Thy Dynamics od α-Anilino Carboxylate and Related Cation Radical α-Heterolytic Fragmentations

Full text of DOI:10.1021/ja963988z

J. Am. Chem. Soc. 1997, 119, 5261-5262
5261
Scheme 1
The Dynamics of r-Anilino Carboxylate and
Related Cation Radical r-Heterolytic
Fragmentations
Zhuoyi Su, Daniel E. Falvey, Ung Chan Yoon,1 and
Patrick S. Mariano*
Table 1. Rate Constants for Decarboxylation and Retro-Aldol
Cleavage of Anilinium Radicals [p-X-C6H4NMeCHRE]•+
Department of Chemistry and Biochemistry
UniVersity of Maryland, College Park, Maryland 20742
fragmentation rates
anilinium radical
k
dec (s-1)a or kra
ReceiVed NoVember 18, 1996
b,c
E
X
R
solvent
(M-1 s-1
)
R-Heterolytic fragmentation reactions of ion-radicals play a
major role in governing the chemoselectivities and efficiencies
of a wide variety of redox processes. The function of these
reactions (eqs 1 and 2) in SET-photochemistry is pivotal since
departure of an electrofugal or nucleofugal group from a
respective cation or anion radical often occurs in competition
with back electron transfer or alternative reaction modes. As a
result, knowledge about the dynamics of ion radical fragmenta-
tion reactions and their dependence on substrate structures and
redox potentials as well as the medium and additives is crucial
to the design of efficient SET-photochemical processes.
CO2NBu4
CO2NBu4
CO2NBu4
CO2NBu4 MeO
CO2NBu4 CF3
CO2NBu4
CO2NBu4
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
H
H
H
H
MeCN
EtOH
MeOH
MeCN
MeCN
1.7 ( 0.2 × 106
H
H
H
H
2.5 × 106
2.8 × 106
8.1 ( 0.3 × 105
1.2 ( 0.2 × 107
1.3 × 106
H
H
H
MeO
CF3
H
Me MeCN
Ph MeCN
H
H
H
Me 60% MeOH-MeCN
Ph 60% MeOH-MeCN
2.6 × 106
60% MeOH-MeCN
60% MeOH-MeCN
60% MeOH-MeCN
4.1 ( 0.6 × 104
2.0 ( 0.3 × 104
3.1 × 105
3.3 × 104
H
3.3 ( 0.7 × 105
a 25 °C. b 25 °C with nBuNOAc as base. c Errors were obtained by
evaluating data from 3-5 independent experiments.
To evaluate the dynamics of the aminium radical decarboxy-
lation and the related retro-aldol cleavage, we explored the SET-
photochemistry of precursors of these transients in order to
ensure that the respective fragmentation reaction pathways were
followed cleanly in these systems. Preparative irradiation (Pyrex
(λ > 300 nm N2, MeCN) of the anilinocarboxylate 1 (2.5 mM)
and DCB (2.5 mM) leads to clean production of a mixture the
diamine 3 (30%), adduct 4 (27%), and recovered DCB (55%)
(Scheme 1). Likewise, adduct 4 (32%) and recovered DCB
(63%) are produced when an MeCN solution containing the
anilino alcohol 2 (2.4 mM), DCB (2.4 mM) and tetra-n-
butylammonium acetate (TBAA) (0.3 M) is irradiated (Scheme
1). The results demonstrate that R-amino radical formation by
decarboxylation and retro-aldol cleavage is dominant for anilino
carboxylate and anilino alcohol cation radical decay.
