[36594-41-9] · C4H12BClO · Monochloroborane Diethyl Etherate · (MW 122.42)
(hydroborating agent providing access to dialkylchloroboranes,2 dialkenylchloroboranes,2 and B-chloro bora heterocycles,3 all useful intermediates for the synthesis of alkenes, dienes, ketones, esters, secondary amines, hydroperoxides, and aminoboranes)
Physical Data: mp -105 to -85 °C.4
Solubility: sol diethyl ether.
Preparative Methods: conveniently prepared by the reaction of Lithium Borohydride with Boron Trichloride in diethyl ether.2 The reaction can be monitored by 11B NMR analysis. A 1.0-1.5 M solution of monochloroborane is obtained, existing in an equilibrium with a small amount of dichloroborane and borane (eq 1).1a The solution cannot be concentrated to obtain a neat product, since it loses diborane when excess ether is removed.5
Analysis of Reagent Purity: concentration of the reagent is determined by the analysis for hydride, chloride, and boron.2,6 If the hydride and chloride are not in the proper ratio of 2:1, a calculated quantity of the solution in deficiency is added. In the 11B NMR spectrum, the resonances for dichloroborane and borane should be of equal intensity; d (ppm) 5.0 (t, JB-H = 131 Hz).2a
Handling, Storage, and Precautions: corrosive, flammable liquid; reacts violently with water. Monochloroborane solutions in diethyl ether are stable for several weeks when kept at 0 °C, protected from moisture and air. Handle and store under nitrogen or argon. Use in a fume hood.
Monochloroborane diethyl etherate reacts readily at 0 °C with alkenes to give dialkylchloroboranes in quantitative yields (eq 2).2 Regioselectivity of the hydroborations is greater than with Borane-Tetrahydrofuran (Figure 1).
The hydroboration of alkenes with monochloroborane diethyl etherate can be controlled to stop at the monoalkylchloroborane stage by the addition of 1-2 equiv. of THF. Monoalkylchloroboranes can be isolated and characterized as complexes with Trimethylamine. However, in diethyl ether solution, monoalkylchloroboranes readily disproportionate.7 The stepwise hydroboration of two different alkenes with monochloroborane diethyl etherate in the presence of THF or Dimethyl Sulfide does not provide the mixed dialkylchloroboranes cleanly.1a Other procedures leading to these products and totally mixed trialkylboranes have been developed.7-11
Dialkylchloroboranes are useful synthetic intermediates. Since they are formed in excellent purity, they can often be used without isolation. Thus dialkylchloroboranes undergo facile reaction with Ethyl Diazoacetate at -78 °C to give the corresponding ethyl alkylacetates in nearly quantitative yield (eq 3).12 Bulky alkyl groups are readily accommodated. Consequently, the reaction provides a general two-carbon homologation under remarkably mild conditions.
Treatment of dialkylchloroboranes with alcohols provides esters of dialkylborinic acids. These compounds react with Dichloromethyl Methyl Ether in the presence of lithium triethylcarboxide (DCME reaction) to give a-chloroborinic esters (eq 4). The esters are converted into alkenes by mild thermal treatment,13,14 while oxidation provides ketones (eq 5).14
Unsymmetrical ketones can be synthesized by the reaction of dialkylchloroboranes with lithium aldimines (eq 6).15
Dialkylchloroboranes react with organic azides to give secondary amines in 50-60% yields (eq 7).16
All reactions described above proceed with retention of configuration of the alkyl group.
B-Chlorodiisopinocampheylborane (Dip-chloride), an asymmetric dialkylchloroborane, can be prepared by hydroboration of a-pinene with BH2Cl.OEt217 (see also Monochloroborane-Dimethyl Sulfide). It is a highly enantioselective reducing agent for ketones,18 and is also used as a chiral catalyst in the asymmetric Diels-Alder reaction (see (+)-B-Chlorodiisopinocampheylborane).19 Other chiral dialkylboranes have been used in the asymmetric aldol reaction.20 Alkyl hydroperoxides can be prepared by oxidation of dialkylchloroboranes with molecular oxygen under controlled conditions.21 The reaction of BH2Cl.OEt2 with silazenes has been used for the synthesis of silylated aminoboranes.22
a,o-Dienes react with monochloroborane to give a mixture of monomeric and polymeric products. Heating the mixture under vacuum provides B-chloroboracycloalkanes in good yields (eq 8).3
Pure B-chloroborinane, B-chloro-3,5-dimethylborinane, and B-chloroborepane are readily isolated. They find application in the synthesis of pheromones.23-26 B-Chloroborocane, an eight-membered ring bora heterocycle, can also be prepared. However, higher medium-ring bora heterocycles are not available by this method. They can be prepared by a stepwise ring enlargement.27
Limonene is transformed into B-chlorolimonylborane which, upon reduction, gives limonylborane, an asymmetric hydroborating agent.28 The monohydroboration of limonene and other cycloalkadienes gives unsaturated alcohols in 30-80% yields.29
The monohydroboration of alkynes with monochloroborane diethyl etherate proceeds cleanly at 0 °C to give dialkenylchloroboranes (eq 9).2
Internal alkynes react in a stoichiometric ratio, whereas 30-40% excess of a terminal alkyne must be used to avoid undesired dihydroboration. Dialkenylchloroboranes undergo the usual reactions of vinylboranes, e.g. protonolysis with Acetic Acid to alkenes and oxidation with alkaline Hydrogen Peroxide to carbonyl compounds. The most useful reactions of dialkenylchloroboranes is their conversion to stereochemically pure (E,Z)-1,3-dienes by the Zweifel reaction,2 and to symmetrical (E,E)-1,3-dienes,30 1,4-dienes, and alkenes via vinylcopper intermediates31 (eq 10).2,30
Dialkenylchloroboranes have been used for the preparation of dialkenylaminoboranes.32
Nicolaus Copernicus University, Torun, Poland
Herbert C. Brown
Purdue University, West Lafayette, IN, USA