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ketene 2 - -1 REARRANGEMENTS NOTES Mechanistic Aspects of...

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- 1 - These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission. www.alchemyst.f2o.org R EARRANGEMENTS N OTES Mechanistic Aspects of Rearrangements Nature of the Rearrangement It can vary from being truly stepwise to migration occurring in concert with initial ionisation. These two situations can be considered as intramolecular analogues of S N 1 and S N 2 respectively. Hence, it has profound stereochemical implications at both the migration origin and the terminus if the centres are chiral. With non-racemic substrates, the generation of a free carbocation at either the migration terminus or origin will result in loss of stereochemical integrity at that centre, whereas a concerted migration with no intermediate will result in inversion of the absolute stereochemistry at the terminus. Migratory Aptitude This is the ease with which any particular group will undergo nucleophilic 1,2-shifts. It is related to electron donating capacity, but such values are not quantifiable as a single group will show different aptitudes for different reactions/conditions. Note that in some instances, for example the Beckmann Rearrangement, only one group ever migrates regardless of the aptitude, simply because of stereoelectronic requirements for the reaction. Likewise, in for example the pinacol rearrangement, the most stable cation controls the reaction pathway, as opposed to the migrating group. A wide range of other factors, such as steric and conformational effects, play subtle roles in determining whether a particular migration is favoured. Generally however, aryl groups migrate more readily than alkyl groups, and within the aryl series, e- donating substituents increase the propensity for migration. However, the position of hydrogen within this framework is highly unpredictable. Neighbouring Group Participation (Anchimeric Assistance) This is where a substituent is capable of stabilising an adjacent carbocation by acting as an intramolecular nucleophile. More details in the Stereoelectronics Notes. Non-classical Carbonium Ions While alkyl groups do not usually undergo σ -participation in acyclic or unstrained ring systems, there is much evidence to suggest that this does occur in strained rings.
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