Iminium cation chemistry

APPLICATIONS OF IMINIUM CATION CHEMISTRY TO ACTIVATED INDOLES... 

Selected Features of the Chemistry of N,N-Bis-Trimethylsiloxy-Substi-tuted Cyclopropanes and Dihydrofurans Scheme 3.217 shows selected features of the chemistry of compounds prepared by intramolecular trapping of bis-N,N-(trimethylsiloxy)iminium cations. 

Overman et al. <1996JA9062> and Kibayashi et al. <2002JOC5517> have utilized the same alkynyl intermediate 222 as a substrate in their chemistry (Scheme 47). While the first group uses a nucleophilic attack of the triple bond on the iminium cation 223, the second group performs a regioselective hydrostannylation, followed by Pd-catalyzed carbonylation, to provide the precursor 224 to the cyclization step. 

Another interesting reaction type detected from the photoreaction of tertiary amine-ketone system which have found use in alkaloids chemistry [172] involves sequential electron transfer and proton transfer followed by back electron transfer to yield iminium cations (Eq. 35) leading to dealkylation products upon aqueous workup. The regiocontrol in these dealkylations is dictated by preferential deprotonation at the less branched amine cation radical a-carbon. 

As can be seen by reviewing the chemistry outlined above, a variety of cleverly devised strategies have been employed for the synthesis of the structurally complex yohimbine alkaloids. One elusive problem in many of these approaches has been the proper adjustment of the C(3) stereochemistry, particularly in approaches to reserpine (2) and deserpidine (3). It is well-known that epimerization at C(3) of the yohimbine skeleton 1 can occur under acidic conditions presumably via a mechanism involving cleavage of either the C(2)-C(3) or C(3)-N(4) bond to afford the respective iminium cation 532 or a-indolylcarbinyl cation 533. In this section, we will review investigations which focus on this epimerization process. 

Mariano and coworkers have exploited this PET bond cleavage chemistry in intramolecular as well as intermolecular cyclization reactions [38]. The combined iminium-benzylsilane functionalities undergo intramolecular PET to provide an intramolecular amine radical/benzyl silane cation-radical pair by exciting either the iminium salt or the arene chromophore [38]. Cleavage of the benzyl C—Si bond presumably takes place with assistance of moderately weak n ldeophiles such as methanol. The diradical couples intramolecularly to provide an indolizidine derivative [38] in 90% yield. 

Numerous reviews have appeared which are relevant to aromatic photochemistry and are also of general interest and application these include accounts of the chemistry of excited-state complexes by Davidson, the photochemical electron-transfer reactions of ethylenes and related compounds by Mattes and Farid, photosolvolyses and photoreactions involving carbo-cations by Cristol and Bindel, conformational flexibility and photochemistry by Wagner, asymmetric photochemistry in solution by Rau, and the photochemistry of iminium salts and related heteroaromatic systems by Mariano." The photochemistry of fragrance materials has been considered by Shibamoto and Mihara, and in their second article they deal with aromatic compounds and phototoxicity. ... 

Jacobsen has also developed an organocatalysed asymmetric addition of nucleophiles with iV-acyl-iminium ions (Pictet-Spengler) and oxocarbe-nium ions, potentially useful reactions in medicinal chemistry (Scheme 14.97). The mechanism involves the formation of an anionic catalyst-chloride complex which acts as a chiral counter ion directing the approach of nucleophiles to one face of the cationic species. 

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