Mechanistic studies of the azomethine ylide-forming photoreactions of N- (silylmethyl)phthalimides and N-phthaloylglycine

Article abstract of DOI:10.1021/ja9841862

In earlier studies we have shown that irradiation of MeCN solutions of N-[(trimethylsilyl)methyl]phthalimide and N-phthaloylglycine in the presence of electron-deficient olefins (e.g., methyl acrylate) results in the production of cycloadducts. In addition, irradiation of these substances in aqueous MeCN leads to formation of N-methylphthalimide. Laser flash photolysis and fluorescence spectroscopy have now been employed to investigate the mechanistic details of these novel excited-state processes. The results of this effort show that azomethine ylides are the key reactive intermediates in these processes. In addition, the investigations provide information about the dynamics of several ylide decay pathways and the nature of the excited states responsible for the ylide-forming silyl-migration (singlet and triplet) and decarboxylation (triplet) reactions. Pulsed irradiations of MeCN solutions of N-[(trimethylsilyl)methyl]phthalimide (1) and N-phthaloylglycine (2) give rise to transients whose absorption and decay properties are consistent with their assignment as azomethine ylides. Kinetic analysis of the decay of the ylides in the presence of dipolarophiles, methyl acrylate and acrylonitrile, provides the rates of the dipolar cycloaddition reactions. Reactions of methyl acrylate with the ylides produced by pulsed irradiation of N-[(trimethylsilyl)methyl]phthalimide (1) and N- phthaloylglycine (2) occur with respective bimolecular rate constants of 8.9 x 10⁶ and 2.7 x 107 M^-1 s^-1. Methanol promotes the decay of the N- [(trimethylsilyl)methyl]phthalimide-derived ylide by a process which is second order in MeOH and has a kinetic OD-isotope effect of 4.3. In contrast, quenching of this ylide by acetic acid is first order in AcOH. The results suggest that the mechanism for MeOH-promoted decay involves initial and reversible formation of a silylate complex via nucleophilic addition of MeOH to the ylide. This is then followed by rate-limiting proton transfer from MeOH to the carbanionic center in the silylate complex either in concert with or preceding desilylation. The mechanism for AcOH-induced ylide decay has these steps reversed; i.e., rate-limiting proton transfer precedes AcOH- induced desilylation. Also, MeOH catalyzes the decay of the ylide derived by irradiation of N-phthaloylglycine by a process which is first order in MeOH and has a kinetic OD-isotope effect of 1.5. Finally, the observations (1) of complete loss of fluorescence of the 1,8- and 2,3-naphthalimide chromophores upon changing the N-substituent from methyl to (trimethylsilyl)methyl and (2) that ylide formation from 1 can be xanthone triplet sensitized suggest that the ylide-forming, silyl-transfer reactions of the (silylmethyl)phthalimides can occur in both the singlet and triplet excited-state manifolds.

Full text of DOI:10.1021/ja9841862

3926
J. Am. Chem. Soc. 1999, 121, 3926-3932
Mechanistic Studies of the Azomethine Ylide-Forming Photoreactions
of N-(Silylmethyl)phthalimides and N-Phthaloylglycine
Yasutake Takahashi,†,‡ Tsutomu Miyashi,† Ung Chan Yoon,§ Sun Wha Oh,§
Maria Mancheno, Zhuoyi Su,| Daniel F. Falvey, and Patrick S. Mariano*,|
Contribution from the Department of Chemistry, Graduate School of Science, Tohoku UniVersity,
Aoba-ku, Sendai 980-8578, Japan, Chemistry Department for Materials, Faculty of Engineering, Mie
UniVersity, Tsu, Mie 514-8507, Japan, Department of Chemistry, College of Natural Sciences, Pusan
National UniVersity, Pusan 609-735, Korea, Department of Chemistry, UniVersity of New Mexico,
Albuquerque, New Mexico 87131, and Department of Chemistry and Biochemistry, UniVersity of
Maryland, College Park, Maryland 20742
ReceiVed December 4, 1998
Abstract: In earlier studies we have shown that irradiation of MeCN solutions of N-[(trimethylsilyl)methyl]-
phthalimide and N-phthaloylglycine in the presence of electron-defecient olefins (e.g., methyl acrylate) results
in the production of cycloadducts. In addition, irradiation of these substances in aqueous MeCN leads to
formation of N-methylphthalimide. Laser flash photolysis and fluorescence spectroscopy have now been
employed to investigate the mechanistic details of these novel excited-state processes. The results of this effort
show that azomethine ylides are the key reactive intermediates in these processes. In addition, the investigations
provide information about the dynamics of several ylide decay pathways and the nature of the excited states
responsible for the ylide-forming silyl-migration (singlet and triplet) and decarboxylation (triplet) reactions.
