Unexpected formation of 1-diethylaminobutadiene in photosensitized oxidation of triethylamine induced by 2,3-dihydro-oxoisoaporphine dyes. A 1H NMR and isotopic exchange study

Article abstract of DOI:10.1021/jo050796q

Photoreduction of oxoisoaporphine dyes occurs via a stepwise mechanism of electron-proton-electron transfer that leads to the N-hydrogen oxoisoaporphine anion. When triethylamine, TEA, was used as the electron donor in anaerobic conditions, 1-diethylaminobutadiene, DEAB, was one of the oxidation products of TEA, among diethylamine and acetaldehyde. DEAB was identified by ¹H NMR and GC-MS experiments by comparison with the authentic 1- diethylaminobutadiene. This is the first report of a butadienyl derivative formed in the dye-sensitized photooxidation of TEA. In addition, isotopic exchange experiments with TEA-d₁₅ and D₂O show that the hydrogens at carbon-2 and carbon-4 of the butadienyl moiety are exchangeable. The observed isotopic exchange pattern could be explained by the head-to-tail coupling of an N,N-diethylvinylamine intermediate that exchanges hydrogens at the C-β via the enammonium ion.

Full text of DOI:10.1021/jo050796q

Unexpected Formation of 1-Diethylaminobutadiene in
Photosensitized Oxidation of Triethylamine Induced by
1
2,3-Dihydro-oxoisoaporphine Dyes. A H NMR and Isotopic
Exchange Study
Julio R. De la Fuente,*,† Carolina Jullian,†,‡ Claudio Saitz,† Vero´nica Neira,†
Oscar Poblete,† and Eduardo Sobarzo-Sa´nchez§
Departamento Quı´mica Orga´nica y Fisicoquı´mica and CEPEDEQ, Facultad de Ciencias Qu´ımicas y
Farmace´uticas, Universidad de Chile Casilla 233, Santiago, Chile, and Departamento de Qu´ımica
Orga´nica, Unidad Asociada al C.S.I.C., Facultad de Qu´ımica, Universidad de Santiago de Compostela,
Santiago de Compostela, Espan˜a
jrfuente@ciq.uchile.cl
Received April 19, 2005
Photoreduction of oxoisoaporphine dyes occurs via a stepwise mechanism of electron-proton-
electron transfer that leads to the N-hydrogen oxoisoaporphine anion. When triethylamine, TEA,
was used as the electron donor in anaerobic conditions, 1-diethylaminobutadiene, DEAB, was one
of the oxidation products of TEA, among diethylamine and acetaldehyde. DEAB was identified by
1H NMR and GC-MS experiments by comparison with the authentic 1-diethylaminobutadiene.
This is the first report of a butadienyl derivative formed in the dye-sensitized photooxidation of
TEA. In addition, isotopic exchange experiments with TEA-d15 and D2O show that the hydrogens
at carbon-2 and carbon-4 of the butadienyl moiety are exchangeable. The observed isotopic exchange
pattern could be explained by the head-to-tail coupling of an N,N-diethylvinylamine intermediate
that exchanges hydrogens at the C-â via the enammonium ion.
Introduction
reaction has been reported for quinone derivatives,3,4
thioindigo dyes,5 quinoxalin-2-ones,6,7 electron-deficient
azaarenes,8 and more recently, for 2,3-dihydro-oxoi-
soaporphines.9 When amines are the electron donor, the
photooxidation products are usually vinylamine, enam-
ine, iminium cation, aldehydes, and dealkylation prod-
ucts.10,11
In the photoreduction of many chromophores by elec-
tron donors, excited-state quenching by electron transfer
leads to transient ion radical pairs which, due to back
electron transfer, give no permanent chemical changes.1
For some compounds, electron transfer quenching gener-
ates basic radical anions that are easily protonated, and
semireduced free radicals accumulate.1,2 In some ex-
amples, a second electron transfer takes place, with
generation of stable or metastable products of two-
electron reduction or dihydro compounds, among oxida-
tion products of the electron donor. This type of photo-
(3) Gan, H.; Whitten, D. G. J. Am. Chem. Soc. 1993, 115, 8031-
8037.
(4) Gan, H.; Zhao, X.; Whitten, D. G. J. Am. Chem. Soc. 1991, 113,
9409-9411.
(5) Schanze, K. S.; Lee, L. Y. C.; Giannotti, C.; Whitten, D. G. J.
Am. Chem. Soc. 1986, 108, 2646-2655.
