|
1. |
Spin trapping by use of nitroso-compounds. Part V. 2,4,6-Tri-t-butylnitrosobenzene: a new type of spin-trapping reagent |
|
Journal of the Chemical Society, Perkin Transactions 2,
Volume 1,
Issue 4,
1973,
Page 369-374
Shigeru Terabe,
Preview
|
PDF (741KB)
|
|
摘要:
1973Spin Trapping by Use of Nitroso-compounds. Part V.l 2,4,6-Tri-t-butyl-nitrosobenzene : a New Type of Spin-trapping ReagentBy Shigeru Terabe and Ryusei Konaka," Shionogi Research Laboratory, Shionogi and Co. Ltd., Fukushima-ku,The utility of 2,4,6-tri-t-butylnitrosobenzene as a spin-trapping reagent has been examined, and two mainadvantages of its use have been clarified. 2.4.6-Tri-t-butylnitrosobenzene has two sites for spin trapping. Primaryalkyl radicals normally attack the nitrogen atom of the nitroso-group to generate nitroxides. Tertiary alkyl radicals,however, attack the oxygen atom of the nitroso-group to form N-alkoxyanilino-radicals which are sufficientlystable for e.s.r. spectra to be recorded. Secondary alkyl radicals usually add both on the nitrogen and oxygenatoms to produce both the corresponding spin adducts, whose ratio varies with the structure of the radical trapped.Solutions of the nitroso-compound produce no detectable paramagnetic species during U.V.irradiation. Therelation between the structure of a spin adduct and the spectral datum is discussed.Osaka, 553, JapanSPIN trapping is an e.s.r. technique developed for theindirect detection and identification of short-lived freeradicals, and has recently received wideReactive free radicals add easily to the diamagneticscavengers (spin traps) to form stable radicals (spinadducts), whose e.s.r. spectra afford information aboutthe structure of the initial free radicals trapped. Twokinds of spin trap, nitroso-compounds and nitrones,which are most commonly utilized, react with variouskinds of free radical to yield relatively stable nitroxides.Previously we employed nitrosobenzene and 2-methyl-(a) Part IV, S.Terabe and R. Konaka, J.C.S. Perkiiz 11,1972, 2136 ; (b) preliminary communication, S. Terabe andR. Konaka, J . Anzer. Chem. Soc., 1971, 93, 4306. * E. G. Janzen, Accynts Chem. Bes., 1971, 4, 31.M. J. Perkins, in Essays on Free-Radical Chemistry,' ed.R. 0. C. Norman, Chem. SOC. Special Publ., 1970, 24, 97.C. Lagercrantz, J . Phys. Chem., 1971, 75, 3466.(a) R. Konaka and S. Terabe, Abstracts, The VIIth E.s.r.Symposium (Japan), Sapporo, 1968, p. 44; Abstracts, TheXXIInd Annual Meeting of the Chemical Society of Japan,Tokyo, 1969, p. 1711; (b) S. Terabe and R.Konaka, J . Amer.Chem. SOC., 1969, 91, 6665; (c) S. Terabe, K. Kuruma, andR. Konaka, Chem. Letters, 1972, 116.See, e.g., (a) A. Mackor, Th. A. J. W. Wajer, and Th. J. deBoer, Tetrahedron, 1968, 24, 1623; (b) C. Lagercrantz and S .Forschult, Natwe, 1968, 218, 1247; K. Torssell, Tetrahedron,1970, 26, 2759; (c) M. J. Perkins, P. Ward, and A. Horsfield,J . Chem. SOC. (B), 1970, 395; ( d ) I. H. Leaver and G. C . Ramsay,Tetrahedron, 1969, 25, 5669.2-nitrosopropane as spin traps to detect and identifyfree-radical intermediates in nickel peroxide oxid-a t i o n ~ . ~ ~ ~ ~ Each of these spin traps has some advantagesand some disadvantages. 