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Carbonyl allylation has been employed in the synthesis of polyketide natural products and other oxygenated molecules with a contiguous array of stereocenters. For example, allylstannanation of a threose-derived aldehyde affords the macrolide antascomicin B, which structurally resembles FK506 and rapamycin, and is a potent binder of FKBP12. [12]
A site adjacent to the unsaturated carbon atom is called the allylic position or allylic site. A group attached at this site is sometimes described as allylic. Thus, CH 2 =CHCH 2 OH "has an allylic hydroxyl group". Allylic C−H bonds are about 15% weaker than the C−H bonds in ordinary sp 3 carbon centers and are thus more reactive.
The rearrangement reaction can be classified into 4 types. Type 1 concerns all 2-heterosubstituted alcohols. Substrates in type 2 rearrangements are allyl alcohols. The carbocation is formed by electrophilic addition to the alkene group with electrophiles such as halonium ions, Brønsted acids and Lewis acids.
A general acyl group (blue) in a ketone (top left), as an acylium cation (top centre), as an acyl radical (top right), an aldehyde (bottom left), ester (bottom centre) or amide (bottom right).
For example, (C 6 H 5)(CH 3) 2 C + is referred to as a "benzylic" carbocation. The benzyl free radical has the formula C 6 H 5 CH 2 •. The benzyl cation or phenylcarbenium ion is the carbocation with formula C 6 H 5 CH + 2; the benzyl anion or phenylmethanide ion is the carbanion with the formula C 6 H 5 CH − 2.
According to the general principle, the Lewis acid first activates the electrophilic carbon in presence of allyltrimethylsilane which then undergoes nucleophilic attack from electrons on the allylic silane. [5] The silicon plays the key role in stabilizing the carbocation of carbon at the β-position.
An allylic rearrangement or allylic shift is an organic chemical reaction in which reaction at a center vicinal to a double bond causes the double bond to shift to an adjacent pair of atoms: It is encountered in both nucleophilic and electrophilic substitution , although it is usually suppressed relative to non-allylic substitution.
The 2-norbornyl cation is one of the best characterized carbonium ion. It is the prototype for non-classical ions. As indicated first by low-temperature NMR spectroscopy and confirmed by X-ray crystallography, [1] it has a symmetric structure with an RCH 2 + group bonded to an alkene group, stabilized by a bicyclic structure.