The kinetics of the reactions of benzhydryl cations with eight diazo compounds 1 a-g were investigated photometrically in dichloromethane. The nucleophilicity parameters N and slope parameters s of these diazo compounds were derived from the equation log k (20 degrees C)=s (E+N) and compared with the nucleophilicities of other pi systems (alkenes, arenes, silyl enol ethers, silyl ketene acetals). It is shown that the nucleophilic reactivities of diazo compounds cover more than ten orders of magnitude, being comparable to that of styrene on the low reactivity end and to that of enamines on the high reactivity end. The rate-determining step of these reactions is the electrophilic attack at the diazo-carbon atom to yield diazonium ions, which rapidly lose nitrogen.
The kinetics of the reactions of arenediazonium ions with arenes, alkenes, allylsilanes, allylstannanes, and silyl ketene acetals have been studied in acetonitrile solution. The reactions follow second-order kinetics, and in several cases ratedetermining attack of the diazonium ion has been proven by kinetic isotope effect studies ( l a + 2b), by the independence of the allylsilane reactivities of the rate of desilylation ( l a + 10a, d) and by the independence of the rate constants of the diazonium counterion. A decrease of the rate constant with increasing solvent donor ability (correlation with Gutmann's donor number) was found. The reactions of diazonium ions with n-nucleophiles roughly follow the correlation lgk (20°C) = s ( E + N), previously derived for the reactions of carbocations with nucleophiles. With the E parameters derived for diazonium ions, rate constants for azo couplings with aromatic and nonaromatic n-nucleophiles can be predicted with an accuracy of =lo2. On the basis of E, the electrophilic reactivities of diazonium ions can be compared with those of carbocations (Figure 9), and the combination with the nucleophilicity parameters N ( Figure 10) gives a first clue on possible azo coupling reactions. Literature reports are discussed within this scheme."Diazonium ions are weak electrophiles and undergo azo couplings only with activated arenes." Statements of that type can be found in most textbooks of organic chemistry, and the question arises whether it is possible to quantify the electrophilic power of diazonium ions in order to make predictions of potential reaction partners. Since Ritchie reported that the reactions of diazonium ions and highly stabilized carbocations with OH-, CN-, N, and related nnucleophiles follow the same constant selectivity relationships"] we were prompted to investigate whether the linear free energy relationships found for the reactions of carbocations with n-nucleophiles[21 also hold for the corresponding reactions of diazonium ions.For this purpose, we have synthesized arenediazonium tetrafluoroborates of variable reactivity (Scheme 1) and studied the kinetics of their reactions with n-nucleophiles"].
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
The kinetics of the reactions of (E)-2-methyl-l,3-~entadiene lgk= s (N+ E ) yielded the nucleophilicity parameters N for (2a), 4-methyl-l,3-pentadiene (2b), and (Z)-1,3-pentadiene these dienes. The influence of methyl groups on the nucle-(2c) with benzhydryl cations have been studied photoophilic reactivities of 1,3-butadienes is discussed, and a sysmetrically in dichloromethane solution. Substitution of tem to estimate reactivities of further alkylated 1,3-dienes is these rate constants into the linear free enthalpy relationship suggested. In order to differentiate stepwise and concerted cycloaddition mechanisms, a knowledge of substituent effects on the one-bond nucleophilicities of 1,3-dienes is needed. Scheme 1 In previous work we have shown that the linear free enthalpy relationship (eq. 1) describes the rates of combination reactions of electrophiles with nucleophiles where only one new o-bond is formed in the rate-determining step. In this equation E represents the reactivities of the electrophiles, while nucleophiles are characterized by the nucleophilicity parameter N and the slope parameter sL5].lgk (20°C) = s ( N + E )(1)As shown in Figure 1, s is equal for different types of nnucleophiles with terminal CH2 groups, i.e., these reactions follow so-called constant selectivity relationships [6] where the relative reactivities of two n-systems are independent of the nature of the reference electrophile. We have now used this relationship to determine the influence of methyl groups on the one-bond nucleophilicities of methylated 1.3-butadienes. E Reactions ProductsWhile the Lewis acid catalyzed reactions of benzhydryl chlorides with 1,3-butadiene, isoprene, piperylene, or 2,3-dimethyl-l,3-butadiene can be terminated at the 1 : 1 product stage when either ZnC12 or BC13 are the BC13 initiated reactions of benzhydryl chlorides with (E)-2-methyl-1,3-pentadiene (2a) and 4-methyl-l,3-pentadiene (2b) do not yield 1 : 1 products but give rise to the formation of polymers. In contrast to the other dienes listed above, which yield terminally mono-and dialkylated allyl cations upon electrophilic attack, compounds 2a and 2b yield terminally trialkyl substituted allyl cations (3a, 3b) (Scheme 2). Since such cations are not efficiently intercepted by
Diazocyclopentadiene reacts with benzhydrylium ions (Ar2CH+) to give 2,5-dibenzhydryl-substituted diazocyclopentadienes. The kinetics have been determined photometrically in dichloromethane under pseudo-first-order conditions using diazocyclopentadiene in excess. Plots of the second-order rate constants (log k 2) versus the electrophilicity parameters E of the benzhydrylium ions gave the nucleophilicity parameter N = 4.84 and susceptibility s N = 1.06 for diazocyclopentadiene according to the correlation log k(20 °C) = s N(E + N). Diazocyclopentadiene thus has a similar nucleophilic reactivity as pyrrole. Previously reported electrophilic substitutions of diazocyclopentadiene are rationalized by these parameters and new reaction possibilities are predicted.
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