ARTICLE pubs.acs.org/joc
Hydrogen Atom Abstraction Reactions from Tertiary Amines by Benzyloxyl and Cumyloxyl Radicals: Influence of Structure on the Rate-Determining Formation of a Hydrogen-Bonded Prereaction Complex Michela Salamone,† Gino A. DiLabio,‡ and Massimo Bietti*,† † ‡
Dipartimento di Scienze e Tecnologie Chimiche, Universita “Tor Vergata”, Via della Ricerca Scientifica, 1 I-00133 Rome, Italy National Institute for Nanotechnology, National Research Council of Canada, 11421 Saskatchewan Drive, Edmonton, AB, Canada T6G 2M9
bS Supporting Information ABSTRACT: A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO•) and benzyloxyl (BnO•) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (kH) were measured for the two radicals (kH(BnO•)/kH(CumO•) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in kH(BnO•)/ kH(CumO•) ratios of 13 to 2027 times were observed. kH approaches the diffusion limit in the reactions between BnO• and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a CH/N hydrogen-bonded prereaction complex between the benzyloxyl R-CH and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD2O•, BnO•-d2) and the 3,5-di-tert-butylbenzyloxyl radical.
’ INTRODUCTION Hydrogen atom abstraction reactions by alkoxyl radicals play an important role in a variety of biological and chemical processes,110 and accordingly a large number of studies have been devoted to the mechanistic investigation of these processes.1126 However, very limited information is available on the role of radical structure on these reactions.11,27 In order to develop our understanding of these processes, we recently carried out a timeresolved kinetic and computational study on the hydrogen abstraction reactions from tertiary amines by the cumyloxyl (PhC(CH3)2O•, CumO•) and benzyloxyl (PhCH2O•, BnO•) radicals.28 Large differences in reactivity between the two radicals were measured. With triethylamine (TEA) a large increase in the rate constant for hydrogen atom abstraction (kH) was observed on going from CumO• to BnO• (kH(BnO•)/kH(CumO•) = 21.5), and a dramatic 3 orders of magnitude increase in kH was observed for the corresponding reactions of the two radicals with 1,4-diazabicyclo[2,2,2]octane (DABCO). On the other hand, comparable reactivities were found for the two radicals in their reactions with triisobutylamine (TIBA), viz., kH(BnO•)/kH(CumO•) = 2.8. These results were explained in terms of the rate-determining formation of a hydrogen-bonded prereaction complex between the relatively acidic BnO• R-CH29 and the r 2011 American Chemical Society
amine lone pair (Scheme 1, path a: R = PhCH2) wherein fast hydrogen atom abstraction occurs (path b). Efficient complex formation is possible only for relatively unhindered amines such as TEA and DABCO. With TIBA, steric hindrance in proximity of the nitrogen lone pair prevents complex formation, and its reaction with BnO• was described as a direct hydrogen atom abstraction (Scheme 1, path c), i.e., a reaction that proceeds through the direct interaction of the radical center with the abstractable hydrogen atom, with no significant prereaction complex formation. With CumO• the presence of R-methyl groups prevents hydrogen bond formation, and its reactions with TEA, TIBA, and DABCO were also described as direct hydrogen atom abstractions (path c). In view of the relevance of these processes and to develop a deeper mechanistic understanding of the role of structural effects in both the abstracting radical and the substrate, we have extended our study to other tertiary amines and diamines, namely, tripropylamine (TPA), triallylamine (TAA), 1-azabicyclo[2,2,2]octane (ABCO), 1,4-dimethylpiperazine (DMP), and 1,2,2,6,6-pentamethylpiperidine (PMP), whose structures are Received: May 20, 2011 Published: June 29, 2011 6264
