Mechanism of the photodimerization of 2-naphthalenecarbonitrile and related reactions

Journal

of Photochemistry

and Photobiology,

A:

Chemistry,41 (1988) 215 - 225

215

MECHANISM OF THE PHOTODIMERIZATION OF 2-NAPHTHALENECARBONITRILE AND RELATED REACTIONS ANGELOALEINIT,ELISA FASANI and ANNA GAMBA Dip. Chimica Organica dell’Universiici,

(ReceivedMay 15,

V. Taramelli 10, 2 7100

Pavia

(Italy)

1987; in revised form July 10, 1987)

Summary

Irradiation of 2-naphthalenecarbonitrile leads to the formation of two cage dimers which involve bonding between substituted rings in one case and a substituted and an unsubstituted ring in the other. In the presence of naphthalene, a cross-cage dimer is formed, and in the presence of anthracene (which is excited) a 9,10-[l/,4’] cross-dimer is formed. Cage naphthalene dimers are formed through two photochemical steps, the first involving the singlet excited state of 2-naphthalenecarbonitriIe and a polar exciplex, and the second being the triplet-sensitized intramolecular cycloaddition of the intermediate adduct. A minor pathway from the same intermediate leads to a binaphthyl,

1. Introduction The photodimerization of anthracene and its derivatives was one of the first photochemical reactions [l] to be discovered and has been a favourite subject of investigation during the last thirty years [2 - 41. A large choice of structural variations is possible. Meso photodimers are normally obtained with no steric hindrance from substituents in position 9, and in certain cases also from 9,10-disubstituted anthracenes. Cross-dimers have been obtained efficiently in some cases [ 5,6]. In substituted anthracenes, head-to-tail and head-to-head orientations are subject to structure and medium effects [6, 7 1. Furthermore, exciplex emission is observed in several cases and helps in mechanistic investigations. In contrast, the photodimerization of naphthalene derivatives is limited in scope. The parent molecule does not dimerize and, with the exception of a l&disubstituted derivative [8] and of some intramolecular examples [9 - 131, photodimerization is limited to 2-substituted naphthalenes, and more precisely to two groups of derivatives, viz. some 2-alkoxynaphthalenes, yielding dimers of structure 1 and 2 [14, 151, and some esters and other +Author

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216

functional derivatives of Z-naphthalenecarboxyhc acid yielding cage dimers of structure 3 [ 8,161.

1

2

3

Dimerization invariably occurs between substituted rings, and, with one exception [ 151, in head-to-tail orientation. Excimer emission is not usually observed. Pursuing our interest in the bimolecular photochemistry of aromatic nitriles [17 - 193, we reinvestigated the photodimerization of 2-naphthalenecarbonitrile (2-NN) [20]. This reaction and some related cross-dimerizations are the subject of the present report. 2. Results Irradiation (Pyrex filtered) of a 1 X IOT2 M degassed solution of Z-PIN in cyclohexane leads to two products in roughly equal amounts. These were separated by chromatography and obtained as crystalline materials. Roth products decompose on melting to give back 2-NN and show elemental analysis and mass spectra appropriate to a dimeric structure. The product eluted first has a melting point and spectroscopic properties corresponding to the only product reported from the irradiation of 2-NN in a previous work by Zweig [21]. In the original paper the structure of 1,4[ 1’,4’] -naphthalenonaphthalene (analogous to formulae 1 or 2) was assigned to this product, but Collin et al. [8] later observed that the reported spectroscopic data fitted those of the cage dimers they had obtained from some 2-naphthalenecarboxy esters, and suggested that this product has indeed a cage structure. To remove residual doubt, we submitted this dimer to acidcatalysed methanolysis and obtained the corresponding dimethyl dicarboxylic ester. This was found to be identical with an authentic sample of the dimer from the methyl ester of 2naphthalenecarboxylic acid, which has been ascertained by X-ray analysis to have the head-to-tail structure 3 [22]. Thus formula 4 is assigned to the first dimer from 2-NN (Scheme I, Table 1). The slower-eluting dimer also has a cage structure and a nuclear magnetic resonance (NMR) spectrum similar to that of compound 4 but showing seven aliphatic hydrogens (one more than in compound 4), which

217

iis@ CN

CN

4

CN

2-NN

CN

6 Scheme

1.

