3566
J. Phys. Chem. 1995, 99, 3566-3573
Photochemical Electron Transfer Initiated Oxidative Fragmentation of Aminopinacols. Factors Governing Reaction Rates and Quantum Efficiencies of C-C Bond Cleavage Hong GanJ Uwe LeinhosJ Ian R. Gould? and David G. Whitten*,+ Department of Chemistry and NSF Center for Photoinduced Charge Transfer, University of Rochester, Rochester, New York 14627 Received: October 14, 1994; In Final Form: December 28, I994@
The fragmentation reactions of the cation radicals of a series of remote diamine-substituted pinacols have been investigated. The cation radicals were generated upon photooxidation with excited 2,6,9,10tetracyanoanthracene in acetonitrile. The products are consistent with cleavage of the central C-C bond. The rate constants for fragmentation were determined both from steady-state quantum yield studies and from time-resolved measurements. In general, the rate constants for fragmentation increase with increasing stability of the radical and cation products. However, the results of temperature dependence studies clearly demonstrate that conformational effects play an important role in the transition state. In some cases, these conformational effects can result in changes in the reactivity order expected from purely thermodynamic considerations.
I. Introduction The reactions which can result from single electron transfer quenching of excited states of organic chromophores are a topic of much interest.’-’ Several facets of this reactivity are intriguing. One is the interrelationship between odd and even electron intermediates and one- vs two-electron redox interconversions. Another is the potential for accomplishing efficient and useful chemical change via capture of the high-energy intermediates (organic ion radicals or ion-radical pairs) produced in the quenching step. A reaction which gives some promise of being fairly general and useful is the specific cleavage of carbon-carbon bonds in photogenerated cation radi~als.1-1~Several of these reactions are very clean processes in which a single carbon-carbon bond is selectively broken in high chemical yield and often with reasonable quantum efficiency. The energetics of the bond cleavage in a photogenerated cation radical can be estimated by use of the relationship for the bond dissociation energy shown in eq 1, where BDE~-R,)
SCHEME 1: Simple Mechanism for Product Formation in Photoinduced Electron Transfer Reactions A*tD
t
\
A’-ID’+
L P
-
A’-+D’+
Products
I J
A+D
for product formation for reactions of this type. When fragmentation is much slower than return electron transfer and ion-radical pair separation, the reaction quantum yield will be limited by the competition between kscp and k-et, i.e. the efficiency with which separated radical ions (A’- D+) are formed from the geminate pair. Quantum efficiencies for product formation which are higher than those for formation of separated radical ions can be obtained if kr is comparable to or larger than k-et and However, this is rarely observed due to a combination of relatively fast return electron transfer, especially when singlet excited state quenching is involved, and slow bond fragmentation. Thus, even in cases where the BDE of the cation radical may be estimated to be very low or even negative, k, can be 2-3 orders of magnitude smaller than k-et, resulting in low quantum yields for product formation. Clearly, the quantum efficiency of a bond cleavage reaction can be improved by either decreasing k-et or increasing ksep or h. The factors which control the rates of return electron transfer and ion-radical pair separation have been studied extensively. Several methodologies have been developed for increasing quantum efficiencies including (a) decreasing k-et by increasing the redox energies of the reactants, taking advantage of the Marcus inverted region effect,*O (b) decreasing k-et by using triplet excited state quenching to generate a triplet ion-radical pair, in which case return electron transfer to ground state neutral reactants is forbidden,10s21*22 (c) using an electron transfer c o s e n ~ i t i z e r ? ~ ~ ~ ~ - ~ ~ (d) using polar solvents to facilitate the ion-radical pair
+
is the bond dissociation energy of the corresponding neutral compound and IP(R-R~) and IP(R.,are the ionization potentials (or oxidation potentials) of the neutral compound and the radical, which when oxidized becomes the cation product of the cation radical fragmentati~n.’~-~~ Large differences in the IP’s of stable organic molecules and radicals may be encountered when oxidation of the radical generates a relatively stable cation (for example R2N-CR’f R2N=CR’2+ e-). As a consequence, the dissociation energies of specific bonds may be drastically lowered in the cation radicals compared to the corresponding neutral compounds, which may be manifested by rapid fragmentation reactions in photogenerated ion radicals or ionradical pairs. In general, the quantum efficiencies of such reactions are determined by the competition among three processes in the initially formed geminate ion-radical pairs (A’-D’+), i.e., return electron transfer (k-et),ion-radical pair separation (ksep), and bond cleavage (kr). These processes are outlined in Scheme 1, which shows the simplest mechanism
-
+
t Department of Chemistry.
*
NSF Center for Photoinduced Charge Transfer. @Abstractpublished in Advance ACS Abstracts, February 15, 1995.
