J . Am. Chem. SOC.1986, 108, 2808-28 13
2808
Radiation Chemical Production and Lifetimes of Trialkoxymethyl Carbocations in Aqueous Solution S. Steenken,*' J. Buschek, and R. A. McClelland2 Contribution from the Max-Planck-Institut fur Strahlenchemie, 0-4330 Mulheim, West Germany. Received June 20, 1985
Abstract: Trialkoxymethyl radicals were produced in aqueous solution by H abstraction from trialkoxymethanes by OH and H. The radicals were then oxidized by one-electron transfer to give trialkoxymethyl carbocations. The reactions of the cations with water and with OH- were studied by using time-resolved conductance techniques. (CH30)&+ hydrolyzes with a rate constant of (1.4 f 0.2) X I O 3 s-' and it reacts with OH- with k = (7 f 1) X IO7 M-' s-l. T h e activation parameters for the reaction with water are EA = 7.6 kcal/mol and AS' = -20.2 eu. Replacement of methoxy in the cation by ethoxy and isopropoxy leads to a pronounced decrease in the rate constants for these processes: the Taft p* values for reaction of (RO)J+ with H20and OH- are 5.7 and 3.7, respectively. With cyclic trialkoxymethyl cations that from the six-membered 1,3-dioxane system is more stable than that from 1,3-dioxolane, the five-atom counterpart. Trialkoxymethyl cations a r e also produced by a @-fragmentation reaction from a-(P-tria1koxy)methoxyalkyl cations.
a-Hydroxyalkyl and a-alkoxyalkyl radicals can be conveniently produced by H abstraction from alcohols, ethers, and acetals. These radicals have long been known to be oxidizable by a variety of organic and inorganic oxidants.M If, as assumed: the oxidation reactions proceed by one-electron transfer, the radicals are converted into carbocations by t h e oxidation process. Generation of a radical by H abstraction from a molecule followed by oneelectron oxidation of t h e radical therefore constitutes a general m e t h o d of carbocation production equivalent t o abstracting a hydride ion from t h e precursor molecule. If radical production and its oxidation can be made t o occur in periods shorter t h a n t h e carbocation lifetime, t h e physical and kinetic properties of t h e carbocation can be studied.'^^ This approach has previously been taken t o generate simple (not substituted by heteroatoms) aliphatic carbocations;' however, their lifetime was too short t o permit their study. In the present paper the method of carbocation formation by consecutive removal of electrons (plus a proton) is applied t o t h e other extreme, Le., t o systems t h a t are multiply stabilized by heteroatoms a t C,:
(RO)$2H
-n'
(RO),C l % (R0)3C+
Trialkoxy carbocations were first prepared a n d isolated by Meerwein and co-workers,I0*" and their physical properties have been studied in nonaqueous solution.I2 Recently, estimates of t h e lifetimes of various mono-, di-, a n d trialkoxy carbocations in aqueous solution have been r e p ~ r t e d . ' ~ . 'However, ~ no direct ~~~~
~
~~
( I ) This author dedicates the paper to Prof. G. Wilke on the occasion of his 60th birthday. (2) Department of Chemistry, University of Toronto, Canada. (3) Adams, G. E.; Willson, R. L. Trans. Faraday SOC.1969, 65, 2981. (4) Asmus, K.-D.; Mkkel, H.; Henglein, A. J . Phys. Chem. 1973, 77, 1218.
(5) Eibenberger, J.; Schulte-Frohlinde, D.; Steenken, S. J . Phys. Chem. 1980, 84, 704.
