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Photochemical tandem addition of water and oxygen to the alkene moiety of m-nitrostyrenes
JournaZ of Photochemistry
and Photobiology,
A:
Chemistry,
48 (1989)
387
PHOTOCHEMICAL TANDEM ADDITION OF WATER OXYGEN TO THE ALKENE MOIETY OF m-NITROSTYRENES PETER
WAN+
Department (Canada) (Received
and MICHAEL
of Chemistry, February
387
AND
J. DAVIS
University
10, 1989;
- 396
of Victoria,
Victoria,
British
Columbia
5% W 2Y2
in revised form April 12, 1989)
Summary A new type of reaction involving the tandem photoaddition of water and oxygen to the alkene moiety of several m-nitrostyrenes in oxygenated aqueous solution via the triplet excited state, to give P-hydroxy-a-hydroperoxides, is reported. In the absence of oxygen, only anti-Markovnikov photohydration was observed. The proposed mechanism involves initial nucleophilic addition of water to the P-carbon of the alkene moiety in the triplet excited state, to generate a m-nitrobenzyl carbanion intermediate, which is subsequently “trapped” by oxygen when it is present in solution or is otherwise protonated by water. The quantum yields for the tandem addition and photohydration were identical (e.g. @ = 0.32 for m-nitrostyrene), suggesting a common primary intermediate in the two pathways.
I. Introduction Nucleophilic addition to unactivated alkenes in the ground state is generally not observed [l, 21. However, if the alkene is activated by electron-withdrawing substituents such as carbonyl or highly electronegative atoms such as fluorine, nucleophilic 1,4-conjugate additions are common [l, 21, in which the nucleophile initially attacks the ,ðylenic carbon, to give a carbanion intermediate, which on subsequent protonation gives the nucleophilic addition product. Such a pathway forms the basis of the useful Michael reaction in organic synthesis in which the incoming nucleophile is usually a carbanion. The photochemical nucleophilic attack of water to the P-carbon of the alkene moiety of m- and p-nitrostyrenes via the triplet
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388
excited state has been reported by us 13, 41, which results in overall antiMarkovnikov
addition
of water (photohydration)
across the alkene (eqn. (1)).
This reaction is mechanistically interesting because the photoaddition of water and alcohols to alkenes generally proceed via initial photoprotona-
Con, to give carbocation intermediates 13 - 51, rather than via a carbanion intermediate, as required for the photohydration of m- and p-nitrostyrenes [ 31. Our recent interest [6 - S] in photo-oxygenation of nitrobenzyl derivatives, where we have proposed that photogenerated nitrobenzyl carbanion intermediates can be efficiently ‘(trapped” by dissolved molecular oxygen, to give isolable hydroperoxides, prompted us to investigate the photohydration of several m-nitrostyrenes in the presence of oxygen, with the aim of trapping the proposed m-nitrobenzyl carbanion intermediates with oxygen. In many photochemical reactions, the presence of oxygen often results in very complicated product mixtures, owing to the operation of a variety of pathways, and hence may not provide useful mechanistic information. However, it will be shown that as far as simple m-nitrostyrenes are concerned, reaction with both water and oxygen is both clean and mechanistically interesting, via an overall tandem addition process.
2. Results and discussion The compounds
chosen for this study were m-nitrostyrene
derivatives
1 - 4. CCCHR
-
lR-H
-2 R -
CH,
-3R=
CH&H,
-4R
= Ph (tram)
The corresponding p-nitrostyrenes were not studied since they reacted much less efficiently than the m-isomers and also tended to give side products. The m-nitrostyrenes were made by dehydrating the corresponding alcohols, which in turn were made via NaBH4 reduction of the corresponding ketones, except for 3, which was made by a Wittig reaction of n-propyltriphenylphosphonium bromide with m-nitrobenzaldehyde. Photolysis of 10m3 M solutions of 1 - 3 in aqueous solution (== 30% CH,CN) with a stream of argon purging through the solution (Rayonet RPR 100 photochemical reactor; 254, 300 or 350 nm lamps; 5 - 10 min) gave anti-Markovnikov alcohol products 5 - 7 in essentially quantitative yield (eqn. (2)).
389
OH L HR
cw
5R=H -
h3 l-3 -
-
-T
(2)
= CH3
ER
“,O/orgo”
CH2CH3
-7R-
NO2
Photolysis of m-nitrostilbene (4) resulted in no observable photohydration but only trans-to-cis photoisomerization, even on extended photolysis of several hours (254, 300 or 350 nm lamps). In addition, triplet sensitization of 4 (ET = 60 kcal mol-‘) with sodium benzophenone-2-carboxylate (8) (ET= 70 kcal malll [9]; Xex,-.it = 350 nm) also resulted in only transto-cis isomerization.
Thus it is clear that for m-nitrostilbene (4) twisting of the alkene to give the orthogonal state, followed by deactivation to the ground state is sufficiently fast that water fails to intercept the triplet state (which is presumed to be polarized) [3, $1. Schulte-Frohlinde and Gijrner [lo] have carried out extensive studies of nitrostilbene photoisomerization in several solvents and have shown that these compounds isomerize cleanly and efficiently, without competing reactions. Thus, it was not unexpected that 4 failed to undergo photohydration. When the same solutions of 1 - 3 were photolyzed with a stream of oxygen purging through the solution, P-hydroxy-cr-hydroperoxides 9 - 11 were observed in essentially quantitative yield in low conversion experiments (< 30%) eqn, (3)). OOH A WCHOH l-3 -
-
h3 H,O/O,
1;
Fi
h3
qCHO
+
$y
(3)
Q
10 T-lR -
NO2
NO,
NO2
12 -
9R-H R = a
(major)
13 -
(minor)
CH, CH,CH,
-I.
