Norbomyl cations structure

Fig. 5.11. Contrasting potential energy diagrams for stable and unstable bridged norbomyl cation. (A) Bridged ion is a transition state for rearrangement between classical structures. (B) Bridged ion is an intermediate in rearrangement of one classical structure to the other. (C) Bridged nonclassical ion is the only stable structure.

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

Fig. 5.12. Crystal structures of substituted norbomyl cations. (A) 1,2,4,7-Tetramethylnorbomyl cation (reproduced from Ref. 154 by permission of Wiley-VCH). (B) 2-Methoxy-l,7,7-trimethyl-norbornyl cation (reproduced from Ref 155 by permission of the American Chemical Society).

Many other cations besides the norbomyl cation have nonclassical structures. Scheme 5.5 shows some examples which have been characterized by structural studies or by evidence derived from solvolysis reactions. To assist in interpretation of the nonclassical stmctures, the bond representing the bridging electron pair is darkened in a corresponding classical stmcture. Not surprisingly, the borderline between classical stmctures and nonclassical stmctures is blurred. There are two fundamental factors... 

A rigorous all-electron, non-empirical ab initio calculation ought to be able to define the bond lengths, angles and charge distribution and hence the structure of an unsolvated norbomyl cation. The most... 

Proton and C-nmr, ESCA, and Raman studies provide a wealth of information which unfortunately is not subject to a unique interpretation. The main conclusion to be drawn therefore is that the structure of the solvent stabilized cation is still unproven. Gas phase estimates of the heat of formation of the norbomyl cation imply a rather marked stability of the stmcture relative to other secondary ions (Kaplan et al., 1970). When combined with other estimates of the heat of formation of the t-butyl cation, however, these data suggest that hydride transfer from isobutane to the norbomyl ion will be endothermic by 6 to 15 kcal mole . This is contrary to experience in the liquid phase behaviour of the ion, and the author s conclusion that their observation of enhanced stability is evidence of stabilization by bridging deserves further scmtiny. 

Comparison of Cl and C6 l3C chemical shifts showed that the ff-participation from the 2-norbomyl ring is significantly reduced in the 2-methyl analogue, whereas in the cyclopropyl and phenyl analogues it has essentially vanished. The STO-3G calculated structures show that the spirocyclopropyl participation is mainly from the exo-C—C bond. The l3C NMR studies of these cations adequately accounted for the vanishingly low values of solvolytic keJkenio rate constants, and show that 3-spirocyclopropyl groups effectively compete with the Cl—C6 ff-bond participation in the 2-norbomyl cation framework. 

The problem of norbomyl cation stabilities vs. solvolysis rate discrepancies in the norbomyl system has been addressed in an important paper.159 The classical and non-classical norbomyl cations do not resemble the 2-endo- and 2-exo -norbomyl solvolysis transition states very closely. The authors conclude that Brown was wrong, but that Winstein was not entirely right either.159 A substituent in the benzene ring has little effect upon the kinetics of the acid-catalysed hydrolysis of 2-exo-norbomyl phenyl ether.160 The FTIR spectra of matrix-isolated 2-methylbenzonorbomen-2-yl cations have been examined at —196 °C the structure can best be represented as (108), rather like a phenonium cation, but at higher temperatures a transition takes place to a structure that is more nearly represented as (109), with some 71-bridging.161 The stereoselectivities of some 7-methyl-7-norbom(en)yl cations have been investigated (110) has a classical structure and reacts in a stereo-random manner, whereas (111) is... 

The X-ray structure of a number of alkoxycarbenium ions has been determined.66 An interesting example is 2-methoxy-l,7,7,-trimethylbicyclo[2.2.1]hept-2-ylium tetrafluroborate 326.630 It is a substituted 2-norbomyl cation and, indeed, the C(2)-C(l)-C(6) bond angle (98.8°) and the C(l)-C(6) bond distance (1.603 A) indicate G-bond charge delocalization, that is, the contribution of the 326b resonance form. 

If the classical structure were correct, the 2-norbornyl cation would be a usual secondary carbocation with no additional stabilization provided by c-delocalization (such as the cyclopentyl cation). The facts, however, seem to be to the contrary. Direct experimental evidence for the unusual stability of the secondary 2-norbomyl cation comes from the low-temperature solution calorimetric studies of Arnett and Petro.75 In a series of investigations, Arnett and Hofelich76 determined the heats of ionization (AHi) of secondary and tertiary chlorides in SbF5-SC>2ClF [Eq. (3.131)] and subsequently alcohols in HS03F-SbF5-SC>2ClF solutions [Eq. (3.132)]. 

Figure 5.50 shows three related molecules, the 7-methyl substituted (the visual orbital progression explained here is not quite as smooth for the unsubstituted molecules) derivatives of the 7-norbomyl cation (a), the neutral alkene norbomene (b), and the 7-norbomenyl cation (c). For each species an orbital is shown as a 3D region of space, rather than mapping it onto a surface as was done in Fig. 5.49. In (a) we see the LUMO, which is as expected essentially an empty p atomic orbital on C7, and in (b) the HOMO, which is, as expected, largely the n molecular orbital of the double bond. The interesting conclusion from (c) is that in this ion the HOMO of the double bond has donated electron density into the vacant orbital on C7 forming a three-center, two-electron bond. Two n electrons may be cyclically delocalized, making the cation a bishomo (meaning expansion by two carbons) analogue of the aromatic cyclopropenyl cation [326], This delocalized bishomocyclopropenyl structure for 7-norbomenyl cations has been controversial, but is supported by NMR studies [327].

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