ISSN 0012�5008, Doklady Chemistry, 2010, Vol. 435, Part 1, pp. 294–296. © Pleiades Publishing, Ltd., 2010.Original Russian Text © N.N. Vlasova, A.V. Vlasov, O.Yu. Grigor’eva, A.I. Albanov, M.G. Voronkov, 2010, published in Doklady Akademii Nauk, 2010, Vol. 435, No. 3,pp. 343–346.

294

The reaction of acylation of aromatic compoundswith acyl chlorides in the presence of AlCl3 was dis�covered back in the 19th century by C. Friedel andJ.M. Crafts and named after the discoverers [1]. In duecourse, the reaction became one of the most impor�tant organic reactions. Later, AlCl3 was replaced insome instances by other metal chlorides, for exampleFeCl3, ZrCl4, and CrCl4 [2]. The Friedel–Crafts reac�tion proceeds by the mechanism of electrophilic sub�stitution. The process is also described to proceedunder photochemical reaction conditions. In thiscase, carboxylic acids, for example MeCOOH andF3CCOOH, were used as acylating agents, as well asacyl chlorides [3, 4].

The possibility to acylate arenes with acyl iodideshas not been not studied until now. We have found forthe first time that the acylation of o� and m�xylenesshowing an enhanced π�electron density is the soledirection of their UV�induced reaction with acetyliodide MeCOI.

3�Acetyl�1,2�dimethylbenzene, bp 100–105°C(yield 61%), is the product of acylation of o�xylenewith MeCOI, Eq. (1).

(1)

For C10H12O anal. calcd. (%): C, 81.04; H, 8.16.Found (%): C, 80.36; H, 8.25.

IR (ν, cm⎯1): 1700 (CO), 1600, 1500 (ring C–C),1000, 850, 780 (ring C–H).

1H NMR (δ, ppm): 2.21 (s, 3H, CH3CO), 2.41 (s,6H, CH3), 7.07–7.46 (m, 1H, H�4,5,6).

Me

Me

Me

Me

COMe+ MeCOI hν

–HI

13C NMR (δ, ppm): 178.07 (C=O), 21.02(CH3CO), 19.99 (CH3Ar), 136.76 (C�3–CH3),125.39 (C�5 arom.), 126.13 (C�6 arom.), 129.17 (C�4arom.), 129.90 (C�2–CH3), 136.76 (C�1–COCH3).

The UV irradiation of a mixture of MeCOI withm�xylene resulted in 2�acetyl�1,3�dimethylbenzene,bp 115°C (yield 55%), Eq. (2).

(2)

For C10H12O anal. calcd. (%): C, 81.04; H, 8.16.Found (%): C, 80.05; H, 8.24.

IR (ν, cm⎯1): 1700 (CO), 1600, 1500 (ring C–C),1000, 850, 780 (ring C–H).

1H NMR (δ, ppm): 7.00, 7.01 (d, 1H, H�3,5), 7.17(t, 1H, H�4), 3J = 7.7 Hz).

13C NMR (δ, ppm): 178.05 (C=O), 21.40(CH3CO), 20.89 (CH3Ar), 126.20 (C�5 arom.),136.20 (C�6–CH3), 128.25 (C�3 arom.), 129.20 (C�4arom.), 129.99 (C�2–CH3), 136.20 (C�6–CH3),137.86 (C�1–COCH3).

It is noteworthy that the photolysis of acetyl iodidein p�xylene medium provides no products of its reac�tion with the arene. However, we failed to recoveracetyl iodide. Only elemental iodine was detected inthe reaction mixture. Under these conditions, MeCOIseems to undergo photochemical decomposition sim�ilarly to that described earlier [5], Eq. (3).

(3)

The assumption that diacetyl is the acetylatingagent in the studied processes was rejected. When amixture of MeCOCOMe with o�xylene was exposed toUV light for 43 h, both reagents were isolated withoutchange.

The photolysis of benzoyl iodide PhCOI in botho�xylene and p�dioxane medium, as distinct from

Me

Me

Me

COMe

Me

+ MeCOI hν

–HI

CH3COI CH3C·

O I•

,+

CH3C·

O C·

H3 CO.+

hν

hν

CHEMISTRY

Acyl Iodides in the Reaction of Photochemical Friedel–Crafts Acylation

N. N. Vlasova, A. V. Vlasov, O. Yu. Grigor’eva, A. I. Albanov, and Academician M. G. VoronkovReceived April 19, 2010

DOI: 10.1134/S0012500810110066

Favorskii Institute of Chemistry, Siberian Branch, Russian Academy of Sciences, ul. Favorskogo 1, Irkutsk, 664033 Russia

DOKLADY CHEMISTRY Vol. 435 Part 1 2010

ACYL IODIDES IN THE REACTION 295

acetyl iodide, led only to the corresponding α�dike�tone PhCOCOPh with mp 95°C (lit.: mp 95–96°C[6]).