Pulsed irradiations of MeCN solutions of N-[(trimethylsilyl)methyl]phthalimide (1) and N-phthaloylglycine
(2) give rise to transients whose absorption and decay properties are consistent with their assignment as
azomethine ylides. Kinetic analysis of the decay of the ylides in the presence of dipolarophiles, methyl acrylate
and acrylonitrile, provides the rates of the dipolar cycloaddition reactions. Reactions of methyl acrylate with
the ylides produced by pulsed irradiation of N-[(trimethylsilyl)methyl]phthalimide (1) and N-phthaloylglycine
(2) occur with respective bimolecular rate constants of 8.9 × 106 and 2.7 × 107 M-1 s-1. Methanol promotes
the decay of the N-[(trimethylsilyl)methyl]phthalimide-derived ylide by a process which is second order in
MeOH and has a kinetic OD-isotope effect of 4.3. In contrast, quenching of this ylide by acetic acid is first
order in AcOH. The results suggest that the mechanism for MeOH-promoted decay involves initial and reversible
formation of a silylate complex via nucleophilic addition of MeOH to the ylide. This is then followed by
rate-limiting proton transfer from MeOH to the carbanionic center in the silylate complex either in concert
with or preceding desilylation. The mechanism for AcOH-induced ylide decay has these steps reversed; i.e.,
rate-limiting proton transfer precedes AcOH-induced desilylation. Also, MeOH catalyzes the decay of the
ylide derived by irradiation of N-phthaloylglycine by a process which is first order in MeOH and has a kinetic
OD-isotope effect of 1.5. Finally, the observations (1) of complete loss of fluorescence of the 1,8- and 2,3-
naphthalimide chromophores upon changing the N-substituent from methyl to (trimethylsilyl)methyl and (2)
that ylide formation from 1 can be xanthone triplet sensitized suggest that the ylide-forming, silyl-transfer
reactions of the (silylmethyl)phthalimides can occur in both the singlet and triplet excited-state manifolds.
Introduction
suggested that it is likely that these processes follow a mech-
anistic route involving generation and endo-selective trapping
of azomethine ylides related to 3 (Scheme 1). Additional
evidence supporting the proposal that ylides are short-lived
intermediates in these pathways comes from the observation
that irradiation of 1 in a D2O-MeCN solution leads to formation
of the N-(monodeuteriomethyl)phthalimide (7).1b This product
is expected to form by ylide protonation-desilylation (or the
reverse).
In recent publications,1 we described a novel class of
photocycloaddition reactions of N-(silylmethyl)phthalimides and
-maleimides that result in the production of functionalized
pyrrolizidines. An example is found in the photoreaction of
N-[(trimethylsilyl)methyl]phthalimide (1)1b with methyl acrylate,
which produces the benzopyrrolizidine 5 exclusively. We have
† Tohoku University.
‡ Mie University.
§ Pusan National University.
A major process involved in the photochemistry of N-
phthaloyl R-amino acids involves decarboxylation to produce
the corresponding N-alkylphthalimide2 (e.g., the transformation
of 2 to 8, Scheme 1). Observations made in our earlier and more
recent2d studies suggest that these photoreactions also proceed
via the intermediacy of azomethine ylides (e.g., 4). Collapse of
4 by a 1,4-hydrogen shift pathway would give rise to the N-alkyl
products. Consistent with this proposal is our finding that
University of Maryland.
| University of New Mexico.
(1) (a) Yoon, U. C..; Kim, D. U.; Kim, J. C.; Lee, C. W.; Choi, Y. S.;
Mariano, P. S.; Lee, Y. J.; Ammon, H. L. Tetrahedron. Lett. 1993, 34,
5859. (b) Yoon, U. C.; Kim, D. U..; Lee, J. G.; Lee, Y. J.; Ammon, H. L.;
Mariano, P. S. J. Am. Chem. Soc. 1995, 117, 2698. (c) Yoon, U. C.; Choi,
S. J.; Lee, Y. J.; Mancheno, M. J.; Mariano, P. S. J. Org. Chem. 1995, 60,
2353. (d) Yoon, U. C.; Lee, C. W.; Oh, S. W.; Mariano, P. S. Unpublished
results.
10.1021/ja9841862 CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/13/1999

Azomethane Ylide-Forming Photoreactions
J. Am. Chem. Soc., Vol. 121, No. 16, 1999 3927
Scheme 1
irradiation of these amino acid derivatives in solutions containing
dipolarophiles (e.g., methyl acrylate) leads to formation of
benzopyrrolizidine cycloadducts (e.g., 6, Scheme 1).1b,d
Our continuing studies of these novel and preparatively
significant photochemical processes have focused on both
mechanistic and synthetic issues. In this paper, we report the
results of photophysical investigations which (1) evidence the
existence of ylide intermediates in the silyl-transfer and decar-
boxylation photoreactions, (2) provide data on the dynamics of
various ylide decay pathways, and (3) suggest the nature of the
phthalimide excited states which are responsible for the silyl-
transfer reaction.
Figure 1. Transient absorption spectrum obtained (200 ns) following
pulsed 266 nm irradiation of an N2-saturated MeCN solution of 1 (5
mM) at 25 °C. The inset is a plot of the observed rate of decay
monitored at 392 nm vs the concentration of acrylonitrile.
