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Laser Flash Photolysis Generation Spectra and Lifetimes of Phenylcarbenium Ions in Trifluoroethanol and Hexafluoroisopropyl Alcohol.

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The radical cation 2 is also able to attack the C-N double
bond in imines in an intermolecular fashion, a reaction not
possible for photochemical 1,3-dipolar cycloaddition.16]The
reaction of azirines with imines leads to the formation of the
N-substituted imidazoles.
The course of the reaction is as follows: Addition of the
radical cation 2 to the imine is followed by ring closure to
form a dihydroimidazole. Under these reaction conditions
the dihydroimidazole is not stable and undergoes subsequent
aromatization, so that, even in the crude product, no dihydroimidazole is detected. Huisgen[71did not find a dihydroimidazole under comparable conditions either. The reaction
of azirines with imines offers a ready approach for the synthesis of N-substituted imidazoles with a wide variety of
substituents.
Since the C-N double bonds of the imine and the azirine
compete for reaction, the yields of imidazoles vary rather
widely (Table 1). The remaining products are the azirine
Table 1. Yields of the new imidazoles 8.
Compd R'
8a
8b
8c
8d
8e
8f
phenyl
phenyl
n-butyl
phenyl
phenyl
n-butyl
R2
R3
R4
Yield
phenyl
H
H
phenyl
H
H
phenyl
phenyl
phenyl
n-propyl
n-propyl
n-propyl
n-propyl
n-propyl
n-propyl
n-propyl
n-propyl
n-propyl
87% [8] [a]
12%
[b]
3%
[c]
25%
[d]
35%
[el
40%
[f]
[a] 9a. 12%; [b] 9b, 85%; [c] 9c, 95%; [d] 9a, 74%; [el 9b, 60%;
57%.
[fl 9c,
dimers. Nevertheless, compared to classical procedures,[*l
the yields of this direct synthesis are quite high. Furthermore, the ready availability of starting materials offers plenty of new synthetic opportunities.
[5] Scavenging by addition of 1 mL of 2,2,2-trifluoroethanol to the reaction
mixture (see Experimental Procedure). The yield of 5 and 6 was 8 % and
7%, respectively. Direct irradiation just led to the formation of 5. When
nucleopbilic alcohols were added, no compound similar to 6 could be
detected.
[6] H. Giezendanner, M. Marky, B. Jackson, H.-J. Hansen, H. Schmid, Helv.
Chim. Acta 55 (1972) 745; H. Giezendanner, H. Heimgartner, B. Jackson,
T. Winkler, H. Schmid, ibid. 56 (1973) 2611; U . Gerber, H. Heimgartner,
H. Schmid, H.-J. Hansen Heterocycles 6(1977) 143.
[7] R. Huisgen, H. Gotthardt, B. 0. Bayer, Chem. Eer. 103 (1970) 2370;
K . Bunge, R. Huisgen, R. Raab, H. J. Sturm, ibid. 105 (1972) 1307.
[8] V. Stoeck, W Schunack, Arch. Pharm. (Weinheim, Ger.) 307 (1974) 922,
ibid.309 (1976) 421.
[9] G. Smolinsky, J. Org. Chem. 27 (1962) 3557; F. W. Fowler. A. Hassner,
L. A. Levy, J. Am. Chem. Soc. 89 (1967) 2077.
[lo] L. E Tietze, T. Eicher: Reaktionen undSvnrhesen, Thieme, Stuttgart 1981,
p. 69.
1111 A. Albini, D. R. Arnold, Can. J. Chem. 56 (1978) 2985.
[12] H. J. Sattler, V. Stoeck. W. Schunack, Arch. Pharm. (Weinheim, Ger.) 308
(1975) 795; ibid. 311 (1978) 736.
Laser Flash Photolysis Generation, Spectra,
and Lifetimes of Phenylcarbenium Ions in
Trifluoroethanol and Hexafluoroisopropyl Alcohol.
