Aminium cation radicals Diels-Alder reactions

The first studies on cation-radical Diels-Alder reactions were undertaken by Bauld in 1981 who showed [33a] the powerful catalytic effect of aminium cation radical salts on certain Diels-Alder cycloadditions. For example, the reaction of 1,3-cyclohexadiene with trans, iraw5-2,4-hexadiene in the presence of Ar3N is complete in 1 h and gives only the endo adduct (Equation 1.14) [33]. 

The aminium salt catalyzed cation radical Diels-Alder reaction... 

This dichotomy has been rigorously established by both theoretical methods (semi-empirical and ab initio calculations) and by experimental measurements for both cation radical and carbocation formation (Scheme 32) [66]. The kinetics of aminium salt-induced cation radical Diels-Alder reactions of these substrates were then studied, and an excellent correlation with the Hammett a values was observed for both the enol ethers and the enol sulfides. The reactions of the aryl propenyl ethers involved 2,3-dimethyl-l,3-butadiene as the dienic component [64], while the reactions of the aryl vinyl sulfides involved cyclopentadiene as the dienic component (Scheme 33) [65]. 

The cation-radical Diels-Alder reactions of cis- and fran5 -l,2-(diaryloxy)ethenes with butadienes are stereospecific, in agreement with a concerted cycloaddition mechanism. " Tris(4-bromophenyl)aminium hexachloroantimonate catalyses the two-step, non-stereospecific cation-radical Diels-Alder reaction of cis- and traui-prop-l-enyl aryl ethers with cyclopenta-1,3-diene in CH2CI2 solution. 

Bauld and coworkers, especially, developed the analogous Diels-Alder (4 + 2) cycloaddition reactions. These reactions are conveniently catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (78) or by photosensitization with aromatic nitriles. The radical cation-catalyzed Diels-Alder reaction is far faster than the uncatalyzed one, and leads to some selectivity for attack at the least substituted double bond for the monoene component (Scheme 18, 79 —> 80), but only modest endo selectivity (e- and x-80) [105]. Cross reactions with two dienes proved to be notably less sensitive to inhibition by steric hindrance of alkyl groups substituted on the double bonds than the uncatalyzed reactions, as cyclohexadiene adds detectably even to the trisubstituted double bond of 2-methylhexadiene (82), producing both 83 and 84. Dienes such as 85 react with donor-substituted olefins (86) to principally give the vinylcyclobutene products 87, but they may be thermally rearranged to the cyclohexene product 88 in good yield [105]. Schmittel and coworkers have studied the cation radical catalyzed Diels-Alder addition of both... 

Luche and coworkers [34] investigated the mechanistic aspects of Diels-Alder reactions of anthracene with either 1,4-benzoquinone or maleic anhydride. The cycloaddition of anthracene with maleic anhydride in DCM is slow under US irradiation in the presence or absence of 5% tris (p-bromophenyl) aminium hexachloroantimonate (the classical Bauld monoelectronic oxidant, TBPA), whereas the Diels Alder reaction of 1,4-benzoquinone with anthracene in DCM under US irradiation at 80 °C is slow in the absence of 5 % TBPA but proceeds very quickly and with high yield at 25 °C in the presence of TBPA. This last cycloaddition is also strongly accelerated when carried out under stirring solely at 0°C with 1% FeCh. The US-promoted Diels Alder reaction in the presence of TBPA has been justified by hypothesizing a mechanism via radical-cation of diene, which is operative if the electronic affinity of dienophile is not too weak. 

Cycloadditions can also be catalyzed by cation radicals generated from aminium salts.537-539 In fact, these are cation-radical chain reactions mimicking Diels-Alder reaction. Interestingly, 2,4-dimethyl-1,3-pentadiene undergoes either a Brpnsted acid-catalyzed cycloaddition [Eq. (6.93)] or cation-radical-catalyzed cycloaddition [Eq. (6.94)] to yield different addition products 539... 

This type of reaction can be induced also by radiolysis [133,134] or by chemical oxidation, particularly with tris-(p-bromophenyl)aminium salts (cation radical catalyzed Diels-Alder reaction) [10]. The scope of this reaction and its synthetic utility have been delineated in detail. The results unambiguously support a free radical cation chain mechanism [10]. 

Gassman first observed the intermediacy of allyl cations as dienophiles in connection with efforts to extend the aminium cation-radical catalyzed Diels-Alder reaction to intramolecular cases. It was found that the cyclization of tetraene (26) performed in the presence of 0.2 equiv. of tris(p-bromophe-nyl)aminium hexachloroantimonate [ArsN+ CleSb ] was in fact catalyzed by protic acid. - A better means of catalyzing the cyclization of (26) involves use of 4 mol % trifluoromethanesulfonic acid at -23 C for 6 min, which provides the trans-fused cycloadduct in 88% yield (98% isomeric purity). Similar treatment of (27), however, provided a mixture of cis- and trans-fused products. A subsequent report revealed that allylic alcohols and allylic ethers are useful precursors to the allyl cation dienophiles with... 

Scheme 5.4 Tris(p-bromophenyl)aminium hexachloroantimonate (4 SbCh ) initiated radical cation chain reaction of phenylvinylsulfide (5) and cyclopentadiene (6) to give the Diels-Alder product 5-(phenylthio)norbornene (7) via the reactive intermediates 5+" and T . ...

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