Results
Laser flash photolysis (LFP) experiments were carried out
with N-[(trimethylsilyl)methyl]phthalimide (1) and N-phtha-
loylglycine (2). Pulsed 266 or 308 nm irradiation of N2-saturated
MeCN solutions of 1 (5 mM) at 25 °C results in the formation
of a transient with an absorption maximum at 392 nm (Figure
1). The UV-absorption and kinetic data (see below) are
consistent with the assignment of the 392 nm transient as the
azomethine ylide 3. Decay of this transient in MeCN follows
second-order kinetics (1.4 × 106 A-1 s-1) (Figure 2). Analyses
of the effects of acrylonitrile (Figure 1, inset) and methyl
acrylate on the decay rates give the second-order rate constants
for trapping of ylide 3 by these dipolarophiles (1.2 × 107 and
8.9 × 106 M-1 s-1, respectively) (Table 1). Methanol (and
MeOD) also promote decay of the 392 nm transient. The
observed rate vs MeOH concentration data best fit a process
that is second order in this alcohol (Figure 3). The rate constants
for MeOH (1.9 × 105 M-2 s-1) and MeOD (4.4 × 104 M-2
s-1) induced decay of ylide 3 correspond to an isotope effect
of 4.3. In contrast, acetic acid facilitates decay of the 392 nm
transient by a process which is first order in AcOH (3.1 × 108
M-1 s-1) (Figure 4).
Figure 2. Experimental decay data (dots) for the 392 nm transient
identified as ylide 3 arising by pulsed irradiation of (silylmethyl)-
phthalimide 1 (solid lines) treated as a (a) first-order and (b) second-
order decay process.
(308 nm) of 2 gives rise to a 392/345 nm absorbing transient
whose decay profile is consistent with its assignment as the ylide
4. Decay of 4 in MeCN follows first-order kinetics (2.9 × 106
s-1) (Figure 5). The rate of decay of 4 is enhanced by the
addition of MeOH (MeOD) in a manner which is first order in
alcohol (2.8 × 105 M-1 s-1 for MeOH and 1.9 × 105 M-1 s-1
for MeOD, giving kOH/kOD ) 1.5). Also, the rate of decay of
this transient is enhanced by the addition of methyl acrylate, in
a process which has a second-order rate constant of 2.7 × 107
The chemical evidence accumulated in our studies of the
photoreactions of N-phthaloyl R-amino acids1b,d suggests that
azomethine ylides related to 4 (Scheme 1) serve as intermediates
in these processes also. The results of LFP studies with the
glycine derivative 2 corroborate this proposal. Pulsed irradiation
(2) (a) Sato, Y.; Nakai, H.; Mizoguchi, T.; Kawanishi, M.; Hatanaka,
Y.; Kanaoka, Y. Chem. Pharm. Bull. 1982, 30, 1263. (b) Griesbeck, A. G.;
Henz, A.; Hirt, J.; Ptatschek, V.; Engel, T.; Loffler, D.; Schneider, F. W.
Tetrahedron 1994, 50, 701. (c) Griesbeck (ref 2d) has also suggested that
the photodecarboxylation reaction may involve excited-state proton transfer,
which would be reasonable if singlets were the reactive excited states of
these substrates. (d) Griesbeck, A. G.; Henx, A.; Peters, K.; Peters, E. M.;
vonScnering, H. G. Angew. Chem., Int. Ed. Engl. 1995, 34, 474.
M-1 s-1
.
It is difficult to obtain direct information about the nature of
the excited state(s) that are responsible for the ylide-forming
photoreaction of (silylmethyl)phthalimide 1 and glycine deriva-

3928 J. Am. Chem. Soc., Vol. 121, No. 16, 1999
Takahashi et al.
Table 1. Spectroscopic and Kinetic Properties of Ylides Derived by Laser Flash Photolysis of Phthalimides 1 and 2a
rate constant
transient
(abs max)
methyl acrylate
(acrylonitrile)
phthalimide
no additive
MeOH (MeOD)
AcOH
1
3 (592 nm)
1.4 × 106 A-1 s-1
2.9 × 106 s-1
8.9 × 106 M-1 s-1
(1.2 × 107 M-1 s-1
2.7 × 107 M-1 s-1
1.9 × 105 M-2 s-1
(4.4 × 104 M-2 s-1
2.8 × 105 M-1 s-1
(1.9 × 105 M-1 s-1
3.1 × 108 M-1 s-1
)
)
)
2
4 (392, 345 nm)
a Error limits for the decay rates in MeCN are estimated to be ( 0.1 × 106. The plots of kobs vs additive concentrations gave r2 values in the
range of 0.94-0.99.4
Figure 3. Observed rates of decay of the 392 nm transient (see Figure
1) formed by pulsed irradiation of 1 vs the squares of the concentrations
of MeOH (empty circles) and MeOD (filled circles).
Figure 5. Experimental decay data (dots) for the 392 nm transient
identified as ylide 4 arising by pulsed irradiation of N-phthaloylglycine
2 (solid lines) treated as a (a) first-order and (b) second-order decay
process.