2-4 Thus 2-met h yl-Znitroso-propane,6 which is one of the best spin traps, is subjectto photolysis to produce di-t-butyl nitroxide whosespectrum may seriously overlap the most interestingregion. The spectra of spin adducts of N-benzylidene-t-butylamine N-oxide! one of the best alternative spintraps, produce some difficulties in that to identify atrapped radical unequivocally a reference spectrum mustbe prepared in another way.Few other nitroso-com-pounds 5 9 9 and nitrones lo have been examined. SomeTh. A. J. W. Wajer, A. Mackor, Th. J. deBoer, and J. D. W.van Voorst, Tetrahedron, 1967, 23, 4021. * See, e.g., (a) E. G. Janzen and B. J. Blackburn, J . Amev.Chem. Soc., 1969, 91, 4481; (b) M. Iwamura and N. Inamoto,Bull. Chem. SOC. Japan, 1967, 40, 702, 703; 1970, 43, 856, 860.Q See, e.g., A. Mackor, Th. A. J. W. Wajer, Th. J . de Boer,and J .D. W. van Voorst, Tetrahedron Letters, 1966, 2115; J. W.Hartgerink, J. B. F. N. Engberts, Th. A. J . W. Wajer, andTh. J. de Boer, Rec. Trav. chim., 1969, 88, 481; R. Stammer,J. B. F. N. Engberts, and Th. J . de Boer, ibid.. 1970, 89, 169;S. Forshult, C. Lagercrantz, and K. Torssell, Acta Chem. Scand. ,1969,23,622; R. Konaka, S. Terabe, and K. Kuruma, Abstracts,The Xth E.s.r. Symposium (Japan), Osaka, 1971, p. 107.lo See, e.g., G. R. Chalfont, M. J . Perkins, and A. Horsfield,J . Chem. SOC. (B), 1970, 401; M. Iwamura and N. Inamoto,ref. 8b370 J.C.S. Perkin IInew types of spin trap have also been reportedl1J2 inthe expectation that a variety of spin traps will be ofuse in applications to mechanistic studies.2We now describe the utility of 2,4,6-tri-t-butylnitroso-benzene (1) as another type of spin trap and certainadvantages of it compared with other spin traps.Therewere three reasons for its choice. (i) The nitroso-compound (1) exists completely in the active monomericform in the solid state as well as in s01ution.l~ (ii) E.s.r.spectra of spin adducts were expected to be simpler thanthose from nitrosobenzene whose ortho- and para-hydrogen coupling constants are large. (iii) Stericeffects would be revealed in spin-trapping reactions.Various radicals were generated from various sourcesin the presence of 2,4,6-tri-t-butylnitrosobenzene (1)(0.01--0405~) in benzene or in substrates themselves togive spin adducts. Radicals were mainly generatedeither by the abstraction of hydrogen atoms from sub-strates by t-butoxyl radicals (formed by the photolysisof di-t-butyl peroxide or the thermolysis of di-t-butylperoxyoxalate) or by the abstraction of halogens fromalkyl halides by tri-n-butylstannyl radicals.It has been shown that the nitroso-compound (1) hastwo main advantages over other spin traps.The first isthat the nitroso-compound (1) has two trapping sites,the nitrogen and oxygen atoms of the nitroso-group.The N-alkoxyanilino-radicals (3) can be distinguishedfrom the nitroxides (2) by g values and nitrogen couplingconstants. The anilino-radical (3) is sufficiently stableBut&N--R ( 2 )Buf A. t- sd&?-on (3)a u t Q:: i ' NO + Rm( 1 1(for several hours) to observe its spectrum which showsthe hyperfine splitting by the hydrogen atom attachedto the carbon atom adjacent to the oxygen atom.It is,therefore, possible to distinguish between attackingsecondary and tertiary alkyl radicals from the spectrumof the anilino-radical alone. The nitroxide (2) is, ofcourse, much more stable than the anilino-radical (3).The second merit of using the nitroso-compound (1) is itsstability even in solution, especially for photolysis.Thus, irradiation of a benzene solution containing thenitroso-compound (1) by a 1 kW high-pressure mercurylamp gave no detectable e.s.r. signal. This fact makesthe nitroso-compound (1) available for many photo-radical reactions.The nitroxides obtained in these experiments arel1 J. G. Pacifici and H. L. Browning, jun., J .Amer. Chem.l2 B. C . Gilbert, V. Malatesta, and R. 0. C . Norman, J. Amer.l3 R. Okazaki, T. Hosogai, E. Iwadare, M. Hashimoto, andSOL, 1970, 92, 6231.Chem. SOL, 1971, 98, 3290.N. Inamoto, Bull. Chem. SOC. Japan, 1969, 42, 3611.FIGURE 1 A typical spectrum of the nitroxide obtained byradical trapping with 2,4,6-tri-t-butylnitrosobenzene. Thisspectrum shows the adduct of methyl