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Amidines are a class of oxoacid derivatives.

The oxoacid from which an amidine is derived must be of the form R_nE(=O)OH, where R is a substituent. The −OH group is replaced by an −NH₂ group and the =O group is replaced by =NR, giving amidines the general structure R_nE(=NR)NR₂.^[1]^[2]^[3]

Carboxamidines

The skeletal formula of acetamidine (acetimidamide)

When the parent oxoacid is a carboxylic acid, the resulting amidine is a carboxamidine or carboximidamide (IUPAC name), and has the following general structure:

Carboxamidines are frequently referred to simply as amidines, as they are the most commonly encountered type of amidine in organic chemistry. The simplest amidine is formamidine, HC(=NH)NH₂.

Examples of amidines include:

DBU
diminazene
benzamidine
Pentamidine
Paranyline

The most common way to make primary amidines is by the Pinner reaction. Reaction of the nitrile with acidic alcohol gives an iminoether; treatment of this with ammonia then completes the conversion to the amidine.

Properties

Amidines are much more basic than amides and are among the strongest uncharged/unionized^[4] bases.^[5]

Protonation occurs onto the sp² hybridized nitrogen. This occurs because the positive charge can be delocalized onto both nitrogen atoms. The resulting cationic species is known as a amidinium ion^[6] and possesses identical C-N bond lengths.

Derivatives

Formamidinium cations

general structure of a formamidinium cation

An notable subclass of amidinium ions are the formamidinium cations; which can be represented by the chemical formula [R₂N-CH=NR₂]⁺. Deprotonation of these gives stable carbenes which can be represented by the chemical formula R₂N-C:-NR₂.^[7]

Amidinate salts

An 'amidinate salt has the general structure M⁺[RNRCNR]⁻ and can be accessed by reaction of a carbodiimide with an organometallic compound such as methyl lithium.^[8] They are used widely as ligands in organometallic complexes.

References

^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "amidines".
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "carboxamidines".
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "sulfinamidines".
^ Roche VF. Improving Pharmacy Students’ Understanding and Long-term Retention of Acid-Base Chemistry. American Journal of Pharmaceutical Education. 2007;71(6):122.
^ Clayden; Greeves; Warren (2001). Organic chemistry. Oxford university press. p. 202. ISBN 978-0-19-850346-0.
^ Schrader, Thomas; Hamilton, Andrew D., eds. (2005). Functional synthetic receptors. Wiley-VCH. p. 132. ISBN 3-527-30655-2.
^ Alder, Roger W.; Blake, Michael E.; Bufali, Simone; Butts, Craig P.; Orpen, A. Guy; Schütz, Jan; Williams, Stuart J. (2001). "Preparation of tetraalkylformamidinium salts and related species as precursors to stable carbenes". Journal of the Chemical Society, Perkin Transactions 1 (14): 1586–1593. doi:10.1039/B104110J.
^ Ulrich, Henri (2007). Chemistry and technology of carbodiimides. Chichester, England: John Wiley & Sons. ISBN 9780470065105.

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