Dichloroborane Diethyl Etherate1


[41156-66-5]  · C4H11BCl2O  · Dichloroborane Diethyl Etherate  · (MW 156.85)

(hydroborating agent providing access to alkyldichloroboranes and alkenyldichloroboranes;2 reagent for deoxygenation of sulfoxides3 and cleavage of cyclic acetals4)

Physical Data: bp 59 °C/1.5 mmHg;5 mp -30 to -25 °C.6,7

Solubility: sol benzene, carbon tetrachloride, diethyl ether; not miscible with pentane.

Analysis of Reagent Purity: the hydride content is determined by hydrolyzing an aliquot and measuring the hydrogen evolved according to the standard procedure.8 The chloride is estimated by hydrolyzing an aliquot and titrating the HCl produced with NaHCO3 using methyl orange as indicator. Coulometric titration has also been used for the determination of BHCl2.OEt2.9a 11B NMR: d, ppm 8.1;9b 7.9 (JB-H = 152 Hz).9c

Preparative Methods: an approximately 1.5 M solution in diethyl ether can be prepared by the reaction of Lithium Borohydride with Boron Trichloride.2 The neat reagent can be isolated by removing excess diethyl ether. It is a colorless liquid, 6.6 M in BHCl2, containing about 4-6 mol % of BCl3.OEt2 as impurity. Less conveniently, BHCl2.OEt2 can also be prepared by reacting BCl3 with Diborane in the presence of diethyl ether.6,7 The deuterated reagent is known.7

Handling, Storage, and Precautions: corrosive, flammable, air- and moisture-sensitive liquid. Reacts violently with water. The reagent is not stable over long periods of time, cleaving the ether solvent at a significant rate, even with storage at 0 °C. The neat reagent is not stable for more than 2 days at rt. Handle in a fume hood.

Hydroboration of Alkenes and Dienes.

The reaction of alkenes with dichloroborane diethyl etherate is slow and accompanied by disproportionation. In contrast, dichloroborane reacts readily with alkenes in the gas phase.10 Apparently, strong complexation with ether significantly decreases the reactivity. This difficulty can be circumvented by the addition of boron trichloride, a strong Lewis acid, liberating dichloroborane from the complex with instantaneous precipitation of BCl3.OEt2 (eq 1).2 (For direct hydroboration of alkenes, see Dibromoborane-Dimethyl Sulfide).

The pentane solution contains essentially pure alkyldichloroborane applicable directly for further transformations. Alternatively, the product can be isolated by distillation. The directive effects of BHCl2.OEt2 are comparable to Monochloroborane Diethyl Etherate, e.g. 1-hexene and styrene react, placing the boron atom at the terminal position with 99.3% and 96.3% selectivity, respectively. Alkyldichloroboranes can also be prepared by the redistribution reaction.11,12

Ethyl Diazoacetate reacts readily with aryl- and alkyldichloroboranes to give the corresponding two-carbon homologated ethyl esters. Essentially quantitative yields are obtained in the reactions of aryldichloroboranes (eq 2).13

The homologations of alkyldichloroboranes proceed in the range of 57-71% yield, so in these cases this procedure has little advantage over that involving the more conveniently synthesized dialkylchloroboranes (see Monochloroborane Diethyl Etherate). A remarkable enhancement of reactivity relative to trialkylboranes is observed in the reaction of alkyldichloroboranes with organic azides. Secondary amines are formed in 84-100% yields (eq 3).14,15

An extension of this reaction to 2-iodoalkyl azides provides b-iodo derivatives of secondary amines which undergo ring closure upon treatment with base, producing the corresponding N-alkylaziridines in 73-94% yields (eq 4).15,16

The procedure is not applicable to 1-azido-2-iodophenylpropane, which undergoes elimination of b-methylstyrene under the reaction conditions.15 In all of the reactions described above the migrating group retains its configuration. Optically active amines can be prepared from optically active alkyldichloroboranes.17 Another useful transformation of alkyldichloroboranes is the controlled oxidation with oxygen, providing hydroperoxides (eq 5).18,19 The reaction is a radical process and the configuration of the alkyl group is not completely retained.

Dihydroboration of allene with dichloroborane diethyl etherate gives a 1,3-dibora product which can be transformed into the 1,2-diborole derivative.20

Hydroboration of Alkynes.

