Reduction of Azobenzene

Article pubs.acs.org/joc

The Effect of Replacing Carbon by Nitrogen in Reductions with SmI 2: Reduction of Azobenzene Chintada Nageswara Rao and Shmaryahu Hoz* Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel S Supporting Information *

ABSTRACT: The reduction of azobenzene by SmI2 in THF to give hydrazobenzene was investigated. The kinetics are first order in the substrate and first order in SmI 2. The kinetic order in MeOH is ca. 0.56, and in TFE it is ca. 0.2. The fractional order in the proton donors is interpreted as being a result of their acting in two opposing manners. In one the proton donor enhances the reaction by protonation of the radical anion, and in the other it slows the reaction by binding to the lone pair electrons of the nitrogen in the azobenzene. This hampers the fast inner-sphere electron-transfer mode. Experiments conducted in the presence of low concentrations of HMPA show rate enhancement suggesting that the SmI2, which is partly coordinated to HMPA molecules, has some free sites to bind to the substrate. When more HMPA is added, it prevents the fast inner-sphere mechanism and the rate decreases. In this system, the increase in the reduction potential of SmI2 caused by HMPA is similar to the rate enhancement by an inner sphere mechanism. In general, the replacement of a skeletal carbon by a nitrogen atom causes a significant rate enhancement.

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INTRODUCTION SmI2 is one of the most popular reducing agents in chemistry today.1 Among its advantages over other reducing agents is its mild reactivity, reducing only substrates having a significant electron affinity. Thus, simple carbon−carbon double bonds are not reduced by SmI2, even when their LUMO is lowered by substitution by aromatic rings as in stilbene.2 In order to activate these compounds, substitution by electron-withdrawing groups such as CN is required.3 In electrophilic aromatic substitution, the incorporation of nitrogen as an amino substituent on the ring leads to activation due to its electrondonating ability. However, when the nitrogen atom is incorporated into the aromatic ring itself, it becomes an electron-withdrawing group with an effect similar to that of a nitro group substituent.4a In some cases, it is even a little stronger than a nitro group (4-chloro-3,5-dinitropyridine versus 1-chloro-2,4,6-trinitrobenzene,4b eqs 1 and 2), whereas in others it is a little weaker (4-chloropyridine versus 1-chloro-4nitrobenzene,4c eqs 3 and 4).We have initiated a quantitative exploration of the effect of substitution of the carbon atoms in a double bond by nitrogen atoms. It should be noted that the high reactivity of the nitro group and its tendency to undergo fast reduction5 make it difficult to study its effect as a substituent. In a previous paper, we studied the reduction of imines 6 (eq 5), in which one of the carbon atoms of the carbon− carbon double bond is replaced by nitrogen. This reaction was shown to be autocatalytic and demonstrated zero-order kinetics because of surface catalysis by microcrystals of SmI3. Thus, replacement of one of the carbon atoms of the double bond of stilbene to give N-benzylidenemethylamine transformed an unreactive substrate (stilbene) into a highly reactive substrate. In the present work, we report on © 2011 American Chemical Society

the study of azobenzene, where the two carbon atoms in the central bond of stilbene were replaced by nitrogen atoms (eq 6).

The product of this reaction is the corresponding hydrazobenzene. This reaction has been previously reported,7 as well as Received: September 2, 2011 Published: October 17, 2011 9438

dx.doi.org/10.1021/jo2018153 | J. Org. Chem. 2011, 76, 9438−9443

The Journal of Organic Chemistry

Article

some crystallographic studies of the formation of various complexes with derivatives of divalent samarium.8

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RESULTS AND DISCUSSION The kinetics of the reactions were followed in a glovebox under nitrogen atmosphere using a stopped-flow spectrometer. The reactions are very fast and the concentrations of the reactants had to be decreased almost to the lower limits of the detection. Typical concentrations were [azobenzene] = 5 mM and [SmI2] = 1 mM unless otherwise noted. Since two electrons are consumed in the reduction of the double bond, the stoichiometric ratio is 1/10, satisfying the pseudo-first-order condition. Proton donors (MeOH or TFE) were added in various concentrations. Hydrazobenzene is the only reaction product. Kagan9 has reported that the single bond of hydrazobenzene, the product of the reaction, can be further cleaved to the corresponding aniline. However, this reduction necessitates an excess amount of SmI2 and is not observed in our case because SmI2 is limited. The reactions were followed at λ = 619 nm (disappearance of the SmI2). The reactions are first order in SmI2 and first order in the substrate (Figure 1 and Table S1, Supporting Information).