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Table 1. Second-Order Rate Constants (kH) for Reactions of Cumyloxyl (CumO•) and Benzyloxyl (BnO•) Radicals with Tertiary Amines kH (M1 s1)a kH(BnO•)/ substrate TEA
•
•
CumO
BnO
kH(CumO•)
2.0 ( 0.1 108b
4.3 ( 0.1 109c
21.5
2.9 ( 0.1 10
9.6 ( 0.1 109d
33.1
8d
1.6 108e
Chart 1 TEA-d15f
1.16 ( 0.02 108c 1.2 108e
4.28 ( 0.05 109c
37
TPA
2.3 ( 0.1 108
3.0 ( 0.1 109
13.0 52
2.1 10
8e
TAA
6.15 ( 0.06 107
3.2 ( 0.2 109
TIBA
1.27 ( 0.02 10
3.51 ( 0.05 108c
ABCO
3.7 106 e,g
7.5 ( 0.4 109
PMP
1.70 ( 0.02 108
4.26 ( 0.07 109
DMP
1.7 10 1.16 ( 0.04 108
8.0 ( 0.1 109
DABCO
9.6 10
1.05 ( 0.05 1010c
8c
2.8 2027 25
8e
6e,g
69 1094
Measured in N2-saturated MeCN solution at T = 25 °C by 266 nm LFP, [dicumyl peroxide] = 10 mM or [dibenzyl peroxide] = 8 mM. kH values were determined from the slope of the kobs vs [substrate] plots, where in turn kobs values were measured following the decay of the CumO• or BnO• visible absorption bands at 490 and 460 nm, respectively. Average of at least two determinations. b Reference 35. c Reference 28. d In N2-saturated isooctane solution. e Reference 17: 308 nm LFP at room temperature in MeCN solution, following the decay of CumO•. f Triethylamine-d15 g The relatively high concentrations of ABCO and DABCO required in these experiments prevented the determination of kH for their reaction with CumO• under the experimental conditions employed. Accordingly, the kH(BnO•)/kH(CumO•) ratios for ABCO and DABCO are based on the kH(CumO•) values given in ref.17 a
position (PhCD2O•, BnO•-d2) and with the 3,5-di-tert-butylbenzyloxyl radical (3,5-DTBBnO•) have also been studied. The reaction of 3,5-DTBBnO• with PMP was also studied.
Figure 1. Plots of the observed rate constant (kobs) against [1,4dimethylpiperazine] for the reactions of the cumyloxyl radical (CumO•, open circles) and benzyloxyl radical (BnO•, filled circles), measured in nitrogen-saturated MeCN solution at T = 25 °C by following the decay of CumO• and BnO• at 490 and 460 nm, respectively. From the linear regression analysis: CumO• + DMP, intercept = 7.31 105 s1, kH = 1.20 108 M1 s1, r2 = 0.9968; BnO• + DMP, intercept = 1.02 106 s1, kH = 7.93 109 M1 s1, r2 = 0.9982.
displayed in Chart 1 together with those for TEA, TIBA, and DABCO. To this end, we carried out a time-resolved kinetic study in acetonitrile solution on the R-CH abstraction reactions from these tertiary amines by BnO•, with comparisons to the more hindered CumO•. With TEA and TIBA, the corresponding reactions with the benzyloxyl radical deuterated in the benzylic
’ RESULTS The reactions of CumO•, BnO•, BnO•-d2, and 3,5-DTBBnO• with the tertiary amines shown in Chart 1 were studied using the laser flash photolysis (LFP) technique. The alkoxyl radicals were generated by 266 nm LFP of nitrogen-saturated MeCN solutions (T = 25 °C) containing the parent symmetric peroxides, as described in eq 1. hν
ROOR sf 2RO• 266nm
PhCðCH3 Þ2 R ¼ PhCH2 , PhCD2 ArCH2
ð1Þ