radical (6) generated bythe reaction of methyl iodide with tri-n-butylstannane inbenzene a t room temperature. The smallest couplings areassigned to the two meta-hydrogensI I I10G i IMFIGURE 2 Spectrum of ally1 2,4,6-tri-t-butylphenyl nitroxide(13). The larger 1 : 2 : 1 triplet reveals coupling to the twoequivalent P-hydrogens, and it is apparent that p-hydrogencoupling is larger than nitrogen coupling 3 k FIGURE 3 A typical N-alkoxyanilino-radical spectrum.Thisspectruin was observed on the irradiation of di-t-butyl peroxidein cyclohexane in the presence of 2,4,6-tri-t-butylnitroso-benzene. The coupling constant by the hydrogen atom on thecarbon atom adjacent to the oxygen atom shows the samemagnitude as meta-hydrogen coupling10G1 1 bl 1aFIGURE 4 Spectrum of a mixture of both the nitroxide (26)and the anilino-radical (31) obtained by the irradiation ofdi-t-butyl peroxide in n-butyl alcohol. The nitroxide (a) haslarger g value and nitrogen coupling constant than the anilino-radical (b). The line widths in the nitroxide spectrum arebroadened by unresolved fine coupling1973 371summarized in Table 1 along with radical sources, g Addition of 1-cyano-l-methylethyl radical generatedvalues, and hyperfine splitting constants. The anilino- from azobisisobutyronitrile to the nitroso-compound (1)radicals observed are summarized in Table 2.The afforded N-( l-cyano-l-methylethoxy)-2,4,6-tri-t-butylTABLE 1Nitroxides [structure (2)] formed by the addition of various radicals to 2,4,6-tri-t-butylnitrosobenzene in benzenea t room temperatureEt(6)Prn( 7)Bun(8)CH,COMe(laj tCH2Ph(16) tCH,*OPh(l7) tCH,.OH(18) tPri(l9)CHMeEt(20)CHMePh(21) tCHMe*[CH,],.Me orCHEt.[CHJ2-Me(22) t 1CH(OH)Me(23) i.CH(OH)Et(24) t $CH(OH)Prn(25) t $Ph (26)S Ph (2 7)Radical source *Thv + DTBPT + Me1(ButO*OCO) ahv + (PhCMe,O),T + EtBrh v + (Et,Sn),T + PrnBrT + BunBrhv + DTBP + Bun,P(Me.[CH,I,o*CO,) 2 T + Ph*[CHJ,.BrT + Cl*[CH,],*BrT + Me,CH*CH,BrT + CH;:CH-CH,BrT + CHX*CH,Brhv + Me,COhv + DTBP + PhMe(ButO-OCO), + PhMehv + DTBP + PhOMehv + DTBP + MeOHhv + PhCOPh + MeOHT + Pr’BrT + EtCHMeBrhv + DTBP + PhEthv + DTBP + Me.[CH,],.Mehv + DTBP + EtOHhv + DTBP + PrnOHhv + DTBP + BunOH(PhCO2) 2Izv + (PhS),* T = Tri-n-butylstannsne, DTBP = di-t-butyl peroxide.Table 2).g Value2.006 12.00602.00602.00602.00612.00612.00612.00612.00612.00602-00622.00602.00612.00642.00602.00602.00612.00602.00602.00602.00602.00612.00572.0066UN11-6513.0313.4613.3913.4413-4613.4513.4013.5713.4013.1413.4713.6212.2313.7313.2913-3313-5913.1612.6012-6912.369-8816.29Hyperfine splitting constants (G)am-H1.030.810.830.820-780.790.780.820.780.840.880.820.830.820.950.760.770.89a0.961-021.050.630-85US-H12.3317.9917.8317.9717-9717.8817.5618.2716.4215.5014-9314-759.8513.7322.1922.2518-7221.7714.4014-3914.37Other12.96 (a-H)0.3 8 (6y-H)0.39 (By-H)0.1 9 (1 2H) bt In solution of substrate.$ Anilino-radical also observed (seea Not observed because of line broadening. b Assignment unknown. c Couplings by 0’- and p’-H, but assignment not established.TABLE 2Anilino-radicals [structure (3)] formed by the addition of various radicals to 2,4,6-tri-t-butylnitrosobenzene in benzeneat room temperatureHyperfine splitting constants (G)RPri (28)CHMeEt(29)Cyclohexyl(30) tCH(OH)Prn(31) tC(CN)Me,(33)SiEt (34)SnEtJ36)COPh( 36)But (32) {Radical source *See Table 1See Table 1hv + DTBP + cyclohexaneSee Table 1T + ButBrkv + DTBP + (EtO),P[Me,C(CN) N:] ,hv + DTBP + Et,SiHhv + DTBP + PhCHOhv + (EtSSn),g Value2.00402.00412.00402.00402.00402.00372.00402.00402.0036aN11-0110.9110.9510.7210.2610.0110.339.9810.53am-H UB-H1.82 1 - 82 ( 1 H)1-66 1*66(1H)1.79 1*79(1H)1.70 1-70( 1H)1.901.981-961-962.07* See Table 1.t In solution of substrate.structures of spin adducts formed are reasonably anilino-radical (33) (Table 2). The structural assign-deduced from their e.s.r.spectra. ment of this spin adduct has been des~ribed.lb.