Kinetic applications of electron paramagnetic resonance spectroscopy

Kinetic Applications of ElectronParamagnetic. Resonance Spectroscopy. XXI. Some Mono-, Di-, and Trialkylhydrazyls1. R. A. Kaba,2 L. Lunazzi,3 D. Linds...
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6162 and references cited therein. (34) J. W. Timberlake, M. L. Hodges, and A. W. Garner, Tetrahedron Len., 3843 (1973). (35) J. B. Hendrickson and I. Joffee, J. Am. Chem. Soc., 95, 4083 (1973). (36) C. G. Overberger, G. Kesslin. and N. R. Byrd, J. Org. Chem., 27, 1568 (1962). (37) P. S.Engel, A. I. Dalton, and L. Shen, J. Org. Chem., 39, 384 (1974). (38) J. C. Martin and J. W. Timberlake, J. Am. Chem. Soc.. 92, 978 (1970). (39) D. Lim. Co//ecf. Czech. Chem. Commun., 33, 1122 (1968). (40) C. E. H. Bawn and S. F. Mellish, Trans. faradaysoc., 47, 1216 (1951). (41) J. D. Rockenfeller and F. D. Rossini, J. Phys. Chem., 85, 267 (1961). (42) D. S. Malament and J. M. McBride, J. Am. Chem. SOC., 92, 4586 (1970). (43) This approximation holds for the cis-trans alkene pairs which have been studied. See J. E. Kilpatrick, E. J. Prosen, K. S.Pitzer, and F. D. Rossini, J. Res. Nat. Bur. Stand., 38, 559 (1946). (44) C. G. Overberger, Rec. Chem. Prog., 21, 21 (1960). (45) B. V. loffe and V. S.Stopskij, TetrahedronLett., 1333 (1968). (46) K. R. Kopecky and T. Gillan, Can. J. Chem., 47, 2371 (1969). (47) K. R. Kopecky and S.Evani, Can. J. Chem., 47,4041 (1969). (48) J. A. Berson, E. W. Petrillo, and P. Blckart. J. Am. Chem. SOC.,98, 636 (1974). (49) P. Dowd, Acc. Chem. Res., 5, 242 (1972). (50) P. S.Engel, J. L. Wood, J. A. Sweet, and J. L. Margrave, J. Am. Chem. Soc., 98, 2381 (1974). (51) P. S.Engel, R. A. Melaugh, A. W. Garner, and F. D. Rossini. M. Mans-

son, J. W. Timberlake, submkted to J. Chem. Thermodyn. (52) H. A. Taylor and F. P. Jahn. J. Chem. Pbys., 7, 470 (1939). (53) G. Geiseler and J. Hoffmann, Z.Phys. Chem. (Frankfurt am Main), 57, 318 (1968). (54) 8. K. Bandlish, A. W. Garner, M. L. Hodges, and J. W. Timberlake, J. Am. Chem. Soc.. 97,5856 (1975). (55) B. H. AI-Sader and R. J. Crawford, Can. J. Chem., 48, 2745 (1970). (56) C. G. Overberger and A. V. DIGiulio, J. Am. Chem. Soc., 81, 2154 (1959). (57) S.F. Nelsen and P. D. Bartlett. J. Am. Chem. Soc.. 88, 137 (1966). (58) P. S. Engel and D. J. Bishop, J. Am. Chem. SOC.,94, 2148 (1972). (59) H. C. Ramsperger, J. Am. Chem. Soc., 51, 2134 (1929). (60) tert-Butyl was taken to be the same as 1-methylcyclopentyl. See ref 18. (61) M. Prochazka, 0. Ryba, and D. Lim, Collect. Czech. Chem. Comrnun.. 38, 2640 (1971). (62) The only exception is benzylazo-a-phenylethane. but both this compound and azo-a-phenylethane were measured at only two temperatures. Thls fact and the problem of t a u t o m e r l ~ r nduring ~ ~ thermolysis lowers our confldence in the AAGt values for benzylazo-a-phenylethane. (63) G. E. Coates and L. E. Sutton, J. Chem. Soc., 1187 (1948). (64) G. F. Hennion and E. 0. Teach, J. Am. Chem. SOC.,75, 1653 (1953). (65) G. F. Hennlon and C. V. DiGiovanna, J. Org. Chem., 30, 2646 (1965). (66) J. C. Stowell, J. Org. Chem., 32, 2360 (1967). (67) C. G. Overberger, T. B. Glbb, S.Chibnik, P. T . Huang. and J. J. Monagle. J. Am. Chem. Soc., 74,3290 (1952).

