Photoinduced Electron-transfer Reaction of Pentafluoroiodobenzene with Alkenes

Abstract

Irradiation of pentafluoroiodobenzene and alkenes gave the corresponding adducts. The presence of single electron-transfer scavengers, (p-dinitrobenzene and t-Bu₂NO) and the free radical inhibitor (hydroquinone) suppressed the reaction. A photoinduced electron-transfer mechanism is proposed.

Introduction

The investigation of pentafluoroiodobenzene, C₆F₅I (1), previously, has been concentrated mainly on its radical generation and reaction with aromatic compounds [1], besides the formation of pentafluorophenyl organometallic compounds [2]. The pentafluorophenyl radical was formed in situ by oxidation of pentafluorophenyl-hydrazine [3], by photochemical decomposition of 1 [2a] or pyrolysis of pentafluorobenzenesulphonyl chlorides [4]. The introduction of the pentafluorophenyl moiety to toluene with [C₆F₅Xe]⁺[AsF ₆]^- was also assumed to be through the pentafluorophenyl radical [5].

Very recently, we found that this radical can be photolytically generated from both pentafluorophenyl per-(poly)fluoroalkanesulphonates (R_FSO₃C₆F₅) and C₆F₅I (1) [2d, 6]. Using this method, the pentafluorophenyl group can be smoothly introduced into benzenes, anilines, pyrroles, indoles, imidazoles, aromatic ethers or phenols. Furthermore a photo-induced electron-transfer (PET) rather than a simple radical mechanism has been proposed. Surprisingly, to our knowledge, there is no report of the reaction of 1 with alkenes in spite of the fact that a palladium catalyzed addition of 1 with alkyne has already been described [7]. Herein, we present the results of irradiation of 1 with simple olefins as well as allyl ethers.

Results and Discussion

Irradiation of 1 with alkenes 2 (1:2=1:10) for 12h gave the 1:1 adducts in moderate yields in addition to a small amount of pentafluorobenzene (4) (Scheme 1).

The reaction temperature, ca. 80°C, was a result of the irradiation, however, at this temperature without irradiation no reaction took place, an indication that irradiation was essential.

Similarly, the reaction of 1 with excess ethyl allyl ether and allyl acetate gave the corresponding adducts in high yields (Scheme 2).

The reaction results are listed in

Table 1. Irradiation reaction of 1 with alkenes^a.

Entry	Olefina	Solvent	Conv.%b	Products	Yield%c	C6F5H%b
1	2a	-	60	3a	30	10
2	2b	-	70	3b	80	10
3	2c	-	70	3c	60	5
4d	2a	-	26	3a	-	10
5e	2a	-	30	3a	-	10
6f	2a	-	35	3a	-	10
7	5b	-	60	6b	60	trace
8	7a	CH3CN	50	8a	36	-
9	7b	CH3CN	79	9a	24	-
				8b	40
10	7c	CH3CN	46	9b	26	-
				8c	21
11	10	-	70	9c	42	8
				11	52
				12	6
12	5a	-	82	6a	70	-
13d	5a	-	46	6a	34	9
14e	5a	-	53	6a	40	10
15f	5a	-	56	6a	46	9
16g	5a	n-C6H14	82	6a	54	9
17g	5a	CH3CN	86	6a	40	25
18g	5a	CH3CN	100	6a	50	36

(a) 1: Alkenes=1:10, for 10-12h; (b) Determined by ¹⁹F NMR; (c) Isolated yields based on 1 consumed; (d) 20mol% p-DNB was added; (e) 20mol% t-Bu₂NO was added; (f) 20mol% HQ was added; (g) 1: 5a=1:3.

.

All the reactions proceeded without extra solvent; when using CH₃CN or CH₃OH as a solvent the yield of 4 was increased (see Table 1, Entry 16-18).

Under similar conditions 1 did not react with electrondeficient alkenes such as CH₂=CHCN. When 1 reacted with arylallylethers (7) in CH₃CN, both the pentafluorinated biphenyl products 8 and the 1:1 adducts 9 were obtained. (see, Entry 8-10, Table 1). Compound 4 was not detected by ¹⁹F NMR spectroscopy in these reactions. The total yield of 8 and 9 was remarkably dependent upon the ratio of 1 and 7 used. For example, when 1:7a was 1:3, the total yield of (8+9) reached 66%, whereas 1:1 only 37%. It was also found that the para substitute (X) in 7 has some influence on the yield of 8 and 9. For example, the reactant with an electron-donating group, e.g. 7b, seemed to give more favorable substitution in the benzene ring than addition to the alkene, whereas the reverse result for 7c was observed (Scheme 3).