(6) De la Fuente, J. R.; Can˜ete, A.; Zanocco, A. L.; Saitz, C.; Jullian,
C. J. Org. Chem. 2000, 65, 7949-7958.
* Corresponding author. Phone: 56-2-9782880. Fax: 56-2-9782868.
† Departamento Qu´ımica Orga´nica y Fisicoqu´ımica, Universidad de
Chile.
(7) De la Fuente, J. R.; Can˜ete, A.; Saitz, C.; Jullian, C. J. Phys.
Chem. A 2002, 106, 7113-7120.
‡ CEPEDEQ, Universidad de Chile.
(8) Go¨rner, H.; Do¨pp, D.; Dittmann, A. J. Chem. Soc., Perkin Trans.
2 2000, 1723-1733.
§ Universidad de Santiago de Compostela. E-mail: esobarzo@usc.es.
(1) Gould, I. R.; Farid, S. J. Am. Chem. Soc. 1993, 115, 4814-4822.
(2) Ci, X.; da Silva, R. S.; Nicodem, D.; Whitten, D. G. J. Am. Chem.
Soc. 1989, 111, 1337-1343.
(9) De la Fuente, J. R.; Jullian, C.; Saitz, C.; Sobarzo-Sa´nchez, E.;
Neira, V.; Gonza´lez, C.; Lo´pez, R.; Pessoa-Mahana, H. Photochem.
Photobiol. Sci. 2004, 3 (2), 194-199.
10.1021/jo050796q CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/30/2005
8712
J. Org. Chem. 2005, 70, 8712-8716

Photochemical Formation of 1-Diethylaminobutadiene
FIGURE 1. Structures of oxoisoaporphines, A, the respective neutral radical, AH‚, and the N-hydrogen anion, AH-.
Result and Discussion
In our recent studies on the photoreduction of oxoi-
soaporphines, A, 2,3-dihydro-1-aza-benzo[de]anthracen-
7-ones (1-3) by amines,9 we have proposed a stepwise
mechanism of electron-proton-electron transfer that
FIGURE 2. Structure of DEAB.
leads to the N-hydrogen oxoisoaporphine anion, AH-,
with quantum yields of nearly 0.1. In that study, when
TEA was the electron donor, the oxidation product was
identified by NMR and GC-MS as 1-diethylaminobuta-
diene, DEAB.
TABLE 1. Chemical Shifts and Coupling Constants for
DEAB
hydrogen δ/ppm
coupling constant/Hz
H-1
H-2
H-3
H-4a
H-4b
H-5
H-6
6.18 J12 ) 13.7; J13 ) -0.5; J14a ) 0.6; J14b ) 0.5
4.90 J21 ) 13.7; J23 ) 10.4; J24a ) -0.5; J24b ) -0.5
6.13 J31 ) -0.5; J32 ) 10.4; J34a ) 10.4; J34b ) 16.8
4.26 J4a1 ) -0.5; J4a2 ) -0.5; J4a3 ) 10.4
4.51 J4b1 ) -0.5; J4b2 ) -0.5; J4b3 ) 16.8
3.08 J56 ) 7.3
The proposed mechanism for photoreduction of triplet
oxoisoaporphines by amines9 involves the formation of
the radical ion pair by single-electron transfer from the
amine to the excited triplet of the oxoisoaporphine, eq 1.
The amine cation radical readily donates a proton to the
radical anion of the oxoisoaporphine to generate the
dehydrogenated neutral radical of the amine, eq 2,
undergoes a second electron transfer, leading to the
iminium cation and the N-hydrogen anion of the oxoi-
soaporphine, AH-, eq 3, where the oxoisoaporphine, A,
the neutral radical, AH‚, and AH, have the structures
shown in Figure 1.
1.08 J65 ) 7.3
formed by hydrolysis of iminium cation by adventitious
water, eq 7. These products, acetaldehyde and diethy-
lamine, were also observed in the photolysis of TEA.
R2N+dC(H)R′ + H2O f R′CHO + R2NH+ (7)
A3 + R2N-CH2R′ f A-• + R2N+•-CH2R′ (1)
Notably, only when TEA was the electron donor in the
photoreductions of any of the used oxoisoaporphine dyes,
A1-A3, was 1-diethylaminobutadiene, DEAB, generated
A
-• + R2N+•-CH2R′ f AH• + R2N-C‚(H)R′ (2)
1
with a quantum yield ca. 0.03, estimated from their H
AH‚ + R2N-C‚(H)R′ f AH- + R2N+dCHR′ (3)
NMR absorptions.