(6) For reviews see: (a) Neta, P. Adu. Phys. Org. Chem. 1976, 12, 2. (b) Henglein, A. Electroanal. Chem. 1976, 9, 163. (c) Swallow, A. J. Prog. React. Kinel. 1978, 9, 195. ( 7 ) Haysom, H. R.; Phillips, J. M.; Scholes, G. J . Chem. SOC.,Chem. Commun. 1972, 1082. Asmus, K.-D.; Deeble, D. J.; Garner, A,; Idriss Ali, K. M.; Scholes, G. Brit. J . Cancer, Suppl. III 1978, 37, 46. (8) Carbocations have also been produced by radiation-chemical methods in nonaqueous solutions. See, e&: Dorfman, L. M.; Sujdak, R. J.; Bockrath, B. Acr. Chem. Res. 1976, 9, 352. Dorfman, L. M.; Wang, Y.; Wang, H.-Y.; Sujdak, R. J. Discuss. Faraday. SOC.1977, 63, 149. (9) Steenken, S.; Neta, P. J . Am. Chem. SOC.1982, 104, 1244. (IO) Meerwein, H.; Bodenbenner, K.; Borner, P.; Kunert. F.; Wunderlich, K. Ann. 1960, 632, 30. Meerwein, H.; Hederich, V.; Morschel, H.; Wunderlich, K. Ann. 1960, 635, I . (1 1) For a review concerning these and other long-lived carbocations see: Olah, G. Chem. Scr. 1981, 18, 97. (12) Ramsey, B. G.; Taft, R. W. J . Am. Chem. SOC.1966, 88, 3058.
0002-7863/86/lS08-2808$01.50/0
measurement of such lifetimes in water seems t o have been carried out. We report here t h e first direct measurement of the rates of reaction of trialkoxy carbocations in aqueous solution. These rates are of interest due t o t h e role of alkoxy carbocations in t h e acid-catalyzed hydrolysis of acetals, ketals, and o r t h o esters.14-17 Evidence is also presented for a novel unimolecular fragmentation reaction of monoalkoxy carbocations.
Experimental Section Acyclic ortho esters were commercially available. They were purified (to a purity 299% (GC)) by fractional distillation and stored in sealed ampules under Ar. 2-Methoxy(or isopropoxy)dioxolane and 2-methoxy-1,3-dioxane were prepared by ortho ester exchange of trimethyl(or isopropyl) orthoformate and 1 equiv of the appropriate diol. Sodium hexachloroiridate (IV), potassium hexacyanoferrate (111), and tetranitromethane were of the highest purity commercially available and were used as received. The triethoxymethyl carbocation was prepared as its BF4- salt by treatment of tetraethyl orthocarbonate with BF, etherate, following cation was standard procedure^.'^^" The 2-methoxy-l,3-dioxolan-2-ylium prepared as a solution in 96% H 2 S 0 , by careful addition of 2,2-dimethoxy-1,3-dioxolane. The latter compound was prepared by the ortho ester interchange reaction of tetramethyl orthocarbonate and 1 equiv of ethylene glycol. First-order rate constants for the hydrolysis of the two cations were determined in aqueous H2S04, 68-75% H2S04 for (CH3CH,0)3C+ and 83-90% H 2 S 0 4for 2-methoxy- 1,3-dioxolan-2-ylium cation, following changes in the N M R spectra. Pulse radiolysis was performed on solutions prepared with reagent grade water from a Millipore Milli-Q system. The solutions were saturated with N 2 0 to convert eaq- into O H , and they were then irradiated with 0.1- to I-ps pulses from a 3 MeV van de Graaff accelerator with dose rates of 300 rads per pulse, thus producing initial radical concentrations of =2 pM. Substrate concentrations were 5-50 mM, oxidant concentrations were 0.08-1 mM, and the pH was varied from 7 to 12 with barium hydroxide. Experiments to obtain rates in nonalkaline solutions were carried out between pH 6.5 and 5 . 5 , the pH being adjusted with HC104. Production and decay of the alkoxyalkyl carbocations was monitored by observing changes in the conductance of the solutions. Conductance experiments were carried out with a 10 MHz AC or a DC method. The signals were fed into a Biomation 8100 transient recorder interfaced with a VAX 11/38 computer via a PDP 11/10, The digitized data were stored and analyzed with the VAX. The data were referenced to three dosimetry systems: (a) a solution at pH 4-5 containing 0.1 M rert-butyl alcohol and saturated with CH,CI for which G(HC1) = 3.14;'' (b) a N 2 0 (13) McClelland, R. A,; Ahmad, M. J . Am. Chem. SOC.1978, 100, 7027, 703 1. (14) McClelland, R. A,; Santry, L. J. Ace. Chem. Res. 1983, 16, 394. (15) Cordes, E. H. Prog. Phys. Org. Chem. 1967, 4, 1. (16) Fife, T. H. Acc. Chem. Res. 1972, 5 , 264. (17) Cordes, E. H.; Bull, H . G. Chem. Rev. 1974, 74, 581. (18) Balkas, T. 1.; Fendler, J. H.; Schuler, R. H. J . Phys. Chem. 1970, 74, 4497.
