Carbonium rearrangement

Usually, the dehydrofluorination process can be carried either by employing a Lewis acid such as HF or BF3 OEt2, or with a basic reagent like NaOH or, better still, MeMgBr. While the former elimination is a fast El process frequently associated with carbonium rearrangements, the basic dehydrofluorination proceeds with a syn-E2 mechanism, which is slow but in many cases results in cleaner non-rearranged products276. 

One of the side reactions that can complicate cationic polymerization is the possibility of the ionic repeat unit undergoing the well-known carbonium ion rearrangement during the polymerization. The following example illustrates this situation. 

The rearrangement of carbonium ions that readily occurs according to the thermodynamic stabiUty of cations sometimes limits synthetic utility of aromatic alkylation. For instance, the alkylation of ben2ene with / -propyl bromide gives mostly isopropylben2ene (cumene) much less... 

When dihydromyrcene is treated with formic acid at higher temperatures (50°C) than that required to produce dihydromyrcenol and its formate, an unexpected rearrangement occurs to produce a,3,3-trimethylcyclohexane methanol and its formate (106). The product is formed by cyclization of dihydromyrcene to the cycloheptyl carbonium ion, which rearranges to give the more stable cyclohexyl compound (107). The formate ester, a,3,3-trimethylcyclohexane methanol formate [25225-08-5] (57) is a commercially avaUable product known as Aphermate. 

Principal terpene alcohol components of piae oils are a-terpiueol, y-terpiueol, P-terpiueol, a-fenchol, bomeol, terpiuen-l-ol, and terpiaen-4-ol. The ethers, 1,4- and 1,8-ciaeole, are also formed by cycli2ation of the p-v( enthane-1,4- and 1,8-diols. The bicycHc alcohols, a-fenchol [512-13-0] (61) and bomeol (62), are also formed by the Wagner-Meerweiu rearrangement of the piaanyl carbonium ion and subsequent hydration. Bomeol is i7(9-l,7,7-trimethylbicyclo[2.2.1]heptan-2-ol [507-70-0]. Many other components of piae oils are also found, depending on the source of the turpentine used and the method of production. 

Carbonylation, or the Koch reaction, can be represented by the same equation as for hydrocarboxylation. The catalyst is H2SO4. A mixture of C-19 dicarboxyhc acids results due to extensive isomerization of the double bond. Methyl-branched isomers are formed by rearrangement of the intermediate carbonium ions. Reaction of oleic acid with carbon monoxide at 4.6 MPa (45 atm) using 97% sulfuric acid gives an 83% yield of the C-19 dicarboxyhc acid (82). Further optimization of the reaction has been reported along with physical data of the various C-19 dibasic acids produced. The mixture of C-19 acids was found to contain approximately 25% secondary carboxyl and 75% tertiary carboxyl groups. As expected, the tertiary carboxyl was found to be very difficult to esterify (80,83). 

When acid-catalyzed ring opening is not synchronous with nucleophilic attack, the intermediate carbonium ion can undergo rearrangement (193,195) (66JOC3941, 73CJC1448). 

The Bamford-Stevens decomposition of tosylhydrazones by base has been applied to steroids, although not extensively. It has been demonstrated that the reaction proceeds via a diazo compound which undergoes rapid decomposition. The course of this decomposition depends upon the conditions in proton-donating solvents the reaction has the characteristics of a process involving carbonium ions, and olefins are formed, often accompanied by Wagner-Meerwein-type rearrangement. In aprotic solvents the diazo compound appears to give carbene intermediates which form olefins and insertion products ... 

It was pointed out earlier that the low nucleophilicity of fluoride ion and its low concentration in HF solutions can create circumstances not commonly observed with the other halogen acids. Under such conditions rearrangement reactions either of a concerted nature or via a true carbonium ion may compete with nucleophilic attack by fluoride ion. To favor the latter the addition of oxygen bases, e.g., tetrahydrofuran, to the medium in the proper concentration can provide the required increase in fluoride ion concentration without harmful reduction in the acidity of the medium. 

The nature and stereospecificities observed in the rearrangement of (68b) to (69a) and (69a) to (70a) suggests that these rearrangements involve two discrete carbonium ion intermediates A and B (see Chart II). 

