Photoinduced Electron-transfer Reaction of Pentafluoroiodobenzene with Alkenes

Ping Cao, Zheng-Yu Long and Qing-Yun Chen*Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China, Tel. +8621 64163300; Fax +86 21 64166128 (chenqy@pub.sioc.ac.cn)Received: 5 December 1996 / Accepted: 15 January 1997 / Published: 29 January 1997Abstract : Irradiation of pentafluoroiodobenzene and alkenes gave the corresponding adducts. The presence ofsingle electron-transfer scavengers, (p-dinitrobenzene and t-Bu


Introduction
The investigation of pentafluoroiodobenzene, C 6 F 5 I (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 [C 6 F 5 Xe] + [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 F SO 3 C 6 F 5 ) and C 6 F 5 I (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.
The reaction temperature, ca.80°C, was a result of the irradiation, however, at this temperature without irradiation Scheme 2.

Entry
Olefin 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 Table1.
All the reactions proceeded without extra solvent; when using CH 3 CN or CH 3 OH 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 CH 2 =CHCN.When 1 reacted with arylallylethers (7) in CH 3 CN, 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 19 F 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 substitu-Table 1. Irradiation reaction of 1 with alkenes a .tion 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-Bu 2 NO 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 C 6 F 5 • either adds to the alkene giving the 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. 1 H 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 (10-40µ) was used for column chromatography.
adduct or attacks the benzene ring producing the corresponding o-and p-biphenyl mixture.