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enter image description here Structure of DBU

DBU is used as a very strong base(which is due to resonance stabilization of amidinium cation) with steric bulk. Is any other compound used instead? If not, why not?

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    $\begingroup$ DBN is used, too. $\endgroup$ – RBW Apr 9 '16 at 13:42
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    $\begingroup$ Caprolactam is $38/kg from Aldrich. Acrylonitrile is $55 for two liters. My guess: it's cheaper to make than similar ring sizes. I know valerolactam is significantly more expensive than $38/kg. $\endgroup$ – SendersReagent Apr 9 '16 at 14:17
  • $\begingroup$ What does the price of lactams have to do with price of amidines ? $\endgroup$ – gluco Apr 10 '16 at 16:15
  • $\begingroup$ @gluco Caprolactam + acrylonitrile = starting materials for DBU. Imagine caprolactam with a nitrogen in place of the oxygen and three more carbons, then cyclized. $\endgroup$ – SendersReagent Apr 13 '16 at 12:50
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DBU is a popular amidine base because it can be deemed to be of lower nucleophilicity than cheaper amidines e.g. formamidine. It has been sometimes erroneously labelled as "non-nucleophilic" but seeing as it reacts with carbon dioxide and sulfonyl halides and anhydrides, this cannot be supported. As has been pointed out, the price of bulk chemicals such as caprolactam (made from cyclohexanone which in turn comes from phenol) and acrylonitrile makes DBU accessible.
The 7-member ring structure isn't special per se but assists with steric bulk to reduce nucleophilicity of the free base and increase the nucleophilicity of the counter anion when acting as a (large) amidinium ion. This occurs through a reduced association of the ion pair. For cost weighed up against basicity, tetramethylguanidine is better value as an organic base if the extra strength is required.

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  • $\begingroup$ I actually just saw an article that suggested that DBU isn't even of low-nucleophilicity, per se, but rather that it just dissociates much in the way pyridine will. $\endgroup$ – SendersReagent Apr 12 '16 at 18:35
  • $\begingroup$ I was referred to an article about 16 years ago that allegedly showed that where DBU reacted with carbon dioxide, DBN did not. I can't place the article unfortunately. $\endgroup$ – Beerhunter Apr 13 '16 at 8:02

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