Cyclopropyl-substituted systems, more

Phenonium ions, of which 105 is the parent species, have long attracted attention, initially concentrated on kinetic and stereochemical evidence for their, generation in solvolysis reactions (equation 10), and more recently with direct observation of the ions in solution " . It has been persuasively argued that these ions are best represented as cyclopropyl-substituted carbocationic systems, with the bisected geometry best suited for TT-donation from the cyclopropyl ring. 

Aliphatic carbocations with more than four carbon atoms 75 Cyclic species without formal tt systems 79 Cyclopropyl-substituted carbocations 83... 

In general, the regiochemistry of the reaction is such that the more highly substituted terminal carbon of the 1,3,5-hexatrienyl moiety (e.g. of 138) ends up as C6 of the bicyclo[3.1.0]hex-2-ene structure (139) while with 2-substituted trienes having identically substituted terminal carbons, such as 140, the cyclopropyl group is formed at the opposite end of the system (cf 141, equation 52)188,210. This is mainly due to steric factors which stabilize one cZt conformer relative to the other. 

It was concluded that the stabilization of the positive charge of 63, by delocalization over the three carbons of the mesomeric propargyl allenyl system, entailed a powerful electron releasing substituent at the allenyl end 13). However, although in theory electron releasing substituents might render 1-substituted cyclopropyl cations more stable than their 2-substituted allyl counterparts 36), the expected ring closure of a 2-substituted allyl cation such as e.g. 64 to the cyclopropyl cation 65 has not been observed experimentally, Eq. (19)37). 

The change in for the comparison of the nortricylyl system (23) relative to 24 is comparable to that observed for 22, indicating that the same attenuation of in the presence of the cyclopropyl group occurs here as well. When the position of the substitution is made more remote, as in the diarylethyl system (25), the influence of the substituent is not so great but the same trend is followed . 

The kinetics of proton exchange and pK measurements for substituted nitromethanes in two solvent systems showed consistently that cyclopropyl was more effective than isopropyl in promoting proton removal (Table 19). However, the acid-strengthening... 

As may be readily appreciated from the foregoing examples, substitution reactions on cyclopropyl systems and their derivatives may take some unexpected routes and lead to a variety of interesting products. Which pathway will actually be followed, or more realistically, what will be the spread of products, may be only determined by performing the experiment and identifying the products. The value of the mechanistic approach is that it will... 

The formation of substituted cyclopropane rings by the reaction of alkenes with Fischer carbenes has been known for some time [58]. More recently, cyclopropyl groups have been produced as parts of bicyclic and tricyclic ring systems by, formally, the reaction of Fischer carbenes with one equivalent of alkene and one equivalent of alkyne. Indeed, this reaction type proceeds through alkene trapping of a metal carbene generated in situ. The various methodologies that have been developed may be divided into three classes the intermolecular reaction of a,co-enynes with Fischer carbenes, the partially intramolecular reaction of Fischer carbene-tethered alkynes with alkenes, and the fully intramolecular reactions of Fischer carbene-tethered enynes. No fiilly intramolecular version of this reaction has been reported. 

The triplet reaction proceeds through the formation of a cyclopropyl diradical intermediate. The less stabilized diradical center utilizes its odd electron density to open the cyclopropane ring and forms a more stabilized 1,3-diradical intermediate. The new 1,3-diradical intermediate gives the cyclopropane derivative as major product of the reaction. Thus, in a 1,4-diene system, a terminus substituted with aryl groups will cyclize in preference to an unsubstituted or alkyl-substituted terminus because of the greater stability of the diradical intermediate by delocalization with aryl groups. The following example of 43 demonstrated the mechanism of the triplet reaction [40]. 

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