763
Communications to the Editor Fragmentation of a-Alkoxyalkyl Radicals. An Electron Paramagnetic Resonance Study Publication costs assisted by the lnsfituffur Strahlenchemie lm Max-Plancklnsfitute fur Kohlen forschung
Sir: From product studies of reactions involving a-alkoxyalkyl radicals it has been i n f e ~ ~ e dthat l - ~ in many cases these radicals undergo a C-0 bond scission ((3 scission) to yield a carbonyl compound and an alkyl radical: ./R1
R,-0-C
--+
R'
R,
+ O=C
/R2
(1)
R ',
I
An analogous fragmentation reaction has been observed to occur with cu-dialkoxyalkyl and a-hydroxy-a-alkoxyalkyl radicah6-lZ So far, however, there have been no attempts to directly demonstrate the occurrence of reaction 1 by physical methods such as EPR. In previous EPR work on ether radicals, emphasis was placed on characterization of the primary radicals formed by H abstraction13-17 and, with one notable exception involving the methyloxiranyl radica1,ls radicals formed by fragmentation of these species were not observed. Product studies on the liquid-phase uv photolysis of tert-butyl methyl etherlg and di-tert-butyl ether4 a t room temperature had indicated that the tert- butoxyisopropyl radical (R1 = tert-butyl, Ra = R3 = methyl) fragments quantitatively according to (1) whereas .the homologous tert-butoxymethyl radical (R1 = tert-butyl, Rz = R3 = H )
is stable with respect to fragmentation under these conditions. EPR studies were expected to yield supporting information on this and analogous systems. Alkoxyalkyl radicals were produced by photolyzing ditert-butyl peroxide (3 vol %) in benzene containing 5-7.5 vol % of the ethers 1-10 (Table I). The photolytically formed tert- butoxy radicals abstract hydrogen from the ethers to yield t-BuOH and a-alkoxyalkyl radicals which may be stable under the experimental conditions or fragment according to (1). The results obtained with various ethers are summarized in Table I. In the series of tertbutyl ethers (1-3) the radical derived from 1 does not fragment a t 30° whereas with 3 only the fragment radical is observed a t the lowest attainable temperature of 0'. 2 occupies its expected place in this series, the primary radical being the only one observable a t 0" whereas considerable fragmentation is found a t 30". It is suggested that (a) F and B straina0 in the primary radical and (b) the stabilizing influence of the alkyl group on the carbonyl bond of the fragmentation product are responsible for this trend. Concerning the latter, it may be estimated by Benson's method of group incrementsz1that the stabilization of the carbonyl group effected by substituting CH3 for H in formaldehyde (Le., in going from formaldehyde to acetone) is of the order of 3.8 kcal/mol. From this it may be inferred that the rate of fragmentation of the primary species derived f'rom 3 should be between 100 and 1000 times larger than that of the primary radical derived from 1. The primary radicals formed on H abstraction from the cumyl ethers 4 and 5 begin to show fragmentation a t 0" (Figure 1). In this series, the fragmentation process is clear-
TABLE I: Radicals Observed on H Abstraction from Various Ethers No.
Ether I-BuOCH, t-BuOCHqCH3
Temp, "C
Radicals detected
Structure
Coupling constants, mT
0 and 30 0
I-BUOCH~ I-BUOCHCH,
= 1.66(2), aCHJ= 0.03(9) na = 1.35, U , = 2.16(3),
30
I -B U O ~ H C H , ,
aCH = 0.02(9) a, = 2.27(9)
(CH3)3k PhC(6H3),0CH3
0 0 and 30
PhC(CH3)20CH,CH,
0 and 30
PhCHqOCHZPh
0 and 30
I-BUOCH(cH,),
PhC(CH,)20bHCH3, Phk(CH,), PhCHOCH, P h
PhC(CH,)?ObHCH,
a, = 2.27 (9) C Z = ~ 1.70(2) 0, = 1.60(6), u,,= 0.47(2), a,,, = 0.16(2), ap = 0.55 u a = 1.42, a, = 2.19(3)
PhkHOCH2Ph
aa =
1.52, noCH2 = 0.14(2),
al = 0.15,, 02 =
0.163,
~3
= 0.46,
04
= 0.51,
a5 = 0.58
P hCHZOCH,
0 and 30
Ph6HOCH3
P hCHq OCHqCH3
0 and 30
