Indian Journal of Chemistry Vol. 40B, June 2001 , pp. 470-474

anti-Markovnikov acetoxyphenylselenation of terminal alkenes

R D Vukievi6* & M Radovi6

Department of Chemistry, Faculty of Science, University of Kragujevac, Radoja Domanovica .12, P.O. BOX 60, 34000 Kragujevac, FR Yugoslavia E-mail: Vuk@knez.uis.kg.ac.yu

Received 18 January 2000; accepted (revised) 7 July 2000

Unexpected anti-Markovnikov acetoxyphenylselenation of terminal alkenes has been achieved by reaction of these . substrates with diphenyl diselenide and lead tetraacetate in benzene under thermal conditions (80cC). The corresponding products, l -acetoxy-2-phenylselenylhydrocarbons accompanied by their isomers, 2-acetoxy- l -phenylselenilhydrocarbons are obtained in moderate to good yields. Acetoxyphenylselenation with same reagents, but in acetic acid as the solvent, yields only expected Markovnikov products.

Although many selenium containing organic compounds have been known for a long time, the use of selenium reagents or intermediates in modem organic synthesis practically begins after the discovery (in the early 1 970' s) that selenoxides, derived by oxidation of compounds containing an areneselenyl group, eliminate to form 0Iefins l ,2. Mild reaction conditions under which this transformation takes place and high selectivity are very attractive for organic synthesis, so that a large number of publications, describing organoselenium olefinations, have appeared over the latter quarter of this century. Also, above mentioned compounds can be transformed by replacement of the areneselenyl group with the hydrogen atom (i.e. by reductionf Of particular interest is the fact that organoselenium compounds to be transformed either by oxidative or reductive processes can contain some other functional groups that do not undergo these reactions. Consequently, it has induced developing of methods for the preparation of compounds containing a C­SeAr fragment in their structure. One of such methods is addition of compounds of structure ArSeX (where X is halide) to simple or funcionalized alkenes, starting by electrophilic attack of an ArSeEf) cation, and giving a-areneselenyl-J3-halogenated derivatives. If the addition reaction proceeds in the presence of some other nucleophiles, many different compounds, like I3-hydroxy-, J3-acetoxy-, J3-alkoxyselenides etc., as well as cyclic ethers (tetrahydrofuran and tetrahydropyran derivatives) can be synthesized2.3 . All these reactions exhibit high regioselectivity, so that in most cases Markovnikov products are obtained. It also was shown that this addition can be directed by reaction conditions: thermodynamically less stable

anti-Markovnikov products can be synthesized only by applying very low reaction temperatures, whereas reactions performed at higher temperatures obey Markovnikov rule4•5 •

An alternative method for the preparation of 13-functionalized selenides, which is sometimes more suitable than addition of areneselenyl halides to alkenes, is the reaction of diaryl diselenides with alkenes in the presence of ammonium peroxydisulphate and nucleophiles6• It is also reported that electrophilic ArSeEf) cation can be produced by electrolysis of the corresponding diaryl diselenide in the presence of halide ions as the mediators7,8, as well as by photo-oxidative cleavage of the mentioned diselenides9•

In continuation of our permanent investigations of electrophilic reactions of simple and functionalized alkenes8• 1O, we now wish to report herein the results obtained by studying the reaction of diphenyl diselenide with terminal alkenes promoted by lead tetraacetate. Our experience gained by lead tetraacetate oxidation of organic compounds and by electrochemical generation of electrophilic selenium species in the presence of unsaturated compounds induced us to try this reaction. Knowing that lead tetraacetate reactions in benzene under thermal conditions proceed mainly by a free radical mechanism, we supposed that selenation of alkenes under these conditions can give results interesting for organic synthesis, at least from mechanistic reasons.