On the UV Spectrum of the Benzyl Cation**
By Robert A . McClelland,* Christopher Chan,
Frances Cozens, Agnieszka Modro, and Steen Steenken *
Dorfman and co-workers have reported reactivities of
Ph,C@, Ph,CH@, and PhCHF with various nucleophiles in
chlorinated hydrocarbon solvents.[' - 3 1 The experiments
were performed using the pulse radiolysis technique, with the
cations obtained upon fragmentation of the initially formed
radical cations. In the case of Ph3C0 and Ph,CH@, there
were excellent matches of spectra of the transients observed
in the pulse radiolysis experiments with ones obtained for
solutions of authentic cations in concentrated acid and there
Experimental Procedure
The azirines were prepared according to[9], the imines according to[lO]. The
synthesis of DCN is described in[ll]. A solution of the azirine (0.3 mmol) and
the imine (l.Ommol, in 10mL of acetonitrile was poured into a Pyrex tube.
1.4-Naphthalenedicarbonitrile(0.1 mmol, 18 mg) was added, the reaction solution deoxygenated by argon bubbling, and the tube sealed. Irradiation was
carried out by means of a Rayonet photochemical reactor and 350-nm radiation
lamps. Irradiation was maintained for 6-10 hours. The solvent was then evaporated and the residue separated by flash chromatography on alumina (Merck
Neutral Act. I) using diethyl ether as eluent. The new compounds gave satisfactory 'H NMR, I3C NMR, and mass spectroscopic data, which are similar to
the data of comparable compounds[l2].
1
A OD
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 10
300
250
350
,i[nm]
450
-
-
400
2 [nm]
Received: March 15, 1991 [Z 4501 IE]
German version: Angew. Chem. 103 (1991) 1352
[l] J. Mattay, Synthesis 1989, 233; H. Tomioka, D. Kobayashi, A. Hashimoto, S. Murata, Tetrahedron Left. 30 (1989) 4685; P. Clawson, P. M. Lunn,
Perkin Trans. 1 1990 153, ibid. 1990, 159;
D. A. Whiting, J. Chem. SOC.
E. Palomino, A. P. Schaap, M. J. Heeg, Tetrahedron Lett. 31 (1990) 6861;
K . Gollnick, M. Weber, ibid. 31 (1990) 4585; N. Ichimose, K. Mizuno.
T. Tamai, Y Otsuji, J. Org. Chem. 5s (1990) 4079; M. Kamata, H. Furukawa, T.Miyashi, Tetrahedron Left. 31 (1990) 681.
[2] R. Huisgen in A. Padwa (Ed.): 1.3 Dipolar Chemisfry, Vol. I, Wiley, New
York 1984, p. 1 ;A. Padwa in W. Horspool (Ed.): Synthetic Organic Photochemistry, Plenum, New York 1984, p. 313; H. Heimgartner, Angew.
Chem. 103 (1991) 271; Angew. Chem. Int. Ed. Engl. 30 (1991) 238.
[3] We assume that the first step is the oxidation of the azirine to the azirine
radical cation, since the energy for the electron-transfer process estimated
from the Weller equation (D. Rehm, A. Weller, Isr. J. Chem. 8 (1970) 259)
is negative and, therefore, the process is thermodynamically allowed.
[4] A. Padwa, J. Smolanoff, J. Am. Chem. SOC.93 (1971) 548; A. Padwa,
M. Dharan, J. Smolanoff, S. I. Wetmore Jr. ibid. 95. (1973) 1945, ibid. 95
(1973) 1954; A. Padwa, J. Smolanoff, A. Tremper, ibid. 97 (1975) 4682.
200
500
550
Fig. I. Absorption spectra of the p-methoxycumyl cation (in TFE. m), the
p-methoxyphenethyl cation (in TFE, A), and the p-methoxybenzyl cation (in
HFIP, e). The precursors p-methoxy-a-methylstyrene, p-methoxystryrene. and
p-methoxybenzyltrimethylammoniumtetrafluoroborate were photolyzed at
248 nm with 20-11s pulses. Spectra were recorded 50- 100 ns after the pulse. The
insert shows the formation and decay of the cation and the (permanent) depletion of parent in the experiment with p-methoxy-a-methylstyrene, at 100 ns,
45 ps, and 500 ps after the pulse.