The monohydroboration of alkynes with dichloroborane diethyl etherate in the presence of an equimolar amount of boron trichloride gives alkenyldichloroboranes (eq 6),2 which can be transformed into alkenylboronates, alkenes, or ketones by alcoholysis, protonolysis, and oxidation, respectively.

a-Silyl ketones are obtained from 1-trimethylsilyl-1-alkynes.21 A 10-40% excess of the silylalkyne is necessary to minimize dihydroboration. The dihydroboration of alkynes with dichloroborane diethyl etherate has been studied only briefly.2,22 1-Alkynes are hydroborated to 1,1-diboraalkanes (eq 7).22

Deoxygenation of Sulfoxides and Cleavage of Cyclic Acetals.

Aliphatic sulfoxides are rapidly deoxygenated to the corresponding sulfides in excellent yields by dichloroborane in THF at 0 °C.3 Other reducible functionalities, such as ketones, esters, and amides, are not affected. Since dichloroborane in THF is a very poor hydroborating agent,23,24 unsaturated sulfoxides might be expected to undergo selective reduction. Cyclic acetals are cleaved under mild conditions4 (see also Monochloroborane Diethyl Etherate and Monochloroborane-Dimethyl Sulfide).

Related Reagents.

Dibromoborane-Dimethyl Sulfide; Dichloroborane-Dimethyl Sulfide; Monochloroborane Diethyl Etherate; Monochloroborane-Dimethyl Sulfide.

1. (a) Brown, H. C.; Zaidlewicz, M. Polish J. Appl. Chem. 1982, 26, 155. (b) Brown, H. C.; Kulkarni, S. U. JOM 1982, 239, 23. (c) Pelter, A.; Smith, K. COS, 1991, 8, 703.
2. Brown, H. C.; Ravindran, N. JACS 1976, 98, 1798.
3. Brown, H. C.; Ravindran, N. S 1973, 42.
4. Bonner, T. G.; Lewis, D.; Rutter, K. JCS(P1) 1981, 1807.
5. Zhigach, A. F.; Sobolev, E. S.; Svistyn, R. A.; Nikitin, V. S. ZOB 1973, 43, 1966.
6. Brown, H. C.; Tierney, P. A. J. Inorg. Nucl. Chem. 1959, 9, 51.
7. Onak, T.; Landesman, H.; Shapiro, I. J. Phys. Chem. 1958, 62, 1605.
8. Brown, H. C. Organic Syntheses via Boranes, Wiley: New York, 1975; p 239.
9. (a) Kulehova, O. D.; Gorbunov, A. Zh. Anal. Khim. 1983, 38, 2031. (b) Brown, H. C.; Sikorski, J. A. OM 1982, 1, 28. (c) Onak, T. P.; Landesman, H.; Williams, R. E.; Shapiro, I. J. Phys. Chem. 1959, 63, 1533.
10. Lynds, L.; Stern, D. R. JACS 1959, 81, 5006.
11. Brown, H. C.; Levy, A. B. JOM 1972, 44, 233.
12. (a) Köster, R.; Grassberger, M. A. LA 1968, 719, 169. (b) Zakharkin, L. I.; Kovredov, A. I. IZV 1962, 12, 2247.
13. Hooz, J.; Bridson, J. N.; Calzada, J. G.; Brown, H. C.; Midland, M. M.; Levy, A. B. JOC 1973, 38, 2574.
14. Brown, H. C.; Midland, M. M.; Levy, A. B. JACS 1973, 95, 2394.
15. Brown, H. C.; Midland, M. M.; Levy, A. B.; Suzuki, A.; Sono, S.; Itoh, M. T 1987, 43, 4079.
16. Levy, A. B.; Brown, H. C. JACS 1973, 95, 4067.
17. Brown, H. C.; Salunkhe, A. M.; Singaram, B. JOC 1991, 56, 1170.
18. Midland, M. M.; Brown, H. C. JACS 1973, 95, 4069.
19. Brown, H. C.; Midland, M. M. T 1987, 43, 4059.
20. Knörzer, G.; Seyffer, H.; Siebert, W. ZN(B) 1990, 45, 1136.
21. Hassner, A.; Soderquist, J. A. JOM 1977, 131, C1.
22. Knörzer, G.; Siebert, W. ZN(B) 1990, 45, 15.
23. Zweifel, G. JOM 1967, 9, 215.
24. Pasto, D. J.; Balasubramaniyan, P. JACS 1967, 89, 295.

Marek Zaidlewicz

Nicolaus Copernicus University, Torun, Poland

Herbert C. Brown

Purdue University, West Lafayette, IN, USA

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