Imminium salts

The most useful general method for the C-acylation of pyrroles is the Vilsmeier-Haack procedure in which pyrrole is treated with the phosphoryl chloride complex (55a, b) of an AiA-dialkylamide (54). The intermediate imine salt (56) is hydrolyzed subsequently under mildly alkaline conditions to give the acylated pyrrole (57). On treatment of the imminium salt (56 R =H) with hydroxylamine hydrochloride and one equivalent of pyridine and heating in DMF, 2-cyanopyrrole (58) is formed (80CJC409). 

Ketoamides can participate in a variation of the Paal-Knorr condensation to yield 5-alkyl-2-aminofurans. Boyd described the cyclization of 1,4-ketoamides 91 upon exposure to acetic anhydride and perchloric acid to yield imminium salts 92 that furnished aminofurans 93 after treatment with triethylamine. ... 

Cleavage of a hydroxyl group of 139 occurred in methanolic hydrogen chloride to give the imminium salt 140 (Scheme 28). Evaporation of the solvent generated O-methylprechilenine (117) in 96% yield. The imminium salt... 

Acidic treatment of prechilenine (139) (Section III,C) afforded the imino keto acid 377 via the imminium salt 140 (Scheme 68). Neutralization and work-up furnished the known y-lactol 371 in 90% overall yield from 139 (96, 181). Kondo et al. (183) obtained 139 from the epidioxide 122 by treatment with pyridine hydrochloride in pyridine along with 377 and norhydrastine. The acid 377 was converted to the known imino ester 373 through N,0-dimethylation. 

A significant limitation, because of which only a few examples of 6-unsubstituted aminopyrans are known, is poor accessibility and lack of stability of starting materials 82 [generated, for example, via imminium salts (01T5591)]. 

Condensation of pelletierine and arylaldehyde affords the imminium salt (143) which is transformed to the ci s-quinolizidone (141) via the unstable trans-fused quinolizidone (144). The cis form (141) comes to equilibrium with the trans isomer (140) via the unsaturated aminoketone (145) by the action of hydroxide anion. 

The title compound, K2 [Rh6(CO)i5C], has been prepared in about 70% yield by the reaction of Rh CO) with sodium hydroxide under carbon monoxide in methanol containing some chloroform.1 The benzyltrimethylammonium and jt-nitrido-bis(triphenylphosphorusX 1 +) [bis(triphenylphosphine)imminium] salts have been prepared by reaction of the corresponding [Rh(CO)4 ] salts with CC14.2... 

The compound K2 [Rh6(CO)15C] is a yellow powder. It is sensitive to air both in the solid state and in solution and is quite soluble in water, methanol, ethanol, acetone, THF, and acetonitrile. The salts of other cations can be obtained by metathesis, in water for the cesium salt and in methanol for the larger tetra-alkylammonium or phosphonium cations. The tetraethylammonium salt is sparingly soluble in THF, whereas the benzyltrimethylammonium and bis-(triphenylphosphine)imminium salts are soluble. All of these salts are soluble in acetone and acetonitrile. The yellow solution of the potassium salt in THF shows characteristic IR bands at 2040 (vw), 1990 (vs), 1885 (vw), 1845 (s), 1830 (sh, m) 1815 (sh, br) and 1775 (vw, br) cm-1. The IR spectral band shapes depend on solvents and cations. The oxidation of K2 [Rh6(CO)i5C] with iron-(III) ammonium sulfate in water under carbon monoxide leads to the octa-nuclear carbido carbonyl cluster Rhg(CO)i9C,6 whereas under nitrogen RhntCO sQ7 or [H30] [Rhls(CO)28C2]8 is obtained. 

An interesting use of removable chiral auxiliaries in photocycloadditidl reactions concerns imminium salts. With cyclic enones, the observed asymmetri induction does not result from an approach of the double bonds in parallel plane because of the triplet nature of the reactive excited state. In contrast, the corn sponding imminium salts react through their singlet excited state, and a ... 

