Cyclopropyl carbinyl system

The rigid geometry of the adamantyl nucleus has also been utilized to quantitatively evaluate the stereoelectronic requirements of cyclopropylcarbinyl cations. The bisected conformation of the cyclopropyl cation, 97, is known to be preferred strongly relative to the perpendicular conformation, 98 280 The adamantyl derivate 99 incorporates a cyclopropyl carbinyl system locked in the... 

Triethylamine as the electron donor was also used by Mattay and co-workers in tandem fragmentation cyclization reactions of a-cyclopropylketones. The initial electron transfer on the ketone moiety is followed by the fast cyclopropyl-carbinyl-homoallyl rearrangement, yielding a distonic radical anion. With an appropriate unsaturated side chain within the molecule both annealated and spi-rocyclic ring systems are accessable in moderate yields (Scheme 41) [62]. 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

The trishomocyclopropenium species 91 has been an important cation in terms of the development of the concept of homoaromaticity. It also provides an example of a different mode of cyclopropyl homoconjugation to that encountered in the systems discussed thus far. In 91, the cyclopropane is formally interacting with the remote positive charge in an edge-on manner. All of the previous examples discussed have involved a cyclopropyl carbinyl-type interaction with the conjugating group being joined to the cyclopropane. 

Alkylation reactions which introduce a double bond adjacent to the cyclopropane ring provide intermediates which may undergo useful rearrangements to the cyclobutanone system. As shown in Scheme 20, reactions of 1-vinylcyclopropanol (108) with acid, positive halogens, peracids or carbonium ions lead to cyclobutanones via the cyclopropyl carbinyl cation 109. 76>... 

The investigations carried out in this area were done primarily to determine the magnitudes of steric and electronic effects on the solvolytic rates and products of reaction in the cyclopropylcarbinyl cation system. The goal of most of these studies was to learn more about the nature of the charge delocalization in the cyclopropylcarbinyl system and of the stereochemistry of the cyclopropylcarbinyl-cyclobutyl and cyclopropyl-carbinyl-allylcarbinyl cation rearrangements. Key papers in these studies were those in 1966 by Schleyer and Van Dine, in 1971 by Majerski and Schleyer and in 1974 by Poulter and Spillner which demonstrated that in the simple cyclopropylcarbinyl system... 

With vinylcyclopropanes thioselenation proceeds via ring-opening of cyclopropyl-carbinyl radicals. An example of the thioselenation of isocyanides by means of a (PhS)2-(PhSe)2 mixed system is shown in Scheme 15.69 the product is a useful building block for TS-lactarn derivatives, e.g. carbacephems. 

In summary, the results discussed above reveal that an ether oxygen in a medium or a bicyclic ring system may offer significant anchimeric assistance in reactions in solvolytic displacement reactions. Because of inductive effects, steric effects, field effects, and internal return, anchimeric assistance, when present, is often difficult to demonstrate kinetically. Products formed from bicyclic and tricyclic oxonium ions probably more often result from participation after carbocation formation. Participation by oxygen apparently occurs to the exclusion of transannular hydride shifts in medium ring systems, but other types of participation such as homoallylic or formation of cyclopropyl carbinyl cations will most likely occur to the exclusion of ether oxygen participation. 

A chiral homoallyl carbocation/cyclopropyl carbinyl carbocation system has been used to prepare ehiral derivatives of the marine natural product, spiniferin-1. Chiral acetals have been used to prepare optically active diterpenoids in biomimetic cationic cyclizations. ... 

Gassman and coworkers have studied possible cyclopropyl participation in the bicyclo[2.1.0]-2-carbinyl tosylate systems 127 and 128. However, the products were explained in terms of cyclobutane ring bond shifts. 

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