Figure 2. Kinetic order in MeOH; [SmI2] = 1 mM, [azobenzene] = 5 mM.

(a) Two competing mechanisms. In one, the order in MeOH is zero and in the other it is one. The relative contribution of the two mechanistic channels determines the experimentally observed order in the reactant. (b) MeOH appears mainly as a dimer and only its monomeric form is active. In this case, the order in MeOH will be half. (c) MeOH acts in two opposing manners. In the one it enhances the reaction and in the other it slows the reaction. We begin with a discussion of the first possibility. For the protonating agent to exhibit a zero kinetic order implies that the first electron transfer is rate determining and that the protonation is a post rate-determining step. An alternative possibility is that two electrons are consecutively added to form the dianion, which then undergoes protonation in a post ratedetermining step. Namely, the addition of the second electron to form the dianion is the rate-determining step. The likelihood of these two mechanisms is very small because of two reasons. To counter the first mechanism we have carried out an experiment in which to a THF solution of the cis isomer of azobenzene a catalytic amount of SmI2 was added. This resulted in an isomerization to the trans isomer showing the reversibility of the electron transfer step (see the Experimental Section). If the trans isomer resembles the cis in this respect, then the first electron transfer cannot be rate-determining step. The formation of the dianion followed by a post rate-determining step protonation would necessitate a second order in the SmI 2, which has not been observed. Hence, a mechanism which is zero order in MeOH is unlikely. Moreover, the relative weight of the two reaction channels should have changed as a function of the MeOH concentration. The path which is zero order in MeOH is not affected by the change of the MeOH concentration while the path which is first order in MeOH will be accelerated by the factor of 40 as the MeOH concentration changes from 0.1−4 M. Therefore, since the contribution of the two mechanistic channels will vary with the concentration of MeOH, the kinetic order in MeOH should also vary over this concentration range. Thus, the straight line in Figure 2 with a slope of 0.56 and r 2 = 0.9946, over this MeOH concentration range, is incompatible with the first mechanistic explanation. The second possibility (dimerization) is also highly unlikely since there is no reason to assume that only in the present case will MeOH be mostly dimeric in THF, and while not in the other numerous reactions investigated to date.

Figure 1. Kinetic order in azobenzene; [SmI2] = 1 mM, [MeOH] = 1 M.

Table 1 and Figure 2 present the results for the kinetic order in MeOH. The data show that the order in MeOH is 0.5. Table 1. First-Order Rate Constants as a Function of MeOH Concentration; [SmI2] = 1 mM, [azobenzene] = 5 mM [MeOH] (M)

k (s−1)

0.1 0.3 1 2 4

24.09 42.07 75.32 122.44 189.71

To the best of our knowledge, a half order in the proton donor over such a wide range of its concentrations has not been observed before. Previous studies using other substrates manifested different orders in MeOH. Among these we found an increasing order,10 varying from zero to four11 and a sigmoid function starting at a zero order and ending in zero order. 3 It is clear that in these cases a slope of half could be identified over a short portion of the log−log plot. However, in the present case, the range was too large to accommodate such an explanation. Possible explanations for the fractional order (smaller than one) are as follows: 9439



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The third possibility seems to be the most attractive one. In a previous paper, we have shown that SmI2 coordinates to the lone pair of the nitrogen in an imine. The inner-sphere electron transfer reduction is much faster than the outer sphere electron transfer. This mode of reaction apparently applies also to azobenzene. Thus, according to the suggested mechanism, MeOH and SmI2 compete to bind to the lone pair of the nitrogen. Because of this, increasing the amount of MeOH in the reaction mixture will slow down the reaction. On the other hand, the protonation of the radical anion whose protonation is the rate-determining step, enhances the reaction. The likelihood of this mechanism was examined using the simulation shown in Chart 1. It should be noted that the parameters

In order to further support the suggested mechanism, we have performed reactions in the presence of TFE rather than MeOH as the proton donor. It is assumed that since this alcohol is more acidic than MeOH, it will also form a stronger hydrogen bond to the lone pair of azobenzene than MeOH. Figure 3 shows the order in the HD used in the simulation

Chart 1

Figure 3. Variation of the kinetic order as a function of Keq in the simulation.

