Azadiene cationic

Keywords imine, iminium cations, nitriles, nitroso compounds, azo compounds, azadienes, nitroso alkenes... 

Ghosez and co-workers (223) illustrated that azadienes could be efficiently used as heterodienes with acryloylimide dienophiles in the presence of chiral cationic Cu(II) catalysts, Eq. 183. The reaction was found to be highly exo- and enantiose-lective, provided the azadiene contains an aryl or vinyl group in the 1-position. Acryloylimide as dienophile affords equal enantioselectivities albeit reduced dias-tereoselectivities (typically 6 1). 

The iron-mediated construction of the carbazole framework proceeds via consecutive C-C and C-N bond formation as key steps [70,71]. The C-C bond formation is achieved by electrophilic substitution of the arylamine with a tricarbonyliron-coordinated cyclohexadienyl cation. The parent iron complex salt for electrophilic substitutions, tricarbonyl[/j -cyclohexadienylium] iron tetrafluoroborate 6a, is readily available by azadiene-catalyzed complexation and subsequent hydride abstraction (Scheme 9). 

Additional examples of these novel SET-promoted photorearrangement reactions were uncovered in our investigations with the 2-azadienes 78 and 82. These substances yield the corresponding A-vinylaziridines 93 and 94, respectively, upon DCA-sensitized irradiation (Structures 93-99). The study was extended to azadiene 73. The DCA-sensitized irradiation of 73 yields the aziridine 95 and the dihydroisoindole 96 [65]. However, under these conditions, 2-azadiene 97 affords, in addition to the expected aziridine 98, the dihydroisoquinoline 99. The formation of the isoindole 96 can be explain by a pathway in which a radical-cation centered on the C—double bond attacks the phenyl ring at C-3, as shown in Scheme 15. The formation of the dihydroisoquinoline 99 can be justified by... 

The influence of diphenyl substitution at C-1 of the 2-azadiene skeleton has been investigated. However, when compounds 84 and 85 are irradiated under the above conditions, alternative reactions occur to afford the corresponding dihy-drobenzoazepines lOOa-lOOb in good yields. The formation of 100 can by justified by attack of a radical-cation centered on the C—double bond onto the phenyl ring present at C-5, as shown in Scheme 17 [65]. 

The use of a range of modified reaction conditions including the use of protic acids or conventional and nonconventional Lewis acid catalysts, pressure-promoted reaction conditions, - cation-radical catalysts, and dry-state adsorption reaction conditions has been employed to accelerate the 4it participation of sensitive heterodienes in thermally slow or problematic Diels-Alder reactions. The former two techniques have proven useful for promoting the typically poor reactions of simple, unactivated 1-oxa-1,3-butadienes or acyclic azadienes. 

The preparation or in situ generation of azabutadienes bearing a formal positive charge, i.e. an im-monium cation, provides a substantial enhancement of the electron-deficient character of the azadiene and in many instances such systems have proven to be effective 4rr components in Ehels-Alder reactions with electron-rich or neutral dienophiles. The most widely recognized class of cationic azadienes shown... 

The acid-catalyzed or Lewis acid-catalyzed (TiCU, BF3-OEt2 -78 C) in situ generation of A -alkyl-aiylimmonium ions in the presence of electron-rich or neutral alkenes has been shown to provide [4 + 2] cycloadducts (IV-methyltetrahydroquinolines) in excellent yields (Scheme 27). More recent efforts have demonstrated that immonium ions derived from the cmidensation of aryl amines and aldehydes in the presence of cyclopentadiene participate as effective 4 ir components of [4 + 2] cycloadditions and complement their demonstrated 2 ir participation in related [4 + 2 ] cycloaddition reactions (Scheme 27). Additional examples of the participation of cationic azadienes in [4 + 2] cycloaddition reactions... 

A select set of cationic heteroaromatic azadienes including acridizi-nium, isoquinolinium, quinolinium, and isoxazolium salts have been shown to participate as useful 47t components of [4+ + 2] cycloadditions. These systems have been discussed in Chapter 9, Section 10. 

Related studies employing (V-protonated vinyl-165 or aryliminium salts,64 161 Lewis salts of aryl-63a and vinylimines,63b 1,3,4-oxadiazolium salts,166 and nitrilium salts167 constitute additional examples of reaction processes proceeding through cationic azadienes which formally participate in [4+ + 2] cycloaddition reactions.168... 

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