TABLE

1

Products isolated from the photodimerization of 2naphthalenecarbonitrile (1 X 10T2 M) and the cross-dimerization with naphthalene (N) and anthracene (A)

Solvent

(2-NN)

Products (W yield)a

Additive (M)

C6H12

MeCN C6H6

MeCN C6H12

M&N MeCN C6H12

MeCN

b4(35), 5(19), Ph2C0, 1.2 x 1O-2 CF&OOH, 1 x 1O-2 N, 4 x 1O-2 N, 4 x 1O-2 N, 4 x 10-2; CFsCOOH, A, 2.5 x 1O-3 A, 2.5 x 1O-3

1 x 1O-2

4(22), 4(18), 4(15), 4(40), 4( 15), 4(30),

aCalculated on reacted Z-NN. Reactions were interrupted b2-NN recovered unchanged. c Calculated based on initial anthracene concentration.

6(5)

5(9) 5(21), 7(44) 5(17), 6(13), 5(20) 5(5), 8(59)C 5(25), 6(20),

7(39)

8(52)c,

9(20)

at 10% - 15% conversion.

can be assigned by means of double-irradiation experiments and computer simulation. Thus one cyano group is on an aromatic ring and structure 5 (or the isomeric structure with a cyano group in position 4) is assigned to this compound. Dimer 4 is less soluble and partially precipitates out during

218

irradiation when formed in high concentration, while dimer 5 is much more soluble, and this is probably why it escaped detection by earlier workers. Irradiation of 2-NN in acetonitrile yields the same two dimers (the ratio 4:5 is now about 2 : 1) and a minor product arising from bimolecular reduction of 2-NN. The chromophore present in this compound corresponds to the 1,4-dihydro form of 2-NN as shown by the UV spectrum, and the two rings are linked in the 1,4’-position, as shown by ‘H and 13C NMR spectra (formula 6). The photochemistry of 2-NN was examined under various conditions in order to obtain mechanistic information. Thus, no reaction takes place in air-equilibrated solutions. 2-NN is unaffected on irradiation in the presence of 1.2 X lo-* M benzophenone in benzene (light absorbed by the ketone). The dimerization is quenched by electron-rich aromatics and alkenes, which are effective quenchers of 2-NN singlet excited state. Addition of lo-* M trifluoroacetic acid to the acetonitrile solution slows the dimerization down by about 40% and quenches the formation of product 6. Irradiation of 2-NN was carried out in the presence of other aromatic derivatives in order to explore the possible formation of cross-dimers. In the presence of a fourfold excess of naphthalene (at least 90% of the light absorbed by 2-NN) the yield of the homodimers 4 and 5 is decreased and a different compound is obtained as the main product. This is a crystalline material showing an NMR spectrum virtually identical with that of homodimer 5 in the aliphatic region and decomposing on melting into a mixture of 2-NN and naphthalene. These and other investigated properties support the structure of the cage cross-cycloadduct 7.

CN

Formation of this product is more strongly quenched by acids than formation of the homodimers 4 and 5. Thus, cross-dimerization is completely suppressed in the presence of lo-* M trifluoroacetic acid. Electron-rich naphthalenes do not yield cross-dimers with Z-NN. Irradiation of a mixture of 2-methoxynaphthalene and 2-NN is ineffective, yielding neither the homodimers which are formed from the irradiation of each compound alone nor a cross-dimer. Irradiation of 2-NN in the presence of 2.5 X 10e3 M anthracene leads, as expected, to the formation of an abundant precipitate of anthracene

219

homodimer 8. Chromatographic separation of the filtrate yields the 2NN homodimers in the case of irradiation in cyclohexane, but from the irradiation in acetonitrile a cross-dimer is obtained and is unambiguously recognized as 9,10-[1’,4’] naphthalenoanthracene 9 on the basis of elemental analysis and spectroscopic properties.

From the photophysical point of view, we detected no change in shape or lifetime of 2NN fluorescence between 5 X lo-’ and 5 X 10e3 M but at higher concentrations and measuring in front-face configuration selfquenching of the monomer fluorescence and weak emission at longer wavelengths corresponding to the excimer are apparent. A Stern-Volmer constant K,= 8 M-’ is evaluated in cyclohexane. The lifetime of 2-NN fluorescence, however, does not show an appreciable change over a large range of concentrations ([B-NN] = 5 X lo-’ M, rF = 29 ns; [ 2-NN ] = 5 X 10q2 M, TF = 32 ns in degassed cyclohexane). Thus the 2-NN excimer is very short lived and yields back reversibly the singlet excited monomer. In the presence of naphthalene, quenching of 2-NN fluorescence takes place (Table 2) and emission from the exciplex is observed (A,,, x 400 nm). This emission is quenched by trifluoroacetic acid, which does not influence TABLE 2 Stern-Volmer

constants for fluorescence quenching and reaction quantum yields

Fluorescer

Quencher

Solvent

2-NN

-

MeCN C6HlZ

2-NN

N

2-NN-N

CF&OOH

A

2-NN

8

C6H12

46 10

MeCN

19

CsH12

11

MeCN

MeCN

o.o05**b 0_002a*b O.OOlC 0.0007=

9

* 2-NN, 1 X 10m2 M; light flux, 1 X 10e6 einsteins min-’ cms2. bThe quantum yield decreases by a factor of 2 when the incident flux is halved. c 2-NN, 1 x 10m3 M; N, 1 X 10e2 M. The reaction observed under these conditions tially cross-dimerization.