Scheme 2
Figure 4. A plot of the observed rates of decay of the 392 nm transient
(see Figure 1) formed by pulsed irradiation of 1 vs the concentration
of acetic acid.
tive 2 owing to the weak or absent fluorescence emission of
these and related N-alkylphthalimides.3 In contrast, N-methyl-
1,8- and -2,3-naphthalimide are known3c to efficiently fluoresce
in fluid solution at 25 °C. In addition, we showed earlier1b that
the (silylmethyl)-1,8-naphthalimide 9 also participates in pho-
tocycloaddition reactions with dipolarophiles, yielding the
naphthoindolizidines 11 along with N-methyl-1,8-naphthalimide
(12) (Scheme 2). In the current investigation, we found that
1
to 15 is based on comparisons of H and 13C NMR data with
irradiation of the related 2,3-naphthalimide in an MeCN solution
those of closely related benzopyrrolizidines (e.g., 5.1b Unlike
containing 0.1 M acrylonitrile results in the formation of the
the photoreactions of the simple N-(silylmethyl)phthalimide 1,
pyrrolizidine 15 (Scheme 3). The stereochemical assignment
where cycloadduct 5 formation is the exclusive reaction pathway
(3) (a) Although the fluorescence spectra and efficiencies of N- detect
reliable fluorescence from N-methylphthalimide. (b) Coyle, J. D.: Harriman,
A.; Newport, G. L. J. Chem. Soc., Perkin Trans. 2 1979, 799. (c) Wintgens,
V.; Valat, P.; Kossanyi, J.; Biczok, L.; Demeter, A.; Berces, T. J. Chem.
Soc., Faraday Trans. 1994, 90, 411.
followed when dipolarophile concentrations are 0.1 M, the
naphthalimides 9 and 13 react under these conditions to form
adducts along with substantial amounts of the respective
N-methyl derivatives 12 and 16.

Azomethane Ylide-Forming Photoreactions
J. Am. Chem. Soc., Vol. 121, No. 16, 1999 3929
Scheme 3
ylides 3 and 4. Although absorption spectra of a variety of
azomethine ylides have been recorded,5 spectroscopic data are
not available for ylides which have the structural features and
substitution patterns found in 3 and 4. Concern that the transients
ascribed to these ylides might really be the triplets of 1 and 2
was alleviated by comparing the data accumulated in this study
with those reported for N-alkylphthalimides by Coyle3b and
Wintgens.3c These workers have shown that the triplets of
N-methylphthalimide and its homologues have (1) absorption
maxima at 320-340 nm, (2) first-order decay profiles, and (3)
decay rates which increase by 4-fold in changing the solvent
from cyclohexane to methanol. These data differ dramatically
from those found in our studies with the phthalimides 1 and 2.
Additional information which is relevant to this issue has
come from transient quenching studies. We observed that
photolysis of phthalimide and its N-methyl derivative gives rise
to transients absorbing at 336 and 337 nm, respectively, and
that these species are quenched at a diffusion-controlled rate
((1.8-2.0) × 1010 M-1 s-1) by ferrocene, a known6 triplet (ca.
38 kcal/mol) quencher. Also, the rates of decay of the 336 and
337 nm transients are not affected by acetic acid (0.1 M). In
contrast, the 392 nm absorbing transient arising by irradiation
of the (silylmethyl)phthalimide 1 is not quenched by ferrocene,
and as described above, its decay rate is greatly facilitated by
the addition of acetic acid. Thus, the accumulated data provide
strong support for the assignments of the 392 nm absorbing
tansients as the azomethine ylides 3 and 4.
Table 2. Fluorescence Quantum Yields of N-CH3, N-CH2TMS,
and N-CH2CO2H Naphthalimides in MeCN at 25 °C
naphthalimide (N-substituent)
λemissiona (nm)
φfb
1,8-Naphthalimides
9 (CH2TMS)
12 (CH3)
17 (CH2CO2H)
0
362, 378
378
0.02c (0.027)d
0.09
2,3-Naphthalimides
13 (CH2TMS)
16 (CH3)
18 (CH2CO2H)
0
368, 380
370, 378
0.3c (0.24)d
0.4
a Excitation at 285 nm in all cases and substrate concentrations
adjusted to ensure equal absorbance at this wavelength. b 2-Aminopy-
ridine was used as the actinometer (ref 4) for these measurements.c This
work. d Reference 3c.
The results of this study confirm our earlier proposal that
azomethine ylides serve as key intermediates in photoreactions
of (silylmethyl)phthalimides and phthalimide derivatives of
R-amino acids. The effort has also unveiled additional informa-
tion about the ylide-forming excited-state processes. On the basis
of the fluorescence quantum yield data given in Table 2, it
appears that N-(silylmethyl)naphthalimide (and perhaps phthal-
imide) singlet excited states are involved in the direct irradiation-
promoted C-to-O silyl-migration reactions that produce the
OTMS-substituted azomethine ylides. This proposal is consistent
with the observation that the singlet-state fluorescence of the
naphthalimides is completely quenched by introduction of the
N-CH2TMS moiety. However, the observation that the triplet
(74 kcal/mol) can sensitize the formation of the OTMS-ylide 3
demonstrates that the triplet state of 1 also participates in the
silyl-migration process.
