Addition and Cycloaddition Reactions of Arenediazonium Ions with 1,3‐Dienes: A Shift From a Concerted to a Stepwise Mechanism

Abstract

Arenediazonium ions 1 undergo [2+ + 41 cycloadditions with (E)-1,3-pentadiene (2a), 2,3-dimethylbutadiene (2b), and (E)-2-methyl-l,3-pentadiene (2c) to give dihydropyridazines or pyridazinium salts. While highly electrophilic diazonium ions and the unsymmetrical dienes 2a and 2c predominantly yield those regioisomers that are expected for a stepwise cycloaddition process, the opposite regioselectivity is found in the cycloadducts of less electrophilic diazonium ions. Kinetic investigations and product studies indicate that all cycloaddition reactions proceed concertedly. The dienes 2d and 2e, in contrast, undergo ordinary azo coupling reactions in acetonitrile/methanol with formation of the hydrazones 17 and the azo compounds 18, respectively. It is demonstrated that the linear free enthalpy relationship lgk = s ( N + E ) can also be used to roughly estimate the rates of ionic cycloaddition reactions.The first report on the reactions of arenediazonium ions with l,3-dienes was published in 1919 by K. H. In the course of our efforts to quantify the electrophilicities of diazonium ionsL4], we had studied the kinetics and products of the reactions of diazonium ions with 1,3-dienes and were surprised by the observation that the reaction of the 2,4-dinitrobenzenediazonium ion (la) with (E)-l,3-pentadiene (2a) predominantly yielded a product with opposite regioselectivity to that reported for less electrophilic diazonium ions by Carlson and Huisgen[*-']. We, therefore, set out to systematically investigate the mechanisms of these reactions by a combined kinetic and product study. Reaction Products 2,3-Diinethyl-l,3-butudiene (2b)This compound has previously been reported to give the 1,6-dihydropyridazine 6d with the p-nitrobenzenediazonium ion (Id), whereas the parent 11 and the halogeno substituted benzenediazonium ions were found to yield the pyridazinium salts 7 (Scheme 3)r21.Since the diene 2b cannot give rise to the formation of regioisomeric products, we have not performed further preparative investigations on the reactions of diazonium ions with this substrate.( E ) und (Z)-1,3-Pentadiene (2a, 2a') Stirring of a suspension of 2,4-dinitrobenzenediazonium tetrafluoroborate (1 a-BF;) in acetonitrile with two equivalents of 2a for five minutes gave a yellow product which was recrystallized from methanol. Its 'H-NMR spectrum showed beneath a singlet at 6 = 2.01 for 4a, a doublet at 6 = 1.32 in accordance with structure 5a. Since

Synthetic Route to Phenyl Diazenes and Pyridazinium Salts from Phenylazosulfonates

“…The regioselectivity of the reactions with the mono-substituted furans 7b,c could be confirmed by comparison of the 1 H NMR spectra with literature data , and by selective nuclear Overhauser effect (NOE) experiments (see the Supporting Information). To distinguish between the different counter ions resulting from conditions A and B , the pyridazinium methanesulfonates are further on labeled as 1′ and the trifluoroacetates as 1″ .…”

“…This is however not astonishing taking into account their yet limited preparative accessibility. Early routes to pyridazinium salts of type 1 were reported by Carlson, Hartnagel, and Himmelspach, either starting from diazonium salts 2 or phenylhydrazines 3 (Scheme ). While the methodology by Hartnagel and Carlson was only applied to a few 1,3-dienes 4 , the strategy by Himmelspach suffers from the drawback that furfuryl alcohol 5 or 1,4-dicarbonyl derivatives are required as precursors, which significantly limits the product scope.…”

One-Pot Assembly and Synthetic Applications of Geminal Acyl/Alkoxy Tetrasubstituted Allenes

“…Hydrazine, after condensation to the hydrazone, proceeded to form the pyrazole 1,5-cyclization product 20 , isolated as a mixture of 20 sans benzyloxy fragment and with the benzyloxy intact at the 4-position (85:15 ratio). On the other hand, the analogous N -tosyl hydrazone underwent 1,6-cyclization to form the corresponding 1,5-dihydropyridazine 21 with the benzyloxy group intact; this type of 5-substituted heterocycle is uncommon in the literature. − Finally, Johnson–Corey–Chaykovsky conditions were used to achieve the epoxidation product 22 . , …”

Aryl diazonium salts and benzene derivatives bearing two electron-donor substituents: chemical and mechanistic behaviours

“…27 Unlike 1,3,5-tris(N,N-dialkylamino)-benzene derivatives, just a few examples are reported for the electrophilic aromatic substitution involving 1,3-N,N-dialkylamino benzenes. [28][29][30][31] Based on these, we decided to study the azo-coupling reaction between these nucleophiles and para-substituted benzenediazonium salts to possibly obtain new compounds of interest for application in elds such as optoelectronics 32,33 and dyes 34 together with further mechanistic information on this kind of reaction.…”

“…Unlike 1,3,5-tris( N , N -dialkylamino)-benzene derivatives, just a few examples are reported for the electrophilic aromatic substitution involving 1,3- N , N -dialkylamino benzenes. 28–31…”