Iminium salts, acyclic

Nuclear magnetic resonance has revolutionized structure determination of iminium salts. A compilation of various resonances for acyclic and cyclic iminium salts are given in Tables 3 and 4 for comparison purposes and for determination of trends. It should be noted that the simplest symmetrically... 

While most of the iminium salts studied are cyclic, several acyclic iminium salts have also been investigated. In 1997, Armstrong and coworkers reported the use of acyclic iminium salt 83 as chiral epoxidation promoter (Fig. 27) [156, 157]. 1-Phenylcyclohexene oxide could be obtained in 100% conversion and 22% ee with stoichiometric amounts of 83. In 2002 acyclic iminium salt 84, prepared from L-prolinol, was investigated by Komatsu and coworkers, and cinnamyl alcohol was epoxidized in 70% yield and 39% ee (Fig. 27) [158]. 

In 2001, Yang and coworkers studied the use of in situ generated acyclic iminium salts as epoxidation catalysts [159]. Epoxidations of a number of aUcenes proceed with 20-50 mol% of amine 85 and aldehyde 86 with Oxone as the primary oxidant (Fig. 28.) Methylstilbene can be obtained in 100% conversion and 59% ee. 

While the addition of organometallic reagents to acyclic or cyclic imines (43) is often compromised by poor imine reactivity, these reagents readily condense with structurally diverse, more electrophilic imini-um salts (44), bearing a positive charge on the nitrogen atom, to provide substituted tertiary amines (45 Scheme 6). A review of the literature describing additions to cyclic iminium salts prior to 1966 is avail-... 

Iminium salts bearing a labile trimethylsilyl group can be generated in situ and undergo nucleophilic addition (see Sections 1.12.4.2 and 1.12.7.3). Bis(trimethylsilyl)methoxymethylamine (75), for example, has been used as a formaldehyde equivalent for the preparation of primary amines. Cyclic imines, such as 3,4-dihydroquinolines, react with trimethylsilyl triflate (TMS-OTf) to provide reactive labile iminium salts (55), which condense with picoline anions. The addition of nonstabilized Grignard and organolithium reagents to acyclic aromatic ketimines and aldimines, however, is often not facilitated by the presence of TMS-OTf ... 

Moderate ees have been achieved by Yang using another exocyclic iminium salt system [15]. These salts are not isolated, but are generated in situ, thus circumventing the difficulties inherent in the preparation and isolation of unstable acyclic iminium salts. A major drawback to this type of system is the necessary high catalyst loadings for an efficient rate of reaction, up to 50 mol% of iminium... 

More recently Bohe, a former co-worker with Lusinchi, has reported an improved achiral catalyst that prevents some of the common side reactions observed in iminium salt-mediated epoxidation [18]. Two factors are known to reduce the catalytic efficiency of the epoxidation process hydrolysis of the iminium salt directly by the reaction medium, which generally only affects the acyclic systems and loss of active oxygen from the intermediate oxaziridinium species, through a reaction that does not regenerate the iminium species, which... 

J-Unsaturated oxazolines can be activated by (V-acylation and the acyl minium salts thus obtained are very reactive dienophilcs. Iminium salts of chiral oxazolines 89 and 101 react with several cyclic and acyclic dienes to give cycloaddncts with a very high diastereomeric excess24. 

A cycloaddition reaction of the halogen-substituted nitrone (289) with olefins in the presence of AgBp4 provides reactive iminium salts in a regio- and stereo-specific manner. These react with various nucleophiles to give 1,2-oxazine derivatives as shown. In a study of the reactions of P-enamino-esters with carbonyl compounds, WamholF et have isolated a series of acyclic and... 

Viehes salt and various acyclic, cyclic, and rigid bicyclic e(-diols give iminium salts which on thermolysis yield /tf-chloro-deoxy-X-N,N-dimethylcarbamates formed in diaxial ring opening reactions (e.g., Scheme 6). 

Armstrong has shown that even acyclic iminium salts can mediate epoxidation by Oxone [73] however, enantiomeric excesses are low [74]. By condensing A-trimethylsilylpyrrolidine 23 with a range of aromatic aldehydes in the presence of trimethylsilyltriflate, Armstrong was able to produce a range of substituted exocyclic iminium salts (Scheme 1.28). 

Cheap and readily available L-proline has been used numerous times for the intermediate and reversible generation of chiral iminium ions from a,/ -unsaturated carbonyl compounds. For example, Yamaguchi et al. reported in 1993 that the rubidium salt of L-proline catalyzes the addition of di-iso-propyl malonate to the acyclic Michael acceptors 40a-c (Scheme 4.13), with enantiomeric excesses as high as 77% [22], With 2-cycloheptenone and 2-cyclohexenone as substrates ca 90% yield and ee of 59% and 49% were obtained. Later the enantioselectivity of this process was increased to a maximum of 88% ee in the addition of di-tert-butyl malonate to the E-pentenone 40a in the presence of 20 mol% Rb-L-prolinate and 20 mol% CsF [23], Taguchi and Kawara employed the L-proline-derived ammonium salts 41a and... 

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