[873-51-8] · C6H5BCl2 · Phenylboron Dichloride · (MW 158.82)
(Lewis acidic reagent for ether cleavage and selective condensations; phenylating agent)
Physical Data: mp 7 °C; bp 175 °C; d 1.194 g cm-3.
Solubility: dec by water, alcohols, or ethers; sol saturated and halogenated hydrocarbons, aromatic solvents.
Form Supplied in: fuming liquid; widely available.
Preparative Methods: by transmetalation between Boron Trichloride and Ph4Sn2 or Phenylmercury(II) Chloride.3 Treatment of phenylboronates with boron trichloride in the presence of catalytic Iron(III) Chloride provides an alternative and efficient process.4 For large scale preparation, a direct boronation of benzene with boron trichloride and Aluminum Chloride has been reported.5
Handling, Storage, and Precautions: stable under N2; should be stored in a sealed ampule since it is highly susceptible to air and moisture.
Phenylboron dichloride has been used in a manner similar to boron trichloride for the cleavage of ethers, but there are no data on their comparative merits.6 Boron enolates, available by treating ketones with PhBCl2 and Diisopropylethylamine at -78 °C, undergo highly syn selective crossed aldol reaction with aldehydes (eq 1).7 The formation of the boron enolates is kinetically controlled and produces (Z)-enolates stereoselectively. Ketones condense with excess ketene in the presence of 2 equiv of PhBCl2 to give stable cyclic boroles and a,b-unsaturated acid chlorides (eq 2).8
The ortho-hydroxyalkylation of anilines is achieved by in situ formation of anilinoboranes which condense with aromatic aldehydes at room temperature (eq 3).9 The boron atom acts as an efficient template to achieve selective ortho substitution via the intermediate (1) through a cyclic transition state. The ortho-hydroxyalkylation of N-alkylanilines and aldehydes may also be performed with BCl3, but the reaction fails with primary anilines because of competing Schiff base formation.10
The reagent is useful for phenylation of azides,11 e.g. in the stereoselective synthesis of N-phenylaziridine (eq 4).12 Similar reactions of phenylboron dichloride and several other arylboron dichlorides with Ethyl Diazoacetate at -25 °C provide quantitative yields of ethyl arylacetates (eq 5).13 The reactions proceed through initial coordination of the azide or diazo group to the boron atom, followed by transfer of the phenyl group from boron to carbon or nitrogen.
A variety of phenylboron derivatives have been synthesized via transmetalation with other organometallic compounds, e.g. the transmetalation with Sn provides borepin derivatives (eq 6).14
Hokkaido University, Sapporo, Japan