1.2- Dimethyl-2-norbornyl cation

Bicyclic and polycyclic carbocations, NMR spectroscopy, 145-150 1, 2-dimethyl-2-norbornyl cation, 149 bicyclobutonium ions, 145-146, 146/ 3- /<7o-trialkylsilylbicyclobutonium ions, 147-148... 

The tertiary 1,2-dimethyl-2-norbornyl cation (732) is clearly unsymmetrical. Solvolyses of the p-nitrobenzoate (730)96) and of the chloride (7ii)518) proceed with partial retention of configuration, i.e., an excess of (734) over its enantiomer (734 ) is produced (57.5 42.5 with (731) in methanol-lutidine). Similar results were obtained with 1,2-dimethyl-5-norbornen-2-yl p-nitrobenzoate (42S)347 and the 5,6-benzo derivatives (733)519 in spite of eventual ir participation. The 1,2-dimethoxy-2-norbomyl cation (735) is unsymmetrical even under stable ion conditions520. The low temperature NMR shows distinct signals for the two methoxy groups, coalescence occurring at +7 C. 

Of particular importance are recent results on the energetics of the exo-2,3-hydride shift in the 2,3-dimethyl-2-norbornyl and the l,2,3,4-tetramethyl-2-norbornyl cations. Line broadening studies reveal a free energy of activation of 6-6 and 7-3 kcal mole for this process in the respective ions (Huang eta/., 1973 Jones et a/., 1974). 

Due to computational savings, the open and closed forms of the 6,6-dimethyl-6-sila-2-norbornyl cation, 42 and 43, the closed 5,6,6-trimethyl-6-sila-2-norbomyl cation (44) and their corresponding benzene complexes 45, 46 and 47, respectively, were optimized... 

Many rearrangements of bicyclic cations in superacids have been reported which are not observable under solvolytic or acidcatalyzed conditions. The equilibration of the arylbicyclooctyl cations (45)—(47)12 and the rearrangement of the rapidly equilibrating 2,3-dimethyl-2-norbornyl cation (48) into the more stable 1,2-dimethyl-2-norbomyl cation (49)73 may serve as examples. These transformations are thought... 

Comparison of the NMR spectrum of the 2-norbornyl cation (Myhre et al., 1990) with that of its 1,2-dimethyl derivative (Myhre et al., 1985) shows, however, a very important difference. In the spectrum of the 2-norbornyl cation, the position of the signal for the C(l) and C(2) atoms is constant in the temperature range 6-120 K, whereas that of the dimethyl derivative is split above 80 K. This observation indicates that, in the latter, an equilibrium exists between two nonsym-metrical, partially bridged, structures (7-34). This equilibrium was already detected by Goering and Humski (1968, 1975 see also Goering and Clevenger, 1972). ... 

In the context of the historical development of our knowledge of the structure of the 2-norbornyl cation since Winstein and Trifan s postulate in 1949, this comparison with the 1,2-dimethyl derivative is very important for aspects provided by the theory of scientific discoveries. As discussed in our book on aromatic diazo compounds (Zollinger, 1994, Chap. 9), verifications are never definitive, in science proofs are not possible (the term proof should only be used in mathematics) falsifications, however, can be definitive. It was shown that molecular orbital calculations and their application to experimental data on the IR and NMR spectra for the 2-norbornyl cation are consistent with a symmetrical, nonclassical cationic intermediate, but not with a rapid equilibrium between two classical intermediates. The... 

In 2,3-dimethyl-2-norbornyl cations the rate of the exo-3,2-hydride shift is far higher than that of the endo-3,2-hydride shift (the difference in activation energies 5.5 kcal/mole Contrary to Olah s data of the two isomeric ions-2,3-endo-dimethyl-2-norbornyl 13S and 2,3-exo-dimethyl-2-norbornyl 139 the more stable one is the former As has been noted earlier, the vacant p-orbital at C interacts mostly with the exo substituent at C this hyperconjugative interaction with the C —H bond is more effective than with the C —CHj bond. 

