Crotyl group
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A crotyl group is an organic functional group with the formula RCH2CH=CHCH3.[1] Systematically, it is called a but-2-en-1-yl group and exhibits geometric isomerism, being either cis (Z) or trans (E). There are many simple compounds in which the crotyl group forms base carbon chain: crotyl alcohol, crotonaldehyde, crotonic acid, and crotyl acrylate are examples.
Synthesis of crotylates
Crotylate (crotyl anions) can be synthesised from 2-butene (either isomer) by deprotonation with highly basic organometallic reagents,[2][3] typically organolithium reagents.[4] The negative charge is delocalised over three of the carbon atoms of the crotylate group.
Crotylation reactions
Crotyl halides, crotyl alcohols,[3] crotyl silanes,[5][6] crotonaldehyde and crotyl boronates are useful reagents for synthesizing crotyl and allyl compounds. Crotyl and allyl compounds are common intermediates in total synthesis[2] due to their ability to undergo sigmatropic rearrangements.
Crotylboronates are easily made from reaction of crotyl anions with alkoxy boronates,[7][8] and are useful reagents in the formation of crotyl and allyl alcohols.[3]
Trans-crotonaldehyde is commonly used to determine the effective Lewis acidity of Lewis acids,[9] determined from the change in 1H NMR shift of the crotonaldehyde handle on binding to a Lewis acid. Limitations of the method include secondary interactions to the 1H NMR handle obscuring the true effect of the Lewis acid, and weak donor strengths of trans-crotonaldehyde resulting in incomplete Lewis acid-base adduct formation.[10]