RCHO
(observed
(major) only for
R -
CH,.
CH,CH,)
No trace of the photohydration products 5 - 7 was observed. Extended photolysis led to increasing yields of m-nitrobenzaldehyde (12) and RCHO, along with a minor amount of cll-hydroxyketone 13, all of which are believed to arise via secondary photochemistry, as confirmed by independent pho-
390
tolysis of 9. The quantum yield for substrate loss (Ca)for 1 (measured by UV spectrophotometry at pH 7 with potassium ferrioxalate [ll] as actinometer) was 0.32 + 0.05, in the presence or absence of oxygen. This value for Q, is identical within experimental error with that previously reported by us using Malachite green leucocyanide actinometry [ 31 for the photohydration of 1. The corresponding measured @ for 2 and 3 were 0.22 and 0.21 respectively (argon or oxygenated solution; pH 7). As anticipated, 4 gave only trans-to-cis photoisomerization when irradiated in oxygenated aqueous solution. Thus it seems clear that oxygen competes only for the initially formed m-nitrobenzyl carbanion intermediate (eqn. (l)), rather than reacting directly with substrate. This proposal is supported by recent studies from our laboratory [8] in which the photogenerated parent m- and p-nitrobenzyl carbanions (formed via a photoretro-aldol type reaction) were shown to react efficiently with dissolved oxygen, to give isolable a-hydroperoxides. In the absence of oxygen, the carbanion intermediates are protonated to give the corresponding nitrotoluenes [S]. The mechanism of reaction of photogenerated m-nitrobenzyl carbanions with molecular oxygen in the present study presumably resembles that of other carbanions [12 - 141, namely via initial electron transfer from carbanion to oxygen, to give a radical ion pair, followed by coupling to give the hydroperoxy anion. The reaction of many carbanions generated under strongly basic conditions with oxygen generally results in carbonyl products owing to facile dehydration of the initially formed hydroperoxide [12 - 141. The advantage of the present reaction is the mild condition used (neutral solution) for generating the carbanion and hence the possible isolation of the hydroperoxide. There was no reaction when 1 was photolyzed in methanol or pure acetonitrile solutions (oxygen or argon saturated). The substrate was recovered unchanged in these experiments. Methanol as a nucleophile is at least as good as (if not better than) water [15]. However, in terms of polarity, water is much more polar than methanol, which may account for the lack of reaction since the mechanism clearly involves a highly polarized species, requiring extensive solvation from the solvent. A similar strong dependence on an aqueous medium was also observed in the photoretroaldol type reactions of nitrobenzyl derivatives [8]. The possibility that the oxygenations may be due to reaction of singlet oxygen (generated via sensitization of oxygen by the substrate) with the m-nitrostyrenes was checked by carrying out Rose Bengal sensitization experiments in aqueous solution. It was found that singlet oxygen generated from Rose Bengal gave no significant reaction for all of 1 - 4. It was possible to sensitize the photo-oxygenation reactions with sodium benzophenone-2-carboxylate (S), albeit an inefficient process because of the competing oxygen quenching of the triplet sensitizer. For a similar reason, triplet quenching experiments were not attempted. However, the well-established triplet state reactivity of nitroaromatic compounds of this type [3,8] would suggest that only the triplet state is responsible for reaction in these substrates.
391
Hydroperoxide 9 was quite stable and can be purified by chromatography on silica gel or crystallized from acetone. Reduction with aqueous Na#Os or KI gave the corresponding 1,2-glycol (14) and hence offers a simple overall procedure for the dihydroxylation of m-nitrostyrene (l), an otherwise difficult oxidative conversion with known thermal chemistry owing to the highly electrondeficient nature of the alkene. OH
Hydroperoxides 10 and 11 were much less stable and readily decomposed on thermolysis (> 30 “C), or on extended photolysis, to give m-nitrobenzaldehyde (12) (> 50%) and low yields (
Ar-
ICCHOH IoH OH,
*
pz;;]
_-Y-J?
(4)
~~~~C.,,,:.:“““”
10 -
13 -
(R -
CH,)
The results presented here along with previous data [3] indicate that the mechanism for water and oxygen addition shown in Scheme 1 is probably operative. That is, the polarized triplet excited state undergoes nucleo-
aide -1
T, ~
[--~~~~~*I H,O 5
~~
H+ -
Ar-CH-CH,OH I
Scheme
1.
Ar-CH,CH,OH
philic attack by solvent water to generate an m-nitrobenzyl carbanion intermediate, which can either be protonated (to give the photohydration product) or “trapped” by dissolved oxygen, to give the ar-hydroperoxides. It was surprising that the relatively low concentration of dissolved oxygen in water (= 1O-3 M) [9] can compete effectively with solvent protons in trapping the carbanion. Either the reaction with oxygen involves an extremely efficient pathway or, alternatively, the protonation step may be relatively slow since these carbanions are stabilized. A lifetime of the order of minutes at pH 7 has been reported for photogenerated p-nitrobenzyl carbanion [ 16 - 181. Although the corresponding m-nitrobenzyl carbanion has not yet been observed spectroscopically, it is reasonable to assume that it also has a significant lifetime in water, which tends to support that the efficient reaction with oxygen observed is due to the longevity of these species. An alternative mechanism for photoaddition of nitrostyrenes is one involving initial photoionization, to give a radical cation, followed by trapping by water at the P-carbon, to a radical, which either abstracts a hydrogen from solvent (to give the usual nitrophenethyl alcohol products) or reacts with dissolved oxygen, to give the P-hydroxy-o-hydroperoxides. OOH ArCH=CH,
hv
+
o2
e- + ArCH%H,
H20
+
ArCH-CH20H
Ar
C H-CH*OH
< ArCHzCHzOH (5)
We have ruled out this mechanism because of the following two points. (1) The nitrophenyl ring system is very electron deficient and would not be expected to undergo electron ionization, to give a radical cation, which would be highly destabilized, Typical substrates which are known to undergo photoionization are methoxybenzenes (which are sufficiently electron rich). (2) The radical intermediate generated via this mechanism eqn. (5) would only dime&e (or disproportionate) and not abstract hydrogen from water. 3. Experimental details 3.1. General ‘H NMR spectra were recorded on Perkin-Elmer R32 or Bruker WM250 instruments, in CDC13 with TMS as internal standard, unless otherwise noted. IR spectra were recorded on a Perkin-Elmer 283 instrument on NaCl discs. Mass spectra were recorded on a Finnegan 3300 instrument. Preparative and semipreparative photolyses were carried out using 200 ml quartz tubes in a Rayonet RPR 100 photochemical reactor equipped with
393
254, 300 or 350 nm lamps. For quantum yield measurements, radiation from an Oriel 200 W Xe-Hg arc lamp was filtered through distilled water and a Corning 7-54 band pass filter prior to further filtration through an Applied Photophysics monochromator set at 254 nm. 3.2. Materials
3.2.1.
