Norbornyl 6,6-dimethyl

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

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... 

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... 

The position numbers for point or points of attachment, double bonds, and substituents in the nor- names will be identical to those in the corresponding systematic bicyclo names. Examples 4-norcaren-2-yl is synonomous to bicyclo 3 [4.1.0 ]hept-4-en-2-yl l,3,3-trimethyl-2-norbornyl to 1,3,3-trimethylbicyclo 3 [2.2.l]hept-2-yl 3,3 dimethyl-2-norbornylidenemethyl to 3,3-dimethylbicyclo3 [2.2.1 ] hept-2-y lidenemethyl. 

Dimethyl-2-methylene- 1-norbornyl (1-Camphenyl, for unsubstituted radical only)... 

The iS-oxide 400 undergoes ring-opening, presumably to the sulfenic acid 401, which cyclizes to five-membered products. The intermediate may be trapped by reaction with trimethylsilyl chloride it reacts with norbornene to give bis-norbornyl sulfoxide, probably via an elimination of 2,4-dimethyl-l,4-pentadien-3-one from an initial adduct. 

A number of papers concerned with the decomposition behaviour of organic halogen compounds in the mass spectrometer and related systems is to be found in the journal Organic Mass Spectrometry. For example, in the case of fluoro-compounds, McCarthy has discussed mass spectral correlations for fluorinated alkanes studies by other workers have included those on aromatic fluoro-com-pounds , fluorine-containing dimethyl esters , and tetrafluoroethanes . In the same journal, studies on other halogen compounds include dichlorocyclo-propanes , norbornyl chlorides and bromides , j8-phenylethyl bromides , and chloro-substituted benzynes . 

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... 

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). 

These observations clearly demonstrate the dominance of steric effects in tertiary norbornyl derivatives but do not necessarily apply to secondary systems. The different response to em-dimethyl substitution in (701)90 and (702)494 as compared with (699) and (700) is a disturbing feature. 

These conclusions are supported by data on the tertiary 2-norbornyl esters (723) 510 and (724)S11) in which the C(3)-D2 effect is nearly independent of configuration. The kH/kD of (724) is much smaller than that of (723), due to charge delocalization into the phenyl ring. 6,6-Dideuteration has a negligible effect on the solvolysis rate of 1,2-dimethyl-2-norbornyl p-nitrobenzoate (725)S12 Although the interpretation of secondary deuterium isotope effects is often difficult, comparison of the various norbornyl derivatives discussed here has uniformly been claimed to support alkyl participation. 

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. 

Martinez et al. have carried out several studies into the Leuckart-Wallach reaction on l-substituted-2-norbomanones.15,16 The expected products result from 2-norbonanone and (li )-Ar-(3,3-dimethyl-2-oxo-l-norbornyl)acetamine, but the reaction of (lR)-N-(7,7-dimethyl-2-oxo-l-norbornyl)acetamine gives rise to an unexpected intramolecular transamidation via a Wagner-Meerwein shift.15... 

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... 

Reaction of several bicyclo[2,2,l]heptenes with phenyl(trichloromethyl)mer-cury reveals the fact that born-2-ene (281) (the synthesis of which has just appeared in Organic Syntheses ) is completely unreactive to this substance, because steric hindrance prevents the cxo-attack required by torsional strain. The labelled bornyl-isobornyl Grignard mixture (282) undergoes cis-exo eliminative transfer to phenyl isopropyl ketone, although to a lesser extent than in the norbornyl system, this probably being a reflection of the fact that the gem-dimethyl group reduces the energy difference between exo- and e/ido-transfer. ... 

It is interesting that though C must formally bear a positive charge, no products from the attack on this atom have yet been isolated. It may be due to an increase in the steric strain brought about by introduction of a double bond into the product also, the migrating atom C is usually less substituted than C and C. Such an attack could probably occur if C were a tertiary carbon atom. Schleyerhowever, when studying the solvolysis of 6,6-dimethyl-2-norbornyl tosylates, showed that the formation of the tertiary carbocation is not yet sufficient to break the C —C bond. The results of Jones and Jones show that only a combination of two factors — the... 

Formation of a tertiary classical ion in solvolyses was clearly shown by Goering in the methanolysis of optically active l,2-dimethyl-2-exo-norbornyl chloride ... 

The reactions studied have been the first examples of solvolysis of 2-norbornyl aryl sulphonates without exclusive formation of exo products. But the comparison of solvolyses between compounds 113,114 and 63 is complicated by different hybridization of the C atom and by the possible formation of an intermediate hemiketal with 113 and 114. These complications are avoided by acetolyzing the isomeric dimethyl ketals 118 and 119 having the sp -hybridized C atom The solvolysis of compound... 

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. 

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... 

S 15 Benzofuran-2,2-dimethyl-5-methoxy-6-(2-aminopropane) A 100-160 N-Hydroxy-N-methyl-3,4-methylene-A A 12-18 3,4-Trimethylene-2,5-dimethoxy-A I 3,4-Tetramethylene-2,5-dimethoxy-A A 14-20 3,4-Norbornyl-2,5-dimethoxy-A A 20-32 3,4-Dimethyl-2,5-dimethoxy-A S 2 1,4-Dimethoxynaphthyl-2-isopropylamine... 

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-... 

Fig. 4. Correlation of acetolysis rates of arenesulfonates and carbonyl frequencies of the corresponding ketones.The numbers represent the following 1-10, cyclohexyl through cyclopentadecyl, respectively 11, 2-propyl 12, 2-butyl 13, 3-methyl-2-butyl 14, 3,3-dimethyl-2-butyl 15, 7-norbornyl 16, enifo-8-bicyclo[3.2.1]octyl 17, 2-adamantyl 18, a-nopinyl 19, jS-nopinyl 20, l,4-a-5,8-j5-dimethanperhydro-9-anthracyl. Reprinted by permission of the American Chemical Society.

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