Neighbouring group participation

Source: Wikipedia, the free encyclopedia.

In

Diels-Alder reaction}) this page is limited to neighbouring group effects seen with carbocations and SN2 reactions
.

NGP by heteroatom lone pairs

In this type of substitution reaction, one group of the substrate participates initially in the reaction and thereby affects the reaction. Due to NGP, the reaction rate gets increased by many folds. [clarification needed] A classic example of NGP is the reaction of a

alkyl chloride without a heteroatom.[citation needed
]

Ph−S−CH2−CH2−Cl reacts with water 650 times faster than CH3−CH2−CH2−Cl. [citation needed]

NGP by an alkene

The π orbitals of an

tosylate
will react more quickly (1011 times faster for aqueous solvolysis) with a nucleophile than the saturated tosylate.

The carbocationic intermediate will be stabilized by

resonance
where the positive charge is spread over several atoms. In the diagram below this is shown.

Here is a different view of the same intermediates.

Even if the alkene is more remote from the reacting center the alkene can still act in this way. For instance in the following

delocalise
the carbocation.

NGP by a cyclopropane, cyclobutane or a homoallyl group

The reaction of cyclopropylmethamine with sodium nitrite in dilute aqueous perchloric acid solution yielded a mixture of 48% cyclopropylmethyl alcohol, 47% cyclobutanol, and 5% homoallylic alcohol (but-3-en-1-ol).[4] In the non-classical perspective, the positive charge is delocalized throughout the carbocation intermediate structure via resonance, resulting in partial (electron-deficient) bonds. Evidently, the relatively low yield of the homoallylic alcohol implies that the homoallylic structure is the weakest resonance contributor.

NGP by an aromatic ring

An

aromatic ring can assist in the formation of a carbocationic
intermediate called a phenonium ion by delocalising the positive charge.

When the following

SN2 reaction forming B, a 48:48:4 mixture of A, B (which are enantiomers) and C+D was obtained [2] [3]
.

The mechanism which forms A and B is shown below.

NGP by aliphatic C-C or C-H bonds

Aliphatic C-C or C-H bonds can lead to charge delocalization if these bonds are close and antiperiplanar to the leaving group. Corresponding intermediates are referred to a nonclassical ions, with the 2-norbornyl system as the most well known case.

External links

References

  1. PMID 461188
    .
  2. PMID 19907773
    .
  3. ISBN 9780120335282
    . Retrieved 2019-12-17.
  4. doi:10.1021/ja01150a029
    .
  1. ^ Advanced organic chemistry, page 314, Jerry March (4th Ed), Wiley-Interscience.
  2. J. Am. Chem. Soc.; 1949; 71(12); 3863-3870. Abstract
  3. J. Am. Chem. Soc.; 1952; 74(9); 2129-2137 Abstract
    .
Retrieved from "https://en.wikipedia.org/w/index.php?title=Neighbouring_group_participation&oldid=1190023211"