The carbonium ion rearrangements that can be observed in halofluonnations are illustrated by the reactions ot norbornene (Table 2) and norbornadiene (Table 3). Product ratios may vary with the different reagent combinations... 

Studies of solvolysis of similar polyfluonnated polycyclic aromatic systems, such as 2,3-(tetrafluorobenzo)bicyclo[2 2 2]octadienes and related compounds, proved the ionic mechanism of this rearrangement [55, 36, 37] (equation 9) Possible nonclassical carbonium ion involvement has been discussed [5S, 39, 40, 41]... 

Esters of diphenylacetic acids with derivatives of ethanol-amine show mainly the antispasmodic component of the atropine complex of biologic activities. As such they find use in treatment of the resolution of various spastic conditions such as, for example, gastrointestinal spasms. The prototype in this series, adiphenine (47), is obtained by treatment of diphenyl acetyl chloride with diethylaminoethanol. A somewhat more complex basic side chain is accessible by an interesting rearrangement. Reductive amination of furfural (42) results in reduction of the heterocyclic ring as well and formation of the aminomethyltetrahydro-furan (43). Treatment of this ether with hydrogen bromide in acetic acid leads to the hydroxypiperidine (45), possibly by the intermediacy of a carbonium ion such as 44. Acylation of the alcohol with diphenylacetyl chloride gives piperidolate (46). ... 

The new carbocation may experience another beta scission, rearrange to a more stable carbonium ion, or react with a hydrocarbon molecule in the mixture and produce a paraffin. 

The third mode of termination which occurs in some carbonium ion polymerizations involves rearrangement of the active carbonium ion into an inactive one which cannot continue the propagation. These reactions can be avoided to a great extent by working at sufficiently low temperatures, and on the whole, they only contribute significantly to the termination reaction in a few systems. 

Van t Hoff t-factors 565 Vinylallenes rearrangement of 748 synthesis of 737 Vinyl carbonium ions 620 17a-Vinyl-17/f-hydroxysteroids, epimerization of 735 Vinyl sulphides, as alkyl sulphoxide reduction products 930, 932 Vinyl sulphones - see also Alkenyl... 

The two main reasons for studying the reversible reaction (3) were (a) to complete the picture of the Koch reaction in terms of quantitative information and (b) to set up a scale of reactivity towards a neutral nucleophile for carbonium ions of different structure. The first item is important from a practical point of view because there are reactions competing with the carbonylation step (3), which can be divided into intramolecular and intermolecular processes. Rearrangement of the intermediate alkylcarbonium ion, e.g. 

As mentioned in the Introduction, rearrangements of the intermediate alkyl cation in the Koch synthesis may compete with the carbonylation. Under the kinetically controlled conditions prevailing in the Koch synthesis of carboxylic acids, the rearrangements occur only from a less stable to a more stable carbonium ion, e.g. from a secondary to a tertiary ion. The reverse rearrangements—from a more stable to a less stable... 

First, the rates of carbonylation of secondary and tertiary alkyl carbonium ions can now be compared quantitatively with the known rates of competing intramolecular rearrangements of these ions. The product distribution in the Koch synthesis of carboxylic acids depends, amongst other things, on these relative rates. 

In contrast to the above case, addition of HCl to 1,1-dimethylallene at —78°C gives at least two thirds and possibly exclusively l-chloro-3-methyl-2-butene, 33, although these results are complicated by rearrangement of the allene to isoprene and the addition of HCl to the isoprene (65). No satisfactory explanation was offered (65) and none is readily available within the carbonium framework to account for the unusual orientation in this addition. Certainly the tertiary carbonium ion, 34, should be more stable than the primary carbonium ion, 35, since neither is stabilized by the adjacent perpendicular n center. This result is all the more surprising since tetramethylallene, 36, behaves as expected... 

One of the most characteristic properties of carbonium ions is their great tendency to undergo rearrangements. These rearrangements include 1,2-alkyl shifts, hydride shifts, cyclopropylcarbinyl rearrangements, Wagner-Meerwein rearrangements, and others. 

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