Stabilizing Aryl Groups

The signals of the aromatic protons Hq and are shifted 1.2 and 0.7 ppm to the low field as compared with their position in the precursor spectrum this is 

In the ion 427 the signals of the and protons are in a far higher field than in other 7-aryl-norbornenyl ions in accordance with the smaller shift of the positive charge on and C.  

The multiplicity of the signal of in the ion 427 is essentially different from that in the spectra of related ions. 

The solutions of all the 7-aryl-7-norbomenyl ions except 427 are stable in fluorosulphonic acid at low temperatures whereas the ion 427 tends to conversions connected with the addition of the acid to the double bond. 

7-nortKnuenyl cations (superadd) and the logarithm of the solvolysis rate constants (log k) for the 7-aryl-7-nor-bomenyl p-nitrobenzoates (solvolysis) witho con mts 

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Section 1115 The simplest alkenylbenzene is styrene (C6H5CH=CH2) An aryl group stabilizes a double bond to which it is attached Alkenylbenzenes are usu ally prepared by dehydration of benzylic alcohols or dehydrohalogena tion of benzylic halides... 

Electrophilic addition (Section 11 16) An aryl group stabilizes a benzylic carbocation and con trols the regioselectivity of addition to a double bond involving the benzylic carbon Markovni kov s rule is obeyed... 

This can be circumvented by choosing alkyl groups with no P H, eg, methyl, neopentyl, trimethylsilylmethyl, phenyl and other aryl groups, and benzyl. The linear transition state for -elimination can also be made stericaHy impossible. The most successful technique for stabilization combines both principles. The pentahaptocyclopentadienyl ring anion (Cp) has six TT-electrons available to share with titanium. Biscyclopentadienyltitanium dichloride... 

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

The diarylmethyl cations listed in Table 5.1 are 6-7pATr+ units less stable than the corresponding triarylmethyl cations. This indicates that the additional aryl group has a cumulative, although not necessarily additive, effect on stability of the carbocation. Primary benzylic cations (monparylmethyl cations) are generally not sufficiently stable for determination of pATr+ values. A particularly stable benzylic ion, the 2,4,6-trimethyl-phenylmefliyl cation, has a pATr+ of — 17.4. 

With 4,4-diarylcyclohexenones, the di-Tc-methane rearrangement occcurs. In compounds in which the two aryl groups are substituted differently, it is found that substituents which stabilize radical character favor migration. Thus, the p-cyanophenyl substituent migrates in preference to the phenyl substituent in 4 ... 

The groups at the termini of the 1,4-pentadiene system also affect the efficiency and direction of the the di-7c-methane reaction. The general trend is that cyclization oceurs at the diene terminus that best stabilizes radical character. Thus, a terminus substituted with aryl groups will cyclize in preference to an unsubstituted or alkyl-substituted terminus ... 

The thionitrosyl group may be stabilized either by a dimethylamino substituent or by the use of a highly bulky ort/io-substituted aryl groups. Ai,A -Dimethylthionitrosoamine M2NNS is obtained as a low melting, deep purple solid from the reaction of 1,1-dimethylhydrazine with sulfur (Eq. 10.1) or by the reduction of Mc2NNSO with LiAlH4. This thermally unstable derivative is monomeric in solution. 

Although bulky aryl groups, e.g., mesityl, are not effective in stabilizing arylselenium (II) azides, the use of intramolecular coordination in 2-Mc2NCH2C6H4SeN3 has enabled the first structural characterization of this reactive functionality. The Se-Ns (azide) bond length is 2.11 A, while the intramolecular Se N distance is 2.20 A, cf. 2.14 A in the arylselenium bromide 15.5, and 2.13 A and 2.17 A, respectively, in the corresponding chloride and iodide. This... 

Reactivity factors in additions to carbon-hetero multiple bonds are similar to those for the tetrahedral mechanism of nucleophilic substitution. If A and/or B are electron-donating groups, rates are decreased. Electron-attracting substituents increase rates. This means that aldehydes are more reactive than ketones. Aryl groups are somewhat deactivating compared to alkyl, because of resonance that stabilizes the substrate molecule but is lost on going to the intermediate ... 

The reaction affords in variable yields (19-85%) a stabilized phosphonium yhde exhibiting two electron withdrawing substituents on the ylidic carbon a to the phosphorus atom, and requires apparently a dialkylarylphosphine oxide incorporating an overcrowded 2,4,6-trisubstituted aryl group. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

Carbenes can also be stabilized by migration of alkyl or aryl groups. 2-Methyl-2-phenyl-1-diazopropane provides a case in which products of both phenyl and methyl migration, as well as intramolecular insertion, are observed. 

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