In MeCN solution, CumO• and BnO• are characterized by a broad absorption band in the visible region of the spectrum centered at 485 and 460 nm, respectively.31,32 No significant spectral difference was observed between BnO• and BnO•-d2, whereas with 3,5-DTBBnO• a ∼40 nm red shift in the position of the visible absorption band was observed (λmax = 500 nm), in line with expected electron-releasing effect of the two tert-butyl groups.31,32 The time-resolved absorption spectra observed after 6265
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Table 2. Second-Order Rate Constants (kH) for the Reactions of Benzyloxyl Radicals with Tertiary Amines
Scheme 2
kH (M1 s1)a substrate
BnO•
BnO•-d2
TIBA
3.51 ( 0.05 108 b
3.64 ( 0.05 108
TEA
4.3 ( 0.1 10
4.7 ( 0.1 10
PMP
4.26 ( 0.07 10
9b
3,5-DTBBnO• 9
9
3.46 ( 0.06 108 3.8 ( 0.1 109 3.60 ( 0.05 109
Measured in N2-saturated MeCN solution at T = 25 °C by 266 nm LFP, [dibenzyl peroxide] = 8 mM. kH values were determined from the slope of the kobs vs [substrate] plots, where in turn kobs values were measured following the decay of the benzyloxyl radicals visible absorption bands at 460500 nm. Average of at least two determinations. b Reference 28. a
266 nm LFP of bis(3,5-di-tert-butylbenzyl) peroxide is reported in the Supporting Information (Figure S1). Under the experimental conditions employed, CumO• decays mainly by CCH3 β-scission,21,32 whereas the decay of the benzyloxyl radicals can be mainly attributed to hydrogen atom abstraction from the solvent.30,33 A number of rate constants (kH) for the reactions of the tertbutoxyl radical (tBuO•) and/or CumO• with tertiary amines, measured under different experimental conditions, are available in the literature.14,17,25,34 It is well established that these reactions proceed by hydrogen atom abstraction from a CH2 and/or CH3 group of the substrate, as described in eq 2 (R0 = H, alkyl). kH
00 _ RO• þ R 0 CH2 NR 2 00 sf ROH þ R 0 CHNR 2
ð2Þ
Our kinetic studies were carried out by LFP in MeCN solution following the decay of the CumO• and benzyloxyl radicals (BnO•, BnO•-d2, and 3,5-DTBBnO•) visible absorption bands at 490 and 460500 nm, respectively, as a function of the amine concentration. The reactions of TEA with CumO• and BnO• were also studied in 2,2,4-trimethylpentane (isooctane) solution. The observed rate constants (kobs) gave excellent linear relationships when plotted against substrate concentration and provided the second-order rate constants for hydrogen atom abstraction from the substrates (kH) by the alkoxyl radicals. One example is provided in Figure 1, which shows the plots of kobs versus [DMP] for the reactions of this amine with CumO• (open circles) and BnO• (filled circles) for measurements carried out in MeCN solution at T = 25 °C. Additional plots for hydrogen atom abstraction reactions by CumO•, BnO•, BnO•-d2, and 3,5-DTBBnO• from the other amines are displayed in the Supporting Information (Figures S2S13). All kinetic data thus obtained are collected in Table 1 together with the pertinent kH(BnO•)/kH(CumO•) ratios. Also included in Table 1 are the rate constants obtained previously under analogous experimental conditions for the reactions of CumO• and BnO• with TEA, TEA-d15, TIBA, and DABCO,28 and the available literature kH values for the reactions with CumO•.17 Where comparisons are possible, our measured kH values for eq 2 for CumO• are in good agreement with the literature values. The hydrogen atom abstraction reactivities of BnO•, BnO•-d2, and 3,5-DTBBnO• with TEA and TIBA and those of BnO• and 3,5-DTBBnO• with PMP are compared in Table 2.