The observed difference in the photochemicalbehavior of acetyl and benzoyl iodides seems to resultfrom steric factors and easier recombination of the

benzoyl radical PhC owing to its higher stability as

compared with the MeC radical.

The process of acylation of aromatic ring was foundto be a sole reaction direction upon photolysis of trim�ethylacetyl and valeroyl iodides RCOI (R =n�Bu, tert�Bu) in toluene medium. The photochemical reactionof toluene with valeroyl iodide and pivaloyl iodidebegins with acylation of the phenyl group. However,aryl alkyl ketones RCOC6H4Me formed at this stageproved to be only intermediates in the studied photo�chemical processes.

In particular, the photolysis of valeroyl iodiden�BuCOI in toluene leads to 1,1�bis(4�methylphe�

nyl)pentane, bp 165°C, 1.5445, as a main reactionproduct (decane�5,6�dione was detected in traceamounts), Eq. 4.

For C19H24 anal. calcd. (%): C, 90.47; H, 9.52.Found (%): C, 89.99; H, 9.96.

IR (ν, cm⎯1): ν(CO) is absent, 3020–2820 (ν(C–H)), 1600, 1500 (ring C–C), 720, 690 (δ(ring C–H)).

1H NMR (δ, ppm): 0.89 (t, 3H, CH3CH2, 3J =

6.91 Hz), 1.39 (m, 2H, CH3CH2), 1.47 (quin, 2H,CH3CH2CH2,

3J = 6.91 Hz), 2.15 (dt, 2H, CH2CH,3J = 7.55, 3J = 7.68 Hz), 2.42 (s, 6H, CH3), 3.96 (t,1H, CHCH2,

3J = 7.68 Hz), 7.20, 7.26 (d, 4H, Ar, 3J =7.94 Hz).

13C NMR (δ, ppm): 14.08 (CH3CH2), 21.04(CH3Ar), 22.82 (CH3CH2), 30.40 (CH3CH2CH2),35.65 (CH3CH2CH2CH2), 50.65 (CH), 127.76 (C�2,6arom.), 129.13 (C�3,5 arom.), 135.39 (C�1–CH),142.76 (C4–CH3).

The mechanism of formation of 1,1�bis(4�meth�ylphenyl)pentane includes the addition of toluene,which behaves as a hydrogen donor in photochemicalprocesses, to the carbonyl group of n�butyl 4�meth�ylphenyl ketone, a product of toluene acylation at thepara position of benzene ring, followed by reduction ofresulting 1,1�bis(4�methylphenyl)pentanol withevolved hydrogen iodide.

(4)

It is well known that ketones readily undergo pho�tochemical reduction in the presence of hydrogendonors, and this reaction provides a basis for the pre�parative synthesis of pinacols, for example, benzopi�nacol [7].

Photolysis of pivaloyl iodide tert�BuCOI in toluenemedium led to a mixture of 3� and 4�(tert�butyl)meth�ylbenzenes (A and B) in 1.5 : 1 ratio (total yield 45%),Eq. (5).

IR (ν, cm⎯1): 3960–2860 (ν(C–H)), 1600, 1480,1460 (ring C–C), 800, 760, 700 (δ(ring C–H)).

1H NMR (δ, ppm): 1.35 (s, 9H, p�Me3C), 1.353 (s,9H, m�Me3C), 2.35 (s, 3H, p�Me), 2.39 (s, 3H,

m�Me), 7.14, 7.32 (d, 2H, 3J = 8.32 Hz, Ar), 7.02, 7.23(m, 4H, Ar).

13C NMR (δ, ppm): 21.61 (Me, A), 31.38 (Me3C),34.25 (MeC), 125.09 (C�2), 128.70 (C�3), 134.73 (C�4), 148.11 (C�1), 20.76 (Me3, B), 31.35 (Me3C), 34.48(MeC), 122.23 (C�6), 126.02 (C�2), 126.11 (C�4),127.92 (C�5), 137.36 (C�3), 151.03 (C�1).

The mechanism of formation of isomers A and Bincludes α cleavage under the action of UV irradiation(the Norrish�type I cleavage [8, 9]) of isomeric 3� and4�methylphenyl tert�butyl ketones, products of photo�chemical acylation of toluene with pivaloyl iodide tert�BuCOI formed at the first stage of the process.