The driving force for the ylide-forming silyl-rearrangement
process can be thought about in several ways. It is well-known3
that phthalimides participate in 2+2-type photocycloaddition
reactions with non-electron-rich alkenes to form benzazepin-
dione products 20 (Scheme 4). These results suggest that the
dipolar structure 19 is a reasonable contributor to the singlet
excited states of these substrates. As such, it is possible to
envisage the ylide-forming reaction of singlet states 21 of
N-(silylmethyl)phthalimides as a rearrangement involving trans-
fer of the silyl group to the silophilic oxyanion carbonyl oxygen
center from the more weakly bonded C-Si position. Relevant
to this are the observations made by Vedejs8 and Padwa,9 which
show that N-(silylmethyl)iminium salts undergo facile desily-
lation to produce azomethine ylides.
To gain insight into the excited-state origin of the ylide-
forming photoreactions, fluorescence measurements were made
with the N-methyl-, -(silylmethyl)-, and -(carboxymethyl)-1,8-
and -2,3-naphthalimides 9, 12, 13, and 16-18. The fluorescence
quantum yields of these substances in MeCN at 25 °C were
determined by using 2-aminopyridine (λexcit 285 nm, λemiss 367
nm, φf 0.6) as the actinometer.4 The data, accumulated in Table
2, show that the placement of a TMS grouping into these
naphthalimides results in complete quenching of their fluores-
cence. In contrast, the carboxylic acid analogues have larger
fluorescence efficiencies compared to the corresponding N-
methylnaphthalimides.
Additional information has been gained about the C-to-O
silyl-migration reactivity of the triplet excited state of 1. We
observed that the 392 nm absorbing transient can also be
generated by use of xanthone triplet-sensitized irradiation of
the (silylmethyl)phthalimide. Pulse irradiation of a solution of
1 and xanthone leads to initial formation of the xanthone triplet
absorbing at 620 nm. This transient is quenched by 1 at a near-
diffusion-controlled rate (6.8 × 109 M-1 s-1) with concomitant
formation of the 392 nm transient assigned to ylide 3.
(5) Das, P. K. In Handbook of Organic Photochemistry; Scaiano, J. C.,
Ed.; CRC Press: Boca Raton, FL, 1989; Vol. II, pp 35-70.
(6) Farmillo, A.; Wilkinson, F. Chem. Phys. Lett. 1975, 34, 575.
(7) (a) Maruyama, K.; Kubo, Y. J. Org. Chem. 1981, 46, 3612. (b)
Mazzochi, P. M.; Bowen, M. J.; Narain, N. K. J. Am. Chem. Soc. 1977,
99, 7063.
(8) Vedejs, E.; Martinez, G. R. J. Am. Chem. Soc. 1979, 101, 6542.
(9) Padwa, A.; Haffmans, G.; Tomas, M. Tetrahedron. Lett. 1983, 24,
4303.
Discussion
Azomethine Ylide Formation. The interpretations given to
the data presented above are based on the proposition that the
392 nm absorbing transients arising by photolysis of phthalimide
derivatives 1 and 2 correspond to the respective azomethine
(4) Rusakowicz, R.; Testa, A. C. J. Chem. Phys. 1968, 21, 2680.

3930 J. Am. Chem. Soc., Vol. 121, No. 16, 1999
Takahashi et al.
Scheme 4
Scheme 6
Scheme 5
Azomethine Ylide Reactions. Product distribution and
reaction quantum yield data accumulated in our earlier studies1b
suggest that the major pathways for the decay of azomethine
ylides, formed by irradiation of N-(silylmethyl)phthalimides,
involve protodesilylation to form N-methylphthalimides and
dipolar cycloaddition with dipolarophiles to form benzopyr-
rolizidines. Aziridine-forming electrocyclic ring closure, a
common reaction open to azomethine ylides derived by pho-
tolysis of aziridines12 and acyclic N-(silylmethyl)imides,1c is not
a major process responsible for the decay of phthalimide-derived
ylides as judged by the absence of ring expansion products in
the preparative photoreaction mixtures. The kinetic data also
show that a, by necessity slow, bimolecular pathway is also
involved in the decay of these intermediates. Information is not
currently available to offer a hint at the mechanism for this
interesting bimolecular decay process.