Additionally, the solvolysis behavior of the norbornyl system is itself subject to substituent effects. The 1,2-dimethylnorbornyl cation is conceptually similar to the norbornyl cation and should afford racemic products on attack by nucleophiles. It has been found, however, that hydrolysis of l,2-dimethyl-exo-2-norbornyl p-... 

Following the clarification of the structure of the norbornyl cation by spectroscopic methods, further study of the alkylnorbornyl cations has now been made. The detailed analyses of low-temperature n.m.r. results show that the 2-methyl-, 1,2-dimethyl-, and 1,2,3,3-tetramethyl-norbornyl cations are all classical species undergoing rapid Wagner-Meerwein rearrangement. A separate study of the hydrochlorination of (456) and subsequent methanolysis of the product (457) unequivocally demonstrates the importance... 

Methyl-2-norbornyl (66) and 1,2-dimethyl-2-norbornyl (67) cations have partially bridged structures. 

In contrast to (74), optically active l,2-dimethyl-2-norbornyl p-nitrobenzoate (79) gives active products the SN1 product (80) is formed with ca. 9% and the El product (81) with ca. 63% retention of configuration96. Interconversion of enantiomeric 1,2-dimethylnorbornyl cations apparently competes with the product forming steps. The different optical purities of (80) and (81) show that they are derived from different intermediates. The authors suggest that most, or all, of the El product is formed from an intimate ion pair and that the SN1 product is formed from a solvent-separated ion pair or a dissociated carbocation. Solvolysis is accompanied by ion pair return which results in racemization of (79) and equilibration of 180-labeled (79). The rate of racemization exceeds that of scrambling of 180 by a factor of ca. 2. Substrate re-formed by ion pair return must be at least as optically active as the El product (81). Therefore, and keq correspond to upper limits of 37% and 20% of the total return, respectively. Scrambling of 180 detects only a small fraction of the total ion pair return in the solvolysis of (79). 

Subsequently the same authors established that the solvolysis of optically active 6,7-dimethoxy-l,2-dimethyl-2-exo-norbornyl chloride or -p-nitrobenzoate is accompanied by anchimeric acceleration as distinct from the unsul tituted analogues 275 and 276 the resulting products are optically active Thus, anchimeric assistance leads to the formation of an unsymmetrical cation which may be the homobenzylic ion 278 while the partial loss of activity is due to the interconversion of enantiomeric ions. The authors have also shown that the accelerating effect of the homopara-methoxyl group is practically the same for the solvolysis in methanol, 50% acetone... 

Cycloheptatriene.—Synthesis. Details have been published on the preparation of specifically substituted cycloheptatrienes by the cycloadditon of cyclopropenes to thiophene-1,1-dioxides followed by expulsion of S02. Substituted tropones have been obtained from the furan adducts of bromo-oxyallyl cations e.g. treatment of l,l,3,3-tetrabromo-4-methylpentan-2-one with di-iron nonacarbonyl in the presence of furan gave adduct (141) which was converted into a-thujaplicin (142). Acid hydrolysis of the norbornyl ketal (143) followed by warming to 70 "C gave 7,7-dimethyl-cycloheptatriene. ... 

The Leuckart reaction (reductive amination of carbonyl compounds with formamide and formic acid) of 2-norbornanone and (lR)-(V-(3,3-dimethyl-2-oxo-l-norbornyl) acetamide furnishes the expected A-(2-norbornyl)formamides. (l/()-AI-(7,7-Dimethyl-2-0X0-l-norbornyl)acetamide, on the other hand, gives a product resulting from a Wagner-Meerwein rearrangement of the usual cationic intermediate followed by an unprecedented transamination, which affords an A,A-diacylammonium ion. Hydrolysis gives (15 )-A-(3,3-dimethyl-2-oxo-l-norbornyl)acetamide, which then undergoes the normal Leuckart reaction. 

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