’ DISCUSSION Starting from the reactions of CumO•, the kinetic data collected in Table 1 show that ABCO and DABCO undergo hydrogen atom abstraction with rate constants that are significantly lower than those measured for the other amines. This behavior was described previously for the reactions of these substrates with both tBuO• and CumO•14a,17,25,28 and was discussed in terms of the operation of a stereoelectronic effect. Hydrogen atom abstraction is most rapid when the R-CH bond being broken can be eclipsed with the nitrogen lone pair. In DABCO and ABCO these bonds are locked in a conformation that reduces overlap, resulting in R-CH abstractions that occur with rate constants that are significantly lower than those measured for the corresponding reactions of conformationally free amines. The rate constant measured for the reaction of CumO• with TAA (kH = 6.15 107 M1 s1) is more than three times lower than those measured for the corresponding reactions of TEA and TPA (kH = 2.0 108 and 2.3 108 M1 s1, respectively), despite the significantly lower R-CH bond dissociation energy of the former amine as compared to the latter ones (BDE = 82.6 and 90.7 kcal mol1, for TAA and TEA, respectively).36 A similar behavior was observed previously by Lalevee34 and Tanko14 for the reactions of TAA and TEA with tBuO• and was explained in terms of an entropic control over these hydrogen abstraction reactions.14a TEA, TPA, TIBA, PMP, and DMP undergo hydrogen atom abstraction by CumO• with comparable rate constants (kH between 1.16 and 2.3 108 M1 s1). However, it is important to point out that PMP has three abstractable R-CH atoms as compared to the six abstractable R-CH atoms in the other four amines.37 The (statistically corrected) 3-fold increase in kH observed on going from DMP to PMP can be again explained in terms of the operation of stereoelectronic effects. Calculations (for details see the Supporting Information) indicate that in DMP rotamer b, where the exocyclic R-CH bond being broken is eclipsed with the nitrogen lone pair, is 3.2 kcal mol1 higher in energy as compared to the most stable rotamer a (Scheme 2). The analogous structures for PMP are separated by 1.5 kcal mol1, indicating that it is easier for PMP to orient its R-CH bond so that it is properly aligned with the nitrogen lone pair in the best suited conformation for hydrogen atom abstraction. An increase in kH was observed in the reaction between TEA and CumO• on going from MeCN to isooctane (kH = 2.0 108 and 2.9 108 M1 s1, respectively). A comparable kinetic solvent effect was measured previously in benzene and chlorobenzene solution (kH = 2.8 108 and 2.7 108 M1 s1, respectively).28 This behavior was explained in terms of a hydrogen-bond interaction between the nitrogen lone pair and the solvent that decreases the degree of overlap between the RCH bond and the nitrogen lone pair in the transition state for hydrogen atom abstraction. Solventsubstrate hydrogen bonding can take place only in MeCN that is a relatively weak 6266
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Figure 2. Representation of the hydrogen-bonded prereaction complex for the reaction between BnO• and DABCO. Key: carbon = cyan, nitrogen = blue, oxygen = red, hydrogen = white.
hydrogen-bond donor (HBD), as measured by Abraham’s R2H parameter (R2H = 0.09),38 and accordingly a decrease in reactivity is observed as compared to benzene, chlorobenzene, and isooctane for which R2H = 0.00. For the reactions of CumO• and BnO• with the amines, very different rate constants were measured. In all cases kH values are higher for BnO• than for CumO•, with kH(BnO•)/kH(CumO•) ratios that range from 2.8 for TIBA to 2027 for ABCO. The difference in reactivity between BnO• and CumO• with TIBA was recently explained by us in terms of a direct hydrogen atom abstraction reaction.28 Comparable rate constant ratios were also measured for other hydrogen atom abstraction reactions from carbon by these two radicals, such as that with 1,4-cyclohexadiene (kH(BnO•)/kH(CumO•) = 1.9)28 and with propanal and 2,2-dimethylpropanal (kH(BnO•)/kH(CumO•) = 1.2 and 1.6, respectively).11 The kH values measured for the reactions of BnO• with the other amines are in all cases at least 1 order of magnitude higher than those measured for the corresponding reactions of CumO•, being close to the diffusion limit (kd = 2.0 1010 M1 s1, in MeCN)39 with the cyclic and bicyclic amines and diamines ABCO, DABCO, and DMP (kH = 7.5 109, 1.05 1010, and 8.0 109 M1 s1, respectively). The significantly larger kH(BnO•)/kH(CumO•) ratios observed for ABCO and DABCO (kH(BnO•)/kH(CumO•) = 2027 and 1094, respectively) as compared to DMP (kH(BnO•)/ kH(CumO•) = 69) and