Lewis acids thermolysis reactions

Danheiser et al. [133] reported a variety of intramolecular [4 + 2]-cycloadditions of a butenyne subunit with a remote acetylene moiety by thermolysis of the substrates, with the best yields being obtained in the presence of phenolic additives. Two examples are presented in Scheme 6.49. Of particular significance with regard to synthetic utility is the observation that protic and Lewis acids were powerful promoters of these reactions. The intermediacy of 1,2,4-cyclohexatriene derivatives, as shown in Scheme 6.49, is highly likely, at least in the non-catalyzed cases. 

Some unusual reactions have been described for 2-(4-chlorophenyl)-2-(3,3-dimethylallyl)-4-phenyl-5(277)-oxazolone 84. This compound undergoes a Lewis acid-catalyzed rearrangement to give a tetrahydrofuropyrrole 85. On the other hand, depending on the reaction conditions, thermolysis of 84 produces the azabicyclohexene 86 or a substituted 2,3-dihydropyridine 87 together with the caged compound 88 formed by dimerization of the 2,3-dihydropyridine and the azabicyclohexene (Scheme 7.21). " ... 

Arenediazonium hexafluorophosphates are less soluble than the corresponding fluoroborates, which permits more thorough washing to remove impurities without greatly affecting the yields. Phosphorus pentafluoride which is eliminated upon thermolysis is not as strong a Lewis acid as boron trifluoride and thus side reactions are minimized. 

The thermal, photochemical and Lewis acid catalysed dissociations of di-azabicyclo[2.2.1]heptenes with side chains of various lengths terminated by aldehyde groups 143 have also been examined [119] (Scheme 31). The course of the reaction depended on the length of the tether. For the precursor with n = 1, tetrahydroindenol 144 was the main isolated product. Thermolysis of... 

Aminoarsine-borane adducts of the type R3 As(NR2 ) (BH3) (R = alkyl n= 1-3 m= 1,2) are stable at low temperatures, but on warming to the ambient undergo thermolysis. The thermolytic products are complex because of side reactions, but among the significant primary breakdown products are reduced arsenic species, e.g. R2ASH. The theoretical interest in these complexes derives from which of the alternative Lewis base sites, arsenic or nitrogen, forms the adduct with the Lewis acid BH3. Krannich and coworkers have investigated such species mainly by variable-temperature multinuclear NMR spectroscopy. 

The Claisen rearrangement can be effectively catalyzed by Lewis acids, Bronsted acids, bases, Rh(I) and Pt(0) complexes as well as by silica . Several reviews were published recently in which the application of zeolites and acid-treated clays as catalysts for the Claisen rearrangement was described Thus, it was shown that the rearrangement conditions for phenolic allyl ethers can be dramatically milder if this reaction is carried out by thermolysis of a substrate immobilized on the surface of previously annealed silica gel for chromatography. For example, the thermolysis of ether 159 on silica gel (in a 159 Si02 ratio of 1 10 w/w) at 70°C gives the phenol 160 in 95% yield after 3.5 hours (equation 70). An additional example is shown in equation 71. ... 

French workers have extensively explored type I intramolecular ene reactions of allenic aldehydes which give dienols. Thermolysis of aldehydes and ketones such as (158) affords ene adducts (159 42%, 150 C, 30 min), (160 75%, 150 "C, 60 min), (161 70%, 200 C, 15 min), (162 52%, 200 C, 60 min), (163 65%, 230 °C, 60 min) and (164). Lewis acid catalysis was investigated with limited success (Scheme 25). Comparison of yields and reaction times indicates that buttressing gem methyl groups and trisubstituted allenes facilitate the reaction, while use of ketones and formation of six- rather than five-membered rings retard it. 

The reports of simple a,j3-unsaturated aldehydes, ketones, or esters participating as 4tt oxabutadiene components of Diels-Alder reactions are limited. Cyclization of aldehyde 7 in the presence of Lewis acid catalysts provides the bridged [4 + 2] adduct 8 exclusively and the cis-enal 9 is considered to be the reactive intermediate. The exclusive formation of the bridged product has been attributed to electronic effects.131 In contrast, mild thermolysis of aldehyde 7 in the gas phase provided the fused [4 + 2] cycloadduct 10, albeit in low yield, and subjecting 7 to more vigorous thermolysis conditions provided mixtures of 8 and 10 [Eq. (34)]131 and intramolecular ene reaction products. 

Another route from five-membered 0-heterocycles to oxepines uses 2,3-dihydrofurans as starting materials and involves their [2 -I- 2] cycloaddition reaction with ethyne or ethylene compounds, followed by cleavage of bicyclic compounds formed by thermolysis or Lewis acid catalysis <83CB1691, 87TL1501,92JOC5102). These transformations are presented in Scheme 29 by starting from dimethyl acetylenedicarboxylate and 2,3-dihydrofuran or its 5-substituted derivatives <87TL1501>. 

Poly (alky 1/aryloxothiazenes), [N=S(0)R]n, have been prepared by the thermolysis of N-silylsulfonimidates, [Me3SiN=S(0)R (OR")]. The thermal condensation is catalyzed by Lewis acids such as Bp3.Et20, AICI3 or by bases such as F" and the phenoxide ion (Fig. 5.22) [29]. These polymers can also be prepared by using an alternative pathway. Thus, the N-silylsulfonimidates can be desilylated to the corresponding free sulfonimi-dates [HN=S(0)R (OR")]. The latter can be converted to the polymers, also by a thermolysis reaction (Fig. 5.22). 

Decomposition of aryl azides in the presence of Lewis acids does not involve arylnitrenes arylnitrenium-Lewis acid complexes are much more likely intermediates. The reaction of phenyl azide with aluminum chloride and aromatic substrates gives fair yields of diarylamines. This reaction is not undergone by phenyl azide alone on thermolysis or photolysis in aromatic solvents (Section II.l.A) ... 

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