J. Photochem. Photobiol. A: Chem., 56 (1991) 35-42

Photosolvolysis acid solution’

of arylmethanols in aqueous sulphuric

35

Barbara Hall and Peter Wantt

Department of Chemist, Universi~ of Vktoria, Kctoria, British Columbia K?W 2X? (Canada)

(Received July 4, 1990)

Abstract

The photosolvolysis of a number of readily available arylmethanols (with the aryl group restricted to unsubstituted phenyl, naphthyl, pyrenyl and phenanthryl) was studied in aqueous sulphuric acid solution in the presence of methanol, ethanol and 2-propanol as external nucleophiies. All the simple arylmethanols studied in this work photosolvolyse only in the presence of acid; they are photostable in neutral solution. Fluorescence quenching by added acid and acid cataIysis of the reaction indicate that it proceeds from Sr. The results provide information on the relative electron-donating powers of these aryl groups in the excited singlet state.

1. Introduction

Photosolvolytic reactions continue to be an area of interest to organic photochemists since they are general reactions which give rise to carbocation intermediates, as opposed to radical intermediates. Photochemical reactions giving rise to radical intermediates have been extensively studied. The understanding of photosolvolysis reactions is accumulating. Cristol and Bindel [l] have written a review summarizing the extent of knowledge of photosolvolytic reactions up to the early 1980s. Recently, we have explored the use of the hydroxide ion as a photochemical leaving group in benzylic systems and have discovered that, in many instances, hydroxide ion functions formally as an exceptionally good leaving group [2-71. These reactions have been termed “photo- dehydroxylations”. Of the systems shown to react via photodehydroxylation, the process has been determined to be via the singlet state and is generally uncomplicated by competing homolytic bond cleavage (as side reactions), so prevalent in many photo- solvolytic reactions where the leaving group is not hydroxide ion (e.g. bromide, acetate and ammonium ions) [l]. Very little is known about the relative ability of simple unsubstituted aromatic compounds to promote photosolvolysis. It is known that simple benzyl esters [l] and 1-pyrenylmethyl esters photosolvolyse efficiently [8,9] via arylmethyl carbocation intermediates. In addition, photolysis of I-(halomethyl)naphthalenes in methanol solution results in products of both carbon-halogen heterolysis and homolysis [lo]. However, the intermediacy of an arylmethyl carbocation in these reactions was not proposed in ref. 10.

‘This work is based, in part, on the Honours Thesis project of B.H. at the University of Victoria.

‘+Author to whom correspondence should be addressed.

IOlO-6030/91/%3.50 @ Elsevier Sequoia/Printed in The Netherlands

36

In this paper, we investigate the photosolvolysis of unsubstituted arylmethanols and related compounds in aqueous solution, in order to obtain information on the photodehydroxylation of these simple aromatic systems, to determine the relative electron-donating powers of these aryl systems in promoting photodehydroxylation and to investigate the use of these systems as clean sources of arylmethyl carbocations on photolysis.

2. Experimental details

2.1. Materials Arylmethyl alcohols, 1, 4 and 7 and hydrocarbon 9 were purchased from Aldrich;

compounds 2,3 and 5 were made via NaBfT4 reduction of the corresponding aldehydes, which were also purchased from Aldrich. Alcohol 6 was prepared by the addition of methylmagnesium bromide to the corresponding aldehyde. The ‘H nuclear magnetic resonance (NMR) spectra and melting points of 2, 3 and 6 were identical to published data for the authentic samples [II]. Alcohol 1 was used as received, whereas all other alcohols were recrystallized before study. Aqueous acid solutions of pH <5 were prepared by diluting stock solutions of standardized HzSO.,, and their pH or & values were obtained by measurement or from standard tables of Hovalues. Standard phosphate buffer solutions were used for the pH 5-7 region.

ArCH20H Ar

OH

1-Pyrenemethanol (5): melting point (m-p.), 125-126 “C, ‘H NMR (relative to tetra- methylsilane (TMS) in CDCIJ) 6 1.78 (broad, lH, exchangeable with DzO), 5.34 (s, 2H), 7.9-8.5 (aromatic, 9H); mass spectrum chemical ionisation (CI) (m/z): 233 (M+ + 1).