m-Nitrostyrene
(I)
Compound 1 was prepared from m-nitroacetophenone by NaBH, reduction and subsequent dehydration over 85% H3P04. The sample obtained was identical with authentic material (from Aldrich).
3.2.2. P-Methyl-m-nitrostyrene (I-(3’-nitrophenyl)propene) (2) Compound 2 was prepared from m-nitropropiophenone: b.p. 104 105 “C (2 mmHg); iH NMR 6 1.9 (m, 3H), 6.4 (m, 2H), 7.3 - 8.2 (m, 4H). 3.2.3.
P-Ethyl-m-nitrostyrene
(I-(3’~nitrophenyl)-1-butene)
(3)
Compound 3 was prepared from a Wittig reaction of n-propyltriphenylphosphonium bromide with m-nitrobenzaldehyde (n-BuLi/THF) : b.p. 120 “C (2 mmHg); ‘H NMR (250 MHz) 6 1.05 (m, 3H), 2.25 (m, 2H), 5.75 (m, 1H), 6.35 (m, lH), 7.3 - 8.2 (m, 4H).
3.2.4. trans-m-Nitrostilbene (4) Compound 4 was prepared from m-nitrobenzyl phenyl ketone [S]: m-p. 102 - 105 “C; ‘H NMR (250 MHz) 6 7.15 (AB quartet, J= 16 Hz, 2H, trans vinyl H), 7.25 - 7.55 (m, 6H), 7.73 - 7.8 (m, lH), 8.05 - 8.10 (m, lH), 8.3 - 8.35 (m, 1H). 3.3. Pho tolysis 3.3.1. Pho tolysis
of I under oxygen
In a typical experiment, 100 mg of the substrate was dissolved in 50 ml CHsCN and mixed with 150 ml distilled water. The solution was then placed into a 200 ml quartz vessel containing a water-cooled cold finger. The solution was purged with oxygen (Linde, 99.9%) for 5 - 10 min prior to photolysis and irradiated for about 10 min with a continuous stream of oxygen purging through the solution. After photolysis, the solution was transferred to an Erlenmeyer flask and NaCl added which was then extracted with 2 X 100 ml CH&12. The crude “H NMR showed a distinctive singlet (exchangeable with D20) at 6 11.1 assignable to a hydroperoxyl proton, in addition to new peaks at S 3.9, 4.2 (exchangeable) and 5.2, all consistent with formation of &hydroxy*-hydroperoxide 9. On extended photolysis (> 10 min), increasing amounts of m-nitrobenzaldehyde (12) appears, as indicated by its characteristic aldehyde singlet at 6 10.2 along with its characteristic aromatic signals. Trace amounts of 13 (R = H) was inferred from a singlet observed at 6 5.0 (-CH,-) and the mass spectrum of preparative TLC fractions. Pure 9 was obtained from preparative TLC (silica/ CH,Cl,): m.p. 72 - 76 “C; ‘II NMR (acetone-de) 6 3.85 (m, 2H), 4.2 (t, J = 8
394
Hz, IH, exchangeable), 5.2 (t, J= 7 Hz, lH), 7.5 - 8.4 (m, 4H), 11.1 (2, lH, exchangeable); IR (cm-i) 3100 - 3700 (broad), 1520 (s), 1360 (s); mass spectrum (CI) (m/z) 182 (M+ + 1 - H20). Elemental analysis was not attempted because of the thermal lability of the compound. Reduction of 9 with aqueous KI or Na#Os gave (3’-nitrophenyl)1,2-ethanediol (14): ‘H NMR (acetone-dd) 6 3.65 (m, 2H), 3.9 (m, exchangeable, lH), 4.65 (m, exchangeable, IH), 4.85 (m, IH), 7.5 - 8.4 (m, 4H), which was identical spectroscopically with an authentic sample made from the hydrolysis of (m-nitrophenyl)ethylene oxide. of I under argon 3.3.2. Photolysis Under the same procedure as above but using argon instead of oxygen, 1 gave m-nitrophenethyl alcohol (5) in quantitative yield. The ‘H NMR spectrum of 5 was identical with an authentic sample from a previous study [31. 3.3.3. Photolysis of 2 under oxygen In a similar procedure as for 1 under oxygen, the ‘H NMR spectrum of the photolysate showed /3-hydroxya-hydroperoxide 10 as the major product (= 60%): ‘H NMR (acetone-de) 6 1.0, 1.15 (two methyl doublets, J = 8 Hz, 3H), 3.2 (broad, exchangeable, lH), 4.05 ( m, lH), 4.9 (m, lH), 7.4 - 8.3 (m, 4H), 11.05 (broad, exchangeable, 1H). Product 10 decomposed in the probe of the NMR spectrometer. (= 35 “C) and hence eluded purification), to give the following products: acetaldehyde (lH NMR 6 2.1 (d, J = 3 Hz, 3H), 9.7 (q, J= 3 Hz, TH)), m-nitrobenzaldehyde (12) and a minor amount of 13 (R = CHs) (‘H NMR 6 1.45 (d, J= 8 Hz, 3H), 5.2 (q, J = 8 Hz, lH), remainder of spectrum obscured by 10). 