Regioselectivity of cyclopropanations

Cu(OTf)2 is an intermediate case (Scheme 9). These findings parallel the catalyst s control over the regioselectivity of cyclopropanation with diazodiphenylmethane 47 (see Sect. 2.1). 

SCHEME 17.23 Examples of regioselectivity of cyclopropane ring opening. 

Table 9. Regioselectivities in the cyclopropanation of 1,3-dienes with EDA in the presence of various catalysts". The yields [%] of cyclopropanation at each of the double bonds are given"... 

When the cis/trans stereoselectivity of cyclopropanation with ethyl diazoacetate in the presence of CuCl P(0-z-Pr)3, Rh6(CO)16 or PdCl2 2 PhCN was plotted against that obtained with Rh2(OAc)4, a linear correlation was observed in every case, with slopes of 1.74,1.04 and 0.59, respectively (based on 22 olefins, T = 298 K) S9). These relationships as well as the results of regioselectivity studies carried out with 1,3-dienes point to the similar nature of the intermediates involved in Cu-, Rh-and Pd-catalyzed cyclopropanation. Furthermore, obvious parallels in reactivity in the transformations of Scheme 45 for a variety of catalysts based on Cu, Rh, Fe, Ru, Re and Mo suggest the conclusion that electrophilic metal carbenes are not only involved in cyclopropanation but also in ylide-forming reactions66. ... 

The [Rh(CO)2Cl]2-induced ring fission of substituted cyclopropanes 8a-b affords the rhodium complexes 9a-b via carbonylation [8]. The regioselectivity of carbonyl group insertion depends on the substituent. Reduction with NaBH4 leads to the corresponding alcohol. (Scheme 4)... 

It is assumed that the reaction proceeds by the oxidative addition of the carbon-carbon bond of the cyclopropane to cobalt species to form a metallacyclic intermediate. The regioselectivity of this reaction is controlled by the relative ease of insertion of the cobalt species into the C1-C2 and the C1-C3 bonds of the cyclopropane (Scheme 7). In the case of the reaction of the (1-R, 2k )-isomer trans-l5a, 5-methyl-3-phenyl-2-cyclopenten-1-one (17a) is produced via TS2 without any serious steric repulsion, while the 4-methyl isomer 16a must be pro-... 

Cycloadditions to [6,6]-double bonds of Cjq are among the most important reactions in fullerene chemistry. For a second attack to a [6,6]-bond of a C q monoadduct nine different sites are available (Figure 10.1). For bisadducts with different but symmetrical addends nine regioisomeric bisadducts are, in principle, possible. If only one type of symmetrical addends is allowed, eight different regioisomers can be considered, since attack to both e - and e"-positions leads to the same product. Two successive cycloadditions mostly represent the fundamental case and form the basis for the regioselectivity of multiple additions. In a comprehensive study of bisadduct formations with two identical as well as with two different addends, nucleophilic cyclopropanations, Bamford-Stevens reactions with dimethoxybenzo-phenone-tosylhydrazone and nitrene additions have been analyzed in detail (Scheme 10.1) [3, 9, 10]. 

For example, adduct 12 was formed from e-4, trans-3-6 and trans-2-7. Consequently, it must involve these three positional relationships. Therefore, its structural assignment is unambiguous. Similarly, various trisadducts carrying C2-symmetrical bis(oxazoline) addends could be isolated and structurally assigned [19, 20]. The regioselectivities of these cyclopropanations strongly depend on the precursor bisadduct. Whereas, for example, all possible trisadducts 9,10, 12,14 that can be obtained from trans-3-6 were formed in about equal amounts the cyclopropanation of trans-4-5 is more selective. Among the four isolated isomers 10 was the most abundant. The fifth isomer, with a Cj -symmetrical tmns-4, trans-4, trans-4- addition pattern, was not found. 

A few examples are available in which regiocontrol in the cyclopropanation of non-conjugated diene is catalyst-dependent. An early example is showed in equation 118. Copper(II) triflate catalysed cyclopropanation of diene 132 with diazomethane occurs preferentially at the less substituted double bond, whereas copper(II) acetylacetonate in contrast promotes cyclopropanation at the more substituted double bond (equation 118)13. Regiocontrol in the cyclopropanation of norbornene derivative 133 is strongly catalyst-dependent (equation 119). When diphenyldiazomethane is used as carbenoid precursor, the regioselectivity of this cyclopropanation is significantly enhanced165. 

Cyclopropanation of unsaturated alcohols.1 The reaction of perillyl alcohol (1) with this reagent results in regioselective cyclopropanation of the isolated double bond and is therefore complementary to the regioselectivity of the Simmons-Smith reaction. The actual reagent is probably diisobutyl(iodomethyl)aluminum. 

The range of alkenes that may be used as substrates in these reactions is vast Suitable catalysts may be chosen to permit use of ordinary alkenes, electron deficient alkenes such as a,(3-unsaturated carbonyl compounds, and very electron rich alkenes such as enol ethers. These reactions are generally stereospecific, and they often exhibit syn stereoselectivity, as was also mentioned for the photochemical reactions earlier. Several optically active catalysts and several types of chiral auxiliaries contained in either the al-kene substrates or the diazo compounds have been studied in asymmetric cyclopropanation reactions, but diazocarbonyl compounds, rather than simple diazoalkanes, have been used in most of these studies. When more than one possible site of cyclopropanation exists, reactions of less highly substituted alkenes are often seen, whereas the photochemical reactions often occur predominantly at more highly substituted double bonds. However, the regioselectivity of the metal-catalyzed reactions can be very dependent upon the particular catalyst chosen for the reaction. 

The cyclopropyl effect has controlled the regioselectivity of the cross-coupling reactions of propargylic/allenylic metallic species with electrophiles afford alkynic cyclopropanes (Scheme 9).70 Cyclopropyl ring strain, which makes the formation of vinylidenecyclopropanes unfavourable, is believed to control the regioselectivity. 

With the bis-malonate 56 containing a vra-disubstituted DB18C6 tether, the regioselectivity of the macrocyclization of C70 via double Bingel cyclopropanation... 

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