O•

O•

nD20

Me

C Bu�OH

Me

Me

HC Bu�

Me

Me

C

O

Bu�n

n�BuCOI

C6H5Me

–HI

hν, C6H5Me+ 2HI

hν

+ H2O + I2.

n�BuCOCOBu�n + I2n�BuCOI

n n

hν

n n n

n + I2

296

DOKLADY CHEMISTRY Vol. 435 Part 1 2010

VLASOVA et al.

(5)

The general mechanism of reactions (4) and (5) iscaused by the fact that, upon excitation of moleculesof initially formed ketones RC(O)Ar (R = n�Bu, tert�Bu; Ar = C6H4Me) by UV light, one electron of theunshared electron pair of oxygen transfers to thehigher antibonding orbital of the π electron system ofthe carbonyl group that acquires biradical character,Eq. (6).

. (6)

The biradical further reacts in two ways: ether viacleavage of C–C bond with neighboring carbonylgroup (α cleavage or the Norrish�type I cleavage) toform free radicals that then recombine as in the case ofR= tert�Bu, Eq. (7),

(7)

or by intermolecular abstraction of hydrogen from ahydrogen donor, such as toluene, in the case of R =n�Bu, equation (4).

It is noteworthy that the UV irradiation of solutionsof RCOI (R = n�Bu, tert�Bu) for 50 h in benzene orwithout it but in the presence of a fine powder of ele�mental copper or yttrium chloride, whose photocata�lytic activity was discovered previously [10], producesno corresponding diacyls (α�diketones) or any otherproducts of photochemical transformations of theseacyl iodides. The acyl iodides were recovered withoutchange. In contrast to this fact, acetyl and benzoyliodides (R = Me, Ph) form corresponding symmetri�cal α�diketones upon UV irradiation not only in tolu�ene but also in benzene medium [11, 12].

The complete absence of diacyls when valeroyl andpivaloyl iodides are exposed to UV irradiation in ben�zene or toluene medium allows us to suppose that theydo not dissociate under these conditions to form freeradicals, while the observed acylation of toluene pro�ceeds via the mechanism of electrophilic photochem�ical substitution, Eq. (8).

(8)

REFERENCES

1. Olah, G.A., Friedel–Crafts and Related Reaction, NewYork: Inerscience, 1964, vol. 3.

2. Heine, H.W., Cottle, D.L., and Wan Mater, H.L.,J. Am. Chem. Soc., 1946, vol. 68, p. 524.

3. Lodder, G. and Havinga, E., Tetrahedron, 1972, vol. 28,no. 22, pp. 5583–5588.

4. Bryce�Smith, D., Cox, G.B., and Gibbert, A., J. Chem.Soc. Chem. Commun., 1971, no. 16, p. 914.

5. Kroger, P. and Riley, S.J., Chem. Phys., 1977, vol. 67,no. 10, pp. 4483–4490.

6. Weigand�Hilgetag, Organisch�chemische Experimen�tierkunst, 3rd ed., Leipzig: J.A. Barth Verlag, 1964.

Translated under the title Metody eksperimenta v orga�nicheskoi khimii, Moscow: Khimiya, 1968.

7. Bachmann, W.E., Org. Synth. Coll., 1943, vol. 11, no. 1,pp. 71–74.

8. Turro, N.J. et al., Acc. Chem. Res., 1972, vol. 5, no. 3,pp. 92–101.

9. Swenton, J.S., J. Chem. Educ., 1969, vol. 46, no. 4,pp. 217–226.

10. Voronkov, M.G., Vlasova, N.N., Bol’shakova, S.A.,et al., Zh. Org. Khim., 1981, vol. 51.

11. Voronkov, M.G., Belousova, L.I., Vlasov, A.V., and Vla�sova, N.N., Zh. Org. Khim., 2008, vol. 44, no. 6, p. 936.

12. Voronkov, M.G., Vlasova, N.N., Vlasov, A.V., et al., Zh.Org. Khim., 2009, vol. 45, no. 11, pp. 1627–1631.

Me

C

O

Bu�

Me

Bu�

Me

Bu�tert�BuCOI + C6H5Me

Me

C

O

Bu�

hν

–HI+ hν

–2CO+

tret

A B

n

n

t

t

C OR

ArC O

R

ArC O

R

Ar––

– –•••

•hν

n → π

O

C

R Ar

O

C

R Ar

–•

•

..hν hν R–Ar + CO,

Me Me Me

RCOI

Me

CORhν(254 nm) * *RCOI

MeH

C R

O

I+

– –

–HI