Results and Discussion Present investigations we started by heating of the

mixture of lead tetraacetate, diphenyl diselenide, the corresponding alkene (molar ratio 1 :0.5: 1 ) and dry

VUKICEVIC et al.: anti-MARKOVNIKOV ACETOXYPHENYLSELENA TION OF TERMINAL ALKENES 47 1

Pb{OAc14 i. 00Ac + 0pb{OAch

AcO 0 c� �R + 0pb{OAch

1 a·f

AcO lAC "Pb�

- 0/ R + \ / ....... C'" /Pb I I _ AcO' \. 0 t=o

�d 0./"-R

II

(PhSe12 A 0 OAc SePh c ,, / ..... I P�

AcO, /o,c_C� Pb -J

+ ACO� g . Pb(OAc12 .. � o� R 'c� �d

III �ePh IV

R = a) CH3(CH2h-: b) CH3(C�)4-: c) CH3(CHVg-: d) Br-(CH2)4-: e) C�=CH(C�h-: f) 0 Scheme I

benzene at 80°C. The reaction products were isolated by column chromatography (Si021 1 5% ethyl acetate in petrol ether) and identified by IR, IH and l3C NMR spectral data. Obtained results are l isted in Scheme I and Table I .

As it can be seen, products of the reaction are 1 -acetoxy-2-phenylselenyl- 2 or 2-acetoxy- l ­phenylselenylhydrocarbons 3, where the first one (the anti-Markovnikov product) predominates. Thus, 1 -hexene la, I -heptene lb, and I -dodecene (lc) were converted into the corresponding derivatives 2a, 2b, 2c (the anti-Markovnikov products) and 3a, 3b and 3c (the Markovnikov products) in overall yield of 54-69%. The ratio of two isomeric products was 2/3 = (75-80): (25-20). Unsaturated bromide 6-bromohex- l ­ene ld yielded by this reaction the corresponding isomers 2d and 3d in the similar overall yield, but the ratio of isomers was different (2d13d = 65 :35).

In the further investigations of this reaction we tested two dienes- l ,5-hexadine Ie and 4-vinylcyclohexene 1f as the substrates. The first one has two equivalent double bonds and under given conditions it gave the products of the acetoxyphenylselenation in which only one of them takes place. The overall yield and the isomer ratio were the same as in the case of alkene la. However, diene 1f, contains two different double bonds, and it can give four different acetoxyphenylselenated regioisomers. As it can be seen from Table I, the reaction proceeded by high regioselectivity: only the vinyl group takes part in the reaction, and only the

Table I-Acetoxyphenylselenation of olefins

Substrate Products Yield Ratio (%)a (2/3)b

la 2a 3a 54 80:20 Ib 2b 3b 68 75:25 Ie 2e 3e 69 75:25 Id 2d 3d 66 65:35 Ie 2e 3e 55 80:20 If 2

1" 3f 88 1 00:0

'Isolated yields, based on the (PhSe)2 consumed; "Determined by 'H NMR spectral data; cA l : I mixture of erythro- and threo­diastereoisomers.

anti-Markovnikov product 2f was formed, in the highest yield (88%). On the basis of IH and l3C NMR spectral data, this compound was formed as the 1 : 1 mixture of the erythro- and threo-diastereoisomers.

Some other cyclic olefins which cannot give isomeric acetoxyphenylselenated products are also tested as the substrates in this reaction. Namely, cyclohexene, cyclooctene and 1 ,5-cyclooctadiene gave l -acetoxy-2-phenylselenylcyclohexane, 1 -acetoxy-2-phenylselenylcyclooctane and 5-acetoxy-6-phenylselenylcyclooctene, respectively, in 67-80% yield.

It is not quite clear what is the mechanism of the reaction described here. As it is mentioned in the introduction section of this paper, j3-functionalized-a­phenylseleno compounds can be derived by electrophilic attack of the PhSeGl ion to the olefinic double bond, whatever was the source of this cation.

472 INDIAN J CHEM, SEC. B, JUNE 200 1

However, in that case the reaction must obey the Markovnikov rule, yielding the product with nucleophilic group bonded to the more substituted olefinic carbon atom, except in the case of very low reaction temperatures. We believe that the reaction starts with a thermal homolytic deacetoxylation of lead tetraacetate, g iving two radical species-acetoxy radical and lead(III) acetate. The first is very unstable and probably does not take place in the reaction, because of its fast decarboxylation. On the other hand, the lead(III) species are electrophilic and probable enough living to attack olefinic bond. Two products of this attack can be derived, namely 1 -triacetoxyleadalk-2-yl I and 2-triacetoxyleadalk- l -yl radical II. The first one is more stable, since it is a Markovnikov product, than the second one. Therefore, this radical must predominate.

