Cyclopropyl carbocations

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. ... 

Besides the carbocations mentioned above, numerous highly stable carboca-tions have been isolated as the salts of inorganic anions. Azulene analogues of triphenylmethylium ion [ll ]-[20 ], [21 j, [22 " ] and [23 j trisubsti-tuted cyclopropenylium ions [l" ] and [24 ]-p6 ] cyclopropyl-substituted tropylium ions [27 ]-[30 ] tropylium ions annelated with bicyclic frameworks [31 ]-[36 ] and [37 ]-[39 ] fulvene-substituted cyclopropenylium [40 ] and tropylium [41 ] ions a tropylium ion condensed with acenaphthylene [42 ] and a cation containing a 147t periphery [43 ] have... 

The -conformation 13 is lower in energy than the Z-isomer 14. These are the smallest cyclopropyl substituted carbocations which can be investigated in solution by high resolution NMR. The corresponding primary cyclopropylmethyl cation 15 cannot directly be observed by high resolution NMR in solution because it is energetically less favorable than the bicyclobutonium ion 16 and thus only a minor isomer in the fast dynamic equilibrium of the cations 15 and 16. 13C- and H... 

Fig. 5 Schematic representation of the linear combination of the filled antisymmetric Walsh orbital of a a-cyclopropyl ring and the vacant p2-orbital of the C + -carbon of a carbocation leading to delocalization of the positive charge in a 4c-2e MO.

The experimental 3J(HH) spin-spin coupling constants for Ha, Hp, and Hp for the -isomer 22 were quite satisfactorily reproduced (A = 0.1 - 1 Hz) by calculations, using a finite perturbation method (FPT level (26), Perdew/IGLO-III at a MP2/6-31G(d)) geometry for the model structure (E)-1 -cyclopropyl-2-(trimethylsilyl)ethyl cation. The calculations confirm the /rexperimentally observed carbocation 22 (20, 27, 28). The (E)- 1-cyclopropy 1-2-... 

It should be noted that analogous secondary a-methylcyclopropylmethyl cations are observable species in superacidic media at low temperatures.21 Observation of the secondary cyclopropylmethyl carbocations under superacidic conditions indicates the superior charge delocalization into the cyclopropyl ring as compared to the cyclobutyl ring. 

Larger changes in bond lengths, as expected, are observed for more localized carbocations. Most of the structures available are for stabilized systems, such as protonated carbonyl compounds [e.g. the protonated cyclopropyl ketones referred to on page 110 (Childs et al., 1990), and dioxacarbocations (Paulsen and Dammeyer, 1973, 1976 Paulsen and Schuttpelz, 1979 Childs et al., 1986, 1991). It is normal to see one of the atoms of the counterion (in most cases MXJ or MX ) packing in the position expected for addition to the activated C=OH(R)+ system, apparently just within the sum of the van der Waals radii for the neutral centres (Childs et al., 1986). This can happen without significant pyramida-lization, however (Childs et al., 1991), and on both sides of the planar carbon centre it tells us little new about reactivity. 

In the first step, Ag+ promotes the departure of Cl- to give a cyclopropyl carbocation. This undergoes two-electron disrotatory electrocyclic ring opening to give the chloroallylic cation, in which the empty orbital is localized on Cl and C3. Then 09 can add to C3 desilylation then gives the product. 

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 cyclopropylmethyl ion 11 is unusually stable and has a 14kcal/mol barrier to rotation about the cyclopropyl-carbocation bond.69 In contrast, the corresponding cyclobutylmcthyl ion 12 quickly rearranges to a cyclopentyl cation. Here, some strain relief occurs in the rearrangement, but this is opposed by the conversion of the stable tertiary carbocation to the less stable secondary ion. Although rearrangement is the normal process for cyclobutylmethyl cations, there is one case 13 in which rearrangement does not occur, and a small rate acceleration is observed.70... 

Also, reactions at vinylic carbons proceed with retention,101 indicating that the intermediate 14 has sp2 hybridization and not the sp hybridization that would be expected in the analogous carbocation. A cyclopropyl anion can also hold its configuration.102... 

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