to the acyclic amines, can be predominantly ascribed to the depressed reactivity of the bicyclic amines in their reactions with CumO•, where as mentioned above, a stereoelectronic effect operates. This effect is clearly not present in the reactions of ABCO and DABCO with BnO•,28 an observation that indicates that in hydrogen atom abstractions from these substrates the cleavage of the R-CH bond does not contribute to the rate-determining step (see below). These observations are in line with our previous mechanistic hypothesis where this behavior was rationalized in terms of the rate-determining formation of a prereaction complex between BnO• and the substrate, wherein hydrogen atom abstraction occurs. The results of computational modeling pointed toward the formation of a relatively stable complex where the amine lone pair is hydrogen-bonded to the acidic benzyloxyl R-CH, as shown in Figure 2 for the reaction between BnO• and DABCO.28
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Additional support to this hypothesis was obtained from the study of the kinetic deuterium isotope effect (KDIE) on the reactions of TEA and TEA-d15 with BnO• and CumO•. No KDIE was observed in the reactions of BnO• (kH/kD = 1.0), whereas kH/kD = 1.7 was measured for the corresponding reactions of CumO• (see Table 1).28 As correctly pointed out by a reviewer, on the basis of the very high kH values measured for the reactions between BnO• and the tertiary amines, the lack of a KDIE in the reaction of BnO• with TEA cannot be taken as conclusive evidence in favor of the mechanistic hypothesis outlined above. However, the 2- to 3-fold increase in reactivity observed in the reactions of BnO• on going from TPA and TEA to ABCO and DABCO is a strong indication that hydrogen atom abstraction does not contribute to the ratedetermining step. If, on the other hand, cleavage of the amine RCH bond was contributing to the overall rate, significantly lower reactivities would be expected for the reactions with ABCO and DABCO on the basis of the operation of the stereoelectronic effect discussed above. The kinetic steps of the proposed mechanism can be represented according to Scheme 3 for the reaction of BnO• with a generic (unhindered) tertiary amine (RCH2)3N, where k1 and k1 are the rate constants for the formation and dissociation of the hydrogen-bonded prereaction complex and k2 is the rate constant for hydrogen atom abstraction within the complex. Based on the discussion outlined above and on the computational indication of a relatively strong hydrogen bond between the acidic BnO• R-CH and the nitrogen lone pair in the prereaction complex,28 k2 . k1 applies. Thus, the reaction rate can be expressed in terms of the rate constant for complex formation k1 as v = k1 [(RCH2)3N] [BnO•], where k1 corresponds to the kH values displayed in Table 1 for the reactions of BnO• with the tertiary amines. Quite importantly, the measured kH values are in all cases below the diffusion-control limit in MeCN.39 Accordingly, the observed differences in kH for the reactions of BnO• (Table 1) reasonably reflect the role of structural and electronic effects in the tertiary amines on the formation of the prereaction complex. Along this line, the very high rate constants measured for the reactions of ABCO, DABCO, and DMP with BnO• are a result of the lack of steric hindrance in proximity of the nitrogen lone pair in these three substrates, a feature that allows for optimal hydrogen bonding with the radical. The decrease in kH observed on going from ABCO, DABCO, and DMP to TEA, TPA, TAA, and PMP (and from TEA to TPA and TAA) reasonably reflects the increased steric hindrance around the nitrogen center that results from the conformational flexibility in the acyclic amines and, with PMP, from the presence of the four methyl groups on the R-carbons.40 In line with these findings is also the observation of a >2-fold increase in kH for the reaction between TEA and BnO• on going from MeCN to isooctane (kH = 4.3 109 and 9.6 109 M1 s1, respectively). This effect is significantly larger than that observed for the corresponding reactions with CumO• (the kH(BnO•)/ kH(CumO•) ratio increases from 21.5 to 33.1 on going from MeCN to isooctane) and points again toward the important role of hydrogen-bond interactions in the formation of the prereaction complex. As mentioned above, MeCN displays a higher HBD ability than isooctane (R2H = 0.09 and 0.00, respectively)38 and can compete with BnO• for the amine lone pair. This competition decreases the efficiency of complex formation, leading to a significant decrease in reactivity as compared to isooctane. Most importantly, the observation of an increase 6267