[*I Prof. Dr. R. A. McClelland, C. Chan, F. Cozens, Dr. A. Modro
Department of Chemistry, University of Toronto
Toronto, Ontario M5S 1Al (Canada)
Prof. Dr. S. Steenken
Max-Planck-Institut fur Strahlenchemie
Stiftstrasse 34-36, W-4330 Mulheim (FRG)
[**I This work was supported by the Donors of the Petroleum Research
Fund, administered by the American Chemical Society, and by NSERC
(Canada).
0 VCH Verlagsgesellschaf~mbH. W-6940 Weinheim. 1991
0570-0833~91~10l0-l337
$3.50+ ,2510
1337
can be no question that the particular cations were being
studied. With PhCH?, however, no spectrum from a "classical" source is available. The transient that was observed had
an absorption maximum at 363 nm. Arguments that this
represented PhCHp were based upon the analogy with the
other two systems, the observation ofthe same transient with
different precursors, and the characteristic quenching by
nucleophiles.['l Using nanosecond laser flash photolysis
(I. = 248 nm) we now have observed a number of substituted
benzyl cations in trifluoroethanol (TFE) and hexafluoroisopropyl alcohol (HFIP). Our results lead to the conclusion
that the 363-nm transient is unlikely to have been the benzyl
cation.
The A,,, and rate constant data for the benzyl cations are
summarized in Table 1, and spectra are shown in Figures 1
and 2 for the parent and p-methoxy series. In the case of the
cumyl (2, R' = Me) and arylethyl (2, R' = H) cations, the
styrenes 1 (R' = Me, H) were the precursors, with cation
generation involving protonation of the electronically excited styrene by the solvent SOH [Eq. (a)]. This process, which
Table 1. Amax [nm] and rate constants (20°C) for reaction of derivatives of benzyl cations
with solvent (k,) and with Bre ( k B , e ) and MeOH (kueoH)in TFE and HFIP. The cations
were generated by 248-nm laser flash photolysis according to Equations (a) and (b).
hv, 248 nm
(a)
RL
RZ
1
2
R' =
H, Me;
X
I
R
B
=
CI, Br, NMe,
I
R
Identification of transients as benzylic cations took several
forms. (1) They were quenched in a manner characteristic of
carbenium ions, with large rate accelerations upon addition
of methanol and bromide (see insert to Figure 2 and Table 1
for the rate constants). Oxygen, on the other hand, had no
effect. (2) The cations were formed in monophotonic processes, which shows that they are not derived from cation
radicals, which could only be generated via biphotonic proces~es.[~l
(3) The appropriate products were observed, for
1338
8 VCH
Cumyl
Phenethyl
340
3.9 x 10'
< 102
1.1 x 10' (TFE) [el
4.5 x 109 (TFE)
9.1 x 10' (TFE)
5.6 x 10, (TFE)
1.6 104
<lo'
has previously been described for p-methoxystyrene in
TFE,[49 has now proved to be a general one for styrenes in
these fluorinated alcohols, HFIP being particularly efficient.
This weakly nucleophilic solvent[61also permits the observation of the more reactive cations.
