[29680-62-4] · C18H15BSe3 · Tris(phenylseleno)borane · (MW 479.02)
(an efficient carrier of benzeneselenolate anion equivalent in selenoacetalization reactions,2-5 in the conversion of O,O-acetals into mixed O,Se-acetals,3 in the opening of epoxides,6 or in the transformation of a,b-unsaturated ketones into 1,1- and/or 1,3-bis(seleno)-2-alkenes;7 a valuable source of phenylselenyl radicals capable of inducing hydroselenation of alkynes and cyclization of enynes;8 used for the deoxygenation of sulfoxides9 and of 1,2-disubstituted epoxides6)
Physical Data: white needles, mp 152-153 °C.1
Solubility: sol carbon disulfide, dichloromethane, chloroform, toluene.
Preparative Methods: in a modified procedure4 of the original,1 Boron Tribromide (14.32 g, 57.1 mM) and dry Carbon Disulfide (100 mL) are placed in a flask cooled in an ice bath under nitrogen. Benzeneselenol (26.95 g, 171.6 mM) in 50 mL carbon disulfide is added to the well-cooled solution from the addition funnel over a period of 4 h. The reaction mixture is warmed to room temperature and stirred overnight. The nitrogen flow is stopped, and the solvent is evaporated. The residual yellow solid is suspended in 100 mL of dry pentane, and the stirred mixture refluxed under a slight static pressure of nitrogen. After cooling to rt, the yellow supernatant is removed by syringe. This washing is repeated with two more 100 mL portions of pentane. The resulting white solid is dried under vacuum, weighed (20.48 g, 74%) and stored protected from moisture.
The analogs tris(p-tolylseleno)borane and tris(p-bromophenylseleno)borane have been similarly prepared.10 Alternatively, B(SePh)3 can be synthesized1 from BBr3 and PhSeNa. Analogous derivatives such as Al(SePh)3,11 Al(Se-1-naphthyl)3,11 Ti(SePh)4,11 Zr(SePh)2,11 Cr(SePh)3,11 Nb(SePh)3,11 Ta(SePh)3,11 W(SePh)4,11 Ni(SePh)2,11 and Co(SePh)2,11 as well as Me3SiSePh,12,13 Me3GeSePh,12b and Me3SnSePh,14 have been prepared.
Handling, Storage, and Precautions: stable to heat, air oxygen, and light, but sensitive to humidity which decomposes them into selenols and boric acid. Therefore they must be stored under nitrogen or argon, and all reactions conducted under anhydrous conditions. Use in a fume hood.
Phenylselenoacetals can be prepared either from carbonyl compounds2-5,15 or from the corresponding O,O-acetals3 by reaction with tris(phenylseleno)borane in the presence of catalytic amounts of proton acids (eqs 1 and 2).
When the amount of B(SePh)3 is reduced and the acid catalyst is omitted, the transacetalization reaction stops half-way, and gives mixed O,Se-acetals in excellent yield (eq 3).3
The opening of 1,2-disubstituted epoxides gives mainly b-hydroxy selenides after short reaction times (1-3 h), but gives 1,2-disubstituted alkenes after longer reaction times (24-54 h) (eq 4).6 Trisubstituted epoxides lead to allylic alcohols.6
Selenoacetalization of a,b-unsaturated carbonyl compounds7 is more complicated for two reasons: 1,4-addition of the phenylselenolate species can compete with 1,2-attack; the well-known 1,3-rearrangement of allyl selenides16,17 also affects the corresponding selenoacetals.18 Thus the nature and composition of the products strongly depends on the substitution pattern of the starting a,b-unsaturated carbonyl derivative7 (eqs 5 and 6).
Hydroselenation of terminal alkynes proceeds best when an electron-attracting group is present on the C&tbond;C triple bond, and when oxygen is not excluded from the medium; (Z)-vinyl selenides are formed with high regio- and stereoselectivity (eq 7).8
In spite of these features, a phenylselenyl radical-based mechanism is proposed mainly because the cyclization of enynes possessing a terminal C&tbond;C triple bond (eq 8) presents all the characteristics of such a mechanism.8 Tris(phenylseleno)borane is also an efficient reagent for the deoxygenation of sulfoxides into sulfides.8
All of these reactions19 can be effected with tris(methylseleno)borane,20 B(SeMe)3, to give products bearing the MeSe moiety.4,7,8,13 It is surprising that the usefulness of the aluminum analogs has not been explored, except for the dimethyl derivative Me2AlSeMe which proved to be an excellent reagent for the conversion of esters into selenolesters.21
It is worth noting the close analogy between selenoboranes and thioboranes, both in their syntheses22 and in their reactivity.22b,c,23
Facultés Universitaires Notre-Dame de la Paix, Namur, Belgium