1
The H NMR spectrum of the photooxidation product
of TEA shows signals between 6.0 and 6.3 ppm and
between 4.2 and 5.0 ppm (see Figure S1 in the Supporting
Information) as well as the 1H NMR signals of the N-H
of the oxoisoaporphine anion, AH-, at 5.4 ppm.9
These results strongly suggest that the photoproduct
of TEA is 1-diethylaminobutadiene, Figure 2. The addi-
tion of authentic DEAB, prepared by the method of
Hu¨ning and Kahanek,12 to the reaction mixture, confirms
the identity of the photoproduct. Table 1 summarizes
experimental chemical shifts and coupling constants for
DEAB.
The identity of the photoproduct was also confirmed
by EI+ mass spectroscopy from the molecular ion m/z+ )
125 and the fragment m/z+ ) 110, corresponding to loss
of CH3, and comparison with the authentic sample mass
spectrum. To further confirm the identity of the photo-
product, we carried out photolysis with perdeuterated
TEA, TEA-d15, and analyzed the products by GC-mass
spectroscopy. As expected, the photoreaction mixture
shows a chromatographic peak with the molecular ion
of m/z+ ) 140, corresponding to perdeuterated DEAB, but
surprisingly, the mayor chromatographic peak has a
The usually found photoproduct of TEA oxidation is the
vinylamine that could be generated through the depro-
tonation of the iminium cation, eq 4, or alternatively by
reaction of the TEA radical cation, deprotonated by excess
TEA, followed by hydrogen atom transfer between two
Et2N-C‚(H)CH3 radicals, eqs 5 and 6.10,11
Et2N+dC(H)CH3 S Et2N-CHdCH2 + H+ (4)
Et2N+•-CH2CH3 + TEA f
Et2N-C‚(H)CH3 + TEAH+ (5)
2Et2N-C‚(H)CH3 f TEA + Et2-N-CHdCH2 (6)
When other tertiary amines, such as N,N-diethylmethy-
lamine, N,N-dimethylethylamine, tri-n-propylamine, or
tri-n-butylamine, were the electron donors, the sole NMR
identifiable products, among AH-, were the respective
aldehyde and the secondary amine dealkylation product,
(10) Cohen, S. G.; Parola, A.; Parsons, G. H., Jr. Chem. Rev. 1973,
73, 141-161.
(11) Chow, Y. L.; Danen, W. C.; Nelsen, S. F.; Rosenblatt, D. H.
Chem. Rev. 1978, 78, 243-274.
(12) Hu¨ning, S.; Kahanek, H. Chem. Ber. 1957, 90, 238-245.
J. Org. Chem, Vol. 70, No. 22, 2005 8713

De la Fuente et al.
FIGURE 3. 1H NMR spectra of the photoproduct mixture: (a) photolysis products with TEA-H15, (b) with D2O added after
photolysis, (c) photolysis with TEA-d15, and (d) photolysis with TEA-H15 in the presence of D2O. For comments and assignments
see the text.
molecular ion of m/z+ ) 137, consistent with exchange
of three deuterium atoms of DEAB by hydrogen. The
relative peak areas for m/z+ 137 (Rt ) 10.04 min) and
m/z+ 140 (Rt ) 10.21 min) was 3.3:1, showing that almost
80% of the formed DEAB has exchanged deuterium by
exchanged with deuterium. To gain more insight into the
isotopic exchange reaction, we performed various pho-
tolysis experiments with TEA-H15 and TEA-d15, whose
1H NMR results are summarized in Figure 3 and Table
2 together with the GC-MS results.
1
hydrogen, claiming for the H NMR experiment using
When D2O is added to the photolyzed mixture with
TEA-H15, only the isotopic exchange of the oxoisoapor-
phine N-H proton is observed, as shown in Figure 3b.
As discussed in preceding paragraph, the photolyzed
1
TEA-d15 and D2O. Nevertheless, in the H NMR experi-
ment with TEA-d15, signals corresponding to the perdeu-
terated DEAB will not be observed, but the isotopic
exchange with adventitious H2O results in signals from
the exchanged hydrogen, allowing their assignment
1
mixture of TEA-d15 shows the H NMR signals corre-
sponding to hydrogens at C2 and C4 of DEAB, due to
isotopic exchange, with hydrogen from adventitious
water, Figure 3c. If D2O is added prior to photolysis with
TEA-H15, Figure 3d, the 1H NMR experiments show only
protons at C1 and C3 of DEAB. These results prove that
the isotopic exchange reaction by hydrogen or deuterium
only occurs at C2 and C4 of the butadienyl moiety of
DEAB at some stage in the photolysis.
