Stable Cations

These initiations were originally reported by Bawn and co-workers. The cations must be used in low concentrations to insure complete dissociation from their respective counterions. Stable organic cations can be formed from olefins, aromatic structures, or compounds with heteroatoms possessing unshared electrons, like oxygen, nitrogen, or sulfur. Some examples of stable cations that can initiate cationic polymerizations are  

The above cations form c stalline salts with anions. These anions are C104, SbCl6, BF4 , PF6 , SbF6 , FeCU , and AsF6.  

The initiation reactions by stable cations, designated as (X ), may proceed by several possible mechanisms  

In (A), the reaction will predominate on the left-hand side. Exceptions appear to be olefins with strong electron-releasing substituents that confer thermodynamic stability to the newly formed cation, -CH2-CHR . This either results from a suitable charge delocalization over the 7r-electron system or from the presence of a heteroatom. Accordingly, only those olefins that possess relatively strong nucleophilic characteristics can be polymerized by stable carbon cations. Such olefins are alkyl vinyl ethers, A -vinyl carbazole, p-methoxystyrene, indene, and vinylnaphthalenes. Styrene and cf-methylstyrene, however, will not polymerize, because they are less reactive. 

The initiation mechanism, as suggested by Ledwith, follows the path of reaction (A) and is a result of a direct addition of the cation to the olefin. This is based on observations of the reaction of the xanthylium cation with 1,1-diphenylethylene. It is also based on the reaction of the tropillium cation with AT-vinyl caibazole. The last may, perhaps, be influenced by steric factors. The high initiating efficiency of the tropillium ion, that has a stable six r-electron system, may be a result of formation of a charge transfer complex  

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More stable cation all atoms have octets of electrons... 

Protonation of the hydroxyl oxygen on the other hand yields a less stable cation... 

The positive charge m this cation cannot be shared by the two oxygens it is localized on one of them Because protonation of the carbonyl oxygen gives a more stable cation that cation is formed preferentially... 

A key feature of the first stage is the site at which the starting ester is protonated Protonation of the carbonyl oxygen as shown m step 1 of Figure 20 4 gives a cation that IS stabilized by electron delocalization The alternative site of protonation the alkoxy oxygen gives rise to a much less stable cation... 

Similar stability and reactivity have also been observed for bridged-Cp systems. The catalyticaHy active (CH2)2Si(C (CH2)4)2ThR2, where R = Cl [89597-06-8] alkyl, CH2CgH [89597-10A] aryls, or H [89597-11-5], Similar to Group 4 transition-metal Zeigler-Natta catalysts, stable cationic Th(IV) species, eg, [Cp 2ThCH2] [108834-69-17, have been isolated with a host of noncoordinatiag/nonreactive anions. MetaHacycle formation has also been... 

The low basicity of pyrrole is a consequence of the loss of aromaticity which accompanies protonation on the ring nitrogen or on carbon 2 or carbon 3 of the ring. The thermodynamically most stable cation is the 2H-pyrrolium ion, and the p/sTa for protonation at C-2 has been recorded as -3.8 the corresponding pK values for protonation at C-3 and at nitrogen are -5.9 and ca. -10 (Scheme 7). 

Measurement of (R /R ) can be accomplished by cyclic voltammetry for relatively Stable species and by other methods for less stable cations. The values obtained for AG -range from 83kcal/mol for the aromatic tropylium ion to 130kcal/mol for destabilized betizylic cations. For stable carbocations, the results obtained by this method correlate with cation stabiUty as measured by pKf.+. Some of these data are presented in Table 5.3. 

Strong acids or superacid systems generate stable fluorinated carbocations [40, 42] Treatment of tetrafluorobenzbarrelene with arenesulfonyl chlorides in nitro-methane-lithium perchlorate yields a crystalline salt with a rearranged benzo barrelene skeleton [43] Ionization of polycyclic adducts of difluorocarbene and derivatives of bornadiene with antimony pentafluonde in fluorosulfonyl chloride yields stable cations [44, 45]... 

The rate constant k iv for solvolysis is assumed to reflect the stability and reactivity of (i.e., faster solvolysis gives a more stable cation, which, therefore, reacts more slowly with nucleophiles). The ratio measured by product distribution... 

A closely related stable cation which also exhibits a strong conformational preference is cyclopropylcarbinyl cation. 

One possible explanation is that adamantyl cation, an intermediate in the reaction, is particularly unstable because it cannot accomodate a planar carbocation center (see Chapter 1, Problem 9). Examine the geometry of adamantyl cation. Does it incorporate a planar carbocation center Compare electrostatic potential maps of adamantyl cation and 2-methyl-2-propyl cation. Which cation better delocalizes the positive charge Assuming that the more delocalized cation is also the more stable cation, would you expect adamantyl tosylate to react slower or faster than tcrf-butyl tosylate Calculate the energy of the reaction. 

Is the stable cation that formed as a result of protonation of the more electron-rich end of the alkene Examine electrostatic potential maps for propene, 2-methylpropene and 2-methyl-2-butene. For each, can you tell whether one end of the 7t bond is more electron rich than the other end If so, does protonation on the more electron-rich end lead to the more stable carbocation ... 