2.2. Product studies Preparative photolyses in MeOH-Hz0 solutions were carried out using approx-

imately 10e3 M substrate in 50% (v/v) MeOH-HzS04(aq) solutions (total volume, 200 ml). Typical photolysis times were 5-30 min at 254 nm using a Rayonet RPR 100 photochemical reactor. All solutions were cooled to about 13 “C during the photolysis

37

and were stirred continuously with a stream of argon. The photolysed solutions were worked up by first adding NaCl and then extracting the solution several times with CH,Cl,. On evaporation of the solvent, the extent of conversion to the methyl ether product was determined by ‘H NMR (comparison of the methylene signals by integration in substrate and product, which have significantly different shifts) and gas chromatography (GC) analyses, which showed excellent (more than 95%) mass balance in all runs. All the photoproducts were separated via preparative thin layer chromatography (TLC) (CH2C12; silica) and had satisfactory ‘H NMR, IR and mass spectra.

2.3. Quan turn yield measurements Quantum yields were measured using potassium ferrioxalate actinometry [12, 131.

Solutions were photolysed at 280 nm using an Oriel 200 W mercury source filtered through an Applied Photophysics monochromator. Product analyses were performed using a Varian 3700 gas chromatograph. Conversions were kept below 30%.

2.4. Steady state and transient. fluorescence measurements Fluorescence measurements were recorded at ambient temperature on a Per-

kin-Elmer MPF 66 spectrofluorometer. Solutions (approximately 10m5 M) were prepared in quartz cuvettes and purged with a stream of argon prior to measurement. The spectra were not corrected. Single photon lifetime measurements were carried out using a Photochemical Research Associates system at the University of Waterloo. Excitation using a hydrogen flash lamp was performed at 260 nm, and emission was monitored at wavelengths longer than 320 nm using band-pass filters.

3. Results and discussion

3.1. Product studies Arylmethanols l-7 were chosen for study. Compounds 1,4 and 7 were commercially

available. Alcohols 2,3 and 5 were made via simple NaBH, reduction of the corresponding aldehydes, which were commercially available. Alcohol 6 was made by addition of methylmagnesium bromide to the corresponding aldehyde. Compound 1 was used as received. All other alcohols were recrystallized prior to study.

Photolysis of low3 M solutions of 1-7 in pure MeOH or in 50% (v/v) MeOH-H,O (aqueous portion at pH 7) in a Rayonet RPR 100 photochemical reactor (deaerated solutions; 254 nm lamps; 5-30 min) failed to give any reaction and the substrates were recovered unchanged. Photolysis in a solution of 50% (v/v) MeOH and 10% (w/w) HzS04 for 5 min gave the corresponding methyl ether products in yields of 15%-38% (eqn. 1)) as calculated by ‘H NMR (Table 1)

R R I

A&H-OH hv

s A&H-OCH, 50%(v/v)MeOH-10% (whu)HzSO4

(1)

yield 15-38%

No reactions were observed when these were left in the dark for the same period of time. Significant thermal solvolysis occurred when acids (more than 20% (w/w) H2S04) were used; photosolvolysis studies were not carried out at these acidities. Compounds 5-7 gave the highest conversion (30%-38%) to the corresponding methyl ethers of the systems studied. Alcohols 6 and 7 were expected to give some of the

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TABLE 1

Percentage yields of methyl ether products observed on photolysis of arylmethyl alcohols l-7 in a solution of 50% (v/v) MeOH and 10% (w/w) H,S04’

Alcohol YieIdb (%)

1 15 2 19 3 15 4 14 5 30 6 33 7 38

?Substrate at 10m3 M. Photolysis time of 5 min at 254 nm (16 lamps were used in a Rayonet RPR 100 photochemical reactor). bConversions to methyl ether product calculated by ‘H NMR integration. Since excellent mass balance was observed, indicating a clean photosolvolytic reaction, the conversions may be taken as actual yields of photosolvolysis products.

corresponding alkene products during photosolvolysis but none was observed. Thus the intermediate secondary carbocations photogenerated in these two systems show no tendency to undergo El elimination (eqn. (2))

“20

8 (not observed)

(2)

The possibility that alkene 8 was formed, but during photolysis was efficiently converted back to 6 (a known photoreaction [14]), was considered but ruled out, since the alkene was not detectable even in low conversion (less than 10%) experiments. The lack of the El pathway for compounds 6 and 7 was confirmed by the fact that alkene 9, which was shown to be photostable in aqueous acid under the photolysis conditions, was not observed on photolysis of alcohol 7.