3.3.4. Photolysis of 2 under argon Under the same procedure as above but using argon instead of oxygen, 2 gave l-(3’-nitrophenyl)-2-propanol (6) in quantitative yield: lH NMR (250 MHz) 6 1.35 (d, J= 7 Hz, 3H), 2.85 (m, 2H), 4.08 (m, lH), 7.4 - 8.1 (m, 4H); IR (cm-‘) 3200 - 2500 (broad), 1520 (s), 1340 (m); mass spectrum (CI) (m/z) 181 (M+ + 1). 3.3.5. Photolysis of 3 under oxygen In a similar manner as for 1 under oxygen, the ‘H NMR of the photolysate showed /3-hydroxy-cw-hydroperoxide 11 as the major product (=70%): ‘H NMR (acetone-d,) 6 1.0 (m, 3H), 1.4 (m, 2H), 3.0 (broad, exchangeable, lH), 3.9 (m, lH), 4.0 (m, lH), 7.5 - 8.4 (m, 4H), 11.05 (broad, exchangeable, 1H). On extended photolysis (> 10 min), 11 decomposes to give the following products: propionaldehyde (‘H NMR 1.05 (t, J= 8 Hz, 3H), 2.4 ( q, J = 8 HZ, 2H), 9.7 (unresolved, lH)), m-nitrobenzaldehyde (12) and a trace of 13 (R = CH,CH,) as inferred by the triplet at 8 5.15 assignable to the methine of 13.
395
3.3.6. Photolysis of 3 under argon Under the same procedure as above but using argon instead of oxygen, 3 gave 7 in quantitative yield: lH NMR (250 MHz), S 0.97 (t, J = 7 Hz, 3H), 1.53 (m, 2H), 1.6 (broad, exchangeable, lH), 2.7 - 2.95 (m, 2H), 3.78 (m, lH), 7.4 - 8.1 (m, 4H); IR (cm-‘) 3200 - 3500 (broad), 1520 (s), 1350 (s); mass spectrum (CI) (m/z) 196 (Ad+ + 1). 3.3.7. Photolysis of 4 under argon or oxygen Photolysis of 4 under argon or oxygen gave the cis isomer, as identified by the appearance of a new AB quartet at higher field, at 6 6.7 (J= 12 Hz) (compared to the AB quartet of the trans isomer at 6 7.15). No other products were observed even on prolonged photolysis. 3.4. Quantum yield measurements Quantum yields for loss of substrate were measured by UV spectrophotometry, by following the loss in optical density of the m-nitrostyrenes. Solutions (10e5 M) were prepared in 3.0 ml quartz cuvettes and purged with oxygen or argon prior to photolysis on an optical bench set-up at Xexcit = 254 nm. Potassium ferrioxalate [ll] was used for chemical actinometry .
Acknowledgment We are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada and the University of Victoria for support of this research. We thank I. McAuley and M.-A. Teo for carrying out some preliminary studies.
References 1 P. Sykes, A Guidebook to Mechanism in Organic Chemistry, Longman, Harlow, Essex, 1986, 6th edn., p. 198. 2 J. March, Advanced Organic Chemistry, Wiley, New York, 1985, 3rd edn., p. 664. 3 P. Wan, S. CuIshaw and K. Yates, J. Am. Chem. Sot., 104 (1982) 2509. 4 P. Wan and K. Yates, Rev. Chem. Intermed., 5 (1984) 157. 5 P. J. Kropp, Org. Photochem., 4 (1979) 1. 6 P. Wan, S. Muralidharan, I. McAuley and C. A. Babbage, Can. J. Chem., 65 (1987) 1775. 7 P. Wan, Tetrahedron Lett., (1985) 2387. 8 P. Wan and S. Muralidharan, J. Am. Chem. Sot., 110 (1988) 4336. 9 S. L. Murov, Handbook of Photochemistry, Dekker, New York, 1973. 10 D. Schulte-Frohlinde and H. Gijrner, Pure Appl. Chem., 51 (1979) 273. 11 C. G. Hatchard and C. A, Parker, Proc. R. Sot. London, Ser. A, 235 (1956) 518. 12 J. C. Stowell, Carbanions in Organic Synthesis, Wiley, New York, 1979, p. 37.