is essen-

220

the fluorescence from 2-NN. Oxygen reduces the intensity of both 2-NN and exciplex emission, without affecting the ratio between the two. Quantum yield of reaction depends on the incident flux. Representative values are reported in Table 2. 3. Discussion Although formation of cage dimers from naphthalene derivatives has already been reported, both in intermolecular cases [ 16, 223 and in intramolecular cases [ 93, the mechanism of this reaction has yet to be clarified. It is expected that two subsequent photochemical steps are involved and in fact the reaction quantum yield is intensity dependent (Table 2). The first step is attributed to the excited singlet state, as triplet sensitization is ineffective. Thus irradiation in the presence of benzophenone in benzene leaves 2-NN unchanged although energy transfer from the ketone to the nitrile (the latter has a triplet energy of 59.8 kcal mol-‘, as deduced from the phosphorescence spectrum) does take place. Cycloaddition from the singlet state involves an intermediate excited complex. (Previous workers [ 151 have proposed that association in ground state complexes is responsible for the observed orientation in 2alkoxynaphthalene dimerization. We do not see any indication that complexes build up under our conditions (the substrate concentration is much lower than in their case).) At least in the case of the cross-dimerization between 2-NN and naphthalene this is supported by the quenching of both exciplex fluorescence and chemical reaction by acids. This quenching shows that the exciplex is distinctly polarized and can be accounted for by reversible protonation of 2-NN. 2-NN1* + N -

(2-NN6-N”+)*

(2-NN”-N”+)* 2-NNH+ -

+ II+ j

2-NNH+ + N

2-NN + H’

‘I’he effect of acids on 2-NN homodimerization is smaller, as no ion pair character develops, but the involvement of an excimer is deduced by analogy. With good donors, such as 2-methoxynaphthalene, complete charge transfer and formation of solvated radical ions of opposite sign takes place. Back electron transfer then leads back to the starting materials and no crossdimerization takes place. (2-NN”-D6+) 2-NN’,,i,

-

2-NN:,,i,

+ Dtolv -

+ D+,,

2-NN + D

The first photochemical step leads to 4 + 4 dimers such as compound 10. This is expected on the basis of theoretical predictions [23] and by analogy with the known photochemistry of 2_alkoxynaphthalenes, which do yield 4 + 4 dimers 114, 151, and with the intramolecular cyclization of di( 1-naphthyl)ether investigated by Todesco et al. [ 121. This reaction is

221

particularly relevant to the present case, in that it was found that formation of the end product, again a cage derivative, involves photochemical 4 + 4 cycloaddition followed by Cope 3 + 3 thermal rearrangementand 2 + 2 photocycloaddition [13] (Scheme II). Although we have no direct evidence for precursors of cage dimers 4, 5 and 7, this literature evidence gives support to the idea that the same mechanism also applies in the present case, and a low steady state concentration of 2 + 2 dimer 11 builds up through the intermediacy of the 4 + 4 dimer 10 and reverts to the starting material (Scheme III). The same obviously applies for cycloaddition between substituted and unsubstituted rings (yielding in the end the cage dimer 5). The latter photochemical step is then understood to be triplet sensitization by the relatively long-lived 2-NN3* (TV = 15 ps under these conditions, as measured by flash photolysis). This is supported by complete quenching of the photodimerization in air-equilibrated solution, whereas quenching of 2-NN singlet excited state or 2-NN-naphthalene exciplex is only partial under these conditions. Furthermore, cage dimers are found from 2-NN and from the esters of 2-naphthalenecarboxylic acid, where a cynnamyl chromophore of low triplet energy is present in the 2 -F2 dimer and thus triplet sensitization is effective, whereas no such reaction takes place from dimers such as 2 (high energy alkoxyethylene chromophore) or in the case of parent naphthalene. Another observed product, uix. compound 6, can be attributed to a sensitized reaction of intermediate 11. Although product 6 might be thought to arise via coupling of a naphthyl radical, its formation does not involve hydrogen abstraction by triplet 2-NN, as it is not formed in the benzophenonesensitized experiment, nor does it involve protonation of the partially

Scheme II.