Kinetic Applications of Electron Paramagnetic Resonance Spectroscopy. XXI. Some Mono-, Di-, and Trialkylhydrazyls' R. A. Kaba,* L. L ~ n a z z i D. , ~ Lindsay, and K. U. Ingold* Contribution from the Division of Chemistry, National Research Council of Canada, Ottawa, Canada, K I A OR9. Received October 29, 1974

Abstract: The kinetics, mechanism, and products of decay of some mono-, 1,2-di-, and trialkylhydrazyls have been examined. 1 -Alkylhydrazyls decay with second-order kinetics at the diffusion-controlled limit. 1,2-Diisopropylhydrazyl undergoes a very rapid second-order decay, which is a P-disportionation to hydrazine and azo compound. According to their structure, N) or by a slow &scission (loss of alkyl trialkylhydrazyls may decay by a fast second-order @-disproportionation (alkyl-H and formation of an azo compound). These results, together with previously reported data on 2,2-dialkylhydrazyi~,'~ are discussed in relation to the possibilities of isolating persistent alkyl hydrazyl radicals. -+

Interest in alkyl hydrazyls has grown dramatically in the 2 years since the uv4 and EPRS spectra of the first of these radicals were reported. Attention has been focused primarily on the EPR Apart from the usual qualitative statements about radical lifetimes, detailed kinetic and product studies have been confined to Nelsen and Landis' work on some bi- and multicyclic trialkylhydrazyls6 and work from this laboratory on a series of 2,2-dialkylhydrazyls. l 4 Further work on alkylhydrazyls is clearly justified when it is remembered that amongst arylhydrazyls is included diphenylpicrylhydrazyl (DPPH), one of the most persistent free radicals known.I5 In this paper, we report on the kinetics and products of the decay of some mono-, di-, and trialkylhydrazyls.

Experimental Section Materials. Methyl hydrazine was obtained from Chemical Intermediates and Research Laboratories, Inc. Benzylhydrazine was prepared from benzyl chloride and hydrazine hydrate.14 1,2-Diisopropylhydrazine was obtained from Fluka, AG, and azotrifluoromethane from Merck Sharpe and Dohme, Ltd. terr-Butylhydrazine was prepared from chloramine and Trialkylhydrazines were prepared by reduction of hydrazones with sodium cyanoborohydride." The hydrazones were prepared by condensation of 1 ,I-dialkylhydrazine with a carbonyl compound. The J o u r n a l of the American Chemical Society

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following preparatidn of l-isopropylamino-2,2,6,6-tetramethylpiperidine'* is fairly typical of a slow and difficult reaction. A mixture consisting of 2.5 g (0.016 mol) of TMPNH2,I9 0.9 g (0.016 mol) of acetone, and 0.07 g (0.0007 mol) of 2-hydroxypyridine (a catalyst) was refluxed 48 hr.20 The reaction mixture was washed with water and ether, and the ether layer was dried and then distilled under vacuum. The hydrazone [TMPN=C(CH3)2] distilled at 85O (12 mm), yield 2.0 g. It was purified by preparative VPC. NMR spectrum in C6D6 (in ppm downfield from Me4Si):21 TMP, (CH3)2 0.84 (s), (CH3)2 1.30 (s), (CH2)3 1.54 (m, broad), =C(CH3)2 1.79 (d, J = 4 Hz). Anal. Calcd for Cr2H24N2 (mol wt 196.34): C, 73.41; H, 12.32; N, 14.27. Found (mol wt (mass spectrometry) 196):.C, 73.37; H, 12.30; N, 14.35. To a stirred solution of 1.2 g (0.006 mol) of TMPN=C(CH3)2 in 25 ml of acetonitrile was added 0.2 g (0.003 mol) of NaBH3CN i n 4 or 5 small portions over 30 min. After a further 45 min, ca. 1 ml of acetic acid was added slowly and the reaction mixture stirred for an additional 6 hr. Then 0.6 ml of concentrated HCI was added, the acetonitrile removed under vacuum, and the oily residue dissolved in 20 ml of " 2 0 , basified with KOH, saturated with NaCI, extracted with ether, and dried over MgSO4. The ether was removed and the yellow oil distilled under high vacuum on a molecular still at room temperature (yield of TMPNHCH(CH3)2, 0.5 8). NMR spectrum in C6D6: TMP, (CH3)4 and (CH2)3 three broad peaks at 0.93, 1.14, and 1.43; CH(CH3)2 1.01 (d, J = 6 Hz), CH(CH3)z 3.14 (septet); NH 2.30 (broad). Anal. Calcd for C12H26N2 (mol wt. 198.35): C, 72.66; H, 13.21; N, 14.12. Found