0 1 9 8 6 A m e r i c a n Chemical Society
Trialkoxymethyl Carbocations in Aqueous Solution
...'......,.,..
......... .................................
J . Am. Chem. SOC.,Vol. 108, No. 11, 1986 2809
-
The results described are explained by eq 1-8. Equation 1
*
H20
H+, OH, H, eaq-, OH-
whereby [H'] = [e,,-]
EC
ea,-
b + U
3
(CH30)$H
U
C
0 0
+(CH30)3COH
-
(1)
+ [OH-]
+ N 2 0 + H 2 0 OH- + O H + N2 H+ + OH- H 2 0 + O H ( H ) 2 (CH,O),C + H20(H2)
-
-
+ H@
b
I (CH,O)2CHOCH,
I1
(2) (3)
+ H2O(H2) (4)
1-hms-l
4 pu Ise
Time
Figure 1. Conductance change observed on producing (CH30)2CHOCH2and (CH,O),C in the presence of 0.5 mM Fe(cN)6p at pH 6.2. [(CH,O),CH] = 10 mM. saturated solution at pH =5 that contained 10 mM dimethyl sulfoxide for which, as compared to solution a, G(H+) = G(CH3SO2-)I9was found to be 6.0; and (c) a N,O saturated solution at pH 5-6 containing 0.1 M methanol and 0.5 mM oxidant (Ox) for which G(H+) = G(Ox'-) = C(OH H) = 6.6. By comparing the amplitudes of the signals from system c with those from system b and taking 3 1 5 and 35 cm2 ohm-' mol-l for the ion mobilities of H+ and CH3SOF, respectively, the dif ference in the ion mobilities between Fe(CN)6p and Fe(CN),+ and that between IrC16"- and IrCI6' was determined to be 10 and 100 cm2 ohm-I mol-I, respectively. These numbers have been used to calculate from the measured conductance changes the equivalents of H+ produced per equivalent of radical.
+
Results and Discussion (1) Trimethoxymethane. Figure 1 shows the buildup of the conductance of an aqueous solution containing 10 mM trimethoxymethane (trimethyl orthoformate) and 0.5 m M Fe(CN):- at pH 6.2 after producing -2 pM OH (90%) H (10%) by a 0.4-ps electron pulse. On the basis of the calibration the overall conductance change is primarily due to production of 1 equiv of H+. It is evident that there are two components: a fast one that accounts for approximately 50% of the total conductance increase, and a slow component that yields the remaining 50%. The rate of the fast component was found to increase with increasing concentration of Fe(cN)6s (measured from 0.02 to 0.4 mM) and from this linear dependence the rate constant for production of conductance by reaction of Fe(CN)63- with the radical (CH30)2CHOCH2(eq 6, see below) was obtained as 1.8 X lo9 M-I s-I. In contrast to the fast component, the rate of conductance increase via the slow component was found to be independent of [Fe(cN),*] in the range 0.1-2 mM. The rate constant measured for this process at pH 6-6.5 is (1.4 f 0.1) X lo3 s-l. After completion of the slow reaction, the total yield of the conductance change was equal within &2% to that from the reaction of C'H20H with Fe(CN)63- in which H+ is quantitatively produced (Experimental Section). When ferricyanide was replaced as the oxidant by hexachloroiridate (IV), a two-step formation of conductance was again seen. As with ferricyanide, the rate of the fast step was dependent on [IrCl:-] (between 0.02 and 0.1 mM), whereas that of the slow component was not (between 0.1 and 1 mM). From the dependence on [IrC162-] of the fast conductance increase the rate constant for prqduction of conductance by reaction of IrC12- with (CH30)2CHOCH2was found to be 3.8 X lo9 M-' s-l. The rate constant for the slow process was measured to be 1.4 X lo3 s-I, Le., equal to that found when ferricyanide was the oxidant.