Time-resolved laser spectroscopy was used to determine the
dynamics of these R-fragmentation reactions. Laser excitation
(308 nm, 6 ns, 50-60 mJ) of solutions containing (Table 1)
tetra-n-butylammonium anilino carboxylates (ANC, 0.5 mM)
and DCB (50 mM) in each case leads to initial generation
(Figure 1) of a 1:1 (known2h molar absorptivities) of two
transients characterized as DCB•- (340(s), 430(w) nm) and
ANC•+ (ca. 460 nm).2h,6 Analysis of the decay profiles for
ANC•+ employed a kinetic treatment which takes into account
competitive diffusion controlled2h SET between ANC•+ and
DCB•- and unimolecular decarboxylation of ANC•+. This
analysis when combined with the known2h,6 molar absorptivities
The importance of this information is evidenced by the
expanding number of investigations in which the rates of ion
radical reactions have been directly measured.2 Our initial
efforts2h in this area focused on the R-deprotonation reactions
of tertiary amine cation radicals and the related R-silylamine
cation radical desilylation process.3 We demonstrated that laser
flash irradiation of solutions containing 1,4-dicyanobenzene
(DCB) and N,N-dialkylanilines generates spectroscopically
detectable anilinium radicals which decay by (1) back electron
transfer with DCB•- at diffusion controlled rates, (2) base
induced R-deprotonation with second order rate constants that
are dependent on base strength and the nature of R- and arene
ring-substituents, or (3) silophile induced R-desilylation. Re-
cently, we initiated a more broad study of aminium radical
R-heterolytic fragmentation reactions in order to determine if
the trends noted earlier are general. Our plan was to determine
the rates of the known,3 but relatively unstudied,4 unimolecular
decarboxylation reactions of R-amino carboxylates (eq 3), to
compare these rates to the bimolecular rates of the related
deprotonation,2h desilylation,2h and retro-aldol fragmentation (eq
3),5,2b and, finally, to ascertain if substituent effects on the rates
of these aminium radical fragmentation can be generalized.
of the transients yields the unimolecular rate constants (kdec
)
for ANC•+ decarboxylation. Decarboxylation rates were mea-
sured for a series ring- and R-substituted systems and in aprotic
and protic solvents (Table 1). In addition, counter cation effects
on the rates of ANC•+ decarboxylation were evaluated. Rep-
resentative data for the metal cation type dependence of kdec
are given in Table 2.
(1) Department of Chemistry, College of Natural Science, Pusan National
University, Pusan 607-728, Korea.
(2) (a) Johnston, L. J.; Schepp, N. P. AdVances in Electron Transfer
Chemistry; Mariano, P. S., Ed.; JAI Press: Greenwich, CT, 1996; Vol. 5,
pp 41-102. (b) Lucia, L. A.; Burton, R. D.; Schanze, K. S. J. Phys. Chem.
1993, 97, 9078. Burton, R. D. Bartberger, M. D.; Zhang, Y.; Eyler, J. R.;
Schanze, K. S. J. Am. Chem. Soc. 1996, 118, 0000. (c) Albini, A.; Fasani,
E.; Freccero, M. AdVances in Electron Transfer Chemistry; Mariano, P. S.,
Ed.; JAI Press: Greenwich, CT, 1996; Vol. 5, pp 103-140. Anne, A.;
Fraoua, S.; Hapiot, P.; Moiroux, J.; Saveant, J.-M. J. Am. Chem. Soc. 1995,
117, 7412. (d) Anne, A.; Fraoua, S.; Moiroux, J.; Saveant, J.-M. J. Am.
Chem. Soc. 1996, 118, 3938. (e) Dinnocenzo, J. P.; Banach, T. E. J. Am.
Chem. Soc. 1989, 111, 8646. (f) Parker, V. D.; Tilset, M. J. Am. Chem.
Soc. 1991, 113, 8778. (g) Maslak, P.; Vallombroso, T. M.; Chapman, W.
H.; Narvaez, J. N. Angew. Chem., Int. Ed. Engl. 1994, 33, 73 and references
therein. (h) Zhang, X.; Yeh, S.-R.; Hong, S.; Freccero, M.; Albini, A.;
Falvey, D. E.; Mariano, P. S. J. Am. Chem. Soc., 1994, 116, 4211.
(3) Davidson, R. S.; Steiner, P. R. J. Chem. Soc. (C) 1971, 1682.
Davidson, R. S.; Harrison, K.; Steiner, P. R. J. Chem. Soc. (C) 1971, 3480.
(4) Photolysis of thiopyridyl esters of R-amino acids can also be used to
generate R-amino radicals, see: Barton, D. H. R.; Herve, Y.; Potier, P.;
Thierry, J. Tetrahedron 1987, 43, 4297.
(5) Davidson, R. S.; Orton, S. P. J. Chem. Soc., Chem. Commun. 1974,
209. For a review of more recent studies see Gaillard, E. R.; Whitten, D.
G. Acc. Chem. Res. 1996, 29, 292.
(6) Jones, G.; Malba, V. Chem. Phys. Lett. 1985, 119, 105. Robinson,
E. A.; Schulte-Frohlinde, D. J. Chem. Soc., Faraday Trans. 1 1973, 69,
707.
