Carbonium ions pinacol rearrangement

The pinacol rearrangement is believed to involve two important steps (i) loss of water from the protonated glycol to form a carbonium ion and (2) rearrangement of the carbonium ion by a 1,2-shift to yield the protonated ketone. 

We have depicted the pinacol rearrangement as a two-step process with an actual carbonium ion as intermediate. There is good evidence that this is so, at least when a tertiary or benzylic cation can be formed. Evidently the stability of the incipient cation in the transition state permits (SNl-lihe) loss of water without anchimeric assistance from the migrating group. This is, we note, in contrast to what happens in migration to electron-deficient nitrogen or oxygen. 

Pinacol Rearrangement.12 Certainly one of the best-known examples of a carbonium ion rearrangement is the pinacol transformation in which polysubstituted ethylene glycols are converted into substituted ketones by the action of acidic reagents such as mineral acid, acetyl chloride, or acetic acid and iodine. In accordance with Whitmore s theory of carbonium ion rearrangements,11 the mechanism of the reaction can be outlined as follows, with pinacol itself as an example ... 

An interesting tertiary alcohol is pinacol (16) in which both the rate of acid catalysed exchange and of rearrangement to pinacolone can be measured. Bunton et al. (1958b) found that both reactions follow the Hammett acidity function H0. The reaction is postulated to proceed through a carbonium ion as follows ... 

Epoxide rearrangements are closely related to the pinacol rearrangement but allow a more general synthesis of carbonyl compounds. On treatment with acids or Lewis acids, even such weak ones as LiBr or MgBr2, epoxides (26) open to give the more stable carbonium ion (27) which rearranges to a carbonyl compound (28). The order for migration is usually H>Aryl> f-Alkyl > Alkyl > />-Alkyl. 

Any other reaction, such as deamination of a /3-aminoalcohol, that generates a carbonium ion of similar structure will normally be followed by a pinacol-type rearrangement. 

Reaction of c/5-l,2-cyclooctanediol with dichlorocarbene yields cyclooctanone, cyclooctene, and the dichlorocarbene adduct of cyclooctene according to equation 3.6 [10]. The formation of cyclooctanone is expected from a pinacol rearrangement of the a-hydroxy carbonium ion formed by decomposition of the a-hydroxyalkoxy-chlorocarbene adduct. On the other hand, reaction of the alkoxychlorocarbene with an adjacent alkoxide anion yields a cyclic dialkoxycarbene which decomposes with loss of carbon dioxide and formation of cyclooctene. Addition of dichlorocarbene to cyclooctene yields 2,2-dichlorobicyclo[8.1.0]-nonane. 

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