Cyclopropanation of alkenes

Many more examples of metal carbonyl catalyzed organic transformations have been summarized in Table 10.4. 

RjSi -OCHjNMej RjSiH I HC(0)NM62 RjSi -O- SiRj + NMej 

Metal carbonyls belong to a unique class of organometallic compounds where carbon monoxide is bonded to the metal atom through the carbon end. They enjoy their relevance in the synthesis of various complex compounds and cluster compoxmds as well as a potential agent in organic transformations and occasionally catalyze some of the unique chemical transformations. 

Manassen, D. D. Whitehurst, In Progress in Catalysis F. Basolo, R. L. Bur well, Jr., Eds. Plenum Press New York, 1973,177 b) Jr. C. U. Pittman, W. D. Honnick, M. S. Wrighton, R. D. Sanner, R. G. Austin, In Fundamental Research in Homogeneous Catalysis M. Tsutsui, Ed. Plenum Press New York, 1979, 603 c) D. Balhvet-Tkatchenko, G. Coudurier, H. Mozzanega, 

Tkatchenko, In Fundamental Research in Homogeneous Catalysis M. Tsutsui, Ed. Plenum Press New York, 1979,257 d) A. K. Smith, F. Hugues, A. Theoher, 

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CIAMICIAN - DENNSTEDT Cyclopropanation Cyclopropanation of alkenes with dichlorocarbene derived Irom CHCI3 and sometimes subsequent rirtg enlargement of fused cyclopropanes 1 Na... 

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. 

The most significant and widely studied reactivity of the ruthenium and osmium porphyrin carbene complexes is their role in catalyzing both the decomposition of diazoesters to produce alkenes and the cyclopropanation of alkenes by diazoesters. Ethyl diazoacetate is used to prepare the carbene complex 0s(TTP)(=CHC02Et)... 

G. Rhodium Porphyrin Carbone Complexes and the Cyclopropanation of Alkenes Catalyzed by Rhodium Porphyrins... 

Both rhodium and osmium porphyrins are active for the cyclopropanation of alkenes. The higher activity of the rhodium porphyrin catalysts can possibly be attributed to a more reactive, cationic carbene intermediate, which so far has defied isolation. The neutral osmium carbene complexes are less active as catalysts but the mono- and bis-carbene complexes can be isolated as a result. 

Kwong and Lee [39] prepared various chiral 2,2 6, 2"-terpyridines and tested them as copper ligands for the cyclopropanation of alkenes. High enantioselectivities were obtained, the presence of bulky alkyl groups at the 8-position of the tetrahydroquinoline ring being crucial (structure 29 in Scheme 17). Thus when = Bu, up to 90% ee for the trans and 94% for the cis isomer were obtained by performing the reaction at 0 °C (transIds = 69/31). 

Scheme 5.3 Cyclopropanation of alkenes and ethyl diazoacetate using Cu-NHC complexes...

Scheme 6.4 Bis(oxazolines)thiophene ligands for Cu-catalysed cyclopropanations of alkenes with EDA.

Since their first introduction by Brunner and McKervey as chiral catalysts for the asymmetric cyclopropanation of alkenes with diazo compounds, chiral dirhodium tetra(A-arylsulfonylprolinates) complexes have been widely used by Davies,in particular, in the context of these reactions. Therefore, the use of... 

Rh-catalysed cyclopropanations of alkenes with phenyldiazoacetate in the presence of sulfonamide ligands. 

As for cyclopropanation of alkenes with aryldiazomethanes, there seems to be only one report of a successful reaction with a group 9 transition metal catalyst Rh2(OAc)4 promotes phenylcyclopropane formation with phenyldiazomethane, but satisfactory yields are obtained only with vinyl ethers 4S) (Scheme 2). Cis- and trans-stilbene as well as benzalazine represent by-products of these reactions, and Rh2(OAc)4 has to be used in an unusually high concentration because the azine inhibits its catalytic activity. With most monosubstituted alkenes of Scheme 2, a preference for the Z-cyclopropane is observed similarly, -selectivity in cyclopropanation of cyclopentene is found. These selectivities are the exact opposite to those obtained in reactions of ethyl diazoacetate with the same olefins 45). Furthermore, they are temperature-dependent for example, the cisjtrcms ratio for l-ethoxy-2-phenylcyclopropane increases with decreasing temperature. 

