Cyclopropanes ylide reactions give

Intramolecular oxonium ylide formation is assumed to initialize the copper-catalyzed transformation of a, (3-epoxy diazomethyl ketones 341 to olefins 342 in the presence of an alcohol 333 . The reaction may be described as an intramolecular oxygen transfer from the epoxide ring to the carbenoid carbon atom, yielding a p,y-unsaturated a-ketoaldehyde which is then acetalized. A detailed reaction mechanism has been proposed. In some cases, the oxonium-ylide pathway gives rise to additional products when the reaction is catalyzed by copper powder. If, on the other hand, diazoketones of type 341 are heated in the presence of olefins (e.g. styrene, cyclohexene, cyclopen-tene, but not isopropenyl acetate or 2,3-dimethyl-2-butene) and palladium(II) acetate, intermolecular cyclopropanation rather than oxonium ylide derived chemistry takes place 334 ). 

In intermolecular cyclopropanations [100], it was found better to use a-bromoesters and amides as ylide precursors and a,/ -unsaturated ketones and esters as electron-deficient alkenes - rather than using a-haloketones as the ylide precursor. (For experimental details see Chapter 14.11.4). The reaction gives access to a range of 1,2-dicarbonyl-substituted cyclopropanes (see Fig. 10.5). The al-kene could have an aryl-, alkyl- or indole-substituted ketone, and a-substitution was also tolerated. Notably, Weinreb amides could be used as the ylide precursor and the product subsequently transformed into a diketocyclopropane. Both enan-... 

If an acceptor-substituted C=C double bond is attacked in the first step of these ylide reactions and a zwitterion is formed whose anionic moiety is an enolate, then a cyclopropane is produced in the ensuing intramolecular SN reaction. On the other hand, if a C=0 double bond is attacked in the first step and an alkoxide results, then the subsequent SN reaction gives an epoxide. 

A detailed mechanistic study of the reaction between stabilised ylides (42) and 1,2-dioxines has been reported by Taylor and coworkers. This reaction provides a facile route to functionalised cyclopropanes (Scheme 12). The same group has also discovered that addition of chiral 3-ketoiminato (43) or cobalt salen (44) complexes, leads to a catalytic asymmetric ring-opening of me so 1,2-dioxines, affording enantio-enriched cis y-hydroxy enones that react with the stabilised ylides to give enantio-enriched cyclopropanes. ... 

A number of cyclopropyl-substituted five-membered heterocycles have been synthesized by addition of various 1,3-dipolar reagents to (alk-l-enyl)cyclopropanes. Most reactions were performed using tricyclo[3.1.0.0 ]hex-3-ene and tricyclo[3.1.0.0 ]hex-3-en-3-yl phenyl sul-fone, giving the corresponding cycloadducts in very good yields on treatment with azides, a carbonyldicobalt complex, nitrile oxides, diphenylni-trilimine, (4-nitrophenyl)benzenecarbonitrile ylide, and diazoalkanes. For example, addition of tricyclo[3.1.0.0 ]hex-3-ene (1) to 4-nitrophenyl azide gave dihydro-1,2,3-triazole 2 in 94 /o yield. ... 

A second common reaction of sulphonium ylides is Michael addition to ajS-unsaturated carbonyl compounds, giving cyclopropanes. This reaction forms the basis of a cyclopentene synthesis starting from sulphonium allylide (14). Other recent examples include the preparation of spiro[cyclopropane-l,4-A -pyrazolin]-5 -one derivatives (15) and cyclopropyl-ulose (16). Un-... 

Dimethylsulfonium methylide is both more reactive and less stable than dimethylsulfoxonium methylide, so it is generated and used at a lower temperature. A sharp distinction between the two ylides emerges in their reactions with a, ( -unsaturated carbonyl compounds. Dimethylsulfonium methylide yields epoxides, whereas dimethylsulfoxonium methylide reacts by conjugate addition and gives cyclopropanes (compare Entries 5 and 6 in Scheme 2.21). It appears that the reason for the difference lies in the relative rates of the two reactions available to the betaine intermediate (a) reversal to starting materials, or (b) intramolecular nucleophilic displacement.284 Presumably both reagents react most rapidly at the carbonyl group. In the case of dimethylsulfonium methylide the intramolecular displacement step is faster than the reverse of the addition, and epoxide formation takes place. 

However, as illustrated in Scheme 2.112, the reaction of ylide 2-490 with 4-hexen-3-one (2-491) did not lead to the expected cycloheptene, but to the cyclopropane derivative 2-493 in 98% yield by a simple addition of the ylide to 2-491 to give 2-492... 

In contrast to ethyl diazoacetate, diethyl diazomalonate reacts with allyl bromide in the presence of Rh2(OAc)4 to give the ylide-derived diester favored by far over the cyclopropane (at 60 °C 93 7 ratio). This finding bespeaks the greater electrophilic selectivity of the carbenoid derived from ethyl diazomalonate. For reasons unknown, this property is not expressed, however, in the reaction with allyl chloride, as the carbenoids from both ethyl diazoacetate and diethyl diazomalonate exhibit a similarly high preference for cyclopropanation. 