396 J. R. Jones, The lonisation of Carbanion Acids, Academic Press, London, 1973. G. A. Russell, A. J. Moye, E. G. Janzen, S. Mak and E. R. Talaty, J. Org. Chem., 32 (1967) 137. 15 C. D. Ritchie, Act. Chem. Res., 5 (1972) 348. 16 J. D. Margerum and C. T. Petrusis, J. Am. Chem. SOC.. 91 (1969) 2467. 17 B. B. Craig, R. G. Weiss and S. J. Athertoh, J. Phys. Chem., 91 (1987) 5906. 18 B. B. Craig and M. D. Pace, J. Chem. Sot., Chem. Commun., (1987) 1144. 13 14
and Photobiology,
A:
Chemistry,
48 (1989)
387
PHOTOCHEMICAL TANDEM ADDITION OF WATER OXYGEN TO THE ALKENE MOIETY OF m-NITROSTYRENES PETER
WAN+
Department (Canada) (Received
and MICHAEL
of Chemistry, February
387
AND
J. DAVIS
University
10, 1989;
- 396
of Victoria,
Victoria,
British
Columbia
5% W 2Y2
in revised form April 12, 1989)
Summary A new type of reaction involving the tandem photoaddition of water and oxygen to the alkene moiety of several m-nitrostyrenes in oxygenated aqueous solution via the triplet excited state, to give P-hydroxy-a-hydroperoxides, is reported. In the absence of oxygen, only anti-Markovnikov photohydration was observed. The proposed mechanism involves initial nucleophilic addition of water to the P-carbon of the alkene moiety in the triplet excited state, to generate a m-nitrobenzyl carbanion intermediate, which is subsequently “trapped” by oxygen when it is present in solution or is otherwise protonated by water. The quantum yields for the tandem addition and photohydration were identical (e.g. @ = 0.32 for m-nitrostyrene), suggesting a common primary intermediate in the two pathways.
I. Introduction Nucleophilic addition to unactivated alkenes in the ground state is generally not observed [l, 21. However, if the alkene is activated by electron-withdrawing substituents such as carbonyl or highly electronegative atoms such as fluorine, nucleophilic 1,4-conjugate additions are common [l, 21, in which the nucleophile initially attacks the ,ðylenic carbon, to give a carbanion intermediate, which on subsequent protonation gives the nucleophilic addition product. Such a pathway forms the basis of the useful Michael reaction in organic synthesis in which the incoming nucleophile is usually a carbanion. The photochemical nucleophilic attack of water to the P-carbon of the alkene moiety of m- and p-nitrostyrenes via the triplet
T Natural Sciences Research Fellow, 1984 lOlO-6030/89/$3.50
and Engineering Research Council (NSERC) of Canada University - 1989. Address correspondence to this author. 0
Elsevier Sequoia/Printed
in The NetherIands
388
excited state has been reported by us 13, 41, which results in overall antiMarkovnikov
addition
of water (photohydration)
across the alkene (eqn. (1)).
This reaction is mechanistically interesting because the photoaddition of water and alcohols to alkenes generally proceed via initial photoprotona-
Con, to give carbocation intermediates 13 - 51, rather than via a carbanion intermediate, as required for the photohydration of m- and p-nitrostyrenes [ 31. Our recent interest [6 - S] in photo-oxygenation of nitrobenzyl derivatives, where we have proposed that photogenerated nitrobenzyl carbanion intermediates can be efficiently ‘(trapped” by dissolved molecular oxygen, to give isolable hydroperoxides, prompted us to investigate the photohydration of several m-nitrostyrenes in the presence of oxygen, with the aim of trapping the proposed m-nitrobenzyl carbanion intermediates with oxygen. In many photochemical reactions, the presence of oxygen often results in very complicated product mixtures, owing to the operation of a variety of pathways, and hence may not provide useful mechanistic information. However, it will be shown that as far as simple m-nitrostyrenes are concerned, reaction with both water and oxygen is both clean and mechanistically interesting, via an overall tandem addition process.
2. Results and discussion The compounds
chosen for this study were m-nitrostyrene
derivatives
1 - 4. CCCHR
-
lR-H
-2 R -
CH,
-3R=
CH&H,
-4R
= Ph (tram)
The corresponding p-nitrostyrenes were not studied since they reacted much less efficiently than the m-isomers and also tended to give side products. The m-nitrostyrenes were made by dehydrating the corresponding alcohols, which in turn were made via NaBH4 reduction of the corresponding ketones, except for 3, which was made by a Wittig reaction of n-propyltriphenylphosphonium bromide with m-nitrobenzaldehyde. Photolysis of 10m3 M solutions of 1 - 3 in aqueous solution (== 30% CH,CN) with a stream of argon purging through the solution (Rayonet RPR 100 photochemical reactor; 254, 300 or 350 nm lamps; 5 - 10 min) gave anti-Markovnikov alcohol products 5 - 7 in essentially quantitative yield (eqn. (2)).
389
OH L HR
cw
5R=H -
h3 l-3 -
-
-T
(2)
= CH3
ER
“,O/orgo”
CH2CH3
-7R-
NO2
Photolysis of m-nitrostilbene (4) resulted in no observable photohydration but only trans-to-cis photoisomerization, even on extended photolysis of several hours (254, 300 or 350 nm lamps). In addition, triplet sensitization of 4 (ET = 60 kcal mol-‘) with sodium benzophenone-2-carboxylate (8) (ET= 70 kcal malll [9]; Xex,-.it = 350 nm) also resulted in only transto-cis isomerization.
Thus it is clear that for m-nitrostilbene (4) twisting of the alkene to give the orthogonal state, followed by deactivation to the ground state is sufficiently fast that water fails to intercept the triplet state (which is presumed to be polarized) [3, $1. Schulte-Frohlinde and Gijrner [lo] have carried out extensive studies of nitrostilbene photoisomerization in several solvents and have shown that these compounds isomerize cleanly and efficiently, without competing reactions. Thus, it was not unexpected that 4 failed to undergo photohydration. When the same solutions of 1 - 3 were photolyzed with a stream of oxygen purging through the solution, P-hydroxy-cr-hydroperoxides 9 - 11 were observed in essentially quantitative yield in low conversion experiments (< 30%) eqn, (3)). OOH A WCHOH l-3 -
-
h3 H,O/O,
1;
Fi
h3
qCHO
+
$y
(3)
Q
10 T-lR -
NO2
NO,
NO2
12 -
9R-H R = a
(major)
13 -
(minor)
CH, CH,CH,
-I.