S0002-7863(96)03988-1 CCC: $14.00 © 1997 American Chemical Society

5262 J. Am. Chem. Soc., Vol. 119, No. 22, 1997
Communications to the Editor
(Mg+2, Ca+2) decrease in kdec. Thus, tight-coordination of the
carboxylate moiety to cations, brought about by the use of less
polar/protic solvents or more oxophilic metals, slows ANC•+
decarboxylation.
Finally, substituents influence the rates of R-heterolytic
fragmentation reaction of anilium radicals. In earlier efforts,2h
we showed that the rate of TBAA-induced R-deprotonation of
[p-X-C6H4NMe2]•+ is a sensitive function of the para-substituent
(kdeprot × 10-5 ) 2.0 (X ) H), 0.8 (X ) MeO), and 8.9 (X )
CF3)). The data in Table 1 demonstrate that this trend holds
for decarboxylation and retro-aldol cleavage. Linear free energy
treatments of the rate data give Fdeprot ) 1.5, Fdec ) 1.8, and Fra
) 1.6 which suggest that anilium radical deprotonation, decar-
boxylation, and retro-aldol cleavage7 reactions occur Via early
transition states which have similar electronic character in the
anilino-moiety region. As discussed earlier,2h,10 the rates of
anilinium radical deprotonation parallel their thermodynamic
acidities which are known to be inversely related to aniline
oxidation potentials. Thus, the current results show that this
relationship (increasing rate with increasing precursor oxidation
potential) also exists for amine cation radical decarboxylation
and retro-aldol cleavage reactions.11
Figure 1. Transient absorption spectra following (1-40 µs) 308 mM
excitation of PhNMeCH2CO2NBu4 and DCB in MeCN at 25 °C.
(Insert: Transient absorption at 460 nm, experimental data (points) and
fit (line)).
Table 2. Salt Effects on [PhMeCH2CO2NBu4]•+ Decarboxylations
added salta
kdec × 106 (s-1
)
added salt
kdec × 106 (s-1
)
none
1.7
1.1
1.9
2.5
RbClO4
CsClO4
Mg(ClO4)2
Ca(ClO4)2
2.8
LiClO4
NaClO4
KClO4
2.9
0.12
0.08
Equally interesting is the observation that the amine cation
radical R-heterolytic fragmentation processes show the same
R-substituent dependence. Accordingly, replacement of one of
the R-hydrogens in [C6H5N(Me)CH2E]•+ by methyl causes a
decrease in the rate of loss of the electrofugal group, while a
rate increase is engendered by phenyl substitution.12 The
rationale presented earlier by Lewis and his co-workers13 for
R-substituent effects on aminium radical deprotonations, based
upon both stereoelectronic and C-E bond dissociation energy
effects, appear to be generally applicable to other R-heterolytic
fragmentation processes.2h,14
In summary, results presented here demonstrate that aminium
radicals generated from R-aminocarboxylate undergo fast de-
carboxylation to produce R-amino radicals and that the decar-
boxylation rates as well as those for related deprotonation,
desilylation, and retro-aldol cleavage can be significantly altered
by changes in solvent, counter cation, and N- and R-substitu-
ents.15
a 25 °C, MeCN with [PhNMeCH2CO2NBu4] ) 0.5 mM and [MClO4]
) 10 mM.
By use of similar methods, the rates of TBAA promoted retro-
aldol fragmentation (kra) of photogenerated cation radicals
derived from the anilinoalcohols (ANA) (Table 1) were
measured. Plots of the observed rates of ANA•+ decay (treated
as two competitive bimolecular decay routes) Vs TBAA
concentration (0.1-0.5 M) provide kra values for cleavage.