The further transformation of radicals I and II proceeds probably by reaction with PhSeSePh, giving adducts III and IV, respectively, which undergo an internal nucleophilic substitution of lead moiety by an acetoxy group, resulting in the final products 2 and 3 and lead(II) acetate. This substitution proceeds probably by a concerted mechanism of an SNi type; an SNi mechanism is less possible, since the non-polar solvent, such as benzene, does not support formation of free ionic species (cf. Scheme I).

An alternative way for the transformation of radicals I and II can be the oxidation to the corresponding carbonium ions by means of either Pb(lll) or Pb(IV) species. These ions can react with diphenyl diselenide to give the same products 2 and 3. However, if carbonium ions are formed, they also must give considerable amounts of diactoxylated products I I by reaction with acetate from lead tetraacetate. Since we have not detected diacetoxylated derivatives in this reaction, it is to say that the reaction of radicals I and II with diphenyl diselenide proceeds much faster than oxidation of it.

Three of above mentioned alkenes Ib, lc, and If­were also acetoxyphenylselenated by the same reagents, but in acetic acid as the solvent, at 60°C. As we expected, a classical Markovnikov addition occurred, and only isomers with phenylselenyl group connected to the terminal carbon atom (3b , 3c, and 3f) were obtained, in a yield similar to the benzene reaction (up to 80%). The mechanism of this reaction is much more clear. Being a polar protic solvent, acetic acid supports oxidation of diphenyl diselenide to either PhSe@ ions or phenylselenyl acetate V. Both these species are able to add to the olefinic bond,

Pb(OAc)4 + (PhSeh - Pb(OAcn + 2 PhSeOAc V 8(1) liS (1) OAc Phse�AC PhSejR Phse�R �R

- VI + -

AcOS 3b,c,f 1b,c,f

Scheme II

giving (via carbocation V, and in agreement with the Markovnikov rule) compounds 3 (cf. Scheme II).

In conclusion, we have demonstrated that lead tetraacetate can be successfully used for unexpected anti-Markovnikov acetoxyphenylselenation of alkenes.

Experimental section All of the substrates used were commercially

available and were used as received. Lead tetraacetate was prepared by reaction of Pb304 with acetic acid and acetanhydride by known procedure l 2• Benzene was dried under the sodium wire, but other solvents were used as received. IR measurements were carried out with a Perkin-Elmer instrument (Model 1 37B). NMR spectra were obtained with a Varian Gemini (model 200) spectrometer (200 MHz) by using TMS as a standard and deuteriochloroform as the solvent.

General procedure for the acetoxyphenyl­selenation in benzene. To a solution of I mmole of the corresponding substrate la-f and diphenyl diselenide (0.5 mmole) in 1 0 mL of dry benzene, 1 1 mmole lead tetraacetate was added and the resulting mixture was stirred and refluxed until disappearance of tetravalent lead (monitored by KI-starch indicator). After cooling to room temperature, the reaction mixture was diluted with 1 5 mL of ether and decanted. Inorganic precipitate was washed with another 1 5 mL of ether, and organic layers were collected, washed with water, saturated sodium bicarbonate, and brine. After drying under anhydrous sodium sulfate, the solvents were disti lled off and the rest was purified by column chromatography (Si021 l5% ethyl acetate in petrol ether).

General procedure for the acetoxyphenyl­selenation in acetic acid. The same procedure was employed as in the benzene reaction, except that the reaction miXture was heated at 60°C. After completion of the reaction, the reaction mixture was diluted with 30 mL of water, neutralized with the 2M solution of sodium hydroxide, and extracted with three 20 mL portions of ether. The further work-up is same as in the case of benzene reaction.