~* E.s.r.l4 T. Hosogai, N. Inamoto, and R. Okazaki, J. Ckem. SOC. are shown in Figures 1 4 .(c), 1971, 3399.Some typical spectraAddition of Alkyl and Substituted Alkyl Radicals.372 J.C.S. Perkin I1spectra of N-alkoxy-N-alkylamino-radicals have beenreported.15Primary alkyl radicals reacted with the nitroso-compound (1) to form exclusively nitroxides (2) in thenormal course of addition, and anilino-radicals (3) werenot detected at all. Secondary alkyl radicals added atboth nitrogen and oxygen atoms to give nitroxides (2)and anilino-radicals (3). The ratio of spin adducts maybe related to steric interference around the radicalcentre in the radical trapped.Thus 1-hydroxypropylradical produced by hydrogen abstraction from n-propyl alcohol by t-butoxyl radical yielded both thenitroxide (24) and the anilino-radical [3; R =CH(OH)Et] in an initial ratio of ca. 3 : 1, while 1-hydroxybutyl radical from n-butyl alcohol yielded bothspin adducts (25) and (31) in an initial ratio of ca. 1 : 2(see Figure 4). Only the nitroxide (23), however, wasobserved in the reaction of l-hydroxyethyl radical.Tertiary alkyl radicals attacked the nitroso-group at theoxygen atom exclusively.Most carbon-centred radicals reacted with the nitroso-compound (1) to give nitroxide (2) and/or anilino-radical(3). Photolysis of di-t-butyl peroxide in chloroform orin methylene dichloride in the presence of the nitroso-compound (1) did not yield any paramagnetic species.This indicates that although hydrogen atoms should beabstracted easily from chloroform or methylene di-chloride by t-butoxyl radical, the radicals formed maynot be scavenged by the nitroso-compound (1).How-ever, perfluoroalkyl radicals are trapped readily.16 Thenitroxide (5) resulting from the p-scission of t-butoxylradical was recognized when hydrogen abstraction byt-butoxyl radical proceeds relatively slowly (see Table 1).The photolysis of di-t-butyl peroxide in isopropylalcohol produced the e.s.r. spectrum of 2,4,6-tri-t-butyl-phenyl nitroxide (4) in high concentration together witha weak signal possibly assigned to the anilino-radical (3;R = CMe,OH).The spectral data of the latter spinadduct, however, could not be determined because of thesuperposition of the strong signal of the nitroxide (4).Formation of the hydrogen atom adduct has beenreported on spin trapping by use of 2-methyl-2-nitroso-propane instead of the nitroso-compound (1) in a similarreaction,6c while the nitroxide formed by the trappingof the l-hydroxy-l-methylethyl radical was referred toin the photolytic reduction of benzophenone in isopropylalcohol.6d Structural assignment of the latter nitroxideseems not to be unequivocal. The formation of theformer spin adduct may be the result of either hydrogen-atom transfer from the 1-hydroxy-l-methylethyl radicalto the nitroso-group 6c or intramolecular proton transferin the l-hydroxy- 1 -rnethyle thy1 adduct followed byelimination of acetone.17l5 W.C. Danen and C. T. West, J. Amer. Chem. SOC., 1971, 98,l6 R. Iconaka and S. Terabe, Abstracts, The Xth E.s.r.l7 P. B. Ayscough, R. C. Sealy, and D. E. Woods, J . Phys.lo R. M. Fantazier and M. L. Poutsma, J. Amer. Chem. SOC.,6682.Symposium (Japan), Osaka, 1971, p. 110.Chem.. 1971, 75, 34S4.1968, 90, 6490.Propargyl nitroxide (14) was the sole adduct obtainedon the reduction of propargyl bromide with tri-n-butyl-stannane, though the product study of the reduction ofpropargyl chloride with the same reagent at 65 "C showedthat propyne and allene were produced in the ratio of5.9 : l.18 The failure to observe the propadienyl-radicaladduct may be due to its instability and/or to its slowaddition to the nitroso-group compared with thepropargyl radical.Additions of Radicals other than Alkyl Radicals.