RCHO
(observed
(major) only for
R -
CH,.
CH,CH,)
No trace of the photohydration products 5 - 7 was observed. Extended photolysis led to increasing yields of m-nitrobenzaldehyde (12) and RCHO, along with a minor amount of cll-hydroxyketone 13, all of which are believed to arise via secondary photochemistry, as confirmed by independent pho-
390
tolysis of 9. The quantum yield for substrate loss (Ca)for 1 (measured by UV spectrophotometry at pH 7 with potassium ferrioxalate [ll] as actinometer) was 0.32 + 0.05, in the presence or absence of oxygen. This value for Q, is identical within experimental error with that previously reported by us using Malachite green leucocyanide actinometry [ 31 for the photohydration of 1. The corresponding measured @ for 2 and 3 were 0.22 and 0.21 respectively (argon or oxygenated solution; pH 7). As anticipated, 4 gave only trans-to-cis photoisomerization when irradiated in oxygenated aqueous solution. Thus it seems clear that oxygen competes only for the initially formed m-nitrobenzyl carbanion intermediate (eqn. (l)), rather than reacting directly with substrate. This proposal is supported by recent studies from our laboratory [8] in which the photogenerated parent m- and p-nitrobenzyl carbanions (formed via a photoretro-aldol type reaction) were shown to react efficiently with dissolved oxygen, to give isolable a-hydroperoxides. In the absence of oxygen, the carbanion intermediates are protonated to give the corresponding nitrotoluenes [S]. The mechanism of reaction of photogenerated m-nitrobenzyl carbanions with molecular oxygen in the present study presumably resembles that of other carbanions [12 - 141, namely via initial electron transfer from carbanion to oxygen, to give a radical ion pair, followed by coupling to give the hydroperoxy anion. The reaction of many carbanions generated under strongly basic conditions with oxygen generally results in carbonyl products owing to facile dehydration of the initially formed hydroperoxide [12 - 141. The advantage of the present reaction is the mild condition used (neutral solution) for generating the carbanion and hence the possible isolation of the hydroperoxide. There was no reaction when 1 was photolyzed in methanol or pure acetonitrile solutions (oxygen or argon saturated). The substrate was recovered unchanged in these experiments. Methanol as a nucleophile is at least as good as (if not better than) water [15]. However, in terms of polarity, water is much more polar than methanol, which may account for the lack of reaction since the mechanism clearly involves a highly polarized species, requiring extensive solvation from the solvent. A similar strong dependence on an aqueous medium was also observed in the photoretroaldol type reactions of nitrobenzyl derivatives [8]. The possibility that the oxygenations may be due to reaction of singlet oxygen (generated via sensitization of oxygen by the substrate) with the m-nitrostyrenes was checked by carrying out Rose Bengal sensitization experiments in aqueous solution. It was found that singlet oxygen generated from Rose Bengal gave no significant reaction for all of 1 - 4. It was possible to sensitize the photo-oxygenation reactions with sodium benzophenone-2-carboxylate (S), albeit an inefficient process because of the competing oxygen quenching of the triplet sensitizer. For a similar reason, triplet quenching experiments were not attempted. However, the well-established triplet state reactivity of nitroaromatic compounds of this type [3,8] would suggest that only the triplet state is responsible for reaction in these substrates.
391
Hydroperoxide 9 was quite stable and can be purified by chromatography on silica gel or crystallized from acetone. Reduction with aqueous Na#Os or KI gave the corresponding 1,2-glycol (14) and hence offers a simple overall procedure for the dihydroxylation of m-nitrostyrene (l), an otherwise difficult oxidative conversion with known thermal chemistry owing to the highly electrondeficient nature of the alkene. OH
Hydroperoxides 10 and 11 were much less stable and readily decomposed on thermolysis (> 30 “C), or on extended photolysis, to give m-nitrobenzaldehyde (12) (> 50%) and low yields (
Ar-
ICCHOH IoH OH,
*
pz;;]
_-Y-J?
(4)
~~~~C.,,,:.:“““”
10 -
13 -
(R -
CH,)
The results presented here along with previous data [3] indicate that the mechanism for water and oxygen addition shown in Scheme 1 is probably operative. That is, the polarized triplet excited state undergoes nucleo-
aide -1
T, ~
[--~~~~~*I H,O 5
~~
H+ -
Ar-CH-CH,OH I
Scheme
1.