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Scheme 3
Scheme 4
in kH on going from MeCN to isooctane, despite an increase in viscosity39a,43 and a corresponding decrease in the diffusioncontrol limit (kd = 2.0 1010 and 1.4 1010 M1 s1, respectively)39,44 represent an additional indication that the observed differences in kH reflect the role of structural and electronic effects in the amines on the formation of the prereaction complex. On the basis of this mechanistic picture, the greater hydrogenbond acceptor (HBA) ability (expressed in terms of the β2H parameter)41,42 of tertiary alkylamines (β2H = 0.580.67) as compared to aliphatic aldehydes (β2H = 0.39) and 1,4-cyclohexadiene (β2H = 0.00) accounts for the significantly larger kH(BnO•)/kH(CumO•) ratios observed in the hydrogen atom abstraction reactions from the former substrates as compared to propanal, 2,2-dimethylpropanal,11 and 1,4-cyclohexadiene.28 In contrast to the results obtained with CumO•, where an almost 4-fold decrease in kH was measured on going from TPA to TAA, very similar kH values were measured for their reactions with BnO•. This observation is again in line with the hypothesis of different reaction pathways for the hydrogen atom abstraction reactions from tertiary amines by these two radicals. It was recently proposed that the reaction between BnO• and tertiary amines could proceed by proton transfer from the acidic benzyloxyl R-CH to the basic amine nitrogen rather than by hydrogen atom abstraction within a hydrogen-bonded prereaction complex.28 This pathway would lead to a ketyl radical anion/ ammonium ion couple that, following proton exchange, would finally lead to the ketyl radical and the amine (Scheme 4). In other words, the amine acting as a base would promote a 1,2-H shift in BnO•. However, we found no experimental or computational evidence to support this hypothesis in our previous study, and it was thus discarded on several grounds, taking moreover into account the previous observation that tertiary amines do not promote 1,2-H shift in BnO•.30 In order to obtain additional information on the possible cleavage of the benzyloxyl R-CH in the rate-determining step of the process, we have studied the reactions of the benzyloxyl radical deuterated at the benzylic positions (PhCD2O•, BnO•-d2) with TEA and TIBA. The data displayed in Table 2 show that with TIBA the reactions with the two radicals are characterized by almost identical rate constants, in line with the operation of a direct hydrogen atom abstraction mechanism since no significant kinetic effect is expected by replacement of H for D in the radical. With TEA a small (∼10%) increase in kH was observed on going from BnO• to BnO•-d2, an observation that is supported by the computational prediction of a slight lowering of the freeenergy barrier of reaction (0.07 kcal mol1) of BnO• with
DABCO that occurs with isotopic substitution.28,45 This observation provides additional evidence (even though not conclusive, due to very high kH values measured for the two reactions) against the hypothesis of a rate-determining proton transfer reaction from the acidic benzyloxyl R-CH to the basic amine nitrogen. The reactions of TEA, TIBA, and PMP with the 3,5-di-tertbutylbenzyloxyl radical (3,5-DTBBnO•) were also studied. The data displayed in Table 2 show that with TIBA the reactions with BnO• and 3,5-DTBBnO• have almost identical rate constants, an observation that is again in line with the operation of a direct hydrogen atom abstraction reaction, as an increase in steric bulk on the aromatic ring is not expected to exert any significant kinetic effect on this process. With TEA and PMP, a relatively small (1520%) decrease in kH is observed on going from BnO• to 3,5-DTBBnO•. This finding appears to be in contrast with the hypothesis of the formation of a n f π complex between the amine lone pair and the benzyloxyl radical aromatic ring, where the aromatic ring could mediate electron transfer from the nitrogen center to the radical, a mechanistic hypothesis that was previously ruled out on several grounds.28 On the other hand, the formation of a hydrogen-bonded prereaction complex like the one described in Scheme 3 is not expected to be significantly affected by the introduction of bulky substituents on the aromatic ring because this interaction involves the benzyloxyl R-CH and the amine lone pair. Along this line, the observed decrease in rate constant can be reasonably accounted for on the basis of the electron releasing character of the tert-butyl substituents. This effect leads to a decrease in the hydrogen-bond donor ability of the benzyloxyl R-CH atoms,30 which results in a weaker interaction