Noteworthy, for 7a, only ortho- and para (8a) (1:2), for 7b and 7c merely ortho-(related to allyoxy group) 8b or 8c were isolated and none of the other regioisomers were detected.

Usually diallyether (DAE, 10) can trap efficiently the poly or perfluoroalkyl radicals resulting in the formation of tetrahydrofuran derivatives (Clock reaction) [8]. However, in this case, besides the major product, 11, a small amount of 1:1 adduct (12) was also obtained (Scheme 4).

In order to elucidate the reaction mechanism, inhibition studies were carried out. For example, the presence of single electron transfer (SET) scavengers, p-dinitrobenzene (p-DNB) and t-Bu₂NO or a free radical inhibitor hydroquinone (HQ), in the reaction system significantly suppressed the reaction of 1 and 2a or 5a (see Entry 4-6, 13-15).

All the results, in addition to our previous work [2d, 6], seem to show that the mechanism may involve a photoinduced electron-transfer cation diradical coupling process, described using 7a as an example, as shown in Scheme 5.

The radical cation of 7a and pentafluorophenyl radical are generated through an electron-transfer under UV irradiation. The resulting C₆F₅• either adds to the alkene giving the adduct or attacks the benzene ring producing the corresponding o- and p-biphenyl mixture.

It is interesting to note that both cyclized (11) and uncyclized (12) products were obtained in the reaction of 1 and 10, indicating that, as very recently reported by Ashby et al, the uncyclized, like cyclized, product, can still be formed via SET pathway [9]. On the other hand, for the formation of the cyclized product 11, the cation diradical coupling may proceed either stepwise or by first transfering DAE+. to the distonic radical cation [10] (Scheme 6).

Experimental

M.P.s are uncorrected. IR spectra were obtained on a Schimadzu-440 instrument in potassium bromide pellets for all solid samples and in films for all liquid samples. ¹H NMR spectra were recorded on a Jeol FX-90Q instrument using tetramethylsilane as an internal standard. ¹⁹F NMR spectra were recorded on a Varian EM-360 instrument at 56.4 MHz using trifluoroacetic acid as an external standard with chemical shifts in ppm positive upfield. Mass spectra was obtained on a Finnigan-4021 instrument. Silica gel (10-40m) was used for column chromatography.

Typical procedure of the reaction of pentafluoroiodoben-zene (1) with alkene (2) and (5)

Under a N₂ atmosphere, a stirred mixture of 1 (2.94g, 10mmol) and 1-hexene (8.4g, 100mmol) in a quartz flask, connected to a condenser, was exposed to a high pressure mercury lamp (300W) at a distance of 8cm for 12h. ¹⁹F NMR, showed that 10% of pentafluorobenzene (4) was formed. The mixture was concentrated under reduced pressure and was subjected to column chromatography, using light petroleumether as eluent to give the product 3a (0.68g, 30%).

2-Iodo-1-pentafluorophenyl hexane (3a)

νmax/cm^-1: 2890, 1530, 1510, 1130, 950-1020; δ_H(CDCl₃): 4.20 (m, 1H), 3.30 (d, 2H, J=6), 1.63 (m, 2H), 1.33 (m, 4H) and 0.88 (s, 3H)ppm; δ_F(CDCl₃): 64.3 (m, 2F), 78.9 (t, 1F), 84.3 (m, 2F) ppm; m/z: 251 (M⁺-I, 31%), 181(C₆F₅CH₂⁺, 100%), 127 (2.7%), 57 (30%); Found: C, 38.18; H, 3.36; F, 25.25%; Calc. for C₁₂H₁₂F₅I: C, 38.10; H, 3.17; F, 25.13%.

2-Iodo-1-pentafluorophenyl octane (3b)

νmax/cm^-1: 2890, 1530, 1510, 1120; δ_H(CDCl₃): 4.27 (m, 1H), 3.33 (d, 2H, J=6), 0.90-2.0 (m, 13H) ppm; δ_F(CDCl₃): 66.0 (m, 2F), 80.0 (t, 1F), 86.2 (m, 2F) ppm; m/z: 279 (M⁺-I, 12.4%), 278 (M⁺-HI, 46.1%), 194 (M⁺-I-C₆H₁₃, 56.8%), 181 (C₆F₅CH₂⁺, 100%), 167 (5.4%); Found: C, 41.38; H, 3.94; F, 23.40%; Calc. for C₁₂H₁₂F₅I: C, 41.39; H, 3.97; F, 23.38%.