The benzyl cations were generated by photoheterolysis of
appropriate precursors, p-methoxybenzyl from p-methoxyp-methbenzyltrimethylammonium f l u ~ r o b o r a t e '1[ ~ and
*
oxybenzyl chloride and p-methylbenzyl from p-methylbenzyl
chloride and bromide [Eq. (b)]. In these cases radicals arising
from a competing photohornoly~is[~~
7 , '1 were also observed
[Eq. (c)l '
I
Cation
360
R - H, Me, MeO; S = CF,CH,, (CF,),CH
CH,-X
example, p-methoxybenzyl trifluoroethyl ether upon photolysis of 4-MeOC,H4CH,N(CH,), .BF, in TFE, and the
Markovnikov adducts of both styrene and a-methylstyrene
with the solvent HFIP. (4) For the p-methylbenzyl cation,
which decays in HFIP with k, = 2 x lo6 s-', a rate constant
'
measured for reaction with broof 1 . 0 1~0 1 O ~ - ' s - was
mide (added as the nBu,N@ salt, Table I), giving kBr4
k, = 5 x lo3 M - l . The solvolysis ofp-methylbenzyl bromide
in HFIP containing nBu,NBr showed pronounced common
ion inhibition, from which the kB,e/ksratio was calculated to
be 5 x lo3 M - ' , identical with the number obtained from
observation of the cation. ( 5 ) For the parent cumyl cation,
the spectrum of the transient in HFIP is virtually identical to
that reported by Oiah et al. for the same cation in SO,-SbF5= 390 nm, E = 1400; I.,,, = 326, E = 11 OOO).['ol
FS0,H
With the latter extinction coefficient, the quantum yield for
formation of the cumyl cation upon irradiation of a-methylstyrene in HFIP was calculated, using a previously described
technique!' '1 as 0.08.
Verlagsgesellschaft mbH, W-6940 Wemherm. 1991
340, -380 [a]
1 x 107
6 x 10'
1.3~109
3.4 x 10' ti]
325, -390 [ f j
>sx107
5 x lo4 [h]
7.2 109
1.1 x 10'
325. -410 [ f j
9 x 103
6.7 x lo9
4.1 x lo6
315. -430 [f]
6 x lo5 [h]
8.2 109
4.3 x 10'
~
Benzyl
320, -360 [a]
4.3 x 106 (9 x 106) [c]
3.2 x 10'
4.8 x 109 (HFIP)
5.8 x 105 (HFIP)
310 [gi
>5x107
2x106
1.0 x 10'0
6.5 107
~~
[a] Shoulder. [b] Units s-I. [c] The rate constant 9 x lo6 s-' was obtained in 1:1 TFE/
acetonitrile with both 4-MeOC,H,CH,N(CH,),.BF4 and 4-MeOC6H,CH,CI as precursors. In 100% TFE and 100 % HFIP, the choride undergoes rapid thermal solvolysis.
[d] Units M - ' s - ' . [el The reaction is reversible (F. Cozens, to be published). [fl1.,,,
approximate, peak is about 10% of major one (see Fig. 2). [g] Weak absorbance also at
higher 1,but this is obscured by other weak transient(s). [h] Extrapolated to zero styrene
concentration. These cations also react with excess styrene remaining after photolysis.
01The k,,,, vs. [MeOH] plot is curved upwards. The number indicated is from a linear
fit in the [MeOH] range 0.1 to 0.35 M.
These experiments establish that simple benzyl cations
such as cumyl and phenethyl do exist as free ions in HFIP.
However, the benzyl cation itself was not observed in this
solvent when precursors such as benzyl halides were used.