November 12, 1975

6763 Table I. Percentage Yields of Products Formed by Oxidation of TMPNHCH(CH,), with Various Reagents Temp, Time, "C hr

Oxidant [(CH,),CON],a [(CH,),CO] z [(CH,),COl z [(CH,),COI, &,OR

HgO a a

In benzene.

50 110 140 25 25 25

48 25 2 25 16 16


45 76 79 5 0



38 8 8 8 6 6 8 22 0 0 No reaction



Table 11. 'H and Decoupled I3C NMR Spectra of A "C NMR

' H NMR 0.85 (d, J = 6 Hz)

9 8 9 65 100 I

69.4 (30.9)

1.18 (s)

(mol wt (mass spectrometry) 198): C, 72.58; H, 13.17; N, 14.22. The NMR spectrum in C6D6 of some of the other hydrazines 68.5 3.68 (septet) and of their dehydro derivatives (hydrazone or azo compound) are 21.3 1.17 ( d , J = 6 Hz) dH3)2 as follows. [(CH3)2CHNH-]2: (CH3)2 0.95 (d, J = 6 Hz); CH 2.73 (septet); NH 2.23 (broad). [(CH3)2CHN=]2: (CH3)2 1.14 (d, J = 6 Hz); CH 3.61 (septet). TMPNHCH3: TMP (CH3)4 1.02 (s); (CH2)3 1.38 (s); CH3NH 2.55 (s), NH 2.03 (broad). yield of ca. 10% based on hydrazine consumed (which amounted to TMPNxCH2: TMP (CH3)4 1.14 (s); (CH2)3 1.45 (s); CH2 6.66 1 mo1/2 mol of tert-butoxy). Other tentatively identifiable product (d) and 6.90 (d, J = 14 Hz). (CH&NNHCH(CH3)2: (CH3)2N peaks in the NMR spectrum implied the presence of an H2C=C 2.20 (s); (CH3)2CH 0.99 (d, J = 6 Hz); (CH&CH 2.83 (septet); 2 Hz) and two, or more, group (4.75, unresolved multiplet, J NH not observed. (CH3)2NN=C(CH3)2: (CH&N 2.38 (s); (CH&CHN=N groups (3.66, 3.68, and perhaps 3.69, apparently (CH3)2C 1.69 (s). overlapping septets with J = 6 Hz). The integrated intensities of Radical Production. The hydrazyl radicals were formed photothese two groups indicated that the concentration of H2C=C was lytically in the cavity of a Varian E-3 EPR spectrometer. All but approximately half of that of all the (CH3)2CHN=N groups toone of the hydrazyls was generated by hydrogen abstraction from gether. VPC analysis on a 12-ft SE-30 column at 200' showed the parent hydrazine by rerr-butoxy radicals.I0 Photolysis of three unknown products, A, B, and C, with retention times of 8.4, CF3N=NCF3 in isopentane yielded the C ~ H S ( C H ~ ) ~ C ( C F ~ )9.0, - and 9.8 min, respectively, together with the hydrazone ( 1 3.3 NNCF3 radicaI.l0 min) and unreacted hydrazine (16.0 rnin). None of the unknown The EPR spectra of the hydrazyls have been reported previousproducts was any of the following: (CH3)2CH(CH2)3C(CH+ I Y . ~ - ' O Photolysis of (CH3)3CNHNH2 in (CH3)3COOC(CH3)3 at CH2 (3.2 min); (CH3)2CH(CH2)2CH=C(CH3)2 (3.5 min); room temperature immediately yields a strong signal due to the TMPH (5.0 min); or TMPN=CHCH3 (10.4 rnin). (CH,),C radical (second-order lines are resolvable). The (CH3)3C The percentage yields of A, B, C and hydrazone are given in radical is also formed when the photolysis is carried out in Table I for the hyponitrite reaction and for oxidation under a vari(CD3)3COOC(CD3)3, and it must therefore come from the hydraety of different conditions. The additional experiments were carzine. After a few minutes of photolysis of either of these mixtures, ried out in order to find conditions where the relative yield of one a second spectrum builds up that certainly belongs to a hydrazyl of the unknown products increased sufficiently to justify an at(oN = 9.45 and 12.8 G, a H ( I H ) = 3.0 G, g = 2.0039). This spectempt at separation by preparative VPC. Compound A was isolattrum is not completely /nconsistent with that which might be exed as a yellowish oil by this technique, following reaction of 2.7 pected from (CH3)3CNNH2 provided splitting by one