Since alcohol 6 gave good yields of the methyl ether product on photosolvolysis in a solution of 50% (v/v) MeOH and 10% HzS04, we studied the photosolvolysis of this substrate using two other alcohol solvents, EtOH and 2-PrOH. The results are shown in Table 2. It is clear from this study that, although both EtOH and 2-PrOH react to give the corresponding ether products, they do so less efficiently than MeOH.

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TABLE 2

Percentage yields of ether products observed for compound 6 with various alcohol nucleophiles in 10% (w/w) H,SOba

Alcohol nucleophile

(ROW

Yieldb (%)

MeOH 85 EtOH 21 2-PrOH 8

aThe volume of ROH used was adjusted to ensure that the same concentration of nucleophite was present in each system, This meant that slightly different effective acidities (due to increase in total volume of the solution) were used in the three experiments. However, the differences in yields observed cannot be accounted for by such a slight change in acidity of the media. bYields calculated by ‘H NMR integration (see footnote b in Table 1).

TABLE 3

Quantum yields of methyl ether formation in a solution of 50% (vhr) MeOH and 10% H2S04

Compound ab”

1 0.066 2 0,083 3 0.066 4 0.061 5 0.14 6 0.15 7 0.17

ah,,= 254 nm. Errors in GP are f 15% of the quoted value,

This is probably due to a steric effect of the nucleophile, as similar results have been observed in a number of other photosolvolytic systems investigated [4, 61.

3.2. ~~u~~~ yields and acid catalysis Quantum yields of photosolvolysis in a solution of 50% (v/v) MeOH and 10%

H,SO, were measured using GC for analysis and potassium ferrioxalate actinometry (A,,- 280 nm). They are given in Table 3. The reactions are strongly catalysed by acid as shown in plots of QP (quantum yield of methyl ether formation) vs. acidity (pH or Ho of the aqueous H2S04 portion of the solvent mixture) for alcohols 2, 6 and 7 (Fig. 1). From this plot, it is ciear that no reaction is observed above pH 3, but GP increases rapidly below this pH. Similar plots were observed for the rest of the alcohols, although the extent of increase varied with the different substrates.

3.3. Steady state and transient fluorescence studies The fluorescence emissions of compounds X-7 were quenched in acid, as shown

by the plot of the relative fluorescence emission intensity (@W+“, where ?I$” is the emission intensity at pH 7 and @Jo is the emission intensity in a more acidic solution) for alcohol 6 (Fig. 1). The absolute fluorescence quantum yield for 6 was about 0.1 in neutral aqueous solution. The increase in the product quantum yield of photosolvolysis

0.0 0.0

-2 0 2 4 6 a

pH or Ho

Fig. 1. Plot of quantum yield of methyl ether formation (@,) for alcohols 2, 6 and 7, and plot of relative fluorescence emission intensity (4pf/@‘) for 6, as a function of acidity. For quantum yields, the acidity quoted is of the aqueous portion only.

TABLE 4

Fluorescence lifetimes as measured by single photon counting for alcohols 2, 5 and 6

Compound 7 (ns)

pH 7” 10% H2SO.,’

2 26.0 f 0.5 6.5fO.l 5 180.0 f 1 160.0 f 1 6 27.0 + 0.5 6.5 10.1

“CH,CN cosolvent (less than 5%) was used. SimiIar lifetimes were observed when MeOH cosolvent was used.

(a,) concurrent with fluorescence quenching for this compound, with almost identical inflection points, is typical of these systems. These observations strongly suggest that it is the singlet state which is reactive, and that the primary photochemical step is proton-assisted photodehydroxylation, as shown for the methoxy-substituted benzyl alcohols [2, 51.

