[63348-81-2] · C2H8BClS · Monochloroborane-Dimethyl Sulfide · (MW 110.43)
Alternate Name: MCBS.
Physical Data: d
Solubility: sol dichloromethane, pentane, diethyl ether.
Form Supplied in: colorless liquid.
Preparative Methods: by redistribution of Borane-Dimethyl Sulfide (BMS) and Boron Trichloride.SMe27 and by the reaction of BMS with CCl4.8 The reagent is in an equilibrium with approximately 15% each of BHCl2.SMe2 and BMS.2,8-10
Analysis of Reagent Purity: active hydride is determined by hydrolysis of an aliquot and measuring the hydrogen evolved according to the standard procedure.11 11B NMR (CCl4; d, ppm) -7.2 (t),9 -6.7 (t, JB-H = 131 Hz).7
Handling, Storage, and Precautions: corrosive liquid; flammable; stench; reacts violently with water. Handle and store under nitrogen or argon. Stable indefinitely when stored under nitrogen at 25 °C. Use in a fume hood.
Monochloroborane dimethyl disulfide is a more convenient reagent than Monochloroborane Diethyl Etherate. It is stable at room temperature, has higher concentration (9.5 M in BH2Cl), and is soluble in various solvents. MCBS hydroborates both terminal and internal alkenes in less than 2 h at 25 °C in dichloromethane, pentane, or diethyl ether.3a The regioselectivity of addition to alkenes is similar to monochloroborane diethyl etherate. Even relatively hindered alkenes produce dialkylchloroboranes, e.g. a-pinene and 2-ethylapopinene are transformed into B-chlorodiisopinocampheylborane (Dip-chloride)12a and B-chlorobis(2-ethylapoisopinocampheyl)borane (Eap2BCl), respectively. Both are highly enantioselective reducing agents for ketones (see (+)-B-Chlorodiisopinocampheylborane).12
With very hindered alkenes, the hydroboration stops at the monoalkylborane stage. Thus 2,3-dimethyl-2-butene gives cleanly Chloro(thexyl)borane-Dimethyl Sulfide (eq 1),2 a useful hydroborating and reducing agent;2,13-17 for example, it reduces carboxylic acids to aldehydes.17
Nopol methyl ether also reacts with MCBS in 1:1 molar ratio, the intramolecular complexation halting further reaction (eq 2).9
Dialkylchloroboranes are useful synthetic intermediates. They are readily methanolyzed, affording methyl dialkylborinates, which can be further transformed into ketones by the Dichloromethyl Methyl Ether (DCME) reaction (eq 3).3
The hydride reduction (hydridation) of dialkylchloroboranes provides access to a variety of dialkylboranes not available by direct hydroboration with borane itself.18 These dialkylboranes are convenient intermediates for the Zweifel synthesis of (E)-19 (eq 4) and (Z)-disubstituted20 alkenes and trisubstituted alkenes.21
Only one R group of R2BCl is utilized in these reactions and an excess of a terminal alkyne has to be used to minimize dihydroboration. These inconveniences can be circumvented by using alkenylborinate intermediates22 or alkyldibromoboranes (see Dibromoborane-Dimethyl Sulfide).
Hydroboration of 1,5-cyclooctadiene4 and 1,3,5,7-cyclooctatetraene23 with MCBS gives B-chloro-9-BBN and the 2,6-diboraadamantane skeleton, respectively. B-Chloro-9-BBN is useful for the preparation of 9-Borabicyclo[3.3.1]nonane derivatives not directly available by hydroboration with 9-BBN, e.g. allenyl-9-BBN, a reagent for the synthesis of homopropargylic alcohols.24
Cyclic hydroboration with MCBS has been used in the synthesis of a hydrozaulene skeleton with three contiguous chiral centers (eq 5).25 MCBS is more reactive than Thexylborane in this transformation.
Oxidation of dialkylchloroboranes derived from terminal alkenes with Pyridinium Chlorochromate and Chromium(VI) Oxide in acetic acid produces aldehydes and acids, respectively26,27 (eq 6).27 Similarly, ketones can be obtained from internal alkenes.26
Aldol reactions of boron enolates are remarkably stereoselective.28 Consequently, generation of stereodefined boron enolates is of primary synthetic importance. Dialkylchloroboranes obtained by hydroboration of alkenes with MCBS and also by other methods are convenient reagents for enolboration of ketones and other carbonyl derivatives;29 for example, dicyclohexylchloroborane gives the (E)-enolate predominantly for diethyl ketone and exclusively for propiophenone (eq 7).30
Computer modeling applied to design boron enolates effective for asymmetric anti-aldol reactions has led to the preparation of [(Menth)CH2]2BCl (eq 8).31
Highly selective formation of (E)-enolates is achieved by treating ketones with (1). Addition to aldehydes gives anti-aldols with diastereoselectivity of 86:14 to 100:0 (anti:syn) in 56-88% ee.31
MCBS chemoselectively cleaves acetals of cyclic ketones, even in the presence of double bonds. Unsaturated aliphatic acetals undergo preferential hydroboration of the double bond5 (for opposite chemoselectivity, see Monochloroborane Diethyl Etherate). Epoxides are stereoselectively cleaved to anti-chlorohydrins6,32 (eq 9).6
Certain dialkylhaloboranes are also effective reagents for the epoxide cleavage, e.g. Dip-chloride cleaves meso-epoxides with high enantioselectivity33 (see also Chlorodiethylborane).
MCBS has been used for the synthesis of metallacarboranes,34 boron-metal cluster compounds35 and boron-containing polymers.36
Nicolaus Copernicus University, Torun, Poland
Herbert C. Brown
Purdue University, West Lafayette, IN, USA