above as a function of the equilibrium constant for its hydrogen-bond formation with the substrate. (All the rate constants, apart from those related to hydrogen bond formation, were retained. The details of the simulation are given in the Supporting Information.). The figure shows that as the equilibrium constant increases, i.e., moving from MeOH to TFE, the kinetic order in the HD will decrease. Thus, we expect that the order in TFE will be smaller than 0.56. Table 2 and Figure 4 show that the order in TFE is 0.22. Namely, it is indeed smaller than that of MeOH.

derived by this simulation were arbitrarily chosen and are not necessarily those of the reaction at hand. This simplistic simulation was performed merely to prove the likelihood of the suggested model. The first step in the model is a rapidly established equilibrium between a substrate (S) and a proton donor (HD). The model also includes an equilibrium between the reagent (R) and S and an intermediate (Int). This step represents the combination of two consecutive equilibria in our reaction; ligation of the SmI2 to a lone pair of azobenzene and the reversible electron transfer step to give the radical anion represented by “Int” in the chart. Int reacts with HD in a rate-determining step to form the product. Shown in this chart are also the rate constants used in each step. The equilibrium constants for hydrogen bond formation between HD and the substrate S were varied in the range 1−10 by varying k2. The simulations were carried out for each equilibrium constant for a series of HD concentrations similar to those used in our reactions. The results were analyzed in terms of first-order disappearance of R. The first-order rate constant for the disappearance of R as a function of HD concentrations gave for a K = 1 a kinetic order of 0.59 in HD over the range of HD concentrations of 0.1−4 M with r 2 = 0.953. This is close to the kinetic order observed for MeOH (0.56).12 In principle, because of the dependence of the concentration of S·HD on the concentration of HD in the first equilibrium, a curved rather than a straight line is expected. However, the smaller the equilibrium constant is, the less pronounced is the curvature. Thus, in the simulation over the range used with TFE (0.1−2 M, see below), r 2 varies from 0.983 for K = 1 to 0.924 for K = 10 (see the Supporting Information). It should be emphasized that because this simulation cannot be used to derive reliable individual rate constants for our reaction, no attempt was made to improve the appearance of the graphs by scanning other rate constants in the simulation.

Table 2. First-Order Rate Constants as a Function of TFE Concentration; [SmI2] = 1 mM, [Azobenzene] = 5 mM [TFE] (M)

k (s−1)

0.1 0.5 1 2

6.81 10.85 10.64 13.82

Figure 4. Kinetic order in TFE; [SmI2] = 1 mM, [azobenzene] = 5 mM.

Lastly, we employed the classical diagnostic tool for distinction between an inner and an outer sphere electron transfer reaction, hexamethylphosphoramide (HMPA). 13 This molecule forms a strong complex with SmI2 and enhances its 9440



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reduction potential. Therefore, the addition of HMPA to a reaction mixture will usually result in rate enhancement. However, if the reaction is of an inner sphere electron transfer nature, detachment of the SmI2 from the substrate due to massive coordination by HMPA molecules may reduce the reaction rate, since the high efficiency of the inner sphere electron transfer is eliminated. Figure 5 shows the effect of

Figure 6. Variation of the first-order rate constant as a function of HMPA concentration in the reaction of tetramethylazobenzene.

enhancement due to the inner-sphere electron-transfer mechanism is similar to the rate enhancement14 induced by increasing the reduction potential of SmI2 by HMPA. Finally, we would like to compare the behavior of azobenzene with that of imines. The initiative to study these two members of the aza family emerged from the wellestablished knowledge that in aromatic nucleophilic substitution, the replacement of a carbon by a nitrogen atom in an aromatic ring is similar, in terms of substrate activation, to ring substitution by a nitro group. The study of the effect of a nitro substituent on the reactions of SmI2 is hampered by the fact that often it is the nitro group itself that is reduced. The data we acquired on the reduction of the series below (I−III), in spite of the difference in the mechanism between the imines and azobenzene reduction by SmI2, clearly suggests that replacing a carbon atom by nitrogen

Figure 5. Variation of the first-order rate constant as a function of HMPA concentration.