-Phenyl radical generated by the photolysis or thermo-lysis of dibenzoyl peroxide added readily to the nitroso-compound (1) to give 2,4,6-tri-t-butyldiphenyl nitroxide(26).The nitrogen coupling constant (9.88 G) issmaller than those of alkyl phenyl nitroxides (10.4-12.3 G),1a95a96 and close to the value for diphenylnitroxide ( a N = 9.77 G).19 The three hydrogens at theortho- and para-positions in alkyl phenyl nitroxides aregenerally equivalent as described previously.la Thesethree hydrogens in the nitroxide (26), however, are notequivalent. Perhaps the two ortho-hydrogens are notequivalent because of the significant steric hindrance tofree rotation of the phenyl group.Phenoxyanilino-radicals (3; R = Ph) were not detected in this reaction.Photolysis of diphenyl disulphide in the presence ofthe nitroso-compound (1) gave a spectrum of a triplet(1 : 1 : 1) of triplets (1 : 2 : 1) whose g value and nitrogencoupling constant are larger than other nitroxides inTable 1. The structural assignment of this spin adduct(27) is supported by the fact that t-butyl alkylthio- andphenylthio-nitroxides 6d,20 show larger g values andnitrogen coupling constants than alkyl t-butyl nitroxides.The S-nitroxide (27) was very unstable and could only beobserved during irradiation. The spectrum decayedrapidly when the light source was removed.The formation of only the anilino-radical (36) frombenzoyl radical is interesting, because acyl radicals havebeen known to produce stable nitroxides by addition tonitroso-compounds.6a The above observation suggeststhat the benzoyl radical is as large as tertiary alkylradicals.No spin adduct was obtained on the oxidation ofcarbazole and N-methylaniline with nickel peroxide, inwhich the presence of nitrogen radical intermediates hasbeen shown p r e v i o u ~ l y .~ ~ ~ ~ ~Triethylsilyl and triethylstannyl radicals exclusivelyadded a t the oxygen atom to the nitroso-compound (1) asdid the tertiary alkyl radicals [see (34) and (35) inTable 21. Steric hindrance in the addition may forcethese radicals to give only the anilino-radicals. Thestrong 0-Si and relatively strong O-Sn bonds 21 make itpossible to form anilino-radicals. In addition, only afew silyl nitroxides are known.22The absence of the tri-n-butylstannyl-radical adductsl9 E.T. Strom, A. L. Bluhm, and J. Weinstein, J . Org. Chem.,2o I. H. Leaver, G. C. Ranisay, and E. Suzuki, Austral. J .22 R. West and P. Boudjouk, J. Amer. Chem. Soc., 1971, 95,1967, 32, 3863.Chem., 1969, 22, 1891.R. A. Jackson, Adv. Free Radical Chem., 1969, 8, 231.69011973 373in spin trapping studies of the reduction of alkyl halideswith tri-n-butylstannane may imply that the rate ofisopropyl nitroxide (19) indicates that the dihedral anglebetween the CB-H bond and the axis of the $,-orbital onthe nitrogen atom comes very close to 0" as shown in + R* (l) structure (I).The fact that the largest 8-hydrogen Bunasp + RX - Bu1I3Sn- + (1) + Arp-OSnBun3R* + Bun,SnH + RH + Bun3Sn*(2)(3)/'R* + (1) --+ ArN(0.)-R (4)Ar = 2,4,6-But,-C,H, Mereaction (1) is much faster than that of reaction (2), Meand that reaction (4) proceeds a t a rate at least com-parable with that of reaction (3). These findings areconsistent with the results of kinetic studies by Carlssonand Ingold 23 that reaction (3) is the rate-controllingstep for the chain propagation in reductions of alkylbromides and iodides with alkylstannanes.In spin-trapping studies of alkyl radicals produced byreduction of alkyl halides by tri-n-butylstannane, eachalkyl halide was mixed with the nitroso-compound (1)in benzene as a control test, and a search for para-magnetic species by e.s.r.was performed, but no radicalspecies could be observed.HyperJine Splitting Constants and Strztctzwal Assign-ment.