0022-365419512099-3566$09.00/0 0 1995 American Chemical Society
ksep.18919
J. Phys. Chem., Vol. 99, No. 11, 1995 3567
Oxidative Fragmentation of Aminopinacols separation, and (e) increasing kscp by minimizing or eliminating the Coulombic attraction between the photogenerated ion radicals, by using a charged species as the donor or acceptor.26 Until recently, however, relatively little attention has been focused on the factors which control k,. While k, is certainly affected by the reaction thermodynamics as indicated in eq 1, the factors that control the activation energy and the transition state for the cation radical fragmentation remain largely undefined. In previous investigations we have examined the fragmentation reactions of 1,2-diheteroatom-substitutede t h a n e ~ ? , ~ - ’ ~ These studies demonstrated that both 1,Zdiamines and aminoalcohols undergo clean C-C bond cleavage reactions upon activation via photoinduced electron transfer. The quantum efficiencies of these reactions depend upon the molecular structure of the donor substrate in addition to the reaction conditions. These results suggest that the conformation of the cation radical intermediates may play an important role in determining the rate of the bond fragmentation and consequently the quantum efficiency of the reaction. Related fragmentation reactions are also observed in “remote” systems in which the heteroatoms are separated from the fragmenting bond by a conjugated group such as benzyl or allyl.27 Although the thermochemical calculations suggest comparable BDE’s in these compounds, preliminary studies indicate that the fragmentation rates may be much slower. As a follow up to our previous studies which focused on “remote” aminoalcohols, we have prepared several symmetric “remote” diamines (1-5), which are also pinacols and the fragmentation products of which should be rapidly and irreversibly oxidized to give stable net oxidation products. In the present paper we report a study of the oxidative fragmentation of these aminopinacols using both steady-state photolysis and time-resolved techniques. Our studies indicate that the fragmentation reactions of the radical cations of these compounds are very different from the corresponding fragmentations of neutral molecules and show a strong dependence on substrate structure. Analogous to the fragmentation reactions of cations of 1,2-diamines,11 the fragmentations of the aminopinacols are characterized by negative entropies of activation, which are characteristic of ordered transition states. ;
N
a
q
o
(
R R
1 R=H 2 R=CHj
3 R=Ph.
A
5
11. Results
A. Synthesis of Aminopinacols. The synthesis of compounds 1-5 is described in detail in the Experimental Section. Pinacol 1 (with R = H) was synthesized by a samariummediated reductive coupling of the corresponding aldehyde (eq 2)?8929 Both diastereomers, meso- and d,l-, were obtained by chromatography.
SCHEME 2: Stepwise Addition of Grignard Reagent to Diketones
- ph,pR MgBr
O H 0
R
R
Ph’MgBr
Yg
Ph’MgBr
* Ph’#Ph’
R R
I
I H+
P h = p-N,N-dimethylphenyl P
h
’ R
HO OH
ttph’
p R
Ph’
R R
Compounds 2-5 were synthesized by Grignard addition to a diketone. In the case of 2,3-butanedione (R = CH3), a small amount of monoaddition product was also isolated, which indicates that the reaction follows the mechanism shown in Scheme 2. For pinacol 2 (R = CH3), the two diastereomers (meso- and d,l-) were also observed and isolated with the d,lisomer as the major product. In the case of compound 2, the purification of the meso-isomer could be achieved only via repeated recrystallization from ethanol. The assignment of meso- and d,Z-isomers is based on their melting points and solubility in ethanol. As with pinacol 1, meso-2 is H
meso-2
d,1-2
is expected to have a higher melting point than that of d,l-2 due to the different inter- and intramolecular interactions in the isomers. For meso-2, the most stable predicted conformation should have strong intermolecular hydrogen bonding (characterized by a high melting point and lower solubility in ethanol), whereas in the d,Z-isomer, intramolecularhydrogen bonding will be predominant. In contrast to the cases for 1 and 2, only one isomer was observed for pinacols 3, 4, and 5. On the basis of the reaction mechanism, the second Grignard reagent should preferentially attack at the least sterically hindered carbonyl, and therefore, d,l-isomers are expected for 4 and 5. The high melting points observed for these compounds are consistent with this assignment. In the case of 3, it is difficult to predict the preferred diastereomer from the Grignard reaction due to the rotation of the central C-C bond and the similar steric size of the phenyl and p-(N,N-dimethy1amino)phenyl groups. We tentatively assign the product to the meso-isomer due to its relatively high melting point and fairly low solubility in absolute ethanol3031 B. Oxidation of Aminopinacols. The pinacols 1-5 are better electron donors than the aliphatic diamines studies p r e v i o ~ s l y . ~ *Cyclic ~ - ~ ~ voltammetry experiments were performed on several of the pinacols in acetonitrile. The observed waves clearly indicated irreversible oxidation of the pinacols, and only peak potentials could be obtained. The average peak potential was ca. 0.72 V vs SCE. The central C-C bond energies are estimated to be ca. 39-40 kcal/mol from Benson’s additivity rules or by comparison with similarly functionalized bibenzyl derivative^.^^ Oxidation of pinacols 1-5 either thermally or photochemically results in products derived from cleavage of the central C-C bond in the cation radical intermediates. For example, p-(N,N-dimethy1amino)benzaldehyde was formed as the major product when 1 was oxidized by an ammoniumyl salt.33 We also found that oxidative cleavage of the C-C bond could also be achieved by thermal reaction
3568 J. Phys. Chem., Vol. 99, No. 11, 1995
Gan et al.