We estimate its lifetime to be in the range 2-20 ns, just beyond the limit of detection of our apparatus. The spectra of
the other eight cations have a major peak with I,,, in the
,, (4range 310-360 nm, with the trend A,, (4-Me0) > 1
Me) > ,
I
,
,
,(H) for a given type of cation (as is also observed
for Ar,C@ and Ar,CH@),[".'21 while for a given para substituent, I,,,(cumyl) > i,,,(phenethyl) > Imax(benzyl).There
is also absorbance at higher wavelengths that is associated
with the cation, as concluded from the identity of the decay
and quenching characteristics with those of the major peak,
and as observed in strong acids with the cumyl cation.["]
This absorbance appears as a shoulder or weak peak with
0570-0833/91/1010-1338$3.50+.25/0
Angeu,. Chem. tnt. Ed. Engl. 30 (1991) No. 10
most of the cations, the exceptions being p-methoxycumyl
and p-methoxyphenethyl. The dependence of E.,
on structure is less precisely defined because of the weakness of this
absorbance, but it appears to be exactly opposite to that seen
in the stronger peaks at lower wavelength. Examination of
the trends in both bands leads to the prediction that the
parent benzyl cation has a strong band with i
,
,
300-310
,
nm
and a weak peak (about 10% of the strong band) at 430460 nm. In contrast, the transient observed in the pulse radiolysis experiments by Dorfman et aL1'I was a single broad
peak with A,,, at 363 nm and weak absorbance at 320 and
420nm (see Figure 2 of Ref. [I]). We conclude that this was
not the benzyl cation. One possibility is that the transient
observed was due to a cyclohexadienyl cation arising from
coupling of an initially generated cation or cation radical
with unreacted precursor. Cyclohexadienyl cations of this
type would absorb in this region.L3I Consistent with a bimolecular mechanism for its formation, the amplitude of the
363-nm transient was larger at higher concentrations of precursor .[ '1
a
0 0 OL
350
A [nm]
08
450
-
550
Fig. 2. Absorption spectra of the cumyl cation ( 0 )and the phenethyl cation (0)
in HFIP. obtained on photolysis of r*-methylstyreneand styrene, respectively,
recorded at c= 100 ns after the pulse. The inserts show the decay of the cumyl
cation at 325 nm (time scale in Ips]), and first-order rates for decay of cation
measured in the presence of Bu,NBr [mM]; the slope of the line yields the rate
constant 6.7 x lo9 M - ' s - ' for the reaction cumyle + Bre,
From an analysis of reactivities with alkenes, M a y et al.
have recently arrived at a similar conclusion; that is, the rate
constants for the 363-nm transient are not consistent with it
being the benzyl
In addition there is a recent example showing that the technique of pulse radiolysis to produce cations is not a general one, particularly for less stable
cations. The radical cation of 9-chlorofluorene1' 51 does not
fragment to the 9-fluorenyl cation.I6I
Received: March 8, 1991 [Z 4484 IE]
German version: Angew. Chem. 103 (1991) 1389
CAS Registry numbers:
p-methoxy-a-methylstyrene,1712-69-2; p-methoxystyrene, 637-69-4; (pmethoxybenzy1)trimethylammonium tetrafluoroborate, 136033-67-5; amethylstyrene, 98-83-9; styrene, 100-42-5; p-methylbenzyl chloride, 104-82-5;
p-methylbenzyl bromide, 104-81-4; 4-methoxycumyl cation, 22666-71-3;
4-methoxyphenethyl cation, 18207-33-5;4-methoxybenzyl cation, 29180-19-6;
4-methylcumyl cation, 20605-66-7; 4-methylphenethyl cation, 31042-87-2;
4-methylbenzyl cation, 57669-14-4; cumyl cation, 16804-70-9; phenethyl
cation, 25414-93-1 ;bromide, 24959-67-9; methanol, 67-56-1.
R. L. Jones. L. M. Dorfman, J. Am. Chem. SOC.96 (1974) 5715.
L. M. Dorfman, R. J. Sujdak, R. Bockrath, Arc. Chem. Rex 9 (1976) 352.
Y. Wang. L. M. Dorfman, Macromolecules 13 (1980) 63.
R. A. McClelland, V. M. Kanagasabapathy, S. Steenken, J. Am. Chem.
So(. 110 (1988) 6913.
[5] R. A. McClelland, F. Cozens. S. Steenken, Tetrahedron Lett. 31 (1990)
2821.
[I]
[2]
[3]
[4]
Angen. Chem. lnr. Ed. Engl. 30 (1991) Nu. 10
ACRL Toxin I: Convergent Total Synthesis
of Its 3-Methyl Enol Ether from D-GIucose**
By Frieder u! Lichtenthaler,* Jurgen Dinges,
and Yoshimasa Fukuda
J
6.:[
[6] R. A. McClelland, N. Mathivanan, S. Steenken, J. Am. Chem. Sac. 112
(1990) 4857.
[7] M. A. Ratcliff, J. K. Kochi, J. Am. Chem. Soc. 93 (1971) 3112.