1
(spectrum shown in Figure 3c). The signals in the H
NMR spectrum correspond to the hydrogens at C2 and
C4 of the butadienyl moiety of DEAB, confirming the
formation of (C2D5)2NCDdCH-CDdCH2 (m/z+ ) 137)
detected as the main photoproduct by GC-MS.
In NMR measurements in which TEA-H15 and D2O
were added prior to photolysis, only the hydrogens of C1
and C3 of the butadienyl moiety of DEAB are observed,
with those corresponding to C2 and C4 being completely
To explain the isotopic exchange pattern we consider
the complete isotopic exchange at C-â of the N,N-
8714 J. Org. Chem., Vol. 70, No. 22, 2005

Photochemical Formation of 1-Diethylaminobutadiene
TABLE 2. Isotopic Exchange Experiments
isotopic exchange on 1,3-butadienyl moiety
conditions
solvent: CD3CN
molecular
ions m/z+
C1
C2
C3
C4
products
TEA-H15/H2O
TEA-d15/H2O
125 (110)a
(CH3CH2)2NCHdCH-CHdC H2
140 (122)b
Xc
Xc
(CD3CD2)2NCDdCD-CDdC D2
(CD3CD2)2NCDdCH-CDdC H2
137 (119)b
TEA-H15/D2O added
prior photolysis
Xd
Xd
(CH3CH2)2NCHdCD-CHdCD 2
a Loss of CH3. b Loss of CD3. c Exchange for H. d Exchange for D.
diethylvinylamine intermediate via the enammonium
ion13-15 in the presence of excess D+ or H+ (from D2O or
H2O, respectively), as indicated in eq 8 for deuterium
exchange.
dienylamines would be expected, but the only identified
products were the respective dialkylamines and alde-
hydes, whose characteristic 1H NMR triplets appear near
9.5 ppm. The lack of formation of DEAB analogues when
photolysis is carried out with these amines may be
explained on the basis of greater steric hindrance that
probably prevents addition of the dialkylvinylamine
1
intermediate. The H NMR experiment with a mixture
of tripropylamine and TEA,22 1:1 v/v, does not show the
signals assigned to DEAB but shows new 1H NMR
signals between 5.8 and 6.3 ppm, at 5.0 ppm, and also
3.0 ppm (see Figure S9 in the Supporting Information)
that could be attributed to other unidentified alkylbuta-
dienes or alkylaminobutadienes likely formed from di-
ethylvinylamine and the vinyl derivative of tripropy-
lamine.
The expected DEAB analogues N,N-dialkyl-1,3-buta-
dienylamine for the photooxidation of N,N-diethylmethy-
lamine or N,N-dimethylethylamine was not formed,
probably due to the well-known deprotonation of radical
cations of amines to yield the least substituted R-ami-
noalkyl radical;10,11,23-25 thus, with the methyl-substituted
trialkylamines the reaction should lead to the methane-
iminium cation, RR′N+dCH2, that by hydrolysis leads to
formaldehyde and the respective dialkylamine.
Two reaction paths would be possible: (A) The head-
to-tail coupling of the resulting â-deuterated vinylamine
with elimination of diethylamine should lead to DEAB
deuterated at C2 and C4 of the butadienyl moiety, eq 9A.
The coupling reaction probably involves a diethylviny-
lamine cation radical,16 likely generated by photoinduced
electron transfer with the dye17 or by the formation of a
[2 + 2] cycloadduct18-20 followed by diethylamine elimi-
nation and formation of (C2H5)2NCHdCD-CHdCD2. (B)
Alternatively, the nucleophilic attack21 of the vinylamine
taking place at the C-R of the iminium cation, followed
by the elimination of diethylamine and H+, eq 9B, should
lead to DEAB with the same isotopic pattern of substitu-
tion over C2 and C4 of the butadienyl moiety. In both
reactions, butadienyl C2 and C4 were the former C-â of
the diethylvinylamine intermediate.
Conclusions
To our knowledge, this is the first report of a butadi-
enyl derivative formed in the dye-sensitized photooxida-
tion of TEA. Isotopic exchange experiments with TEA-
d15 and D2O show that the hydrogens at C2 and C4 of
the butadienyl moiety are exchangeable. The observed
isotopic exchange pattern could be explained by the head-
to-tail coupling of the diethylvinylamine intermediate
that exchanges hydrogen at the C-â via the enammonium
ion. The isotopic exchange reaction reported here may
provide a method for obtaining 1-diethylaminobutadiene
isotopically labeled at C2 and C4 or at C1 and C3 of the
butadienyl moiety.