HMPA on the first order reaction rate constants of a reaction of 1 mM SmI2, 5 mM azobenzene and 25 mM TFE. In the absence of HMPA k = 9 s−1, and at 4 mM HMPA, it increases to 46 s−1 and then decreases to a rate constant of 11 s−1 at 32 mM HMPA. The initial increase in rate is probably due to a partial coordination of the SmI2 by HMPA. This partial coordination still enables coordination of the SmI2 to the nitrogen lone pair, and at the same time enhances, to a certain degree, the reduction potential of the SmI2. When more HMPA is added, its coordination to the SmI2 leaves no free sites for bonding to the lone pairs of azobenzene leading to a rate decrease. In contradistinction to this result, the effect of HMPA on the reduction of 2,6,2′,6′- tetramethylazobenzene (eq 7) was rate enhancement as shown in Table 3 and Figure 6. In this case,

in the central double bond significantly enhances the reaction rate. Stilbene (I) does not react with SmI2 (under normal conditions) unless the stilbene is activated by a cyano group for example. The overall reaction times, under similar conditions, are 10 s for the imino substrate (II), which is much longer than the 0.1 s noted for azobenzene (III). It is worth noting that unlike imines, which display an autocatalytic reaction profile due to surface catalysis by the SmI3 microcrystals produced in the course of the reaction, azobenzene presents a rather normal kinetic behavior. This suggests that the interaction with Sm3+ on the surface of the microcrystals may not be due to an interaction with the lone pair, which is common to the two systems.15 It seems more likely that the interaction with the catalyst is therefore that of the π system, which differs significantly between the two systems.

Table 3. First-Order Rate Constants As a Function of HMPA Concentration in the Reaction of Tetramethylazobenzene (5 mM), [SmI2] = 1 mM, [TFE] = 25 mM [HMPA] (mM)

k (s−1)

0 4 8 16

0.017 3 40 155

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coordination to either lone pair on the nitrogen atoms is obstructed due to steric hindrance: It is interesting to note that at 32 mM of HMPA, where the SmI2 is practically fully coordinated by HMPA molecules, the rate constant is almost equal to that of the reaction with no HMPA added (Figure 5). This implies that the rate

SUMMARY AND CONCLUSIONS We have shown that azobenzene reacts with SmI2 by an innersphere electron-transfer mechanism. The first step in the process is coordination of the SmI2 to the lone pair of the nitrogen. Proton donors, which compete with the SmI2 by hydrogen bonding to this lone pair, impede the reaction rate. 9441



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Partial coordination by HMPA enhances the reaction rate. This is apparently because there are enough free sites left on the SmI2 which enable its coordination to the lone pair, and the HMPA partial coordination increases the reduction potential of SmI2. Further addition of HMPA leads to the occupation of all free sites on the samarium retards the reaction rate. The activation energy lowering effect of the inner sphere mechanism is of the same magnitude as that of the addition of HMPA to the reaction mixture. Finally, we have shown that similar to nucleophilic aromatic substitution, replacement of a carbon atom by nitrogen increases significantly the rate of reduction by SmI2.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS

We thank Dr. Moshe Ben-Tzion for the kinetic simulations.