VUKICEVIC et al. : anti-MARKOVNIKOV ACETOXYPHENYLSELENA TION OF TERMINAL ALKENES 473

Spectral data of new compounds l-Acetoxy-2-phenylselenylhexane 2a: IR (film):

3057, 2928, 1 743, 1 236 em- I ; IH NMR (200 MHz; CDCI3) : 8 0.9 1 (t, J = 6_5 Hz, 3H), 1 .25- 1 .50 (m, 6H), 2.0 (s, 3H), 3.25-3 .40 (m, I H), 4. 1 5-4.35 (m, 2H), 7.20-7.3 1 (m, 3H), 7.50-7.6 1 (m, 2H); I3C NMR (200 MHz; CDCI3) : 8 1 3 .9, 22.4, 29.7, 3 1 .6, 43.4, 67.5, 77.2, 1 27.7, 1 29.0, 1 32.8, 1 34.9, 1 70.5.

l-Acetoxy-2-phenylselenylheptane 2b: IR (film): 3057, 293 1 , 1 742, 1 237 em- I ; IH NMR (200 MHz; CDCI3) : 8 0.90 (t, J = 6.5 Hz, 3H), 1 .20- 1 .75 (m, 8H), 1 .99 (s, 3H), 3.25-3 .40 (m, IH), 4. 1 5 -4.32 (m, 2H), 7.25-7.3 1 (m, 3H), 7.5 1 -7.6 (m, 2H); 13C NMR (200 MHz; CDCI3) : 8 14.0, 20.8, 22.5, 27.2, 3 1 .5, 3 l .9, 43.4, 67.4, 1 27.7, 1 29.0, 1 29. 1 , 1 34.9, 1 70.8.

l-Acetoxy-2-phenylselenyldodecane 2c: lR (film): 3059, 2926, 1 743, 1 236 em- I ; IH NMR (200 MHz; CDCI3) : 8 0.88 (t, J = 6.5 Hz, 3H), 1 .22- l .80 (m, 1 8H), 1 .99 (s, 3H), 3 _25-3.40 (m, 1 H), 4. 1 0-4.42 (m, 2H), 7.23-7.3 1 (m, 3H), 7.50-7.60 (m, 2H); I3C NMR (200 MHz; CDCI3) : 8 14. 1 , 20.7, 22.6, 27.5, 28.9, 29. l , 29.3, 29.4, 29.5, 3 1 .9, 32.2, 43.4, 67.5, 1 28.0, 1 29.2, 1 30.0, 1 35 .3, 1 70.5.

l-Acetoxy-6-bromo-2-phenylselenylhexane 2d: IR (fi lm): 3057, 2938, 1 734, 1 237 em- I ; IH NMR (200 MHz; CDCl3) : 8 1 .40-2.05 (m, 6H), 2.01 (s, 3H), 3.20-2.46 (m, 3H), 4. 1 1 -4.34 (m, 2H), 7.23-7.33 (m, 3H), 7 .5 1 -7.6 1 (m, 2H); I3C NMR (200 MHz; CDCh): 8 1 3 .9, 20.8, 22.4, 29.7, 3 1 .6, 43.4, 67 .5, 125.4, 1 27 .9, 1 29. 1 , 1 35 . 1 , 1 70.7.

6-Acetoxy-5-phenylselenylhex-l-ene 2e: IR (film): 3058, 3020, 2957, 1 740, 1 236 em- I ; IH NMR (200 MHz; CDCI3) : 8 1 .40- 1 .80 (m, 2H), 2.00 (s, 3H), 2. 1 5-2.45 (m, 2H), 3 .25-3 .40 (m, I H), 4. 1 2-4.35 (m, 2H), 4.90-5. 1 2 (m, 2H), 5 .65-5 .90 (m, 11-1), 7.23-7 .33 (m, 3H), 7.5 1 -7.6 1 (m, 2H); 13C NMR (200 MHz; CDCI3) : 8 20.8, 30.9, 3 1 .6, 42.7, 67.4, 1 1 5 .5, 1 27.9, 1 29. 1 , 132.8, 1 35 . 1 , 1 37.4, 1 70.7.

erythro- and threo-2-Acetyl-l-cyclohex-3-enyl-l­phenylselenylethane 2f: IR (film): 3058, 3023, 292 1 , 174 1 , 1 235 em- I ; IH NMR (200 MHz; CDCI3): 8 1 .7-2.35 (m, 7H), 1 .98 and 2.00 (two singlets, 3H), 3.25-3.45 (m, I H), 4.28-4.49 (m, 2H), 5.68 (m, 2H), 7.25-7.32 (m, 3H), 7.55-7.62 (m, 2H); 13C NMR (200 MHz; CDCI3) : 8 20.8 (CH3-CO-), 25 .4, 25.5, 25.9, 27.7, 28.7, 30.0, 65.5, 65.6 (8 CH2 groups of both isomers), 35.3, 35 .5 (C-CH-C-), 50.8, 50.0 (-CH-Se-), 125.8, 1 26. 1 , 126.9, 1 27.0, 1 27.5, 1 29. 1 , 1 34.3, 1 34.4 (olefinic and aromatic CH groups).