222

2-NN" A

-/ 2-NN

(2-NNi*

11

hv /

/

2-NN3*

Scheme III.

negative 2-NN ring in the singlet exciplex (on the contrary, its formation is quenched in the presence of acids). This reaction can be better understood as a minor pathway from the triplet sensitization of intermediate 11, which undergoes homolysis of the cyclobutane ring and hydrogen abstraction (Scheme III). As for the reaction of 2-NN in the presence of anthracene, monochromatic irradiation experiments show that the cross-dimer 9 arises from a reaction of excited anthracene, not excited 2-NN, as it is formed by irradiation at 366 nm, whereas the other dimers are obtained only by irradiation at 313 nm. Here again cross-dimerization involves a significantly polarized exciplex and is observed only in polar media.

4. Conclusions Every known photodimerization of a naphthalene derivative is an inefficient process. The quantum yield is low, @ < lo-*, for Z-NN; from preliminary measurements even lower values apply for 2alkoxynaphthalene and 2-carboxynaphthalene derivatives under comparable conditions. There are two reasons for this inefficiency. (i) Excimers, which could otherwise act as a reservoir and facilitate the access to the pericyclic minimum intervening in the cycloaddition process, undergo only limited stabilization. Thus, excimers are only reversibly formed

223

(note the low constant for steady state self-quenching of fluorescence) and a tiny fraction of the excited molecules proceed towards chemical reaction. (ii) Cycloadducts formed in the first photochemical step are unstable and tend to cleave back to the starting material. Indeed intermolecular adducts have been isolated in only two instances, viz. from 2alkoxynaphthalenes where highly polar and insoluble products are formed and crystallize out without undergoing further change, and from 2-cyanonaphthalenes and 2carboxynaphthalenes, as here triplet sensitization is energetically possible and yields irreversibly cage dimers such as products 4, 5 and 7 through a second photochemical step. This complex mechanism makes it difficult to relate the product distribution actually obtained to a preferential orientation for the initial excimer and the existence of different channels for the different modes of cycloaddition observed (1 and 2 from 2-alkoxynaphthalenes, 4 and 5 from 2-NN). At any rate, the formation of dimer 5 and cross-dimer 7 shows that the bonding does not take place exclusively between substituted rings, as was thought previously [ 15,161. Advantage can be taken of the formation of charge transfer exciplexes between naphthalenes of different ionization potential for the preparation of cross-dimers. This strategy has limited application, however, as when the difference is too great the exciplex cleaves with complete charge transfer into solvated radical ions and no net chemistry results, as in the case of 2-NN in the presence of 2-methoxynaphthalenes. Furthermore, the actual isolation of the cross-dimers from different naphthalenes is again limited to the conditions discussed above. A cross-dimer between anthracene and 2-NN, involving less resonance energy loss than for the naphthalene-naphthalene dimers, is obtained easily. 5. Experimental details Commercial samples of 2-NN and other aromatic derivatives were purified by chromatography on alumina and recrystallized. Spectroscopic grade solvents were used as received. The UV spectra were recorded on a Perkin-Elmer 200 spectrophotometer, the IR spectra on a Perkin-Elmer 257 spectrophotometer and the ‘H and 13C NMR spectra on Brucker instruments (80 or 270 MHz) using (CH3)4Si as internal standard. Mass spectra were obtained by means of a Du Pont 492-B instrument. Melting points were uncorrected. New products gave satisfactory (to &0.3%) elemental analysis (carbon, hydrogen and nitrogen). 5.1. Irradiation procedure A solution of 2-NN (1.53 g, 1 X lo-’ M) in 1 1 acetonitrile was brought to the boil in the irradiation vessel and cooled while being purged with argon. Irradiation was carried out by means of a Helios Italquartz 500 W medium pressure mercury arc through Pyrex for 30 h. Evaporation of the solvent