Additional data supporting the proposal that TBAA induced
retro-aldol fragmentation is the major, if not exclusive, pathway
for decay of ANA•+ are an OH/OD isotope effect of 2.27 and
an NCH3/NCD3 isotope effect of 1.3 for PhN(CH3)CH2CH2-
OH.8
A number of interesting features of anilinium radical R-frag-
mentation reactions can be gleaned by inspection of the
accumulated data. Firstly, ANC•+ decarboxylations are among
the most rapid of amine cation radical fragmentation processes.9
Owing to their large rate constants (ca. 106-107 s-1) and
unimolecular nature, decarboxylations dominate other modes
of aminium radical decay when high concentrations of bases
or silophiles are absent. For comparison purposes, the rate
constant for TBAA deprotonation of [PhN(Me)2]•+ is 2.0 × 105
M-1 s-1 and for MeCN (solvent) desilylation of [PhN(Me)CH2-
Acknowledgment. The authors express their appreciation to the
National Science Foundation (PSM CHE-9420889 and INT-9511989
and DEF CHE-9521601) and the Korea Science and Engineering
Foundation (UCY, CBM-Postech and International Cooperative Pro-
gram) for their generous support of this research.
JA963988Z
TMS]•+ is 2.0 × 104 s-1
.
(10) We showed earlier (ref 2e) that log kdeprot is well correlated with
both σ and E1/2(+) values of para-substituted anilines.
(11) Schesener, C. T.; Amatore, C.; Kochi, J. K. J. Am. Chem. Soc. 1984,
106, 3567, 7472. Sankararaman, S.; Kochi, J. K. J. Am. Chem. Soc. 1989,
11, 2263.
Secondly, the decarboxylation rates are moderately sensitive
to solvent, nearly doubling in moving from the less polar/aprotic
MeCN (ET ) 47) to the more polar/protic MeOH (ET ) 56).
This effect can be attributed to greater solvation by MeOH of
the butylammonium cation and carboxylate anion components
resulting in a greater rate of free carboxylate decarboxylation.
This simple reasoning is compatible with the observed metal
cation effects (Table 2). Metal perchlorate salts appear to
influence the decarboxylation rates in two ways. Addition of
perchlorates of non-oxophilic metal cations such as Rb+ and
Cs+ leads to an increase in the decarboxylation rates, a likely
result of a general salt effect. In contrast, the presence of highly
oxophilic metal cations results in a modest (Li+) to dramatic
(12) For comparison purposes, the rate constants of deprotonation of
[Ph2N-CH2R]•+ by TBAA are 9.5 × 105 (R ) H), 2.3 × 105 (R ) Me),
and 3.2 × 106 (R ) Ph) M-1 s-1 (ref 2h).
(13) Lewis, F. D. Acc. Chem. Res. 1986, 19, 401. Lewis, F. D.; Ho, T.
I.; Simpson, J. T. J. Am. Chem. Soc. 1982, 104, 1924.
(14) (a) The R-substituent effects argue against a two-step mechanism
for conversion of anilium carboxylates to R-amino radicals, involving rate
limiting SET to generate R-amino-carboxy radicals followed by fast (ref
13b) decarboxylation. (b) Falvey, D. E.; Schuster, G. B. J. Am. Chem. Soc.,
1986, 108, 7419; Bockman, T. M.; Hubig, S. M.; Kochi, J. K. J. Am. Chem.
Soc., 1996, 118, 4502.
(15) An example of how this knowledge can lead to the design of efficient
SET-photochemical processes is seen in the results of a laser spectroscopic
study (unpublished) which show that a change in the N-substituent Z in
[PhN(Z)CH2TMS]+• from Me to CO2Et causes the rate of MeOH promoted
desilylation to increase from 7.0 × 105 s-1 to 1.8 × 107 s-1, and those of
a parallel photochemical study which show that the quantum yield for SET-
photocyclization of the phthalimides (i f ii) increases from 0.05 to 0.22
when Z is changed from Me to Ac.
(7) Retro-aldol cleavage of related anilino-alcohol cation radicals has
been analyzed from this same perspective by Schanze and his co-workers
(ref 2b).
(8) Even though the rates of TBAA induced R-CH deprotonations of
[PhNMeCH2R]•+ are in the range of those measured for retro-aldol
fragmentation, the current data show that R-CH deprotonation is not
competitive in decay of [PhNMeCH2CH2OH]•+
.
(9) (a) Earlier (ref 9b) we observed that the rates of R-heterolytic
fragmentation of benzylic cation radicals varies in the order CO2- > TMS
> CO2H > H. (b) d’Alessandro, N.; Albini, A.; Mariano, P. S. J. Org.
Chem. 1993, 58, 937.