TABLE 1: Quantum Yields for Pinacol Fragmentation Initiated by Photoinduced Electron Transfer to Excited 2,6,9,10-Tetracyanoanthraceneand Rate Constants for Fragmentation and Return Electron Transfer in Ion-Radical Pairs in Acetonitrile at Room Temperature pinacol 10-9~ (s-1) 10*@,,b lO*@iNd,C interceptd x sloped 10-4k: (s-1) 10-9t: (s-1) 1 12 0.032 2 12 2.5 3.5 28 2.9 19 14.0 3 11 2.1 3.5 26 9.4 5.5 13.0 4 10 4.1 5.5 18 2.4 15 8.6 5 15 0.0075 Rate constant for quenching of fluorescence of tetracyanoanthracene. Quantum yield measured at 0.2-4.7% conversion of the pinacol. Quantum yield extrapolated to zero conversion, equivalent to @ions (see text). From plots of quantum yield data according to eq 5 (see text). e From analysis according to eq 6, with kw = 5 x lo8 s-I and kdiff = 2 x 1Olos-l (see text). of 2,3-dichloro-5,6-dicyanobenzoquinone(Eln(DDQ,DDQ'-) = -0.51 V vs SCE) with the pinacols. p-(NJV-Dimethylamino)acetophenone is the only cleavage product detected when 2 is mixed with DDQ in methylene chloride or CDCl3 under thermal conditions. All of the aminopinacols are found to decompose very slowly in benzene or acetonitrile solution upon standing at room temperature, to form ketones (2-5) or aldehyde (l), presumably due to slow oxidation by oxygen. Even as a solid, compound 1 was found to slowly decompose to p-(NJVdimethy1amino)benzaldehyde. Consequently, all studies in this investigation were made using freshly prepared and vacuumdegassed solutions. 2,6,9,10-Tetracyanoanthracene(TCA) was chosen as the lightabsorbing electron acceptor for the steady-state photooxidation studies. Unlike the commonly used 9,lO-dicyanoanthracene (DCA), the anion radical primary reduction product of TCA, TCA'-, is stable under vacuum in a ~ e t o n i t r i l e .The ~ ~ reduction potential of TCA in acetonitrile has been determined to be -0.45 V vs SCE.2O Oxidation of all of the pinacols by the TCA excited singlet state is expected to be thermodynamically favorable, whereas the redox reactions between pinacols and ground state TCA are disfavored. Consistent with this expectation, the TCA fluorescence is found to be quenched with rate constants near to the solvent-controlled diffusion limit by all pinacols (Table 1).
All of the pinacols were found to fragment cleanly to carbonyl products upon photolysis at 436 nm in the presence of TCA (eq 3).Over the time period required for photolysis of the
pinacols, no reaction could be detected in the presence of TCA in the dark. Only TCA absorbs at the excitation wavelength. The products formed are carbonyl compounds (from the pinacol) and the TCA anion radical. As indicated previously, the TCA anion radical is stable in degassed a ~ e t o n i t r i l e .The ~ ~ presence of the TCA anion radical was clearly indicated by its blue/gray color, which persisted after the photolysis. The extent of formation of the colored T C R - was used to determine the quantum yields for fragmentation of the cation radicals. The fragmentation quantum yield was taken to be half that for formation of TCA'-, eq 3.34 Importantly, the fragmentation quantum yields were found to decrease with increasing conversion of TCA to TCA'-. The measured quantum yields for small conversion and also those extrapolated to zero conversion (see Discussion) are summarized in Table 1. No differences in reactivity could be detected for the meso and d,Z-isomers of 1 and 2, although the reaction efficiency for 1 was low, thus making comparison difficult in this case (see further below). Compound 4 was observed to have the highest quantum yield for reaction. In addition to the steady-state studies, time-resolved measurements of the rates of fragmentation of the cation radicals were
TABLE 2: Kinetic Parameters for Fragmentation of Pinacol Cation Radicals from Time-Resolved Experiments in Acetonitrile pinacol 10-4k, (s-l) E. (kcal mol-') A 9 (e.u.) 1