-Of interest is the finding that the order ofmagnitude of the @-hydrogen couplings in the nitroxidesobserved is secondary > primary > methyl for theinitial radicals trapped (see Table l), in contrast to thatfound in the usual nitroxides.a* 25 @-Hydrogen couplingsin alkyl phenyl nitroxides, for example, are 10.4, 8.3, and2.1 G when alkyls are methyl, ethyl, and isopropyl,respectively.% 8-Hydrogen coupling is known to dependon its relative orientation to the $,-orbital containing theodd electron 26 and it is considered that the conformationof an alkyl group is largely governed by steric repulsionbetween the phenyl group and the N-alkyl group inalkyl phenyl nitroxide~.~~ Thus the preferred dihedralangle between the Cg-H bond and the $,-orbital in alkylphenyl nitroxides has the largest value in isopropylphenyl nitroxide, leading to the smallest value for thep-hydrogen coupling as expected from the cos20 law.26In the case of alkyl 2,4,6-tri-t-butylphenyl nitroxides,however, the orientation of p-hydrogen atoms may besubstantially affected by the non-bonded interactionbetween the N-alkyl groups and the two ortho-t-butylgroups.Steric hindrance from the two ortho-t-butylgroups would constrain the N-0 bond to twist at about90" from the plane of the phenyl group. The largenitrogen coupling (13.0-13-7 G) compared with that ofalkyl phenyl nitroxides (104-12-3 G) la convincinglydemonstrates the above view.Such a steric situation isbest understood by inspection of molecular modelsconsisting of atomic models having dimensions pro-portional to van der Waals radii. Such a model of23 D. J. Carlsson and K. U. Ingold, J . Amer. Chem. SOC.,1968, 90, 7047.G. Chapelet-Letourneux, H. Lemaire, R. Lenk, M.-A.Mardchal, and A. Rassat, Bull. SOC. chim. France, 1968, 3963;A. R. Forrester, J. M. Hay, and R. H. Thomson, ' OrganicChemistry of Stable Free Radicals,' Academic Press, London,1968, chap. 6.coupling constant observed appears in isopropyl nitroxide(19) supports the above consideration. In the case ofthe nitroxide (6) the average dihedral angle predicted is30" which is consistent with the observed [3-hydrogenDistinction among nitroxides (6)-( 10) is difficult(see Table 1).However, in nitroxides (11)-(18) themethylene group shows different p-hydrogen couplings.The differences of the @-hydrogen coupling constants inthese series of nitroxides seem to reflect mainly theelectronegativity effect of the substituent on the f3-carbon atom rather than a change of the relative con-formation of the @-methylene group to the nitroxylgroup. This tendency could be investigated moreclosely by a study of the temperature-dependence of thee.s.r. spectra of these nitroxides.16In the case of secondary alkyl-radical adducts theelectronegativity effect is also probably an importantfactor influencing the P-hydrogen coupling constant.Thus, the secondary alkyl nitroxides (19), (21), and (23)can be successfully distinguished by the difference in theP-hydrogen coupling constants, but distinction betweennitroxides (19) and (20), or among (23), (24), and (25) isalmost impossible from spectra of the nitroxides alone.However, the observed ratios of nitroxides (2) andanilino-radicals (3) are also available for identification asdescribed above.The e.s.r. spectra of the anilino-radicals (33)-(36) were little different (Table 2).Unfortunately it seems that 2,4,6-tri- t-but ylnit roso-benzene (1) is not a good spin trap for radicals other thanalkyl. However, for alkyl radicals it is better thanother spin traps because the spectrum of the spin adductis more susceptible to variation in the nature andstructure of the radical trapped.Further, the nitroso-compound (1) has good prospects for applications tophotoradical reactions owing to its stability towardsphotolysis.couplings.EXPERIMENTALE.s.r. Spectra.