Cyclopropane system

In the absence of hydroxyl group assistance, methylenation of exocylic-A -5a-H compounds and the 24,25-double bond of lanosteroP gives the cyclopropane systems (22)-(24). 

Das aus dem (a-Halogen-alkyl)-cyclopropan-System mit Organo-zinnhydriden gebildete Radikal kann sich zum entsprechenden Allyl-Radikal umlagern, oder es stellt sich ein Gleichgewicht ein z. B.6 ... 

Cleavage of C—H bonds is also well documented for mononuclear complexes (66-68, 147), and seems likely for dinuclear systems [(84, 386) also Section II,A, Eq. (7)] but apart from some cyclopropane systems (387), cleavage of C—C bonds has not been demonstrated at mononuclear centers. Equation (65) shows such cleavage at a cluster center (388) ... 

The [5 + 2]-cycloadditions of tethered alkyne-VCPs that are 1,2-disubstituted on the cyclopropane ring 5j—1 have been studied and a mechanism has been advanced to explain the regio- and stereoselectivities of the reactions.37 In most cases, the product resulting from cleavage of the less-substituted (sterically less encumbered) carbon-carbon bond is obtained. The [5 + 2]-reaction is stereospecific in that a /ram-rclationship of the substituents on the cyclopropane leads to a m-relationship of the substituents in the product and vice versa (Equations (4) and (5)). For some tethered alkyne-VCPs which contain a functional group that weakens the carbon-carbon bond of the cyclopropane system, the more substituted (weaker) carbon-carbon bond can be cleaved selectively depending on the choice of catalyst. Thus far, the rhodium(l)-catalysts are more selective catalysts than the mthenium(0)-catalysts in the [5 + 2]-reaction of these substituted alkyne-VCPs (Scheme 7).38... 

No results have yet been reported for the kinetics of the pyrolysis of vinylcyclobutane though there is some indirect evidence that one of the reaction paths would yield cyclohexene. Kinetic results are available for isopropenylcyclobutane and by analogy with cyclopropane systems the behaviour of this compoimd should be very similar to vinylcyclobutane. It has been reported (Ellis and Frey, 1963) that the pyrolysis of isopropenylcyclobutane gives rise to ethylene, isoprene and 1-methyl-cyclohexene. These products arise by two simultaneous first-order processes which are both homogeneous ... 

The same stereochemical principles are going to apply to both acyclic and cyclic compounds. With simple cyclic compounds that have little or no conformational mobility, it is easier to follow what is going on. Consider a disubstituted cyclopropane system. As in the acyclic examples, there are four different configurational stereoisomers possible, comprising two pairs of enantiomers. No conformational mobility is possible here. 

RADICAL CATIONS OF CYCLOPROPANE SYSTEMS CONJUGATION AND HOMOCONJUGATION WITH ALKENE FUNCTIONS... 

A technique related to EPR, electron nuclear double resonance (ENDOR), allows the assignment of the individual hfcs to particular nuclei and, with reasonable assumptions, will also identify the sign of the interaction. The only obvious drawback of this technique lies in the fact that it requires sophisticated instrumentation, which is, so far, available in only a few laboratories. Applications to strained ring systems, viz., cyclobutene, bicyclobutane, or a tricyclic derivative, have been reported. Howcvct, applications to simple cyclopropane systems have not been reported to date. 

ELECTRON TRANSFER PHOTOCHEMISTRY OF CYCLOPROPANE SYSTEMS RADICAL CATION REACTIVITIES... 

However, electron transfer-induced photoreactions in the presence of nucleophiles have attracted by far the greatest attention a rich variety of cyclopropane systems have been subjected to these reaction conditions. We will consider several factors that may affect the structure of the radical cations as well as the stereo- and regiochemistry of their nucleophilic capture. Factors to be considCTcd include (1) the spin and charge density distribution in the cyclopropane radical cation (the educt) (2) the spin density distribution in the free-radical product (3) the extent of... 

Ohe, Uemura et al. further developed this electrocyclization into the [3,3]-sigmatropy of a cyclopropane system substituted with ethynyl and an acyl or an alkenyl group [33]. Thus, treatment of cis-l-acyl-2-ethynylcyclopropanes 125 with 5 mol% Cr CO)5(thf) in the presence of EtgN at rt induced isomerization to phenol derivatives through the [3,3]-sigmatropy of the vinylidene intermediate 126 to give... 

Substituted cyclopropane systems also undergo nucleophilic addition of suitable solvents (MeOH). For example, the photoinduced ET reaction of 1,2-dimethyl-3-phenylcyclopropane (112, R = Me) with p-dicyanobenzene formed a ring-opened ether by anti-Markovnikov addition. The reaction occurs with essentially complete inversion of configuration at carbon, suggesting a nucleophilic cleavage of a one-electron cyclopropane bond, generating 113. The retention of chirality confirms that the stereochemistry of the parent molecule is unperturbed in the radical cation 112 " ". 

Due to the strain associated with the cyclopropane system, a variety of ring-opening transformations can occur. As this material has been extensively reviewed in recent years,6-8 106-108 this section will concentrate only on the most useful synthetic processes by means of some illustrative examples. 

As discussed earlier in Sect. 5.3.3 of this review, because of the incremental strain imparted by fluorine substitutents to cyclopropane systems, the rate of ring-opening of the 2,2-difluorocyclopropylcarbinyl radical is substantially enhanced with respect to the already very fast analogous hydrocarbon system [174],... 

CF2 groups within a carbocyclic ring system are unremarkable, generally absorbing slightly downheld from those contained in a straight chain acyclic system, with the remarkable exception of cyclopropane systems, the fluorines of which exhibit a characteristic ( 40 ppm) upheld shift to absorb at about -139 ppm (Scheme 4.4). 

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