Ar-CH,CH,OH
philic attack by solvent water to generate an m-nitrobenzyl carbanion intermediate, which can either be protonated (to give the photohydration product) or “trapped” by dissolved oxygen, to give the ar-hydroperoxides. It was surprising that the relatively low concentration of dissolved oxygen in water (= 1O-3 M) [9] can compete effectively with solvent protons in trapping the carbanion. Either the reaction with oxygen involves an extremely efficient pathway or, alternatively, the protonation step may be relatively slow since these carbanions are stabilized. A lifetime of the order of minutes at pH 7 has been reported for photogenerated p-nitrobenzyl carbanion [ 16 - 181. Although the corresponding m-nitrobenzyl carbanion has not yet been observed spectroscopically, it is reasonable to assume that it also has a significant lifetime in water, which tends to support that the efficient reaction with oxygen observed is due to the longevity of these species. An alternative mechanism for photoaddition of nitrostyrenes is one involving initial photoionization, to give a radical cation, followed by trapping by water at the P-carbon, to a radical, which either abstracts a hydrogen from solvent (to give the usual nitrophenethyl alcohol products) or reacts with dissolved oxygen, to give the P-hydroxy-o-hydroperoxides. OOH ArCH=CH,
hv
+
o2
e- + ArCH%H,
H20
+
ArCH-CH20H
Ar
C H-CH*OH
< ArCHzCHzOH (5)
We have ruled out this mechanism because of the following two points. (1) The nitrophenyl ring system is very electron deficient and would not be expected to undergo electron ionization, to give a radical cation, which would be highly destabilized, Typical substrates which are known to undergo photoionization are methoxybenzenes (which are sufficiently electron rich). (2) The radical intermediate generated via this mechanism eqn. (5) would only dime&e (or disproportionate) and not abstract hydrogen from water. 3. Experimental details 3.1. General ‘H NMR spectra were recorded on Perkin-Elmer R32 or Bruker WM250 instruments, in CDC13 with TMS as internal standard, unless otherwise noted. IR spectra were recorded on a Perkin-Elmer 283 instrument on NaCl discs. Mass spectra were recorded on a Finnegan 3300 instrument. Preparative and semipreparative photolyses were carried out using 200 ml quartz tubes in a Rayonet RPR 100 photochemical reactor equipped with
393
254, 300 or 350 nm lamps. For quantum yield measurements, radiation from an Oriel 200 W Xe-Hg arc lamp was filtered through distilled water and a Corning 7-54 band pass filter prior to further filtration through an Applied Photophysics monochromator set at 254 nm. 3.2. Materials
3.2.1.
m-Nitrostyrene
(I)
Compound 1 was prepared from m-nitroacetophenone by NaBH, reduction and subsequent dehydration over 85% H3P04. The sample obtained was identical with authentic material (from Aldrich).
3.2.2. P-Methyl-m-nitrostyrene (I-(3’-nitrophenyl)propene) (2) Compound 2 was prepared from m-nitropropiophenone: b.p. 104 105 “C (2 mmHg); iH NMR 6 1.9 (m, 3H), 6.4 (m, 2H), 7.3 - 8.2 (m, 4H). 3.2.3.
P-Ethyl-m-nitrostyrene
(I-(3’~nitrophenyl)-1-butene)
(3)
Compound 3 was prepared from a Wittig reaction of n-propyltriphenylphosphonium bromide with m-nitrobenzaldehyde (n-BuLi/THF) : b.p. 120 “C (2 mmHg); ‘H NMR (250 MHz) 6 1.05 (m, 3H), 2.25 (m, 2H), 5.75 (m, 1H), 6.35 (m, lH), 7.3 - 8.2 (m, 4H).
3.2.4. trans-m-Nitrostilbene (4) Compound 4 was prepared from m-nitrobenzyl phenyl ketone [S]: m-p. 102 - 105 “C; ‘H NMR (250 MHz) 6 7.15 (AB quartet, J= 16 Hz, 2H, trans vinyl H), 7.25 - 7.55 (m, 6H), 7.73 - 7.8 (m, lH), 8.05 - 8.10 (m, lH), 8.3 - 8.35 (m, 1H). 3.3. Pho tolysis 3.3.1. Pho tolysis
of I under oxygen
In a typical experiment, 100 mg of the substrate was dissolved in 50 ml CHsCN and mixed with 150 ml distilled water. The solution was then placed into a 200 ml quartz vessel containing a water-cooled cold finger. The solution was purged with oxygen (Linde, 99.9%) for 5 - 10 min prior to photolysis and irradiated for about 10 min with a continuous stream of oxygen purging through the solution. After photolysis, the solution was transferred to an Erlenmeyer flask and NaCl added which was then extracted with 2 X 100 ml CH&12. The crude “H NMR showed a distinctive singlet (exchangeable with D20) at 6 11.1 assignable to a hydroperoxyl proton, in addition to new peaks at S 3.9, 4.2 (exchangeable) and 5.2, all consistent with formation of &hydroxy*-hydroperoxide 9. On extended photolysis (> 10 min), increasing amounts of m-nitrobenzaldehyde (12) appears, as indicated by its characteristic aldehyde singlet at 6 10.2 along with its characteristic aromatic signals. Trace amounts of 13 (R = H) was inferred from a singlet observed at 6 5.0 (-CH,-) and the mass spectrum of preparative TLC fractions. Pure 9 was obtained from preparative TLC (silica/ CH,Cl,): m.p. 72 - 76 “C; ‘II NMR (acetone-de) 6 3.85 (m, 2H), 4.2 (t, J = 8
394
