Thermal Uncatalyzed Reactions

In contrast to thermal uncatalyzed reactions of /V-acylsulfinylamines with aryl isocyanates which give rise to azoarenes,188 the cobalt or iron carbonyl-catalyzed process gives additionally 3,5-dioxo-l,2,4-triphenyl-1,2,4-triazolidine (Scheme 124).189 The only possible restriction on this simple urazole synthesis would be the expectation that the substituents on the reactants must be the same to prohibit exchange. 

Desimoni and coworkers84 probed the catalytic effect of metal perchlorate salts on the rate of the Diels-Alder reactions between malonates 88 and cyclopentadiene (equation 27). They found that especially magnesium perchlorate was able to catalyze the reaction by binding two malonates in a bidentate fashion. Reaction times were shortened up to 1000 times. The endo/exo selectivity was inverted from 89/90 = 40/60 (n =4) and 17/83 (n = 5) for the thermal uncatalyzed reactions to 89/90 = 60/40 (n = 4) and 80/20 (n = 5) for the magnesium perchlorate catalyzed reactions. 

The most important consequence of the change in mechanism in organometallic catalysts is that the regiochemical, stereochemical, or selectivity outcome can be completely different (and possibly even tunable) in the catalyzed case. This is a very great advantage in organic synthesis, where the normal rules can be inverted. For example, hydroboration can be made to go in a Markovnikov manner, inverting the selectivity seen for the thermal (uncatalyzed) reaction. 

Some diazoalkanes cyclopropanate olefins in the absence of any catalyst [658-660]. Thus, for instance, upon generation from A -cyclopropyl-A -nitrosourea at 0 °C diazocyclopropane spontaneously cyclopropanates methylenecyclopropanes [658]. Thermal, uncatalyzed cyclopropanations of unactivated olefines with aryldiazome-thanes can already occur at only slightly elevated temperatures (e.g. at 80 °C with 1-naphthyldiazomethane [661]). Henee, for enantioselective cyclopropanations with a chiral catalyst, low reaction temperatures should be chosen to minimize product formation via the uncatalyzed pathway. 

Interestingly, the uncatalyzed reaction takes place at 80°C and is a rare case of a thermal [2+2] cycloaddition. However, the catalytic process offers advantages in terms of higher cis/trans ratios. It should be noted that the turns isomer can be isomerized into the thermodynamically favored cis product by extended heating to 80°C in acetonitrile. The utility of the products was demonstrated by the synthesis of amino acid 20 (Scheme 2.6). 

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... 

Different chitosan-based catalysts are compared in Table 3. The reactions were performed at 70°C due to the thermal stability of the catalyst. The first ones focus on the influence of the drying procedure in comparison with an uncatalyzed reaction and a known heterogeneous catalyst (Table 3, entries 1-5). Lyophilized chitosan (Cl) does not display any activity when the aerogel is as efficient as the functionalised silica (C3). This result illustrates the accessibility to the amino groups of chitosan in its aerogel form. 

If these reactions occur in uncatalyzed processes where bond breaking and bond formation are taking place concertedly, and not in two-step pathways via ionic or diradical intermediates, then the stereochemistry of these sigmatropic shifts can be predicted in a qualitative manner 1 -4. According to the Woodward-Hoffmann rules the thermally allowed reaction should take place in an antarafacial fashion across the allylic framework. The shifting hydrogen has to move from one side of the allylic plane to the other as depicted below. 

A large amount of work has been done on the thermal (uncatalyzed) oxidation of acetylene. Since it did not seem practical to use this method for the present purpose because of the relatively high temperatures involved, and since its mechanism is apparently quite different from that of the heterogeneous reaction, it will not be discussed here. It is worth noting, however, that the catalytic process seems to be the simpler of the two in regard to the products of reaction though peroxide formation with the eventual production of rather complex molecules—... 

Catalyzed Diels-Alder Reactions. The uncatalyzed thermal intramolecular Diels-Alder reaction of 5,5 -oxyhis[( )-l,3-penta-diene] nonstereoselectively generates four isomeric 4-vinylcyclo-hexenes (eq 37). The major product has a trails ring fusion, in contrast to the single cis ring-fused isomer generated in the cop-per(I) triflate-catalyzed photoreaction of the same tetraene (eq 25). Copper(I) triflate also catalyzes a thermal Diels-Alder reaction of 5,5 -oxybis[( )-l,3-pentadiene] that proceeds under milder conditions than the uncatalyzed reaction. The stereoselectivity is remarkably enhanced, generating mainly the major isomer of the uncatalyzed thermal reaction and a single c/s-fused isomer (eq 37) that is different than the one favored in the photochemical reaction (eq 25). 

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