Fluorescence lifetimes were measured via single photon counting for selected compounds in neutral solution and in 10% HzSO + The results are shown in Table 4. All the fluorescence decays observed were fitted to good first-order decays, with lifetimes shorter in acid than in neutral solution, confirming that a dynamic quenching mechanism is operative in the observed steady state fluorescence quenching by acid. This is consistent with a mechanism involving a proton-assisted primary photochemical

41

step. The moderate change in r observed for compound 5 on going from pH 7 to 10% H,SO, is due to a much lower catalytic effect of photosolvoiysis by acid for this compound compared with the other systems.

The results of this study and from previous work on photodehydroxylation 12-61 support the reaction mechanism shown in Scheme 1. Assuming the validity of Scheme 1, estimates of kH (and k,) can be made from the following analysis. Excitation of the substrate ArCH20H to the singlet excited state (Scheme 1) may result in fluorescence emission, intersystem crossing, or reaction to give [ArCH*+], via a solvent(HtO)- assisted process (k,) and a hydronium ion catalysed route (kn[H+]). Capture of the photogenerated carbocation by Hz0 (kHzo) or added nucleophile N (&[N]) gives the starting material or product respectively. In previous work [2], we have shown that

DP = 0.31&+ (3)

describes the relationship between @,, (quantum yield of methyl ether formation) and &+ (quantum yield of carbocation [ArCH*‘] formation) in 50% (v/v) MeOH-H,O. Substituting &+ =k,[H+]r (d erived via a steady state assumption for [A&H,+]), we obtain

kH= @R+ 0.31[HC]r

which gives an expression for kH for photodehydroxylation. Substituting appropriate values for G$, [H’] and r for compound 6 in a solution of 50% (v/v) MeOH and 10% (w/w) H,SO, (the reasonable assumption is made that the water contents in pure water and in 10% H2S04 are about the same), we calculate kH= 6 X lo7 M-’ S - ‘. Using only fluorescence quenching data for compound 6, a linear Stern-Volmer plot was observed, from which we obtained kH = 5 x 10’ M-’ s-r, which is in reasonable agreement with the value obtained via the product quantum yield using eqn. (4). For comparison, kH = (O-37-1.4) X lOlo M-’ s-l for methoxy-substituted benzyl alcohols [Z]. Estimates of k, are more difficult to make since these substrates do not show photoreactivity in neutral solution. However, assuming an upper limit of sP,=O.OOl

neutral solution, we can estimate that k,< 1 X lo5 s-r. For comparison, K= (1.4-1.7) x 10' s-' for methoxy-substituted benzyl alcohols [Z].

Although it is not possible to rule out an S,2 mechanism in these reactions without results from flash photolysis studies, the results of such studies for related benzyl alcohol systems [15-191 have shown the intermediacy of carbocations formed via an SN1 pathway. It would appear that as far as benzylic alcohols are concerned, photosolvolysis is via a simple photodehydroxylation (S,l) step, which may be catalysed by added acid.

ArCH2+ + 1

-OH

‘%#‘I

\ ArCH2N

Scheme 1.

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4. Conclusions

The results show that all of the relatively simple arylmethanols studied in this work photosolvolyse only in the presence of acid (pH ~3). They are otherwise photostable in neutral solution. Previous studies [2, 61 have shown that, in general, methoxy- substituted benzyl alcohols photosolvolyse without acid, but become more reactive when acid is present. It would appear that without methoxy substituents, simple aryl groups do not have sufficient electron-donating power to effect solvolysis with a poor leaving group such as hydroxide ion. However, these substrates show substantial photosolvolytic reactivity in acid solution of pH 2 or less. This indicates that there is a degree of charge polarization in the excited singlet states of these systems which, in the presence of sufficient acid as catalyst, can initiate C-OH bond heterolysis. Thus simple arylmethyl carbocations can be photogenerated cleanly, enabling these relatively simple cations to be studied further via flash photolysis, where there is much activity [15-191.

Acknowledgments

This research was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada. Additional support came from the University of Victoria. We thank Mr. Erik Krogh for assistance in carrying out the lifetime measurements,

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