The results of our earlier chemical and current LFP studies
demonstrate that the decay of (silylmethyl)phthalimide-derived
ylide is enhanced by methanol and acetic acid owing to reactions
that lead to the production of N-methylphthalimides. The
differences between the molecularity of these protodesilylation
reactions with MeOH vs AcOH suggest the operation of two
different, but perhaps related, mechanisms. A two-step route
(Scheme 6, top) involving preequilibrium formation of the
silylate complex 24 best explains the kinetic order and large
(4.3) OD-isotope effect associated with MeOH-promoted decay
of ylide 3. Collapse of 24 by proton transfer from MeOH is
expected to be rate limiting in this case owing to the high
silophilicity and low acidity of this alcohol. A similar mecha-
nism might be responsible for acetic acid-promoted decay of
the OTMS-azomethine ylide intermediate. In this case, addition
of AcOH to form a silylate complex analogous to 24 should be
rate limiting (i.e., protonation of the silylate complex by AcOH
should be fast). Consequently, the reaction would be, as is
observed, first order in AcOH. Alternatively, the ordering of
the protonation and desilylation steps might be reversed in the
AcOH-induced decay process. In this route (Scheme 6, bottom),
the first step would be reversible AcOH-promoted C-protonation
to form intermediate iminium salts 25. Deprotonation of 25 by
acetate should be slow relative to desilylation. Thus, the first
Alternatively, the singlet and triplet excited-state silyl-transfer
processes might be driven by intramolecular electron transfer
from the σC-Si donor to the high-energy phthalimide singlet or
triplet excited states. From this perspective, C-to-O silyl
migration would occur in the short-lived zwitterionic diradical
22. Support for this proposal is found in our recent studies with
(silylalkyl)phthalimides,10 in which we have found that facile
intramolecular SET from σC-Si donor sites to phthalimide
excited singlets promotes silyl transfer to silophiles such as
water.
The emission data presented in Table 2 show that the presence
of a carboxylic acid function in the N-naphthoylglycines 17 and
18 leads to a slight enhancement of the fluorescence efficiencies.
This phenomenon might be due to intramolecular hydrogen-
bonding between the side chain carboxyl and imide carbonyl
groups and its effect on the energetic ordering of naphthalimide
n-π* vs π-π* singlet excited states. Hydrogen-bonding
interactions should also exist in related N-phthaloylglycine
derivatives.11 Importantly, the fluorescence data show that the
presence of carboxylic acid side chains in the naphthalimides
17 and 18 does not bring about singlet-state quenching. This
observation is consistent with the findings of Kanaoka2a and
Griesbeck2b,c which suggest that photodecarboxylation reactions
of N-phthaloyl R-amino acids are initiated by triplet excited-
state H-atom abstraction followed by rapid decarboxylation of
the formed carboxy radicals 23 (Scheme 5). Intersystem crossing
needed to reenter the ground-state singlet ylide manifold can
occur at any stage following diradical 23 formation and
decarboxylation.
(10) Lee, Y. J.; Ling, R,; Mariano, P. S.; Yoon, U. C.; Kim, D. U.; Oh,
S. W. J. Org. Chem. 1996, 61, 3304.
(11) For a general review see: Turro, N. J. Modern Molecular
Photochemistry; Benjamin-Cummings: Menlo Park, CA, 1978; pp 41-43
and 165-70. For the specific example of fluorenone see: Caldwell, R. A.;
Gajewski, R. P. J. Am. Chem. Soc. 1971, 93, 532 and references therin.
(12) Hermann, H.; Huisgen, R.; Mader, H. J. Am. Chem. Soc. 1971, 93,
1779; Padwa, A.; Hamilton, A. J. Heterocycl. Chem. 1967, 4, 118.

Azomethane Ylide-Forming Photoreactions
J. Am. Chem. Soc., Vol. 121, No. 16, 1999 3931
reported as m/z (relative intensity), were recorded by using electron
impact ionization (EI).
Scheme 7
Photochemical reactions were conducted by using an apparatus
consisting of a 450 W medium-pressure mercury lamp surrounded by
a glass filter and within a quartz, water-cooled well that was purged
with O2-free N2 both before and during irradiation. Photochemical
1
reaction progress was monitored by TLC or H NMR.
Laser Flash Photolysis Experiments. Nitrogen-saturated MeCN
solutions of 1 and 2 at 25 °C were subjected to pulse irradiation by
using either the fourth (266 nm) harmonic output from a Q-switched
Nd:YAG laser (6-8 ns pulse width) or an XeCl (308 nm) excimer
laser (6-10 ns pulse). The probe assembly for measurement of time-
resolved spectra consists of either a 150 W xenon lamp and a
polychromator equipped with an image intensifier coupled with an
image sensor or a 450 W xenon lamp and a digital oscilloscope.
N-[(Trimethylsilyl)methyl]-2,3-naphthalimide (13). A suspension
of 0.18 g (4.6 mmol) of NaH (60% dispersion) and 2,3-naphthalimide
(0.3 g, 1.5 mmol) in 5 mL of DMF was stirred for 20 min at 0 °C, and
0.90 g (4.56 mmol) of (trimethylsilyl)methyl iodide was added. The
mixture was stirred for 12 h at 25 °C, diluted with water, and extracted
with ether. The ethereal extracts were dried and concentrated in vacuo,
giving a residue which was subjected to silica gel chromatography
(hexane-diethyl ether), giving 0.3 g (75%) of 13: mp 128-129 °C
(hexane); 1H NMR δ 8.19 (2H, s, ArH), 7.93 (2H, m, ArH), 7.59 (2H,
m, ArH), 3.21 (2H, s, CH2), 0.06 (9H, s, TMS); 13C NMR δ 168.1
(CO), 135.3 (C), 130.2 (CH), 129.0 (CH), 128.1 (C), 124.1 (CH), 29.5
(CH2), -1.8 (TMS); IR (CHCl3) 1760, 1694, 1640, 1605 cm-1; UV
(MeCN) 259 (113 000), 358 (7800) nm; EIMS m/z (rel intens) 283
(M+, 48), 282 (100), 268 (56), 74 (49); HRMS m/z 283.1016 (C16H17-
NO2Si requires 283.1028).
step in this process would be rate limiting, and therefore, the
overall reaction would be first order in AcOH.
