Cydopropanations selectivity

In a long-term research project, Hossain and coworkers investigated the usefulness of the CpFe(CO)2+ fragment [35-38] in the cydopropanation reaction of alkenes by a carbene transfer utilizing diazo esters as the carbene source (Scheme 9.17). The cydopropanation products of styrene derivatives could be obtained in good yields of up to 80% and excellent cis selectivity by using an excess of the alkene, whereas the cydopropanation of aliphatic alkenes was less effective, yielding the desired cyclopropane derivative in up to 51% yield. 

From 1995 to 2000, catalyst profiles of several ruthenium catalysts bearing pyridine-diimide 1 [13], diiminocarbene 2 [14], diamine-arene 3 [15],phos-phino-arene 4 [16], and substituted cyclopentadienyl 5 and 6 [17, 18] were shown to have good activity for the cydopropanation (Fig. 1). At the relatively high reaction temperature of 60-100 °C,they also gave moderate-to-high yields over 90%. It is interesting in that the dipyridine-diimide complex 1 and the p-cymene-carbene complex 2 show high trans selectivity, 86 14 and 82 18, respectively. 

In 1994, asymmetric cydopropanation (ACP) with ruthenium catalysts was first reported by Nishiyama and coworkers [ 19,20] by adoption of their chiral bis(oxazolinyl)pyridine (Pybox) ligands. The reaction profiles of Ru Pybox catalysts reveal extremely high trans selectivity with high enantioselectivity (or di-astereoselectivity) of cyclopropane products at the relatively low reaction temperatures (around 20-50 °C) so far reported for ruthenium catalysts. After 1997,... 

The same complex is a suitable catalyst for the cydopropanation of 1,1- and 1,2-disubstituted alkenes with trimethylsilyldiazomethane (Eq. 9) [18]. High exo selectivity was obtained when cyclohexene was used as an olefin. 

Trans selectivity in chrysanthemates was obtained by using a bulky copper catalyst (36) together with a sterically4iindered dia/oester (1-methyl diazoacetate), whereas cu-pyrethraies can be obtained by dehalogenation of the cti-cyclopropane resulting from cydopropanation of 2Hmethyl-5,5,5-trichlorO 2-pentene 50. ... 

With particular cupper catalysts bearing very weak ligands, Kochi observed unusual selectivities during the first stages of competitive cydopropanation of Ntexene and tetrameihylclhyienc [37], Two different mechanisms were thus recognized lor the copper carbenoid cydopropanation reactions. 

Heterosubstituted cydopropanes can be synthesized from appropriate olefins and car-benes. Since cydopropane resembles olefins in its reactivity and is thus an electron-rich carbo-cycle (p. 76ff.), it forms complexes with Lewis adds, e.g. TiCU, and is thereby destabilized. This effect is even more pronounced in cyclopropanone ketals. If one of the alcohols forming the ketal is a silanol, the ketal is stable and distillable. The O—Si-bond is cleaved by TiCl4 and a d3-reagent is formed. This reacts with a1-reagents, e.g. aldehydes or ketals. Various 4-substituted carboxylic esters are available from 1-alkoxy-l-siloxycyclopropanes in this way (E. Nakamura, 1977). If one starts with l-bromo-2-methoxycyclopropanes, the bromine can be selectively substituted by lithium. Subsequent treatment of this reagent with carbonyl compounds yields (2-methoxycyclopropyl)methanols, which can be transformed to / ,y-unsaturated aldehydes (E.J. Corey, 1975B). 

A Cu-Box complex supported on monolith (51) was developed for the enantio-selective cydopropanation of ethyl diazoacetate. The flow reactions using (51) provided an increase in enantioselectivities of about 20% relative to those for the homogenous batch process (Scheme 7.38) [142]. Pyridine-oxazolidine based monoliths (52) and (53), whose central metals were Ru and Cu, respectively, were also developed [143,144]. Mesoporous silica was utilized as a support for the Cu-Box complex for asymmetric cydopropanation in a flow reador. Aza(bisoxazoline) was easily immobilized on siliceous mesocellular foam MCF) microparticles, which are... 

In the competition between allylic intramolecular cydopropanation and macrocyclization (Eq. 5.20), the more electrophilic catalyst favors macrocyclization. Doyle has explained this differential selectivity as due to the formation of an intermediate ii-complex between the C-C double bond and the carbene center. The more electrophilic the carbene carbon or the more electron-rich the double bond, the more that this 7t-complex is favored and the more favorable the pathway to macrocyclization [97]. However, thus far few systems have been examined... 

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