-General experimental procedures havebeen described.la An inverted U-type mixing cell 27 wasused for all experiments. In halogen abstractions, benzenesolution (ca. 1 ml) containing 0.02-0-001~-2,4,6-tri-t-butylnitrosobenzene (1) and O.S~-alkyl halide was mixed26 E. G. Janzen, Topics Stereochem., 1971, 6, 177.26 See, e.g., D. H. Geske, Progr. Phys. Org. Chem., 1967, 4,2' G. A. Russell, E. G. Janzen, and E. T. Strom, J . Amer.125.Chem. SOC., 1964, 86, 1807374with a benzene solution (ca.1 ml) containing O.S~-tri-n-butylstannane at room or somewhat higher (60 "C) tem-peratures. For hydrogen abstractions, ca. 2 ml of a sub-strate containing 0-0 1-0~005~-nitroso-compound (1) anddi-t-butyl peroxide (0.2 ml) was irradiated in situ in thecavity of the spectrometer at 250 f. 10 nm. For photolysisor thermolysis of azobisisobutyronitrile (at 365 f 10 nm ora t 60 "C), dilauroyl peroxide (at 250 f 10 nm or at 70 "C),hexaethyldistannane (at 310 f 10 nm for the generation oftriethylstannyl radical and at 250 f 10 nm for the gener-ation of ethyl radical), dicumyl peroxide (at 250 f 10 nm),diphenyl disulphide (at > 3 10 nm) , di-t-butyl peroxyoxalate(at room temperature), and dibenzoyl peroxide (at 80 "C),benzene solutions (ca.2 ml) of 0~005~-nitroso-compound (1)and 0-01--0-1~ of the radical source were used. Benzo-phenone (0.005~ in benzene) and acetone (neat) wereirradiated a t 365 3 10 and 250 & 10 nm, respectively.For irradiation, the output from a Shimazu-Bausch andLomb high-intensity grating monochromator with a high-pressure mercury lamp source was focused on the samplewith a quartz lens.Spectra were recorded on a Varian V-4502-15 X-bandspectrometer with 100 kHz magnetic-field modulation in aflat quartz cell for polar substrates or in a cylindrical quartzcell for non-polar substrates at room temperature. The28 P. D. Bartlett, E. P. Benzing, and R. E. Pincock, J . Amer.Claem. SOC., 1960, 82, 1762.J.C.S. Perkin I1determination of hyperfine splitting constants and g valueshas been given,la and the values listed in Tables 1 and 2were averages of a t least three measurements. Spectralsimulation was not carried out because the simple spectradid not require it.Reagents.-Most of the compounds used were com-mercial, and purified by crystallisation or distillation ifnecessary. Alkyl halides were passed through a column ofactive aluminium oxide immediately before use. Thenitroso-compound (1) was prepared by the oxidation of2,4,6-tri-t-butylaniline with m-chloroperbenzoic acid.13 Di-t-butyl peroxide distilled a t reduced pressure gave one peakon g.l.c., and di-t-butyl peroxyoxalate was obtained by themethod of Bartlett et aLa8 Tri-n-butylstannane was pre-pared by the reaction of hexa-n-butyldistannoxane withpolymethylhydrosiloxane.*Q Hexaethyldistannane wassupplied by Professor Y. Sat0 of Nagoya City University.We thank Dr. K. Nishikida and Mr. S. Sakata for assist-ance in the e.s.r. experiments, Mr. K. Kuruma for assistancein preparative work, and Professor E. G. Janzen of theUniversity of Georgia for discussions.[2/1680 Received, 17th July, 197212D K. Itoi and S. Kumano, J. Chem. SOC. Jopan, Ind. Chem.Sect., 1967, 70, 82; K. Hayashi, J. Iyoda, and I. Shiihara, J.OrganometaZZic Chem., 1967, 10, 81
ISSN:1472-779X
DOI:10.1039/P29730000369
出版商:RSC
年代:1973
数据来源: RSC
|
|