[24964-91-8] · C18H12Br3Cl6NSb · Tris(4-bromophenyl)aminium Hexachloroantimonate · (MW 816.46)
Physical Data: mp 142 °C.2
Solubility: sol acetonitrile, dichloromethane.
Form Supplied in: dark blue crystalline solid.
Purification: one method is as follows: a solution of the title reagent (1) in dichloromethane (1 part) is poured into diethyl ether (3 parts) to precipitate the salt; the resulting blue needles are collected and dried under vacuum.2
Handling, Storage, and Precautions: is an irritant and is moisture sensitive.
p-Methoxybenzyl protected alcohols can be deprotected with (1) to give the alcohol in excellent yield in moist acetonitrile. Benzyl ethers are not affected by this reagent but can be removed with the analogous tris(2,4-dibromophenyl)aminium hexachloroantimonate.3,4 The conversion of thioacetals to the corresponding aldehyde or ketone is effected by (1) in acetonitrile containing aqueous sodium bicarbonate.5
Alkyl, aryl, or xanthate glycosides undergo glycosidation reactions with primary or secondary alcohols of O-glycosides to give b-O-linked disaccharides. This reaction is only observed when acetonitrile is used as solvent (eq 1).6
As a Diels-Alder catalyst, (1) has been shown to increase the rate of this reaction and give improved selectivities.7 For example, the uncatalyzed Diels-Alder reaction of cyclohexadiene with tetramethylbutadiene proceeds in low yield at 200 °C. With a catalytic amount of (1) present the reaction proceeds at -25 °C with improved selectivity (eq 2).
Also, (1) has been found to be a good catalyst for single electron acceptors and shown to provide a highly selective route for cycloadditions of neutral or electron-rich acceptors to conjugated dienes (eq 3).7,8
Under anhydrous conditions the cyclization of vinylbiphenyls gives the corresponding fluorene (eq 4), but if a trace of water is added the expected electrocyclization product is formed in excellent yield.9
These reactions are readily effected in acetonitrile to give cyclopentanes under mild conditions (eqs 5 and 6).10
Epoxides are oxidatively rearranged to ketones or esters in dichloromethane by what is thought to be a radical-type mechanism. For example, tetraphenylethylene oxide is rearranged to 2,2,2-triphenylacetophenone,11 and the adamantyl derived epoxide in eq 7 is rearranged in quantitative yields to either the corresponding ketone or ester, depending upon the reaction temperature.11
A similar reaction is observed with the epoxide of b-ionone, where a cyclohexane ring can be contracted to a cyclopentane (eq 8).11
b-Ionone also undergoes allylic oxidation to give the allylic alcohol with (1).12 Ketones are oxidized by (1) to the a-hydroxy ketone and, at higher temperatures, the a-diketone.13 When methanol is added to the solvent (acetonitrile), the a-methoxy ketone is formed.14
In conjunction with Selenium(IV) Oxide, (1) epoxidizes alkenes and enol ethers;15 in oxygen saturated solutions of dichloromethane, (1) can be used to make dioxetanes16 from alkenes or enol ethers. The oxidation of 2-nitrophenylsulfenamide protected amines or amino acids to the corresponding sulfenimine is readily brought about by (1) in the presence of a base such as 2,6-Lutidine (eq 9).17 Other methods for this conversion have included electrochemical oxidation.
A stable adduct of (1) is formed in the presence of 2,3-diazabicyclo[2.2.2]oct-2-ene.18 a,a-Dimesitylenol ethers undergo oxidative rearrangement to the monomesitylbenzofuran with (1),19 and vinylferrocene undergoes dimerization upon treatment with (1).20
Martyn J. Earle
The Ohio State University, Columbus, OH, USA