Phenylselenium Trichloride


[42572-42-9]  · C6H5Cl3Se  · Phenylselenium Trichloride  · (MW 261.43)

(for introduction of unsaturation into organic molecules by hydrolytic selenoxide elimination; chlorinating agent)

Alternate Name: benzeneselenenyl trichloride.

Physical Data: mp 133-134 °C (dec).

Solubility: insol ether, hexanes, carbon disulfide; sol DMSO, acetonitrile; slightly sol chloroform, methylene chloride.

Form Supplied in: pale yellow solid; not widely available.

Preparative Methods: conveniently prepared on a large scale by chlorination of Diphenyl Diselenide with Sulfuryl Chloride (eq 1).1

Handling, Storage, and Precautions: unstable under normal laboratory conditions. However, if the freshly prepared material is rapidly dried and kept in a freezer (-20 °C), it can be stored and used for several months without any visible decomposition. Organoselenium compounds are reputed to be toxic. Use in a fume hood.

Hydrolytic Selenoxide Elimination.

Phenylselenium trichloride readily introduces a PhSeCl2 group into the a-position of ketonic substrates with loss of HCl (eq 2).1,2 The resulting selenium(IV) dichlorides are crystalline and easy to purify. Upon mild hydrolysis (aq NaHCO3), the SeCl2 group is hydrolyzed to the corresponding selenoxide, which undergoes the usual thermal elimination reaction to give an enone (eq 2). Thus, phenylselenium trichloride is complementary to Benzeneseleninic Acid and Benzeneseleninyl Chloride for the introduction of unsaturation into organic molecules via direct selenation with selenium(IV) compounds.

For the preparation of certain enones, the hydrolytic selenoxide elimination reaction has definite advantages (easier operation and purification, higher overall yields) compared to methodology relying on introduction of divalent selenium3 (see Benzeneselenenyl Bromide, Benzeneselenenyl Chloride, and 2-Pyridineselenenyl Bromide) followed by oxidation-elimination. To some extent, depending on the substrate, phenylselenium trichloride acts as a chlorinating agent towards ketones.1 The benzeneselenenyl chloride formed in this process reacts readily with ketones to give an a-phenylselenenyl ketone (eq 3). For efficient utilization of the selenium reagent it is therefore desirable to use an excess (50-100%) of the ketone and to treat the reaction mixture with sulfuryl chloride to convert the a-phenylselenenyl ketone formed to the desired selenium(IV) dichloride (eq 3).

Unfortunately, the reaction of phenylselenium trichloride with unsymmetrical ketones usually shows a poor regioselectivity.1 Thus ethyl methyl ketone afforded a 57:43 mixture of methyl- and methylene-substituted products (1) and (2). For this reason, the hydrolytic variation of the selenoxide elimination reaction is best suited for the conversion of symmetrical ketones or aryl alkyl ketones to their corresponding enones (eq 4).

The chlorination of divalent organoselenium compounds, followed by hydrolysis to effect selenoxide elimination, has been used as an alternative to other methods of oxidation-elimination. In this way, 2-chloro-1-alkenes4 and allylic and vinylic acetates5 were successfully prepared.

Phenylselenium trichloride undergoes a stereospecific anti addition to alkenic compounds to give (b-chloroalkyl)phenylselenium dichlorides.6 The regiochemistry of addition is highly substrate dependent. Whereas styrene affords only the Markovnikov addition product, 1-butene produces an 81:19 mixture of anti-Markovnikov and Markovnikov products. The presence of an oxygen substituent (acyloxy or aryloxy group) in the allylic position of the alkene causes addition of PhSeCl3 to occur anti-Markovnikov (eq 5). When the addition products are submitted to the usual conditions for hydrolytic selenoxide elimination, allylic and/or vinylic chlorides are formed. Chloroselenation products derived from alkenes containing a directing allylic substituent show a high preference for elimination away from the oxygen substituent to afford an (E/Z) mixture of vinylic chlorides (eq 5). Other alkenes, including cyclic ones, afford mixtures of allylic and vinylic chlorides in varying proportions. Symmetrical alkenes without allylic hydrogens are converted to vinylic chlorides with retention of the alkene geometry.

Chlorinating Agent.

Selenium(IV) dichlorides (3), prepared from PhSeCl3 and methyl ketones, undergo a Pummerer-like reaction7 when treated with Pyridine in methylene chloride (eq 6).8 The resulting a-chloro-a-phenylselenenyl ketones (4) can be chlorinated on selenium and the Pummerer reaction repeated to afford a,a-dichloro-a-phenylselenenyl ketones. The chlorinating properties of phenylselenium trichloride have also been used in the preparation of mono-, di-, tri-, and tetrachlorinated porphyrins.9

1. Engman, L. JOC 1988, 53, 4031.
2. Engman, L. TL 1985, 26, 6385.
3. Reich, H. J.; Renga, J. M.; Reich, I. L. JACS 1975, 97, 5434 and refs cited therein.
4. Engman, L. TL 1987, 28, 1463.
5. Engman, L. JOC 1989, 54, 884.
6. Engman, L. JOC 1987, 52, 4086.
7. Nakayama, J.; Sugihara, Y. CL 1988, 1317.
8. Engman, L.; Persson, J.; Tilstam, U. TL 1989, 30, 2665.
9. Ali, H.; van Lier, J. E. TL 1991, 32, 5015.

Lars Engman

Uppsala University, Sweden

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.