all of the asymmetric variants developed for the cyclopropanation. However, more information regarding the consequences of this coordinative interaction would be required before the appearance of a catalytic, asymmetric method. The first steps in this direction are found in studies of chiral auxiliary-based methods. 

Although the rationalization of the reactivity and selectivity of this particular substrate is distinct from that for chiral ketals 92-95, it still agrees with the mechanistic conclusions gained throughout the study of Simmons-Smith cyclopropa-nations. StOl, the possibility of the existence of a bimetallic transition structure similar to v (see Fig. 3.5) has not been rigorously ruled out. No real changes in the stereochemical rationale of the reaction are required upon substitution of such a bimetallic transition structure. But as will be seen later, the effect of zinc iodide on catalytic cyclopropanations is a clue to the nature of highly selective reaction pathways. A similar but unexplained effect of zinc iodide on these cyclopro-panation may provide further information on the true reactive species. 

This chiral modifier provides one of the only methods for selective cyclopropa-nation of substrates which are not simple, allylic alcohols. In contrast to the catalytic methods which will be discussed in the following section, the dioxaborolane has been shown to be effective in the cyclopropanation of a number of allylic ethers [67]. This method has also been extended to systems where the double... 

There are three main criteria for design of this catalytic system. First, the additive must accelerate the cyclopropanation at a rate which is significantly greater than the background. If the additive is to be used in substoichiometric quantities, then the ratio of catalyzed to uncatalyzed rates must be greater than 50 1 for practical levels of enantio-induction. Second, the additive must create well defined complexes which provide an effective asymmetric environment to distinguish the enantiotopic faces of the alkene. The ability to easily modulate the steric and electronic nature of the additive is an obvious prerequisite. Third, the additive must not bind the adduct or the product too strongly to interfere with turnover. 

For a reaction as complex as catalytic enantioselective cyclopropanation with zinc carbenoids, there are many experimental variables that influence the rate, yield and selectivity of the process. From an empirical point of view, it is important to identify the optimal combination of variables that affords the best results. From a mechanistic point of view, a great deal of valuable information can be gleaned from the response of a complex reaction system to changes in, inter alia, stoichiometry, addition order, solvent, temperature etc. Each of these features provides some insight into how the reagents and substrates interact with the catalyst or even what is the true nature of the catalytic species. 

As part of an independent study of catalytic asymmetric cyclopropanation, Denmark et al. described a systematic investigation of the effect of addition order, stoichiometry and catalyst structure on sulfonamide-catalyzed Simmons-Smith cyciopropanations. Although early studies had shown promising levels of enantios-electivity, higher selectivity would be required for this to be a synthetically useful transformation. The principal issues that were addressed by this study included ... 

The 1,2-cyclohexanediamine-derived sulfonamide is not unique in its ability to afford enantiomerically enriched cyclopropanes. The efforts at improving the original protocol led not only to higher selectivity, but to a deeper understanding of the nature of the catalytic process. 

O Connor, S.P. Catalytic, Enantioselective Cyclopropanation of Allylic Alcohols PhD Thesis, University of Illinois, Urbana-Champaign, 1993. 

The catalytic asymmetric cyclopropanation of an alkene, a reaction which was studied as early as 1966 by Nozaki and Noyori,63 is used in a commercial synthesis of ethyl (+)-(lS)-2,2-dimethylcyclo-propanecarboxylate (18) by the Sumitomo Chemical Company (see Scheme 5).64 In Aratani s Sumitomo Process, ethyl diazoacetate is decomposed in the presence of isobutene (16) and a catalytic amount of the dimeric chiral copper complex 17. Compound 18, produced in 92 % ee, is a key intermediate in Merck s commercial synthesis of cilastatin (19). The latter compound is a reversible... 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

Cyclopropanes can be cleaved by catalytic hydrogenolysis. Among the catalysts used have been Ni, Pd, and Pt. The reaction can often be run under mild condi-tions." ° Certain cyclopropane rings, especially cyclopropyl ketones and aryl-substituted cyclopropanes," can be reductively cleaved by an alkali metal (generally Na or Li) in liquid anunonia." Similar reduction has been accomplished photo-chemically in the presence of LiC104." ... 

Larock has developed a new catalyst system for the Pd-catalyzed cyclization of olefinic tosylamides. Whereas typical conditions require either stoichiometric amounts of Pd(II) salts or catalytic amounts of Pd(II) in the presence of benzoquinone as a reoxidant, the new catalyst system utilizes catalytic Pd(OAc)2 under an atmosphere of O2 in DMSO with no additional reoxidant <96JOC3584>. Although o-vinylic tosylamides 76 can be cyclized to Af-tosylindoles 77 using this catalyst system, PdCla/benzoquinone is more effective for such cyclizations. Interestingly, in the case of o-allylic tosylanilides, the cyclization can be modulated to afford either dihydroindole or dihydroquinoline products. In a related approach involving a common 7i-aUyl Pd-intermediate, 2-iodoanilines were coupled with vinylic cyclopropanes or cyclobutanes in the presence of a Pd catalyst to afford dihydroindoles <96T2743>. 

Ruthenium porphyrin complexes are also active in cyclopropanation reactions, with both stoichiometric and catalytic carbene transfer reactions observed for Ru(TPP)(=C(C02Et)2> with styrene. Ru(Por)(CO)orRu(TMP)(=0)2 catalyzed the cyclopropanation of styrene with ethyidiazoacetate, with aiiti.syn ratios of 13 1... 

These two compounds with S configuration on their oxazohne rings were tested as copper(I) catalysts for the cyclopropanation of styrene, the hgand 9 with S axial chirality being much more enantioselective than 10 with the R configuration. Thus, the catalytic system CuOTf-(S,S)-bis(oxazolyl)-binaphthyl (9, R = Bu) led to excellent enantioselectivities, particularly for the cyclopropanation of styrene with (-menthyldiazoacetate 95% ee for the trans-cyclopropane and 97% ee for the cis, with trans/cis = 68/32. 

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