of the mmol of hydrazine with di-tert-butyl peroxide (2.7 mmol) at 140' amino H's is less than the line width,22 AHpp 1.1 G. However, for 7.5 hr (3 half-lives for the peroxide). It was not sufficiently the fact that this hydrazyl signal does not reach a steady level until pure (the major impurities being B and bleeding from the VPC 5-8 min of continuous photolysis is very suspicious for a primary column) for elemental anaysis. The ' H and decoupled I3C NMR radical product and, on the basis of the nitrogen.splittings, we spectra of A (in C6&, chemical shifts in ppm downfield from therefore suggest23 that this radical is (CH3)3CNNHC(CH3)3 Me4Si) are consistent with those of the acyclic azo compound produced as secondary product after a number of reactions, about shown in Table 11. The I3C chemical shifts were assigned by comwhich we prefer not to speculate. parison with azoisopropane (CH, 68.1; CH3, 21 . I ) and 2,2'-azoisoThe kinetic procedure used to follow radical decays has been debutane (C, 66.5; CH3, 27.6). taking into account possible 8- and scribed in previous papers in this ~ e r i e s . ' . ' ~ , ' ~ y-substitution effects.24 The other I3C resonances were not asProduct Studies. Three trialkylhydrazines and one 1,2-dialksigned because of problems associated with the impurities present. ylhydrazine were allowed to react with ca. 50 mol % of thermally The presence of an azo linkage is supported by the uv spectrum of generated tert-butoxy radicals (from tert-butyl hyponitrite) in deA: A, 360 mg (c 14) in n-pentane. For comparison, azoisoprogassed C6D6 at 50' for 48 hr in the dark.I4 Since the decay kinetpane has: A, 356 mp (e 14) in the same solvent. The ir spectrum ics of monoalkylhydrazyls could not be properly examined (see of (neat) A [2900 (v broad), 1467 (s), 1458 (shoulder), 1379 (s), later), no product studies were undertaken on these radicals. The 1363 (s), 1309 (m), 1260 (w), 1171 (m), 1033 (w), and 804 ( w ) reactions were carried out in NMR tubes in the presence of a trace cm-'1 is also very similar to that of azoisopropane [2968 (s), 2927 of Me4Si. The hydrazine concentrations were ca. 0.4 M and the (s), 2893 (s), 2864 (s), 1466 (s), 1458 (shoulder), 1378 (s). 1364 hyponitrite ca. 0.1 M . The NMR spectra were recorded immedi(s), 1310 (m). 1126 (s) cm-'1. The mass spectrum of A did not ately before and after the reaction. The NMR analysis of the prodshow a parent peak. I t has as its maximum m/e 126,corresponding ucts was confirmed, whenever possible, by VPC. presumably to the molecular ion C9H IS+ produced perhaps as (i) [(CH3)2CHNH--]2 gave only [(CH3)2CHN=]2, and this shown: was formed in an amount equal to the initial hyponitrite concentration. That is, 1 mol of azo compound was formed for every 2 mol of rerr-butoxy (Le., for every two hydrazyl radicals). (ii) TMPNHCH3 gave only TMPN=CH2, 1 mo1/2 mol of tertbutoxy. (iii) (CH3)2NNHCH(CH3)2 was, unfortunately, rather impure and, to judge from the NMR spectrum and VPC trace, gave a vaA riety of different products, but none in overwhelming yield. After The mass spectral cracking pattern of A is identical with that for showing that (CH3)2NN=C(CH3)2 was not formed in significant amounts, this reaction was abandoned. (CH3)2CH(CH&CH=C(CH3)2. The mass spectra of B and C (combined VPC, mass spectrometer) both have maximum n r / ~ (iv) TMPNHCH(CH,)2 gave, after reaction, a complex NMR 124. Combination of this fact with the observed yields of A. B. and spectrum, in which TMPN=C(CH& could be recognized in a