+
(19) Veltwisch, D.;Janata, E.; Asmus, K.-D. J. Chem. Soc.,Perkin Trans. 2 1980, 146.
(CH3O)jC + OX -+ (CH30)3C+ I+ (CH30)2CHOCH2+ Ox
-
(CH30)3C+
-
(CH30)3COH
(5)
+
(6)
+ H+
(7)
(CH3)2CHOC+H2 Ox'11+
+ H20 (CH30)2CHOC+H2 + H2O (CH,O)$+
+ OX'-
(CH30)2CHOCHZOH
+ OH- -.+(CH30)jCOH
+ H+ (8) (9)
identifies the radicals and ions produced in the radiolysis of water; after completion of reaction 2 (half-life < 5 ns), [H+] = [OH-]. The rate of the neutralization reaction, eq 3, depends on pH: at pH 6 its half-life is 4 ps, which means that at pH 6 only processes with reaction periods longer than =5 pus can be kinetically studied by the conductance technique. In eq 4 it is shown that OH (and H ) react with (CH30),CH by H abstraction from the central and from the external carbons. The radicals I and I1 are produced with equal yields. This is deduced from the experimentally observed ratio of the fast and the slow component expressed in terms of equivalents of H+. Taking into account the difference in number of hydrogen atoms in the two groups the 1 to 1 ratio translates into a 9:l preference for abstraction of the central hydrogen. This type of selectivity for H-abstraction reactions of OH (and possibly H) is in agreement with earlier dataS relating to ethers and acetals. Both radicals formed in eq 4 are oxidized to give the carbocations I+ and 11' (eq 5 and 6). The monoalkoxylated cation II+ reacts rapidly with water to give H+ and a hemiasetal. This process (eq 8) is responsible for the fast rise of conductance. Since the rate of this fast component is dependent on the oxidant concentration, the hydration of the monoalkoxylated carbocation is obviously not rate determining. Since, due to the neutralization reaction, a t pH 6 rates larger than = l X los s-I cannot be measured, this value can be identified as the lower limit for the rate of reaction 8. In order to get a more precise estimate of the lifetime in water of a monoalkoxy carbocation, the 2-isopropoxypropan-2-yl radical was produced at pH 4 by H abstraction from =O, 18 M diisopropyl ether in the presence of IrC162-,and the conductance o! the solution was monitored. The reaction between (CH3)2CHOC(CH3)2and IrC162-proceeds by one-electron transfer with k = 3.6 X. lo9 M-' s-I.~ It was found to yield 1 equiv of H+ per (CH3)2CHOC(CH3)2. The rate of H+ production by hydrolysis of the 2-isopropoxypropan-2-ylium carbocation increased linearly with increasing [IrC162-] up, to 2 mM, where the rate of oxidation of s-I = 7.2 (CH3)2CHOC(CH3)2is equal to (3.6 X 109)(2 X X 106 s-l. The rate constant for hydrolysis of the carbocation must therefore be larger than this number. Taking into account the experimental error limits and signal-to-noise, the rate constant is estimated to be 2 2 X lo7 s-l. The oxidation of the organic radicals (eq 5 and 6) can also be studied by observing the corresponding reduction in the concentration of the oxidant. In the case of ferricyanide its bleaching was monitored at X = 420 nm with solutions buffered to pH =8 with I 1 m M phosphate.20 There appeared to be only one com-
2810 J . Am. Chem. Soc., Vol. 108, No. 11, 1986
Steenken et al.