REFERENCES

(1) (a) Namy, J. L.; Girard, P.; Kagan, H. B. New J. Chem. 1977, 1, 5−7. For reviews see: (b) Kagan, H. B. Tetrahedron 2003, 59, 10351− 10372. (c) Molander, G. A. Chem. Rev. 1992, 92, 29−60. (d) Molander, G. A.; Harris, C. R. Chem. Rev. 1996, 96, 307−338. (e) Molander, G. A.; Harris, C. H. Tetrahedron 1998, 54, 3321−3354. (f) Nicolaou, K. C.; Ellery, S. P.; Chen, J. S. Angew. Chem., Int. Ed. 2009, 48, 7140− 7165. (g) Krief, A.; Laval, A.-M. Chem. Rev. 1999, 99, 745−777. (h) Gansäuer, A.; Bluhm, H. Chem. Rev. 2000, 100, 2771−2788. (i) Nakata, T. Chem. Soc. Rev. 2010, 39, 1955−1972. (j) Ichikawa, S. Chem. Pharm. Bull. 2008, 56, 1059−1072. (k) Soderquist, J. A. ́ Aldrichimica Acta 1991, 24, 15−23. (l) Concellón, J. M.; RodriguezSolla, H.; Concellón, C.; Amo, V. Chem. Soc. Rev. 2010, 39, 4103− 4113. (m) Edmonds, D. J.; Johnston, D.; Procter, D. J. Chem. Rev. 2004, 104, 3371−3403. (n) Procter, D. J.; Flowers, R. A., II.; Skrydstrup, T. Organic Synthesis Using Samarium Diiodide: A Practical Guide; The Royal Society of Chemistry, Cambridge, 2010. (2) Unless special methods are used: Dahlén, A.; Hilmersson, G. Tetrahedron Lett. 2003, 44, 2661−2664. (3) Amiel-Levy, M.; Hoz, S. J. Am. Chem. Soc. 2009, 131, 8280− 8284. (4) (a) Terrier, F. Nucleophilic Aromatic Displacement; VCH Publishers, Inc.: New York, 1991; pp 11−13; (b) Murto, J. Acta Chem. Scand. 1966, 20, 310−322. (c) Miller, J. Aromatic Nucleophilic Substitution; Elsevier: Amsterdam, 1968. (5) (a) Kende, A. S.; Mendoza, J. S. Tetrahedron Lett. 1991, 32, 1699−1702. (b) Yacovan, A.; Hoz, S. J. Org. Chem. 1997, 62, 771− 772. (c) Dumez, E.; Faure, R.; Dulcère, J.-P. Eur. J. Org. Chem. 2001, 2577−2588. (d) Chen, X.; Zhong, W.; Zhang, Y. Heteroat. Chem. 2002, 13, 302−306. (e) Anderson, J. C.; Blake, A. J.; Howell, G. P.; Wilson, C. J. Org. Chem. 2005, 70, 549−555. (f) Ankner, T.; Hilmersson, G. Tetrahedron Lett. 2007, 48, 5707−5710. (6) Rao, C. N.; Hoz, S. J. Am. Chem. Soc. 2011, 133, 14795−14803. (7) (a) Z hang, Y.; Lin, R. Synth. Commun. 1987, 17, 329−332. (b) Brady, E. D.; Clark, D. L.; Keogh, D. W.; Scott, B. L.; Watkin, J. G. J. Am. Chem. Soc. 2002, 124, 7007−7015. (8) (a) Takats, J.; Zhang, X. W.; Day, V. W.; Eberspacher, T. A. Organometallics 1993, 12, 4286−4288. (b) Yuan, F.; Liu, X. Appl. Organometal. Chem. 2005, 19, 877−878. (c) Fu-Gen, Y.; Xiu-Juan, L.; Yong, Z. Chin. J. Chem. 2005, 23, 749−752. (d) Yuan, F.; Qian, H.; Min, X. Inorg. Chem. Commun. 2006, 9, 391−393. (e) Turcitu, D.; Nief, F.; Ricard, L. Chem.Eur. J. 2003, 9, 4916−4923. (9) Souppe, J.; Danon, L.; Nomy, J. L.; Kagan., H. B. J. Organomet. Chem. 1983, 250, 227−236. (10) (a) Chopade, P. R.; Prasad, E.; Flowers, R. A. II. J. Am. Chem. Soc. 2004, 126, 44−45. (b) Tarnopolsky, A.; Hoz, S. J. Am. Chem. Soc. 2007, 129, 3402−3407. (11) Tarnopolsky, A.; Hoz, S. J. Org. Biomol. Chem. 2007, 5, 3801− 3804. (12) It should be emphasized that this simulation was performed merely to examine the likelihood of the suggested mechanism. Therefore, it was simplified by not including factors such as the MeOH complexation to SmI2 or the fact that azobenzene has two binding sites (which is simply equivalent to doubling the reactant concentration), etc. (13) (a) Ankner, T.; Hilmersson, G. Tetrahedron 2009, 65, 10856− 10862. (b) Enemærke, R. J.; Daasbjerg, K.; Skrydstrup, T. Chem. Commun. 1999, 343−344. (c) Prasad, E.; Flowers, R. A. II. J. Am. Chem. Soc. 2002, 124, 6895−6899. (d) Enemærke, R. J.; Hertz, T.; Skrydstrup, T.; Daasbjerg, K. Chem.Eur. J. 2000, 6, 3747−3754. (e) Shabangi, M.; Kuhlman, M. L.; Flowers, R. A. II. Org. Lett. 1999, 1,