The thermally generated imminium salts 866 and 868 undergo intramolecular cyclization followed by aromatiza-tion. The former affords 5-methylthieno[3,2-/ ]carbazole 867 and the latter gives the [2,3- ] isomer 869 (Scheme 134) <1995JOC3707>. The thermally induced electrocyclization of enamines 870 affords benzo[ ]thio-phenes 871 or pyrrothienoazepine 872 <1997JOC7744>. 

Dendrobine was oxidized with NBS to the imminium salt 120 (Scheme 9). A Reformatsky reaction then converted the imminium salt into dendrine (88) and... 

Bulman Page and coworkers have optimized conditions for the use of their 5-amino-2,2-dimethyl-4-phenyl-l,3-dioxane derived imminium salt catalyst 66, and introduced two new analogues (Figure 26) for asymmetric epoxidation. 1-Phenyl cyclohexene and 1-phenyl 3,4-dihydronaphthalene are epoxidized in up to 89% ee <2007JOC4424, 2007T5386>. 

Cyclopropenyl imminium salts can be prepared by the reaction of cyclopropenones with substituted ammonium saltsor by the reaction of ethoxycyclopropenyl salts with amines The ease of rotation about the carbon-nitrogen double bond shows that the positive charge is not located solely on the nitrogen as in 67 but is shared with the cyclopropene ring as in 67a . 

The reaction sequence 73a -> 77 (Scheme 2) was claimed as a new route to imminium salts, and the first entry into transannular reactions of secondary amines. Compounds 76a (R = COPh) and 76b (R = CH2Ph), can also... 

Formylation of 4,6,8-trimethylazulene occurs at the 1-position and the isolated imminium salt is readily cyclized with base (Scheme 2).9 Ring-activated styrenes such as 2 undergo side-chain formylation with subsequent... 

The exploitation of the full potential of a,P-unsaturated aldehydes for synthetic purposes is often handicapped by the complications reported above. These disadvantages can be overcome when the unprotected a,P-unsaturated aldehydes are replaced by oc,p-unsaturated aldimines, which are activated as imminium salts (Scheme 5.24). ... 

Their main findings were (1) The actual active catalyst is the (bis)phosphine Ni complex. (2) For allylammonium salts the reaction proceeds by an associative rather than a dissociate mechanism involving coordination of the allylammonium cation to the metal center followed by oxidative addition of the C-N bond to Ni(0). The resulting NH, remains coordinated to Ni(II) in a pentacoordinated intermediate, and finally a phosphine is lost to generate the final product. The reaction of the imminium salts, on the other hand, follows a dissociative mechanism. 

Thiazolium salts also catalyze several reactions related to the benzoin condensation. An example is the reaction of aldehydes and imminium salts (509) to form a-amino ketones (510) (Equation... 

Langlois and coworkers [331, 339, 1008] proposed a,P-unsaturated oxa-zolines 9.70 bearing the bomane skeleton as dienophiles for asymmetric Diels-Alder reactions. Activation with trifluoroacetic anhydride generates N-acyi-imminium salts in situ. Their cycloadditions with a number of dienes take place with a high selectivity between -78°C and -15°C (Figure 9.37). However, the reaction with 2,3-dimethylbutadiene is not sdective. The removal of the chiral auxiliary is accomplished by sequential treatment of the products with aC00CH2Ph and dilute NaOH. 

The structure of 7a-(3-pyrrolidinyl)-hexahydro-l//-pyr-rolizine (8) can be conceptualized as a hybrid between the individual enantiomers of nicotine. Pyrrolizidine 8 was synthesized in straightforward fashion by addition of the pyridyl lithium anion to the readily prepared imminium salt (Eq. (1)) (Wasicak et al., 1998). Compound 8 displays subnanomolar binding affinity (K = 0.32 nM) for a4 2 receptors, which is a 3-fold increase in binding affinity over (S)-nicotine (1) and a 60-fold increase relative to the less potent (/ )-nicotine enantiomer. One advantage with heterocyclic substituted pyrrolizidines series over their pyrrolidine counterparts is that no stereochemical issues come into play since they are symmetrical molecules. 

An isolable imminium salt 3 for a-methylenation of carbonyl compounds, analogous to the Mannich reaction (see 1st edition). 

The imminium salt (10) gave the oxaziridinium salt (11) on-treatment with p-nitroperbenzoic acid, but was inert to hydrogen... 

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