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When an aldehyde or a ketone undergoes reductive amination when a primary amine is acting,the mechanism follows a pathway through hemiaminal and imine,what happens when a secondary amine is acting on the given carbonyl ? There will be no hydrogen on the amine nitrogen in this case to facilitate loss of water with the hydroxyl group of the hemiaminal.How does the reaction proceed then?

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    $\begingroup$ Secondary amines undergo reductive amination perfectly well via the iminium ion. This explains it clearly chemistryscore.com/reactions/reductive-amination $\endgroup$ – Waylander Jul 25 at 12:35
  • $\begingroup$ Did you read @Waylander's link? The mechanism is there. $\endgroup$ – user55119 Jul 25 at 14:34
  • $\begingroup$ Yes I did , actually I was not able to follow the answer there , what I was able to gather from there was that the reducing agent , LiAlH4 in my case , will substitute the OH and nitrogen will grab a hydrogen from water $\endgroup$ – ADITYA PRAKASH Jul 25 at 14:37
  • $\begingroup$ Sodiumtriacetoxyborohydride in DCM is my reagent of choice for reductive aminations. LiAlH4 will just reduce your ketone before it reacts with the amine $\endgroup$ – Waylander Jul 25 at 15:06
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In modern organic synthesis, reductive amination of aldehydes and ketones is considered one of the important and principal ways to make secondary and tertiary amines. It is a powerful and reliable strategy for the formation of $\ce{C–N}$ bonds. One of the advantages of the reductive amination is it can avoid the problem of over-alkylation that is always the problem with direct alkylation of amines with alkyl halides.

Reductive amination can be achieved following either a one-step or a two-step procedure. In both procedures, an amine and a carbonyl compound condense to form an imine (with primary amine) or iminium ion (with secondary amine). This imine or iminium ion is then reduced in situ (one-step reaction) or separated and subsequently reduced (two-step reaction) to form an amine product. Following is the suggested mechanism for reductive amination:

Reductive Amination

Most common condition is sodium triacetoxyborohydride ($\ce{NaB(OAc)3H}$) in THF or dichloroethane (DCE) under slighly acidic conditions (common acid used is acetic acid)(Ref.1).

Reference:

  1. Ahmed F. Abdel-Magid, Kenneth G. Carson, Bruce D. Harris, Cynthia A. Maryanoff, Rekha D. Shah, “Reductive Amination of Aldehydes and Ketones with Sodium Triacetoxyborohydride. Studies on Direct and Indirect Reductive Amination Procedures,” J. Org. Chem. 1996, 61(11), 3849-3862 (https://doi.org/10.1021/jo960057x).
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    $\begingroup$ A good answer. A useful additive for reductive aminations that proceed in poor yield (or not at all) is Titanium (IV) alkoxide - Ti(OEt)4 or Ti(iOPr)4. $\endgroup$ – Waylander Jul 26 at 6:28

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