EXPERIMENTAL SECTION

General Methods. THF was dried over Na wire, in the presence of benzophenone, and distilled under an argon atmosphere. The freshly distilled THF was used for all kinetic experiments as well as for the preparative reactions. TFE and MeOH were dried according to known procedures.16 Water content was determined to be lower than 20 ppm. SmI2 solutions were prepared as needed from a freshly prepared 0.1 M THF solution.17 The concentration of the SmI2 solution was spectroscopically determined (λ = 619 nm; ε = 635). Commercial azobenzene of high purity (confirmed by 120−121 °C and 13C NMR) was used without further purification, for the preparative as well as for the kinetic experiments. The 2,6,2′,6′tetramethylazobenzene was prepared from 2,6-dimethylaniline by following literature procedure.18 The identity of the products were confirmed by 1H (300 MHz), 13C (75 MHz) NMR, and HRMS analyses and compared with the literature values.19 Kinetics. The kinetics of the reactions were followed using a stopped flow spectrophotometer in a glovebox under nitrogen atmosphere at room temperature. The reactions were monitored at the λ max of the SmI2 (619 nm). Whenever, a proton donor was used, it was mixed with the substrate solution. Each set of experiments was repeated two to three times. Within a set, each measurement was routinely repeated three times. At the end of each series, the first measurement was repeated to ensure reproducibility within a set. The deviation usually observed was ca. 5%. First-order kinetics were analyzed using Kinet Asyst (v. 2.2 Hi-Tech Ltd.). General Procedure for Product Preparation under Conditions Similar to the Kinetic Measurements. A freshly prepared solution of SmI2 (0.1 M) in THF was added in the glovebox to a homogeneous solution of the azobenzene or 2,6,2′,6′-tetramethylazobenzene and MeOH in dry THF. The total volume of the reaction was 100 mL, and the final concentrations were [SmI2] = 11 mM; [azobenzene] = 5 mM and [MeOH] = 0.1 M. After 5 min, the solvent was evaporated under reduced pressure at 30 °C. The crude reaction mixture was dissolved in DCM (40 mL) and washed with potassium dihydrogen phosphate buffer (20 mL) followed by brine (10 mL) and dried over anhydrous Na2SO4. The solvent was evaporated under reduced pressure. Under the above reaction conditions, hydrazobenzene or 2,6,2′,6′-tetramethylhydrazobenzene, respectively, are only the products observed. Cis to Trans Isomerization of Azobenzene. Freshly prepared THF solution of trans-azobenzene (0.5 mM) was isomerized to the corresponding cis-azobenzene by irradiation at λ max = 365 nm over a period of 45 min using a UV lamp (UVSL-25; 254/365 nm).20 The immediate measurement of the spectrum showed an absorption at λ max = 317 nm indicating the formation of cis-azobenzene. When the cis-azobenzene (0.5 mM) was treated with a catalytic amount of SmI 2 (0.008 mM) for 30 s in the dark, the absorption of the cis isomer disappeared almost completely.

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ASSOCIATED CONTENT * Supporting Information Table S1 and simulation data. This material is available free of charge via the Internet at http://pubs.acs.org. S

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2133−2135. (f) Prasad, E.; Knettle, B. W.; Flowers, R. A. II. J. Am. Chem. Soc. 2002, 126, 6891−6894. (g) Miller, R. S.; Sealy, J. M.; Shabangi, M.; Kuhlman, M. L.; Fuchs, J. R.; Flowers, R. A. II. J. Am. Chem. Soc. 2000, 122, 7718−7722. (14) Shabangi, M.; Flowers, R. A. II. Tetrahedron Lett. 1997, 38, 1137−1140. (15) If this interaction is responsible for the autocatalysis, one should then assume that surface catalysis is barred in azobenzene because the steric effect of the lone pair on the α nitrogen is larger than that of the H in the C−H unit of imines. (16) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergamon Press: New York, 1989. (17) Girard, P.; Namy, J. L.; Kagan, H. B. J. Am. Chem. Soc. 1980, 102, 2693−2698. (18) Ortiz, B.; Villanueva, P.; Walls, F. J. Org. Chem. 1972, 37, 2748− 2750. (19) (a) Sydnes, L. K.; Elmi, S.; Heggen, P.; Holmelid, B.; MaltheSørensen, D. Synlett 2007, 11, 1695−1698. (b) Pinkus, J. L.; Goldman, L. S. J. Chem. Educ. 1977, 54, 380−381. (20) (a) Hartley, G. S. Nature (London) 1937, 140, 281. (b) Otruba, J. P. III.; Weiss, R. G. J. Org. Chem. 1983, 48, 3448−3453.

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Reduction of Azobenzene

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