2-Acetoxy-l-phenylselenylhexane 3a: IR (film): 3059, 2930, 1 740, 1 235 em- I ; IH NMR: (200 MHz; CDCh) 8 0.88 (t, J = 6.5 Hz, 3H), 1 .25- 1 .50 (m, 6H), 1 .95 (s, 3H), 3.07 (d, J = 6.0 Hz, 2H), 4.95-5 . 1 0 (m, I H) , 7.20-7.3 1 (m, 3H) , 7.50-7.6 1 (m, 2H); I3C NMR (200 MHz; CDCh): 8 1 4.0, 2 1 .3 , 28.5, 30.7, 38.5, 68. 1 , 73.5, 1 27.7, 1 29.0, 1 32.8, 1 34.9, 1 70.5.

2-Acetoxy-l-phenylselenylheptane 3b: IR (film): 3062, 2929, 1 739, 1 240 em- I ; IH NMR (200 MHz; CDCI3) : 8 0.9 1 (t, J = 6.5 Hz), 1 .20- 1 .75 (m, 8H), 1 .94 (s, 3H), 3 .07(d, J = 6.0 Hz, 2H), 4.4,95-5 . 10 (m, I H), 7.2-7.7 (m, 5H); I3C NMR (200 MHz; CDCl) : 8 1 3 .9, 20.9, 22.5, 24.9, 3 1 .6. 3 1 .9, 33.7, 73.4, 1 27.0, 1 28.3, 1 30.8, 1 32.8, 1 70.7.

2-Acetoxy-l-phenylselenyldodecane 3c: IR (film): 3058, 2930, 1 74 1 , 1 235 em- I ; IH NMR (200 MHz; CDCl) : 8 0.88 (t, J = 6.5 Hz, 3H), 1 .22- 1 .80 (m, 1 8H), 1 .94 (s, 3H), 3.07 (d, J = 6.0 Hz, 2H), 4.95-5 . 10 (m, I H), 7.23-7.3 1 (m, 3H); 7.50-7.60 (m, 2H); I3C NMR (200 MHz; CDCh): 8 14. 1 , 20.9, 25. 1 , 27.5, 28.6, 29. 1 , 29.3, 29.4, 29.5 , 3 1 .6, 32.0, 33.7, 73.4, 1 27. 1 , 1 29. 1 , 1 32.8, 1 36.8, 1 70.7.

2-Acetoxy-6-bromo-l-phenylselenylhexane 3d: IR (film): 3057, 2939, 1 736, 1 235 em- I ; 'H NMR (200 MHz; CDCl) : 8 1 .40-2.05 (m, 6H), 1 .95 (s, 3H), 3.07 (d, J = 6.0 Hz, 2H), 4.97-5 .09 (m, I H), 7.23-7 .33 (m, 3H); 7.5 1 -7.6 1 (m, 2H); I3C NMR (200 MHz; CDCl3) : 8 1 4.8, 20.9, 22.8, 29. 1 , 30.5, 38.7, 73.3, 1 27. 1 , 1 29.8, 1 32.8, 1 33.8, 1 70.5.

5-Acetoxy-6-phenylselenylhex-l-ene 3e: IR (fi lm) : 3058, 3023, 2955, 1 739, 1 238 em- I ; IH NMR (200 MHz; CDCl) : 8 1 .40- 1 .80 (m, 2H), 1 .94 (s, 3H), 2. 1 5-2.45 (m, 2H), 3.08 (d, J = 6.0 Hz, 2H), 4.90-5 . 12 (m, 3H), 5 .65-5 .90 (m, I H), 7.23-7.33 (m, 3H); 7.5 1 -7.6 1 (m, 2H); l3C NMR (200 MHz; CDCl) : 8 20.9, 29.5, 3 1 .6, 32.8, n.8, 1 1 5 .4, 1 27. 1 , 1 29. 1 , 1 32.8, 1 35 . 1 , 1 37.4, 1 70.4.

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