Bridgehead cations

Bridgehead bicyclo[4.4.0]decyl, bicyclo[4.3.0]nonyl, and bicyclo[3.3.0]octyl cations are found to be rapid equilibrating ions (see Section 3.5.2.1).188 The isomeric bridgehead congressane (diamantane) cations 52 and 53 have been prepared and observed.182 The diamant-4-yl cation 52 rapidly rearranges to the diamant-l-yl cation 53 at —60°C, possibly through intermolecular hydride shifts. Bridgehead bicyclo[3.3.3]undec-l-yl cation 54 has also been observed by 1H and 13C NMR spectroscopy.189 

Bridgehead bicyclo[2.2.1]hept-l-yl cation (1-norbornyl cation) has not yet been directly observed 1-chloronorbomane yields the stable 2-norbornyl cation in SbF5-SO2 solution.192 Thus, ionization to the bridgehead carbocation must be followed by a fast shift of hydrogen from C( 1) to C(2) (either intramolecular or intermolecular), the driving force for which is obviously the tendency to relieve strain in the carbocation. 

Starting with dodecahedrane or substituted derivatives, Olah, Prakash, Paquette, and coworkers prepared dodecahedryl cation 59 under superacid conditions193,194 

Previously discussed spiro[2.5]oct-4-yl cation 34148 is also a cyclopropylmethyl cation with substantial positive-charge delocalization into the cyclopropyl group. Other representative cyclopropylmethyl cations that have been prepared in the 

The cyclopropylmethyl cation has recently been generated in the gas phase from both homoallyl chloride and cyclopropylmethyl chloride and studied using collisional activated dissociation mass spectrometry.216 Interestingly, cyclobutyl chloride, which yields the cyclopropylmethyl cation in the condensed phase, gives a substantial amount of the bicyclobutonium ion in the gas phase. 

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Adamantylamine is prepared from the corresponding alcohol or bromide by bridgehead cation generation in the presence of acetonitrile (49). Selective hydrolysis of the resultant acetamide to the rigid cycloahphatic amine by acid or base is difficult. 

These variations in the ease of formation of bridgehead cations are attributed to steric effects 9S>105>187l Carbonium ions prefer planarity strongly 187>292). 

The importance of transannular interaction for the stabilization of the bridgehead cation 19 was revealed in numerous calculations" and is obvious from the interbridgehead distance in this species, calculated to be 1.525 A at the MP2/6-31G levef This can be compared with bicy do [1.1. IJpentane (2), for which this level of calculation yields 1.872 A (recall that the reported experimental values are 1.845 and 1.874 A) and with [1.1. Ijpropellane (la), for which 1.592 A is calculated at this level of theory and 1.60 A is observedThe bridgehead cation 19 clearly contains a central CC bond and it is probably best to view it as a protonated [1.1.1 jpropellane" . The calculated (HF/4-31G footnote 24 in Reference 82) proton affinity of [1.1. Ijpropellane is 229 kcalmol", very close to that of ammomia. The cation can also be viewed as a CH -trimethylmethylene complex ... 

The natural product longifolene to which II is closely related, is specially prone to yielding the unorthodox bridgehead cation. See among others, S. Dev, Acc. Chem. Res., 14, 82 (1981). 

Products resulting from photolysis of alkyl iodides indicate that according to Scheme 58 homolysis of the C—bond is followed by electron transfer, which results in an ion pair in a solvent cage (Kropp, 1984). In the case of norbornyl iodide (171) and other bridgehead iodides, bridgehead cations re-... 

The relative reactivity of tertiary bridgehead systems toward solvolysis is well correlated with the increase in strain that results from conversion of the ring structure to a carbocation, as calculated by molecular mechanics. " This result implies that the increased energy associated with a nonplanar carbocation is proportional to the strain energy present in the ground state reactant. The solvolysis rates also correlate with bridgehead cation stability measured by gas phase hydride affinity andMP2/6-31 IG MO calculations. ... 

Other bridgehead cations this is not so. Two factors may lead to hyperconjugative stabilization of bridgehead carbocations being particularly favorable increased p character of C—C bonds as a result of increased strain and the cationic carbon having a tetrahedral geometry that was calculated... 

Dehydration of compounds (653 R = H or Ph) by toluene-p-sulphonic acid in benzene proceeds by homo-allylic rearrangement to give (654 R = H or Ph) some unrearranged olefin is obtained in the dehydration of (653 R = Ph). The cycliza-tion of ethyl (cyclo-oct-4-enylidene) acetate with BFajOEtj in hot benzene affords (655) and the corresponding acid (656). The bridgehead cation (657) is the presumed reaction intermediate. Aluminium chloride-catalysed cyclization of the lactone (658) resulted in the stereospecific formation of the carboxylic acid (660) the epimeric lactone (659) produced a mixture of a hexahydrofluorene carboxylic acid and the epimeric benzobicyclononene acid (661) mechanisms are discussed. 

Since they are difficult to form, bridgehead cations are highly reactive. Irradiation of hahdes 8 in a variety of media provided a convenient means of exploring the remarkable reactivity of the 1-norbornyl cation (18). It was efficiently trapped by diethyl ether and THE to afford the ethers 19 and 20. Even in aqueous THE, a substantial portion of the butenyl ether 20 was obtained. Trapping by t-butyl alcohol afforded some of the ditertiary ether 21 but was accompanied by formation of 1-norbornanol (22) and 2-methylpropene. Cation 18 was trapped by CHjClj to afford chloride 23, and in aqueous CH3CN the Ritter product 24 was formed. It should be noted that radical abstraction from CH2CI2 occurs at H and ionic abstraction at CL... 

Another cation that, hke bridgehead cations, is difficult to form by conventional methods is the 7-norbornyl cation. The problem in this case is angle strain. Thus, extended treatment of 7-iodonorbornane (52) with methanohc AgN03 afforded only a trace of ether 53 (R = CH,). By contrast, irradiation in CH3OH readily afforded ether 53 (R = CHj), along with smaher amounts of the reduction product norbornane (9), the unprecedented 1,3-ehmination product 54 and the rearrangement products 55-57. The 2-chloro derivatives 58 underwent syn,anti interconversion in competition with formation of the reduction product 60, the ethers 61 and 63, and the 1,3-elimination product 62. By contrast, treatment... 

Preparation of Carbocations, Onium Ions, and Their Salts. SbFs is a preferred medium for the preparation of carbocations and onium ions. In fact, the first observation of stable carbocations was achieved in this medium. - By dissolving /-butyl fluoride in an excess of SbFs, the /-butyl cation was obtained (eq 26). Subsequently, many alkyl cations have been obtained in SbFs (either neat or diluted with SO2, SO2CIF, or SO2F2). The 2-norbomyl cation, one of the most controversial ions in the history of physical organic chemistry, was prepared from exo-2-fluoronorbomane in SbFs/S02 (or SO2CIF) solution (eq 27). Bridgehead cations such as 1-adamantyl, 1-trishomobarrelyl, and 1-trishomobullvalyl cations have also been similarly prepared with the use of SbFs. ... 

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