Carbocations initially formed upon addition of an electrophile to an alkene may be able to undergo skeletal rearrangement depending on whether or not a more stable cation exists and, if it does exist, whether or not it can be reached via a low-energy pathway. Consider addition of HBr to 3-methyl-1-butene, the product of which is 2-methyl-2-butyl bromide. 

Draw Lewis structures for the possible carbocations resulting from protonation of the double bond in 3-methyl-1-butene, and decide which is favored. (Check your result using available energy data for C5H11 carbocations.) What would be the product of bromide addition to the more stable cation Is this the observed product ... 

The first step in the addition of an electrophile such as HBr to an alkyne involves protonation and subsequent formation of an intermediate vinyl cation. Where does propyne protonate Compare energies of 1-methylvinyl and 2-methylvinyl cations. Which is more stable Why Measure CC bond distance in the more stable cation. Does the cation incorporate a full triple bond (as in propyne) or a double bond (as in propene). Examine atomic charges and electrostatic potential maps to locate the positive charge in the two cations. Is the more stable ion the one in which the charge is better delocalized Use the charges together with information about the ions geometry to draw Lewis structures (or a series of Lewis structures) for 1-methylvinyl and 2-methylvinyl cations. 

Is the location of positive charge in the more stable cation also where the lowest-unoccupied molecular orbital (LUMO) is most concentrated Rationalize what you observe. Does attack by a nucleophile (bromide) lead to the Markovnikov or anti Markovnikov product ... 

HCl addition usually proceeds through a carbocation intermediate, with the dominant product resulting from the most stable cation. Compare energies for the possible cations that can be obtained from isoprene (isoprene+H+). Rank them from most to least stable. Examine the geometries of the cations, and sketch one or more Lewis stmctures for each which are consistent with its geometry. Be sure to point out any unusual features. What factors might be responsible for the ordering of cation stability ... 

Obtain the energy of each cation that might be generated by electrophilic addition of Br to biphenyl (biphenyl+Br+). Which one is most stable Are there others of comparable stability Examine the structure of the most stable cation(s), and draw all of the resonance contributors needed to describe this ion(s). Predict the product(s) of biphenyl bromination. Will the reaction be highly selective, moderately selective or unselective ... 

The reactivity of the methyl group in 4-methylcinnoline ethiodide indicates that the structure of this compound is 5, and this evidence has also been interpreted to mean that N-1 is the basic group in cinnolines. However, evidence of this type is only indicative since the formation of quaternary salts is subject to kinetic control, whereas protonation yields predominantly the thermodynamically more stable cation. The quinazoline cation has been shown to exist in the hydrated, resonance-stabilized form 6 7 by ultraviolet spectro-... 

The water molecule was found to add across the 7,8-positions in 6-hjr-droxypteridine by Brown and Mason. These authors showed that the neutral species of 6-hydroxypteridine exists mainly as 6,7-dihydroxy-7,8-dihydropteridine (4) in equilibrium with a little of 3. The stable cation is largely derived from 4 and the stable anion largely from 3. 

On reaction with acid, 4-pvrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hiickel 4n 4- 2 rule, explain why the protonated product is so stable. 

Electrophilic substitutions normally occur at C2, the position next to the nitrogen, because reaction at this position leads to a more stable intermediate cation having three resonance forms, whereas reaction at C3 gives a less stable cation with only two resonance forms (Figure 24.6). 

The use of liquid salts, based on anodi-cally stable cations such as 1,2-dimethyl-3-propylimidazolium (Dmpi) without added solvent, allows the investigation of the electrochemical stability of anions [75],... 

When dienones 39 and 40 are photolyzed in sulfuric acid they both rearrange to the same product, 2-methyl-5-hydroxybenzaldehyde (41) (Filipescu and Pavlik, 1970). The mechanism for this photorearrangement is consistent with that of the protonated cyclohexadienones already discussed, i.e., disrotatory closure to afford the intermediate bicyclic cations 42 and 43. In this case it is conceivable that the electron-withdrawing effect of the dichloromethyl group forces the subsequent thermal cyclopropyl migration entirely in the direction of the most stable cation 44 to yield the observed product. 

The photolysis of protonated 2,6-cycloheptadienone (73) and its 5-methyl derivative (74) at — 78° in FHSOs yields as a single product in each case the corresponding protonated vinylcyclopentadienones 75 and 76, wliich subsequently isomerize thermally to the more stable cations 77 and 78 (Noyori et al., 1972). 

Various quantitative methods have been developed to express the relative stabilities of carbocations. One of the most common of these, though useful only for relatively stable cations that are formed by ionization of alcohols in acidic solutions, is based on the equation ... 

Table 4. M=P bond distances, M=P-R bond angles, and P NMR chemical shifts of stable cationic terminal phosphinidene complexes ...

In view of the ease of dissociation of the a bond into resonance-stabilized hydrocarbon ions, it is expected that an authentic hydrocarbon salt could be isolated from an extraordinarily stable cation and anion. The first synthesis of a salt composed solely of carbon and hydrogen was achieved on the combination of tris(3-guaiazulenyl)cyclopropenylium ion [1" ] and tris(7//-dibenzo[c,g]fluorenylidenemethyl)methanide ion [2 ] (Okamoto et al., 1985). 

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