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]

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