in the prereaction complex. In conclusion, the results discussed above clearly show that the structure of both the amine and the alkoxyl radical can play a very important role in hydrogen atom abstraction reactions, providing in particular very different mechanistic pictures for the reactions of CumO• and BnO•. An increase in rate constant is observed in all cases on going from CumO• to BnO•, with the kH(BnO•)/ kH(CumO•) ratios that vary from 2.8 for the reactions with TIBA to 2027 for those with ABCO. With BnO• a 30-fold decrease in reactivity is observed on going from the most reactive (unhindered) amine (DABCO, for which kH = 1.05 1010 M1 s1) to the least reactive and sterically hindered one (TIBA, for which kH = 3.51 108 M1 s1). Most importantly, the experimental results clearly show that with this radical structural effects in the amine can induce a change in mechanism from a rate-determining formation of an hydrogen-bonded amine/radical complex followed by intracomplex hydrogen atom abstraction in the case of unhindered amines to a direct hydrogen atom abstraction in the case of hindered amines. With CumO• steric effects appear to play a minor role and the up to 60-fold decrease in reactivity observed on going from acyclic amines to ABCO mostly reflects the operation of stereoelectronic effects. Accordingly, the reactions of this radical with tertiary amines can 6268
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’ EXPERIMENTAL SECTION Materials. Spectroscopic grade acetonitrile and 2,2,4-trimethylpentane (isooctane) were used in the kinetic experiments. Triethylamine (TEA), tripropylamine (TPA), triallylamine (TAA), triisobutylamine (TIBA), 1,4-dimethylpiperazine (DMP), 1-azabicyclo[2,2,2]octane (ABCO), and 1,2,2,6,6-pentamethylpiperidine (PMP) were of the highest commercial quality available (g99%). TEA, TPA, TAA, TIBA, DMP, and PMP were further purified prior to use by filtration over neutral alumina. ABCO was further purified by sublimation. The purity of the substrates was checked by GC prior to the kinetic experiments and was in all cases >99.5%. Dicumyl peroxide was of the highest commercial quality available and was used as received. Dibenzyl peroxide, dibenzyl peroxide-d4, and bis(3,5-di-tert-butylbenzyl) peroxide were prepared in small portions by reaction of KO2 with the pertinent benzylic bromide in dry benzene, in the presence of 18-crown-6 ether, according to a previously described procedure.30,47 Details on the synthesis and characterization of the products are given in the Supporting Information. Laser Flash Photolysis Studies. LFP experiments were carried out with a laser kinetic spectrometer using the fourth harmonic (266 nm) of a Q-switched Nd:YAG laser, delivering 8 ns pulses. The laser energy was adjusted to e10 mJ/pulse by the use of the appropriate filter. A 3.5 mL Suprasil quartz cell (10 mm 10 mm) was used in all experiments. Nitrogen-saturated solutions of dicumyl peroxide and dibenzyl peroxides (10 and 8 mM, respectively) were employed. These concentrations were chosen to ensure, in the presence of amines, prevalent absorption of the 266 nm laser light by the precursor peroxides. The photochemical stability of the amines at the laser excitation wavelength (266 nm) was checked by LFP of acetonitrile solutions containing substrate concentrations comparable to the highest concentrations employed in the kinetic experiments. The high concentrations of ABCO and DABCO required for the kinetic study of their reactions with the cumyloxyl radical prevented the determination of the rate constants for these reactions under the experimental conditions employed (see Table 1). All the experiments were carried out at T = 25 ( 0.5 °C under magnetic stirring. The observed rate constants (kobs) were obtained by averaging 48 individual values and were reproducible to within 5%. Second order rate constants for the reactions of the cumyloxyl and benzyloxyl radicals with the amines were obtained from the slopes of the kobs (measured following the decay of the cumyloxyl and benzyloxyl radicals visible absorption bands at 490 and 460500 nm, respectively) versus [amine] plots. Fresh solutions were used for every amine concentration. Correlation coefficients were in all cases >0.992. The given rate constants are the average of at least two independent experiments, typical errors being e5%.
’ ASSOCIATED CONTENT
bS
Supporting Information. Details on the syntesis and characterization of the dibenzyl peroxides. Plots of kobs versus amine concentration for the reactions of CumO •, BnO•, BnO•-d2, and 3,5-DTBBnO•. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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