Ph6HOCH2CH3, PhtH,
PhbHOCH,
(la
= 1.51, noCH = 0.14(3), 0.16, a, = 0.45,
al = 0.15, a2 = 3 ab =
10
Ph3COCH,
0 and 30
5
= 0.155, Uq = 0.165, = 0.45, 04 = 0.50, a5 = 0.58 f l u = 1.63(2), U , = 0.51(2), a, = 0.18(2), np = 0.62 ~a 11.51, u O C H ( = 0.10, 01
Ph6Hz PhCHqOCH(CH3),
0.50. a5 = 0.57
= 1.52, aoCHZ = 0.14(2),
PhCHOCH(CH,),, PhCHq
PhCHOCH (CH,),
Ph,b
Ph3d
a C H= j O.Ol(6) = 0.155, = 0.165, a3 = 0.45, a4 = 0.50, 0 5 = 0.58 U , = 0.25(6), a, = 0,11(6), CI, = 0.28(3) The Journal of Physicai Chemistry. Voi. 79, No. 7 . 1975
Communications to the Editor
764
Figure 1. Radicals observed on H abstraction from cumyl ethyl ether in benzene at 0': (a) spectrum of PhC(CH3)2 OCHCH3 recorded at 1-mW microwave power and 0.05 mT modulation amplitude: (b) central part of the spectrum of PhC(CH3)2 showing second-order structure. The spectrum was recorded using a microwave power of 0.2 mW and a modulation amplitude of 0.004 mT. ly facilitated by resonance stabilization of the product radical which amounts to more than 11.2 kcal/mol. 22,23 The effect is even more pronounced with the radical produced from 10 which shows complete fragmentation a t 5". With the primary radical from 5 the activation energy of fragmentation was determined, using the method described by Hamilton and F i ~ c h e rto , ~be ~ 8 f 1 kcal/mol. In the series of benzyl ethers (6-9) fragmentation does not seem to occur if the primary radical is of the benzyl type. With 7, exclusive H abstraction a t the benzylic carbon is observed. With 8 and 9, however, H abstraction from the ethyl and isopropyl groups, respectively, seems to take place in addition to H abstraction from the benzylic carbon. This is concluded from the presence of the fragmentation products of the expected primary radicals. The latter radicals could not be identified unambiguously due to interference with the intense lines from the benzyl type radicals. References and Notes (1)C Walling and M F. Mintz, J. Am. Cbem. Soc., 89, 1515 (1967). (2)C.Walling and J. A. McGuiness, J. Am. Cbem. SOC.,91, 2053 (1969). (3)J. W. Timberlake and M. L. Hodges, Tetrahedron Lett., 4147 (1970). (4)H.-P. Schuchmann and C. von Sonntag, Tetrahedron, 29, 3351 (1973). (5) M. E. Snook and G. A. Hamilton, J. Am. Chem. Soc., 96, 860 (1974).
(6)E. S.Huyser, J. Org. Cbem., 25, 1820 (1960). (7)E. S . Huyser and D. T. Wang, J. Org. Chem., 27, 4816 (1962). (8)E. S.Huyser and D. T. Wang, J. Org. Chem., 29, 2720 (1964). (9)V. Hartmann, C. von Sonntaq, and D. Schuite-Frohlinde, 2. Naturforsch. 6, 25, 1394 (1970). (IO) B. Mailiard, M. Cazaux, and R. Laiande, Bull. SOC. Cbem. Fr., 467 11971) j . - . .,.
(11) C. von Sonntag and M. Dizdaroglu, 2. Naturforsch. 6,28, 367 (1973). (12)C. von Sonntag, K. Neuwaid, and M. Dizdarogiu, Radiat. Res., 58, 1 (1974). (13)P. L. Kolker, J. Chem. SOC.,5929 (1964). (14)P.L. Kolker and W. A . Waters, J. Cbem. SOC., 1136 (1964). (15)W. T. Dixon, R. 0. C. Norman, and A. Buley, J. Cbem. Soc., 3625 (1964). (16)L. E. Burchill and P. W. Jones, Can. J. Cbem., 49, 4005 (1971). (17)A. Hudson and K. D. J. Root, Tetrahedron, 25, 531 1 (1969). (18)G. Behrens and D. Schulte-Frohiinde, Angew. Cbem., 85, 993 (1973); Angew. Cbem., int. Ed. Engi., 12, 932 (1973). (19)H.-P. Schuchmann and C. von Sonntag, Tetrahedron, 29, 181 1 (1973). (20)C. Ruchardt, Angew. Chem., 82, 845 (1970). (21)S. W. Benson, "Thermochemical Kinetics", Wiley, New York, N.Y., 1968,p 180. (22)D. J. Bogan and D. W. Setser, J. Am. Chem. SOC., 96, 1950 (1974). (23)R. Walsh, D. M. Golden, and S.W. Benson, J. Am. Cbem. SOC.,88,650 (1966). (24)E. J. Hamilton and H. Fischer, Heiv. Cbim. Acta, 56,795 (1973).
lnstitut fur Strahlenchemie im Max-flanck-lnstitut fur Kohlenforschung 0-433 Mulheim/Ruhr West Germany Received October 10, 1974
S.Steenken' H.-P. Schuchmann C. von Sonntag