Recently, Ohe and IJemura reported a novel approach to the catalytic cyclopropanation of alkenes via 2-furyl178 179 or 2-pyrrolyl carbenoids180 that originate from the intramolecular nucleophilic attack of a carbonyl oxygen or an imine nitrogen (ene-yne-ketone and ene-yne-imine precursor, respectively) on a 7t-alkyne complex or a cationic cr-vinyl complex. Initially, the group 6 complexes like Cr(CO)s were used. Soon it was found that a series of late transition... 

Among methods of preparing optically active cyclopropane compounds, the Simmons-Smith reaction, first reported in 1958, is of significance. This reaction refers to the cyclopropanation of alkene with a reagent prepared in situ from a zinc-copper alloy and diiodomethane. The reaction is stereospecific with respect to the geometry of the alkene and is generally free from side reactions in contrast to reactions involving free carbenes. 

Stereoselective inns-cyclopropanation. Rhodium(II) carboxylates are generally the preferred catalysts for cyclopropanation of alkenes with diazoacetates (7,313 9,406,10,340) even though they show only low tram-selectivity. The tram-selectivity can be markedly enhanced by use of rhodium(II) acetamide. Use of rhodium(II) 2,4,6-triarylbenzoates favors ds-stereoselectivity.1... 

Activation. Erdik1 has reviewed the methods used since 1970 for activation of zinc and of organozinc reagents. Although chemical activation is still useful, ultrasound activation is being used increasingly. Thus sonic activation allows use of ordinary zinc for cyclopropanation of alkenes with CH2I2 in 67-97% yield and for Reformatsky-type reactions at room temperatures. 

The development of this reaction over the subsequent 50 years placed it, along with the Rh(II) variant, as the method of choice for the catalytic cyclopropanation of alkenes. A number of reviews have recently appeared detailing the advances in cyclopropanation (5-10). This reaction remains one of the most recognizable copper-catalyzed asymmetric transformations as evidenced by the plethora of publications utilizing it as a testing ground for new ligands. 

Scheme 1. General mechanism for the copper-catalyzed cyclopropanation of alkenes using diazoesters.

Scheme 2. Stereochemical model proposed by Aratani for the cyclopropanation of alkenes using 16. [Adapted from (14).]...

Ito and Katsuki (55) examined the use of chiral bipyridine (bpy) compounds as ligands in the asymmetric cyclopropanation of alkenes. Moderate diastereoselectivities and excellent enantioselectivities were observed in the cyclopropanation of vinyl arenes, Eq. 38. This catalyst system afforded very high ee values of the cis isomer. 

Carbenes are both reactive intermediates and ligands in catalysis. They occur as intermediates in the alkene metathesis reaction (Chapter 16) and the cyclopropanation of alkenes. As intermediates they carry hydrogen and carbon substituents and belong therefore to the class of Schrock carbenes. As ligands they contain nitrogen substituents and are clearly Fischer carbenes. They have received a great deal of attention in the last decade as ligands in catalytic metal complexes [58], but the structural motive was already explored in the early seventies [59],... 

Fig. 1.9. Possible mechanism of the cyclopropanation of alkenes with electrophilic carbene complexes [28].

Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(IV) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction these include tetrafluoroboric acid [457-459], trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460], In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [178,462] (see Section 2.1.7). 

The transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [617] their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(I) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(II) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(II) or palladium(II) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically. 

Fig. 3.32. Catalytic and stoichiometric cyclopropanation of alkenes with carbene complexes.

Table 3.3. Cyclopropanation of alkenes with titanium carbene complexes generated in situ [33],...

Diazoalkanes are the carbene complex precursors most commonly used for the catalytic cyclopropanation of alkenes. Reactions involving this type of ylide will be discussed in this section. 

The transition metal-catalyzed cyclopropanation of alkenes with diazomethane is a valuable alternative to Simmons-Smith methodology [645]. Because of the mild reaction conditions under which this reaction takes place, diazomethane is the reagent of choice if sensitive olefins are to be cyclopropanated [646-648]. 

For cyclopropanation of alkenes devoid of base-sensitive functional groups a one-pot procedure has been developed [649]. In this procedure diazomethane is generated in a biphasic system from A-methyl-A-nitrosourea and potassium hydroxide in the presence of a palladium complex (e.g. Pd(acac)2, (PhCN)2PdCl2, or Pd[P(OPh)3]4) and the alkene. In this way the handling of diazomethane is elegantly avoided. 

Likewise, PEG-supported bisoxazoline (40) can be used as a ligand for copper-mediated enantioselective reactions such as cyclopropanations of alkenes, [2-1-4] cycloadditions as well as ene reactions. Best results were obtained in case of the latter reactions as products were formed in yields up to 96% and ee s up to 95% (Scheme 4.25) [117]. 

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