[8] B. Foster, B. Gaillard, N. Mathur, A. L. Pincock, J. A. Pincock, C. Sehmbey, Can. J. Chem. 6S (1987) 1599.
[9] S. Steenken, R. A. McClelland, J. Am. Chem. Suc. f If (1989) 4967.
[lo] G. A. Olah, C. U. Pittman, R. Waack, M. Doran, J. Am. Chem. Soc. 88
(1966) 1488.
[ l l ] J. Bartl, S. Steenken, H. Mayr, R. A. McClelland, J. Am. Chem. Soc. I12
(1990) 6918.
[12] R. A. McClelland, V. M. Kanagasabapathy, N. Banait, S. Steenken, J. Am.
Chem. Soc. 111 (1989) 3966.
I131 a)G. P. Smith,A. S. Dworkin, R. M. Pagni, S. P. Zingg, J. Am. Chem. SOC.
111 (1989) 525; b) S . Steenken, R. A. McClelland, ibid. 112 (1990) 9648.
[14] H. Mayr. R. Schneider, B. Irrgang, C. Schade, J. Am. Chem. Soc. 112
(1990) 4460.
[IS] E. Gaillard, M. A. Fox, P. Wan, J. Am. Chem. Soc. 111 (1989) 2180.
0 VCH
A pathogenic type of the fungus Alternaria citri, found
host-specifically on rough lemon and Rangpur lime citrus
species, produces a number of toxins, which cause a rapidly
spreading, harvest-threatening necrosis (brown spot disease)
on the leaves and fruits of infected plants."' The main component of this mixture of toxins and, at the same time, the
substance with the highest biological activity is ACRL Toxin
I (1 a), which, after laborious isolation, was characterized as
a light orange oil.t2.3a1 Since it is present as a mixture of keto
and enol tautomers ('H NMR) and, moreover, readily undergoes decarboxylation, 1 a is converted for convenience
into the stable methyl enol ether 1 b. Intensive CD and NMR
spectroscopic investigation of 1 b, together with X-ray structure analysis of the decarboxylation product, established its
constitution and absolute config~ration.[~l
Our strategy for the total synthesis of l b involves the
preparation of enantiop~re,'~'
connective segments from Dglucose. The key feature of the retrosynthetic analysis was
the expectation that the dihydro-a-pyrone ring could be introduced via a suitable acetoacetic ester-derivative. Thus, the
molecule was dissected into the segments A, B, and C
(Scheme 1). The advantage of this segmentation is the possibility of carrying out a convergent reaction sequence, since
both the twofold methyl-branched C, chain of synthon A
and the adipaldehyde building block B are derivable+ach
in the correct absolute configuration-from the 1 ,2-acetonide of 3-deoxy-3-C-methyl-a-~-a~~ofuranose
(2), which,
in turn, is accessible from D-glucose by a known route.
In order to synthesize the key starting material 2 on a scale
of 50-100 grams, the diacetone glucose 3 was first oxidized
with pyridinium dichromate/acetic anhydride.['' The resulting dose 5, formed in practically quantitative yield, was
converted into 4 by reaction with methyl(tripheny1)phosphonium bromide. Hydrogenation of 4 over Pd/C followed by
[*I Prof. Dr. F, W. Lichtenthaler, Dip1.-Ing. J. Dinges, Dr. Y. Fukuda[+]
Institut fur Organische Chemie der Technischen Hochschule
Petersenstrasse 22, W-6100 Darmstadt (FRG)
['I Research fellow on leave of absence from Meiji Seika Kaisha, Kawasaki,
Japan (1988-1989).
[**I Enantiopure [4] Building Blocks from Sugars, Part 13. This work was
supported by the Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie. Part 12: F. W. Lichtenthaler, Zuckerindusrrie
(Berlin) 115 (1990) 762.
VerlagsgesellschaftmbH. W-6940 Weinheim. 1991
0570-0833/91/1010-1339$3.50+.2S5/0
1339
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photolysis, trifluoroethanol, flash, generation, phenylcarbenium, hexafluoroisopropyl, ions, alcohol, laser, lifetime, spectral
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