2-Iodo-1-pentafluorophenyl heptane (3c)

νmax/cm^-1: 2890, 1530, 1500, 1120; δ_H(CDCl₃): 4.27 (m, 1H), 3.33 (d, 2H, J=6), 0.90-2.20 (m, 11H) ppm; δ_F(CDCl₃): 65.0 (m, 2F), 79.0 (t, 1F), 85.2 (m, 2F) ppm; m/z: 265 (M⁺-I, 2.4%), 264 (M⁺-HI, 16.1%), 194 (M⁺-I-C₅H₁₁, 100%), 181 (C₆F₅CH₂⁺, 35.2%), 167 (7.4%); HRMS (for C₁₃F₅H₁₃, M⁺-HI), Calc: 264.0937, Found: 264.0922.

C₆F₅CH₂CH(I)CH₂OC₂H₅ (6a)

νmax/cm^-1: 2900, 1740, 1500, 1220; δ_H(CDCl₃): 3.10-4.24 (m, 7H), 1.80 (t, 3H)ppm; δ_F(CDCl₃): 65.7 (m, 2F), 79.5 (t, 1F), 85.3 (m, 2F) ppm; m/z: 353 (M⁺-I, 34.0%), 224 (M⁺-IC₂H₅, 29%), 181 (C₆F₅CH₂⁺, 100%), Found: C, 34.70; H, 2.56; F, 24.50%; Calc. for C₁₁H₁₀F₅IO: C, 34.74; H, 2.62; F, 25.00%.

C₆F₅CH₂CHICH₂OAc (6b)

νmax/cm^-1: 2900, 1740, 1500, 1220; δ_H(CDCl₃): 4.37 (m, 3H), 3.37 (m, 2H), 2.13 (s, 3H)ppm; δ_F(CDCl₃): 66.7 (m, 2F), 80.3 (t, 1F), 86.7 (m, 2F) ppm; m/z: 395 (M⁺+1, 0.77%), 335 (M⁺-AcO, 29%), 267 (M⁺-I, 100%), 207 (M⁺-AcO-HI, 67%), 181 (C₆F₅CH₂⁺, 49%), 43 (AcO⁺, 84%), Found: C, 33.56; H, 1.99; F, 24.52%; Calc. for C₁₁H₈F₅IO₂: C, 33.50; H, 2.03; F, 24.52%.

Typical procedure of the reaction of pentafluoroiodoben-zene (1) with aryl allyl ether 7

Under a N₂ atmosphere, the mixture of 1 (2.94g, 10mmol), 7a (10g, 0.1mol) and CH₃CN(20ml) was irradiated for 12h as above. After work-up, the excess of 7a was then distilled off in vacuum, the oily residue was subject to column chromatography using petroleum ether as eluent to give product 8a (0.6g, 36%) and 9a (0.4g, 24%).

8a (Formula see Scheme 7)

νmax/cm^-1: 1640, 1520, 1480, 1280, 1060, 860, 820; δ_H(CDCl₃): 7.0 (m, 4H), 5.9 (m, 1H), 5.25 (m, 2H), 4.5 (m, 2H) ppm; δ_F(CDCl₃): 63.7 (m, 2F, p-), 65.7 (m, 2F, o-), 78.7 (t, 1F), 85.0 (m, 2F) ppm; m/z: 300 (M⁺, 100%), 260 (M⁺-C₃H₄, 18%), 231 (M⁺-CO-C₃H₅, 46%), 205 (M⁺-C₃H₅-C₂H₂, 24%), 181 (C₆F₅CH₂⁺, 70%), HRMS (for C₁₅H₉F₅O): Calc: 300.2220, Found: 300.1599.

9a (Formula see Scheme 7)

νmax/cm^-1: 1580, 1500, 1230, 980, 750, 680; δ_H(CDCl₃): 7.10 (m, 5H), 4.42 (m, 1H), 4.20 (m, 2H), 3.40 (m, 2H) ppm; δ_F(CDCl₃): 66.3 (m, 2F), 78.7 (t, 1F), 85.0 (m, 2F) ppm; m/z: 427 (M⁺, 18.0%), 300 (M⁺-I, 100%), 181 (C₆F₅CH₂⁺, 70%), 94 (C₆H₅OH⁺, 25.0%), Found: C, 42.32; H, 2.39; Calc. for C₁₅H₁₀F₅IO: C, 42.08; H, 2.35.