ponent in the bleaching at 420 nm, with the yield of ferricyanide Table I. Rate Constants for Reaction of IrCI:-, Fe(CN)63-, and consumption being equal to that of the initial yield of OH H. C(N02)4with Radicals Derived from Orthoformates and This means that the radicals I and I1 react with ferricyanide with OrthocarbonatesQ the same rate constant (k(5)= k(6), determined to be 1.7 X lo9 k / M " s-' M-' s-l, Le., essentially equal to k ( 6 ) as obtained from the conductance experiments). radical IrCIG2-* F e ( C N L - ? C(NO,)ad When IrC16'- was used as the oxidant instead of ferricyanide, (CH30)3C (1) 3.8 x 109 1.7 x 109 =2 x 109 also only one kinetic component in its depletion (monitored at X 3.8 x 109 1.7 x 109 = i x 109f (CH30)2CHOCH2 (11) = 490 nm) was seen. The amount of bleaching was equal to that 3.8 x 109e 1.8 x 109' (CzH@)X (111) 2.9 x 109 1.6 x 109 found when CH,OH was used to reduce IrC16'- (=loo%). As (C2H,0)2CHOCHCH3 (IV) 2.9 X IO9 1.6 X lo9 with ferricyanide, it can therefore be concluded that with IrC16'2.8 x 109e the rate constants for reactions 5 and 6 are the same, which means 2.8 x 109 2.7 x 109 ((CHAzCHO)iC (VI) that IrC16'-, like Fe(CN)63-, does not differentiate kinetically ((CH3)2CHO)zCHOC(CH3)2 2.8 X lo9 2.7 X lo9 between the strong reductant (CH30)3Cand the weaker elec(W tron-donor (CH30),CHOCH2. The rate constants for reaction (CH30)3COCH2(XII) 3.0 X lo9 3.8 X IO8 5 and 6 with Ox = IrC16'- were determined as 3.8 X lo9 M-' s-l, 4.0 x 1 0 9 e similar to those of other9 a-alkoxyalkyl radicals and in agreement (C,H,O),COCHCH, (XIII) 2.2 x io9 1.8 x io9 with the value for k ( 6 ) from conductance. 3.4 x 109' As a third oxidant, tetranitromethane (TNM) was used. Its a [Substrate] 10 mM, [oxidant] = 0.05-0.5 mM, 20 "C. *From optical redox potential is not known, due to lack of reversibility in the experiments at A = 490 nm and pH 7-8. 'From optical experiments at A = one-electron reduction. However, T N M has been found an ef420 nm and pH 7-8. dFrom optical experiments at A = 350 nm and pH ficient yet selective oxidant for a variety of organic r a d i ~ a l s . 4 - ~ * ~ ~7.9. * ~ ~From conductance experiments. /Measured at A = 300 nm. When radicals I and I1 were produced in the presence of 0.1-1 mM T N M at pH 7.9, the formation of the nitroform anion (monitored at X = 350 nm) was observed. It occurred in two steps: 3.2 a fast one (half-lifes 510 ps) the rate of which depended on the T N M concentration and whose yield was 4 0 % of that of OH U H, and a slow one (half-life 184 ps), which accounted for the u) n remaining 50% and whose rate was independent of the T N M y" 3.0 concentration. The fast process is assigned to reaction of 01 (CH30)3Cwith T N M by one-electron transfer (eq 10) with a 0
+
ET
+
(CH30),C
+ (NO2)&
k
4
(CH30)3C++ NOz.
+ C(N02)< (10)
measured k of =2 X lo9 M-I s-l, whereas the slow, unimolecular formation of C(NOJ3- is suggested to be due to heterolytic decomposition of the nitroxide formed by addition of radical I1 to T N M , cf. reactions 11 and 12, followed by rapid hydrolysis of 11'. Oxidation by T N M of a-monoalkoxyalkyl radicals via an
-
(CH30)2CHOCH2+ C(NO2I4
(CH30)2CHOCH20N(O.)C(NOJ3(1 1) ad
-
ad
+
(CH30)2CHOC+H2 NOz.
11+
+ C(N02)
