Intermolecular Addition Reactions of N-Acyliminium Ions (Part I)

Abstract

I n t e r m o l e c u l a r A d d i t i o n R e a c t i o n s o f N -A c y l i m i n i u m I o n s ( P a r t I )Abstract: This review highlights the advances in the literature up to July 2008 on the intermolecular reactions of acyclic and cyclic Nacyliminium ions. This is an update of an earlier review in 2000 on this topic and does not include intramolecular addition reactions to N-acyliminium ions which was recently reviewed. This review is presented in two parts, with the first part dealing with acyclic and pyrrolidinone-based N-acyliminium ions. Part II continues with other five-membered heterocyclic derivatives and higher systems. IntroductionThis review highlights the advances in the literature up to July 2008 on the intermolecular reactions of acyclic and cyclic N-acyliminium ions. This is an update of an earlier review in 2000 2 on this topic and does not include intramolecular addition reactions to N-acyliminium ions which was recently reviewed. 3 A review article on addition reactions to related, but less reactive, N-acylimines has also been recently published. 4The highly reactive nature of N-acyliminium ions require that they are generated in situ usually in the presence of the other reactive, electron-rich, nucleophilic partner (NuY, Y = metal, SiR 3 , SnR 3-, etc.). In general these intermediates are generated from more stable and isolatable asubstituted N-acylamines of the type 1 by treatment with a Lewis acid or sometimes a protic acid (Scheme 1). The reaction of 2 with a nucleophilic species (NuY) then gives a-substituted N-acylamine 3. Compounds 1-3 can be acyclic systems or R 1 and R 2 , R 2 and R 3 , R 1 and R 3 can be taken together to form part of a ring system as shown in the general structures 4, 5, and 6.Compounds like 1 (X = OH and NHCO 2 R) are most likely formed in situ from the Lewis acid promoted threecomponent, one-pot coupling reactions of carbamates, aldehydes (or acetals) and silyl nucleophiles or electronrich aromatic nucleophiles (Scheme 2). 5,6N-Acyliminium ions like 11 can also be generated in dichloromethane solution, in the absence of nucleophiles, by the electrochemical oxidation of N-trimethylsilylmethyl carbamates like 10 (Scheme 3). These intermediates have been characterised spectroscopically and were subDownloaded by: Florida International University. Copyrighted material.

Catalytic Asymmetric Additions of Enol Silanes to In Situ Generated Cyclic, AliphaticN‐Acyliminium Ions

“…Since early reports in the 1950s, N ‐acyliminium ions have been used as highly reactive intermediates in a plethora of carbon−carbon‐bond‐forming approaches to natural products, and other biologically active molecules [1–6] . Despite significant advances in this field, however, stereocontrol of such reactions has remained challenging, and catalytic enantioselective approaches, with few notable exceptions, require arene stabilization [7–19] .…”

“…Since early reports in the 1950s, N-acyliminium ions have been used as highly reactive intermediates in a plethora of carbonÀ carbon-bond-forming approaches to natural products, and other biologically active molecules. [1][2][3][4][5][6] Despite significant advances in this field, however, stereocontrol of such reactions has remained challenging, and catalytic enantioselective approaches, with few notable exceptions, require arene stabilization. [7][8][9][10][11][12][13][14][15][16][17][18][19] Intermolecular catalytic asymmetric reactions proceeding via cyclic, aliphatic Nalkoxycarbonyliminium ions are particularly difficult to control, arguably due to a lack of conjugated π-systems, which facilitate ionization and catalyst-substrate interactions.…”

Katalytische asymmetrische Additionen von Enolsilanen an in situ erzeugte zyklische, aliphatischeN‐Acyliminiumionen

“…Acyliminiumionen als hochreaktive Zwischenprodukte in einer Vielzahl von Kohlenstoff-Kohlenstoff-bindungsbildenden Reaktionen auf dem Weg zu Naturstoffen und andere biologisch aktive Moleküle verwendet. [1][2][3][4][5][6] Trotz bedeutender Fortschritte auf diesem Gebiet ist die Stereokontrolle solcher Prozesse immer noch eine Herausforderung, und katalytische enantioselektive Ansätze erfordern, von wenigen Ausnahmen abgesehen, eine Stabilisierung durch Arene. [7][8][9][10][11][12][13][14][15][16][17][18][19] Intermolekulare katalytisch asymmetrische Reaktionen, die über zyklische, aliphatische N-Alkoxycarbonyliminiumionen ablaufen, sind besonders schwer zu kon-trollieren, was vermutlich auf das Fehlen konjugierter π-Systeme zurückzuführen ist, deren Gegenwart die Ionisierung und die Wechselwirkungen zwischen Katalysator und Substrat erleichtern würde.…”

1-(N-Acylamino)alkyltriarylphosphonium Salts with Weakened Cα-P+ Bond Strength—Synthetic Applications

“…The presence of a positively charged nucleofugal phosphonium moiety in the close surroundings of the N -acyl group determines its unique chemical properties such as high reactivity in α-amidoalkylations [1,2,3,4,5,6,7,8,9,10]. This type of reactions has enjoyed unflagging interest for years as a synthetic method with great potential, especially valuable for C-C and C-heteroatom bond formation [1,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]. …”