For the photooxidation of tri-n-propylamine or tri-n-
butylamine, the DEAB analogues 2,4-dialkyl-1,3-buta-
Experimental Section
(13) Ellenberg, M. R.; Dixon, D. A.; Farneth, W. E. J. Am. Chem.
Soc. 1981, 103, 5377-5382.
(14) Ellenberg, M. R.; Eades, R. A.; Thomsen, M. W.; Farneth, W.
E.; Dixon, D. A. J. Am. Chem. Soc. 1979, 101, 7151-7154.
(15) Lammertsma, K.; Prasad, B. V. J. Am. Chem. Soc. 1994, 116,
642-650.
(16) Ledwith, A. Acc. Chem. Res. 1972, 5, 133-139.
(17) Padwa, A. Chem Rev. 1977, 77, 37-68.
(18) Arrieta, A.; Cossio, F. P.; Lecea, B. J. Org. Chem. 1999, 64,
1831-1842.
(19) Hall, H. K., Jr.; Abdelkader, M.; Glogowski, M. E. J. Org. Chem.
1982, 47, 3691-3694.
(20) Hall, H. K., Jr.; Ykman, P. J. Am. Chem. Soc. 1975, 97, 800-
807.
Amines were stored over potassium hydroxide pellets and
vacuum distilled, trap-to-trap, sealed into glass tubes at 10-4
mm of Hg, and stored at -18 °C. Before each experiment, a
(21) We thank one of the reviewers for letting us see way B of the
coupling reaction.
(22) We thank one of the reviewers for the suggestion to use an
amine mixture.
(23) Lewis, F. D.; Ho, T.; Simpson, J. T. J. Am. Chem. Soc. 1982,
104, 1924-1929.
(24) Pandey, G.; Reddy, P. Y.; Bhalerao, U. T. Tetrahedron Lett.
1991, 32, 5147-5150.
(25) Goez, M.; Frisch, I. J. Photochem. Photobiol., A 1994, 84, 1-12.
J. Org. Chem, Vol. 70, No. 22, 2005 8715

De la Fuente et al.
new tube was opened to ensure freshness of the amine.
Triethyl-d15-amine, 99% D from Sigma Aldrich, was used as
received.
1-Diethylaminobutadiene, DEAB, was prepared by a
literature-reported method12 in 42% yield: bp 54 °C/8 mm of
Hg (lit.12 64-66 °C/10 mm of Hg). 1H NMR data are shown in
Table 1. The compound decomposes in air.
hydrogen of DEAB against the integration of the methylene
hydrogen of the N-hydrogen oxoisoaporphine anion.
GC-MS analyses were performed in a capillary column,
starting at 80 °C for 1 min with a 10 deg/min ramp up to 180
°C. EI+ detection mode was used, scanning m/z+ from 47 to
200 with a scan time of 0.15 s and leaving an interscan time
of 0.16 s.
NMR Spectral Measurements. Reactions were carried out
by direct photolysis (in the NMR tube sealed with a septum)
of N2-purged solutions containing ca. 1.5 mg/mL of the 2,3-
dihydroxoisoaporphine in CD3CN. Immediately after purging,
an aliquot, 3-10 µL, of pure amine was added through the
septum. The solutions were photolyzed with light of 366 nm.
Acknowledgment. We thank FONDECYT Grant
No. 1030963 and Proyecto Facultad-CEPEDEQ for
financial support and Professors Frank Quina and
Clifford Bunton for the critical reading of this manu-
script.
1
During photolysis, several H NMR spectra were recorded in
order to test for maximum concentrations of the photoproduct.
COSY spectra were taken when no more changes were
detected in the NMR spectra. It was not possible to obtain a
13C NMR spectrum of the irradiated samples due to the low
concentration of the photoproducts.
Supporting Information Available: Authentic 1-diethy-
laminobutadiene 1H NMR and mass spectra and mass spectra
for deuterated DEAB are provided. This material is available
free of charge via the Internet at http://pubs.acs.org.
DEAB quantum yield estimation was made by compar-
1
ing the integration for the H NMR signals of the methylene
JO050796Q
8716 J. Org. Chem., Vol. 70, No. 22, 2005