8b (Formula see Scheme 7)

νmax/cm^-1: 1640, 1600, 1580, 1500, 1280, 1220, 1120, 980, 910, 800; δ_H(CDCl₃): 6.95 (m, 3H), 5.9 (m, 1H), 5.20 (m, 2H), 4.40 (d, 2H), 2.21 (s, 3H) ppm; δ_F(CDCl₃): 63.0 (m, 2F), 79.0 (t, 1F), 85.3 (m, 2F) ppm; m/z: 314 (M⁺, 100%), 299 (M⁺-CH₃, 36%), 273 (M⁺-C₃H₅, 12%), 245 (M⁺-COC₃H₅, 16%). HRMS (for C₁₆H₁₁F₅O): Calc: 314.2543, Found: 314.0768.

9b (Formula see Scheme 7)

M.p.: 71-73°C. νmax/cm^-1: 2860, 1520, 1500, 1370, 1220, 1120, 980, 960; δ_H(CDCl₃): 6.90 (AA’BB’, 4H), 4.4 (m, 1H), 4.20 (s, 2H), 3.40 (m, 2H), 2.23 (s, 3H) ppm; δ_F(CDCl₃): 65.0 (m, 2F), 78.0 (t, 1F), 85.3 (m, 2F) ppm; m/z: 442 (M⁺, 26%), 315 (M⁺-CH₃, 36%), 181 (C₆F₅CH₂⁺, 82%), 108 (p-MeC₆H₄OH⁺, 100%). Found: C, 43.32; H, 2.39; Calc. for C₁₆H₁₂F₅IO: C, 43.44; H, 2.71.

8c (Formula see Scheme 7)

νmax/cm^-1: 2960, 1640, 1590, 1580, 1520, 1480 1280, 1220, 1010, 980; δ_H(CDCl₃): 7.17-6.83 (m, 3H), 5.90 (m, 1H), 5.30 (m, 2H), 4.62 (s, 2H) ppm; δ_F(CDCl₃): 63.0 (m, 2F), 78.3 (t, 1F), 86.0 (m, 2F) ppm; m/z: 334 (M⁺, 100%), 293 (M⁺-C₃H₅, 32%), 265 (M⁺-CO-C₃H₅, 45%). HRMS (for C₁₅H₈ClF₅O): Calc: 334.6640, Found: 334.6216.

9c (Formula see Scheme 7)

M.p.: 59-60°C. νmax/cm^-1: 1600, 1540, 1510, 1498, 1240, 1000; δ_H(CDCl₃): 7.27 (AA’BB’, 4H), 4.40 (m, 2H), 4.25(d, 1H, J=4Hz), 3.43 (m, 2H) ppm; δ_F(CDCl₃): 65.0 (m, 2F), 78.3 (t, 1F), 84.7 (m, 2F) ppm; m/z: 462 (M⁺, 50%), 335 (M⁺-I, 25%), 181 (C₆F₅CH₂⁺, 100%), 128 (p-ClC₆H₄OH⁺, 25%), Found: C, 39.25; H, 1.81; F, 20.40%; Calc. for C₁₅H₉ClF₅IO: C, 38.96; H, 1.94; F, 20.56%.

11 (Formula see Scheme 7)

M.p.: 42-44°C. νmax/cm^-1: 1520, 1500, 1120, 980; δ_H(CDCl₃): 4.00 (d, 2H, J=6Hz), 3.70 (m, 4H), 3.26 (m, 2H), 2.80 (m, 2H) ppm; δ_F(CDCl₃): 66.0 (m, 2F), 79.7 (t, 1F), 85.3 (m, 2F) ppm; m/z: 392 (M⁺, 19%), 265 (M⁺-I, 60%), 235 (M⁺-I-CH₂O, 22%), 181 (C₆F₅CH₂⁺, 100%), Found: C, 37.51; H, 2.57; F, 24.45%; Calc. for C₁₂H₁₀F₅IO: C, 37.42; H, 2.55; F, 24.23%.

C₆F₅CH₂CHIOCH₂CH=CH₂ (12)

νmax/cm^-1: 2800, 1650,1500, 1110, 980; δ_H(CDCl₃): 5.80 (m, 1H), 5.23 (m, 2H), 4.32 (m, 1H), 4.00 (d, 2H, J=4Hz) ppm; δ_F(CDCl₃): 65.0 (m, 2F), 79.0 (t, 1F), 85.0 (m, 2F) ppm; m/z: 392 (M⁺, 0.68%), 335 (M⁺-C₃H₅O, 63%), 181 (C₆F₅CH₂⁺, 100%), 127 (I⁺, 4.5%), Found: C, 37.74; H, 2.55; F, 24.23%; Calc. for C₁₂H₁₀F₅IO: C, 37.42; H, 2.55; F, 24.23%.