The OH-azomethine ylides formed by irradiation of N-
phthaloyl R-amino acids are readily transformed to N-meth-
ylphthalimides by intramolecular proton transfer. Interestingly,
MeOH plays a catalytic role by serving to enhance the rate of
this reaction. The first-order MeOH involvement and small (1.5)
OD-isotope effect associated with enhanced decay of ylide 4
suggest that MeOH may simply act as a bridge (e.g., 26) in the
proton-transfer relay (Scheme 7). The small OD-isotope effect
associated with the MeOH- vs MeOD-promoted decay of the
OH- vs OD-ylide 4 might also be a consequence of the slower
rate of intramolecular D vs H transfer, leading to the N-
deuteriomethyl vs N-methyl product.
The LFP results have clarified several observations made in
our earlier studies1 of the preparative photochemistry of the
azomethine ylide precursors. Specifically, we found that ir-
radiation of the N-(silylmethyl)phthalimide (1) in MeCN solu-
tions containing 0.1 M methyl acrylate leads to exclusive and
high-yielding formation of the cycloadduct 5 (Scheme 1). Under
these conditions, none of the N-methylphthalimide (8) is formed.
In contrast, reactions of the N-phthaloyl R-amino acids such as
the glycine derivative 2 in MeCN with 0.1 M methyl acrylate
lead to lower yielding production of the cycloadducts along with
commensurate generation of N-methyl products.1 We now know
that one reason for the differences in the reactivity profiles of
the OTMS- and OH-ylides 3 and 4 is that the latter intermediate
undergoes rapid (2.9 × 106 s-1) unimolecular 1,4-hydrogen
transfer to produce the N-methyl product in competition with
bimolecular dipolar cycloaddition with methyl acrylate (2.7 ×
107 s-1 M-1). In contrast, the decay of the OTMS ylide 3, owing
to its bimolecular nature, is slow relative to cycloadditions with
dipolarophiles.
Photochemistry of N-[(Trimethylsilyl)methyl]-2,3-naphthalimide
(13). An N2-purged solution of 13 (220 mg, 0.78 mmol) and 0.7 mL
(10.5 mmol) of acrylonitrile at 25 °C in 110 mL of MeCN was irradiated
using Pyrex-filtered light for 22 h (100% conversion). The photolyzate
was concentrated in vacuo, giving a residue which was subjected to
chromatography on silica gel (hexanes-ether) to give 17 mg (10%) of
15, 4 mg (3%) of 2,3-naphthalimide, and 34 mg (14%) of the known12
phthalimide 16 (hexane-ether).
Data for 15: mp 177-178 °C; 1H NMR δ 8.26 (1H, s, ArH), 7.97
(1H, s, ArH), 7.91 (2H, d, J ) 7.9, ArH), 7.56 (2H, d, J ) 7.9, ArH),
3.95 (1H, dd, J ) 9.4, J ) 9.1, CHN), 3.51 (1H, t, CHN, J ) 9.4),
3.42 (1H, d, J ) 6.6, CHCN), 2.85 (1H, m, CH2), 2.61 (1H, m,CH2),
-0.18 (9H, s, TMS); 13C NMR δ 169.4 (CO), 139.2 (C), 135.6 (C),
134.2 (C), 130.0 (CH), 129.8 (CH), 128.7 (CH), 128.4 (CH), 127.4
(CH), 124.8 (CH), 122.7 (CH), 117.4 (CN), 98.0 (C-OTMS), 41.1
(CH2), 40.2 (CH), 32.1 (CH2), 0.6 (TMS); IR (CHCl3) 2361 (CN), 1714
(CO) cm-1; EIMS m/z (rel intens) 336 (M+, 24), 283 (54), 282 (100);
HRMS m/z 336.1290 (C19H20N2O2Si requires 336.1293).
N-(Carbomethoxymethyl)-1,8-naphthalimide (17). A suspension
of 0.6 g (0.015 mol) of NaH (60% dispersion) and 1,8-naphthalimide
(1.0 g, 0.01 mol) in 5 mL of DMF at 0 °C was stirred for 20 min.
After addition of 1.6 mL (0.015 mol) of ethyl bromoacetate the mixture
was stirred for 12 h at 25 °C, diluted with water, and extracted with
ether. The ethereal extracts were dried and concentrated in vacuo, giving
1.4 g (98%) of N-(carbomethoxymethyl)-1,8-naphthalimide (17): mp
Summary. The studies described above have led to a clearer
understanding of the mechanisms for azomethine ylide formation
and decay in the photochemistry of N-(silylmethyl)phthalimides
and N-phthaloyl R-amino acids. With the detailed knowledge
provided by these efforts, we are better positioned to design
synthetically relevant photoreactions of these substrates.