Hz, IH, exchangeable), 5.2 (t, J= 7 Hz, lH), 7.5 - 8.4 (m, 4H), 11.1 (2, lH, exchangeable); IR (cm-i) 3100 - 3700 (broad), 1520 (s), 1360 (s); mass spectrum (CI) (m/z) 182 (M+ + 1 - H20). Elemental analysis was not attempted because of the thermal lability of the compound. Reduction of 9 with aqueous KI or Na#Os gave (3’-nitrophenyl)1,2-ethanediol (14): ‘H NMR (acetone-dd) 6 3.65 (m, 2H), 3.9 (m, exchangeable, lH), 4.65 (m, exchangeable, IH), 4.85 (m, IH), 7.5 - 8.4 (m, 4H), which was identical spectroscopically with an authentic sample made from the hydrolysis of (m-nitrophenyl)ethylene oxide. of I under argon 3.3.2. Photolysis Under the same procedure as above but using argon instead of oxygen, 1 gave m-nitrophenethyl alcohol (5) in quantitative yield. The ‘H NMR spectrum of 5 was identical with an authentic sample from a previous study [31. 3.3.3. Photolysis of 2 under oxygen In a similar procedure as for 1 under oxygen, the ‘H NMR spectrum of the photolysate showed /3-hydroxya-hydroperoxide 10 as the major product (= 60%): ‘H NMR (acetone-de) 6 1.0, 1.15 (two methyl doublets, J = 8 Hz, 3H), 3.2 (broad, exchangeable, lH), 4.05 ( m, lH), 4.9 (m, lH), 7.4 - 8.3 (m, 4H), 11.05 (broad, exchangeable, 1H). Product 10 decomposed in the probe of the NMR spectrometer. (= 35 “C) and hence eluded purification), to give the following products: acetaldehyde (lH NMR 6 2.1 (d, J = 3 Hz, 3H), 9.7 (q, J= 3 Hz, TH)), m-nitrobenzaldehyde (12) and a minor amount of 13 (R = CHs) (‘H NMR 6 1.45 (d, J= 8 Hz, 3H), 5.2 (q, J = 8 Hz, lH), remainder of spectrum obscured by 10). 3.3.4. Photolysis of 2 under argon Under the same procedure as above but using argon instead of oxygen, 2 gave l-(3’-nitrophenyl)-2-propanol (6) in quantitative yield: lH NMR (250 MHz) 6 1.35 (d, J= 7 Hz, 3H), 2.85 (m, 2H), 4.08 (m, lH), 7.4 - 8.1 (m, 4H); IR (cm-‘) 3200 - 2500 (broad), 1520 (s), 1340 (m); mass spectrum (CI) (m/z) 181 (M+ + 1). 3.3.5. Photolysis of 3 under oxygen In a similar manner as for 1 under oxygen, the ‘H NMR of the photolysate showed /3-hydroxy-cw-hydroperoxide 11 as the major product (=70%): ‘H NMR (acetone-d,) 6 1.0 (m, 3H), 1.4 (m, 2H), 3.0 (broad, exchangeable, lH), 3.9 (m, lH), 4.0 (m, lH), 7.5 - 8.4 (m, 4H), 11.05 (broad, exchangeable, 1H). On extended photolysis (> 10 min), 11 decomposes to give the following products: propionaldehyde (‘H NMR 1.05 (t, J= 8 Hz, 3H), 2.4 ( q, J = 8 HZ, 2H), 9.7 (unresolved, lH)), m-nitrobenzaldehyde (12) and a trace of 13 (R = CH,CH,) as inferred by the triplet at 8 5.15 assignable to the methine of 13.
395
3.3.6. Photolysis of 3 under argon Under the same procedure as above but using argon instead of oxygen, 3 gave 7 in quantitative yield: lH NMR (250 MHz), S 0.97 (t, J = 7 Hz, 3H), 1.53 (m, 2H), 1.6 (broad, exchangeable, lH), 2.7 - 2.95 (m, 2H), 3.78 (m, lH), 7.4 - 8.1 (m, 4H); IR (cm-‘) 3200 - 3500 (broad), 1520 (s), 1350 (s); mass spectrum (CI) (m/z) 196 (Ad+ + 1). 3.3.7. Photolysis of 4 under argon or oxygen Photolysis of 4 under argon or oxygen gave the cis isomer, as identified by the appearance of a new AB quartet at higher field, at 6 6.7 (J= 12 Hz) (compared to the AB quartet of the trans isomer at 6 7.15). No other products were observed even on prolonged photolysis. 3.4. Quantum yield measurements Quantum yields for loss of substrate were measured by UV spectrophotometry, by following the loss in optical density of the m-nitrostyrenes. Solutions (10e5 M) were prepared in 3.0 ml quartz cuvettes and purged with oxygen or argon prior to photolysis on an optical bench set-up at Xexcit = 254 nm. Potassium ferrioxalate [ll] was used for chemical actinometry .
Acknowledgment We are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada and the University of Victoria for support of this research. We thank I. McAuley and M.-A. Teo for carrying out some preliminary studies.
References 1 P. Sykes, A Guidebook to Mechanism in Organic Chemistry, Longman, Harlow, Essex, 1986, 6th edn., p. 198. 2 J. March, Advanced Organic Chemistry, Wiley, New York, 1985, 3rd edn., p. 664. 3 P. Wan, S. CuIshaw and K. Yates, J. Am. Chem. Sot., 104 (1982) 2509. 4 P. Wan and K. Yates, Rev. Chem. Intermed., 5 (1984) 157. 5 P. J. Kropp, Org. Photochem., 4 (1979) 1. 6 P. Wan, S. Muralidharan, I. McAuley and C. A. Babbage, Can. J. Chem., 65 (1987) 1775. 7 P. Wan, Tetrahedron Lett., (1985) 2387. 8 P. Wan and S. Muralidharan, J. Am. Chem. Sot., 110 (1988) 4336. 9 S. L. Murov, Handbook of Photochemistry, Dekker, New York, 1973. 10 D. Schulte-Frohlinde and H. Gijrner, Pure Appl. Chem., 51 (1979) 273. 11 C. G. Hatchard and C. A, Parker, Proc. R. Sot. London, Ser. A, 235 (1956) 518. 12 J. C. Stowell, Carbanions in Organic Synthesis, Wiley, New York, 1979, p. 37.
396 J. R. Jones, The lonisation of Carbanion Acids, Academic Press, London, 1973. G. A. Russell, A. J. Moye, E. G. Janzen, S. Mak and E. R. Talaty, J. Org. Chem., 32 (1967) 137. 15 C. D. Ritchie, Act. Chem. Res., 5 (1972) 348. 16 J. D. Margerum and C. T. Petrusis, J. Am. Chem. SOC.. 91 (1969) 2467. 17 B. B. Craig, R. G. Weiss and S. J. Athertoh, J. Phys. Chem., 91 (1987) 5906. 18 B. B. Craig and M. D. Pace, J. Chem. Sot., Chem. Commun., (1987) 1144. 13 14