Allyl acetals154). Allyl ethers give no or only trace amounts of ylide-derived products in the Rh2(OAc)4-catalyzed reaction with ethyl diazoacetate, thus paralleling the reactivity of allyl chloride. In contrast, cyclopropanation must give way to the ylide route when allyl acetals are the substrates and ethyl diazoacetate or dimethyl diazomalonate the carbenoid precursors. 

The dominant role of the traditional copper catalysts, generally used under heterogeneous conditions, has not been challenged as yet. Only a few reports shed light on the efficiency of alternative catalysts. Copper(II) triflate allows high-yield intramolecular cyclopropanation of y,8-unsaturated diazoketone 182160) it is superior to CuS04 (53 % yield 192 ) or Rh2(OAc)4160). The solvent is crucial for an efficient conversion If the reaction is carried out in ether, the solvent competes with the double bond for the electrophilic metal carbene to give 184, presumably via an oxonium ylide intermediate. 

The reaction of chloromethyl aryl ethers with nucleophilic reagents has been described by Barber et al Thus, by reaction with thiourea, potassium thiocyanate, or sodium cyanide, there arc obtained aryloxyalkylisothiouronium salts, aryloxyalkyl thiocyanates, and aryloxyalkylacetonitriles, respectively. The reaction of chloromethyl aryl ethers with butyllithium leads to an aryloxycarbene which on reaction with olefins gives aryloxy-cyclopropanes. The ethers react with triphenylphosphine and a base to give phcnoxymethylene ylides which arc useful in con-... 

Wenkert and Khatuya (51) examined the competition between direct insertion of a carbene into furan (via cyclopropanation) and ylide formation with reactive side-chain functionality such as esters, aldehydes, and acetals. They demonstrated the ease of formation of aldehyde derived carbonyl ylides (Scheme 4.30) as opposed to reaction with the electron-rich olefin of the furan. Treatment of 3-furfural (136) with ethyl diazoacetate (EDA) and rhodium acetate led to formation of ylide 137, followed by trapping with a second molecule of furfural to give the acetal 138 as an equal mixture of isomers at the acetal hydrogen position. 

Sulfur ylides are among the most interesting carbon nucleophiles and their synthetic importance has been recently reviewed.One especially interesting use of these ylides is their application to the synthesis of cyclopropane derivatives using unsaturated oxazolones. For example, stabilized sulfur yhdes react with unsaturated oxazolones 629 via a Michael reaction to give oxazolone spirocyclopropanes 630 as shown in Scheme 7.202 and Table 7.46 (Fig. 7.57), whereas the less stabilized sulfur ylides give ring-opened products 631 as the major compounds (Scheme 7.202). ... 

Similar catalytic asymmetric approach has been successfully used in the reaction of a sulfonium ylide with electron-deficient imines" " and alkenes," " giving aziridines 135 and cyclopropane 136 with high enantioselectivity, respectively (Scheme 15). 

Carbenoid sources other than those derived from diazo precursors for catalytic cyclopropanation reactions are currently limited. Inter- and intramolecular catalytic cyclopropanation using iodonium ylide have been reported. Simple olefins react with iodonium ylides of the type shown in equations 83 and 84, catalysed by copper catalysts, to give cyclopropane adducts in moderate yield127 128. In contrast to the intermolecular cyclopropanation, intramolecular cyclopropanation using iodonium ylides affords high yields of products (equations 85 and 86). The key intermediate 88 for the 3,5-cyclovitamin D ring A synthon 89 was prepared in 80% yield as a diastereomeric mixture (70 30) via intramolecular cyclopropanation from iodonium ylide 87 (equation 87)1 0. 

Ylides of other elements have been used much less commonly than sulfur ylides in cyclopropanations. Rather, other ylides are better known for their uses in other types of reactions, the best example being the use of phosphonium ylides in the Wittig reaction with carbonyl compounds to give alkenes. Nonetheless, some cases of cyclopropanations have been reported with phosphonium ylides and the related arsenic derivatives. Examples are given in Table 9. 

The production of buta-1,3-dienes (37) by reaction of 1,2-diarylcyclopropenes with dihalocarbenes is thought to involve electrophilic attack of the carbene to give a dipolar intermediate (38).51 The addition of carbene to CO and H2C=0 has been studied by MNDO calculations.52 Photolysis of diethyl diazomalonate in a CO matrix at low temperature gave rise to ketenes by immediate trapping of the postulated carbene (39).53 The major products of reaction between dichlorocarbene and cyclone were CO and the gem-dichloro species (40).54 The predominance of this pathway over formation of the dichlorooxirane or the cyclopropane is attributed to the aromatic nature of the carbonyl ylide and its twist geometry. 

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