1
162-163 °C (hexane-ether); H NMR δ 8.61 (2H, d, J ) 7, ArH),
8.23 (2H, d, J ) 8.1, ArH), 7.76 (2H, t, J ) 7.0, ArH), 4.95 (2H, s,
CH2), 4.25 (2H, q, J ) 7.0, CH2), 1.30 (3H, t, J ) 7.0, CH3); 13C
NMR δ 168.1 (CO), 163.8 (CO), 134.3 (ArH), 132.6 (ArH), 131.6
(ArH), 128.3 (C), 126.9 (ArH), 122.2 (C), 61.6 (CH2), 41.4 (CH2), 14.1
(CH3); IR (CHCl3) 1746, 1700, 1663 cm-1; EIMS m/z (rel intens) 283
(M+, 32), 210 (100); HRMS (EI) m/z 283.0839 (C16H13NO4 requires
283.0844).
A suspension of 1.0 g (3.5 mmol) of this ester in 30 mL of 15%
aqueous HCl was stirred at reflux for 6 h and concentrated in vacuo,
giving 0.88 g (97%) of 17: mp 250 °C dec (acetone-ether); 1H NMR
(acetone-d6) δ 8.56 (2H, d, J ) 7.2, ArH), 8.43 (2H, d, J ) 8.6, ArH),
7.86 (2H, t, J ) 7.2, ArH), 4.87 (2H, s, CH2); 13C NMR (acetone-d6)
δ 169.3 (CO), 164.1 (CO), 135.3 (CH), 132.7 (C), 131.72 (CH), 128.8
Experimental Section
General Procedures. All reported reactions were run under a dried
nitrogen atmosphere. Unless otherwise noted, all reagents were obtained
from commercial sources and used without further purification. All
1
compounds were isolated as oils and shown to be >90% pure by H
and/or 13C NMR unless otherwise noted.
1H NMR and 13C NMR spectra were recorded by using CDCl3
solutions unless otherwise specified and chemical shifts are reported
1
in parts per million relative to residual CHCl3 at 7.24 ppm (for H
NMR) and 77.0 ppm (for 13C NMR). 13C NMR resonance assignments
were aided by the use of the DEPT technique to determine numbers of
1
attached hydrogens, and H NMR coupling constants (J-values) are
reported in hertz. Infrared data are reported in units of inverse
centimeters. Low (MS) and high (HRMS) resolution mass spectra,
(13) Rigaudy, J.; Sparfield, D. Tetrahedron 1978, 39, 2263.

3932 J. Am. Chem. Soc., Vol. 121, No. 16, 1999
Takahashi et al.
(C), 127.9 (CH), 121.0 (C), 41.4 (CH2); IR (CHCl3) 3300, 1720, 1651,
1644 cm-1; UV (MeCN) 218 (46 000), 318 (13 600), 335 (11 100) nm;
EIMS m/z (rel intens) 255 (M+, 10), 211 (100), 210 (57); HRMS (EI)
m/z 255.0534 (C14H9NO4 requires 255.0531).
EIMS m/z (rel intensity) 255 (M+, 23), 211 (81), 210 (100); HRMS
m/z 255.0538 (C14H9NO4 requires 255.0531).
Acknowledgment. The authors appreciate the financial
support generously provided by the NSF (P.S.M., Grants CHE-
9707401 and INT-9796064; D.E.F., Grant CHE-9521601),
KOSEF (U.C.Y., Grant 965-0300-002-2, CBM Postech 1997
and 1998), and Korea Research Foundation (UCY, Grant 1998-
15-D00177).
N-(Carboxymethyl)-2,3-naphthalimide (18). A solution of glycine
(0.49 g, 6.67 mmol), 2,3-naphthalic anhydride (1.2 g, 6.05 mmol), and
30 mL of triethylamine in 30 mL of xylene was stirred at reflux with
azeotropic removal of water for 15 h and concentrated in vacuo, giving
a solution which was acidified with 10% aqueous HCl. The solid
obtained was filtered, washed with water, and dried, giving 1.36 g (88%)
Supporting Information Available: Spectroscopic data in
the form of 1H and 13C NMR spectra for all previously
unreported compounds (13, 15, 17, and 18) (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
1
of 18: mp 240 °C dec (acetone-ether); H NMR (acetone-d6) δ 8.46
(2H, s, ArH), 8.05 (2H, m, ArH), 7.77 (2H, m, ArH), 4.45 (2H, s,
CH2); 13C NMR (acetone-d6) δ 168.8 (CO), 167.6 (CO), 136.5 (C),
131.2 (CH), 130.26 (CH), 128.8 (C), 125.6 (CH), 39.4 (CH2); IR
(CHCl3) 1736, 1713 cm-1; UV (MeCN) 258 (59 800), 358 (6000) nm;
JA9841862