lngold et al.


Mono-. Di-,and Trialkylhydrazyls

6764 C and the relative intensities of the H2C=C and (CH3)2CHN=N groups in the hyponitrite oxidation leads us to assign the following structures to B and C.

While fast reactions rarely exhibit large deuterium isotope effects, it is quite possible that the reaction actually proceeds by a rate-controlling radical coupling to form a tetrazane. The observed products would then be formed by a subsequent slower 1,3-hydrogen transfer. 2 (CH,),CHNHNCH(CH,

B C This assignment receives some support from the relative VPC retention times of B and C and of (CH&CH(CH2)3C(CH+CH2







This mechanism is analogous to that previously established and (CH3)2CH(CH2)2CH=C(CH3)2. for ( C H h N N H , (CH3CH2)2NNH, and ( C ~ H S C H ~ ) ~ N Results NH radi~a1s.I~ 2,2-Dialkylhydrazyls. The decay kinetics and products Monoalkylhydrazyls. Methy!- and benzylhydrazine react have with tert-butoxys to give CH3NNH2 and C ~ H S C H ~ N N H ~ , been described p r e v i o u ~ l y . ~ ~ Trialkylhydrazyls. (i) TMP-1-aminomethyl decayed with re~pectively.~ The large number of lines in the EPR spectra clean second-order kinetics. Within the limits of experimenof these radicals made the intensity of any individual line tal accuracy, the rate constant was unaffected by temperatoo low for kinetic study. However, the radical concentrature in the range -30 to 30’ so the activation energy for tions were proportional to the square root of the light intendecay is probably 1 2 kcal/mol. sity, which implies that they decay by radical-radical processes. From the radical concentrations under steady illumik 2 E p , = l . Z ( i 0 . 2 ) X 10’ W 1SeC-’ nation and the known rate of tert-butoxy formation, the biThe formation of the hydrazone as the sole oxidation prodmolecular rate constant for radical decay was estimated to uct implies that the observed reaction is a disproportionabe (1.0 f 0.5) X lo9 M-l sec-’ at 25’; i.e., decay occurs at tion involving alkyl hydrogen. essentially the diffusion-controlled limit. 2 TMPkCH, TMPNHCH, + TMPN=CH, BCH,Ik”, products (ii) 2,2-Dimethyl-l-isopropylhydrazylwas slightly longer The formation of the (CH3)3C radical when tert-butlived. It decayed with second-order kinetics, and the rate ylhydrazine reacts with tert-butoxys, suggests that constant was independent of the temperature from -30 to (CH3)3CNHNH rather than (CH3)3CNNH2 (which is the 30’. type of hydrazyl formed from less hindered monoalkyl hyk Z m , = 2 . 1 ( i 0 . 4 ) x 10‘K‘ sec-’ d r a z i n e ~ may ~ ) be formed initially but is not detected either because it undergoes an unexpectedly rapid @-scission or Since hydrazone was not formed, the expected disproporbecause it is an unusually powerful hydrogen donor,25e.g.: tionation must be inhibited by steric factors. The most likely reaction would seem to be formation of a tetrazane that (CH,),CNHfiH ( C H 3 ) , t + HN=NH subsequently decays to yield a variety of products. (iii) TMP-1-aminoisopropyl gave more problems than all (CH3),CNHkH + (CH3),COOC(CH3)3 the preceding hydrazyls. Brief irradiation of a fresh sample [(CHS),CN=NH + (CH3)3CO* + (CH3),COH] gave radicals that decayed rapidly (e.g., 7112