224

under reduced pressure and chromatography of the residue yielded 1.4 mg unreacted starting material and the following products. 1,14-Dicyano-l,2,2a,7,8,12b-hexahydro-1,8:2,7-ethanediylidenedibenzo[a,e] cyclobuta[c] cyclooctene (4) (455 mg, 35%) colourless crystals from acetone, melting point 213 - 220 “C (225 “C in ref. 211, decomposes on melting into 2-NN; IR, 2224 cm- ‘; NMR (C,D,), 6 = 3.12 (2-H, 13-H), 3.42 (2a-H, 8-H), 3.78 (7-H, 12b-H), J2+ = Js_13 = 8 Hz; J,_-8 = J2a_-12b= 12.1. 1,5-Dicyano-1,2,2~,7,8,12b-hexahydro-1,8:2,7-ethanediylidenedibenzo[a,e] cycIobuta[c] cyclooctene (5) (250 mg, 19%), colourless crystals from benzene-cyclohexane, melting point 188 - 190 “C, decomposes on melting into 2-NN; IR, 2227, 2236 cm-l; NMR (C6D6, values obtained from computer simulation) 6 = 2.44 (14-H), 2.86 (13-H), 2.88 (2-H), 3.28 (7-H), 3.41 (2a-H), 3.57 (8-H), 3.94 (12&H), J13+ = 7.2 Hz, J2_-14= 7.9, J7_-14 = 7.6, Jza_14 = 1, JS-14 = 0.9; J7--13 = 1, J2.+= 8.1, J,_, = 0.6, JS13 = 7.6, J,-s = 11.6, J20_-12b = 11.9. l-[ 4-(1,4-Dihydro-2-cyanonaphthyl)] -1,4-dihydro-2-cyanonaphthalene (6) (65 mg, 5%), colourless crystals from benzene-cyclohexane, melting point 155 - 156 “C; IR 2220 cm- ‘; *H NMR (CDC13), 6 = 2.9 and 3.35 (AB system,

l’-CH,),

AB system

centered at 3.25

(4-CH2),

4 and 4.1 (4’-H and

l-H), 6.4 (3’-H), 6.6 (3-H); 13C NMR (CDC13) 6 = 31 t, 48.2 d, eight distinguishable doublets between 127 and 143. The other preparative experiments were carried out analogously, as detailed in Table 1. New compounds are the following. 1-Cyano-1,2,2a,7,8,12b-hexahydro-1,8 :2,7_ethanediylidenedibenzo[u,e] cyclobuta[c]cyclooctene (7), colourless crystals from benzene-cycloNMR (C,D,), hexane, melting point 177 - 179 “C; IR 2220 cm-l; 6 = 2.67 (14-H), 2.94 (13-H), 3.06 (Z-H), 3.58 - 3.78 (7-H, 2a-H, 8-H), 4.06 (12b-H), J13e14= 7 Hz, J2_& = J2+ = 8, J2_14 = J7_-14 = 7, J2a_-12b= 12; mass spectrum M+ (281), 252 (M+-H-HCN), 153 (2-NN), 128 (N). 12-Cyano-9,10,11,14-tetrahydro-9,10-[1’,4’]naphthalenoanthracene (9), colourless crystals from benzene-cyclohexane, melting point 169 - 170 “C; IR 2220 cm-‘; NMR (C&D,), 6 = 3.32 (14-H), 3.7 (11-H), 3.88 (9-H), 3.98 (10-H), 6.27 (13-H), J13e14= 7.6 Hz, J+10 = 11, J11_-13= 1, J,o-ll = 10.8. 5.2. Methanolysis of dimer 4 50 mg of 4 in 5 ml methanol containing 0.5 ml concentrated hydrochloric acid were refluxed for 5 h. Evaporation of the solvent and recrystallization from acetone gave a product melting at 202 - 203 “C, identical (spectroscopic properties, admixture melting point) with the dimer obtained from the irradiation for 18 h of 2-carboxymethylnaphthalene (1.5 g) in 80 ml MeCN (27% conversion, 71% yield on the converted material). The properties of this material were identical with those reported in ref. 16. 5.3. Measurements The quantum yield of the photoreactions was determined at 313 nm by irradiating dilute solutions contained in spectrophotometric 1 cm or 0.1 cm

225

cells with a super-high-pressure mercury lamp (Osram 200 W/2, Schott PIL interference filter). The samples were degassed by repeated freeze-pumpthaw cycles before irradiation. Fluorescence spectra were measured on an Aminco-Bowman spectrophotofluorometer using similarly degassed samples, Linear Stem-Volmer plots were obtained for fluorescence quenching in each case. The phosphorescence spectrum of 2-NN was determined at 77 K in EPA glass. Triplet lifetimes were measured by conventional flash photolysis using an Applied Photophysics instrument. Singlet lifetimes were measured by Dr. F. Masetti, Perugia, by means of a single-photon-counting instrument.

Acknowledgments

This work was supported by Consiglio Nazionale delle Ricerche, Progetta Finalizzato Chimica Fine e Secondaria, Rome. We thank Drs. G. G. Aloisi and F. Masetti for experimental help and discussions.

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