634
Organometallics 1983, 2, 634-637
Addition Compounds of Alkali-Metal Hydrides. 23. Preparation of Potassium Triisopropoxyborohydride in Improved Purity Herbert C. Brown," Behrooz Nazer,la and James A. Sikorskilb Richard B. Wetherill Laboratory, Purdue University, West La fayette, Indiana 47907 Received August 4, 1982
Commercial potassium triisopropoxyborohydride, K(i-Pr0)3BH,or the usual material prepared at 25 OC from potassium hydride and triisopropoxyborane in tetrahydrofuran contains a significant impurity, detectable in the "B NMR spectrum. This impurity, probably potassium tetraisopropoxyborohydride, significantly decreases the yield when the reagent is used to hydride thexylmonoalkylchloroboranes, ThxBRICl, for the synthesis of "mixed" thexyldialkylboranes. This impurity can be removed by refluxing a THF solution of the impure potassium triisopropoxyborohydride over potassium hydride. Alternatively, the reaction of triisopropoxyborane with excess KH in refluxing THF gives a product essentially free of the impurity. Storage of the solution KIPBH over a small excess of potassium hydride (- 10%)maintains the product essentially free of the impurity. Use of this material permits the preparation of essentially quantitative yields of the desired "mixed" thexyldialkylboranes, ThxBRIR2,via the hydridation of the intermediate ThxBRICl in the presence of a second olefin. Early attempts to prepare the lithium and sodium triisopropoxyborohydride proved difficult.2 The rate of reaction of triisopropoxyborane with lithium hydride and sodium hydride was very slow so that reaction could be achieved only a t elevated temperatures (eq 1). In those pre-NMR days we could not be confident of the homogeneity of the product produced under such vigorous conditions.
itd"-"" +
+ NaH 7 Na(i-PrO)3BH
KH
THF + sec-Bu3B x K-sec-Bu3BH
(2)
KH
THF + (i-PrO),B x K(i-PrO),BH
(3)
derivatives: proved to be a highly stereoselective reducing agent.4a Somewhat surprisingly, K(i-PrO),BH proved to be a very gentle reducing agent.4c The characteristics of K(i-Pr0)3BH appeared to make it the ideal reagent for our new synthesis of mixed thexyldialkylboranes6 (eq 4 and 5). The reaction worked satisfactorily. However, the yields were in the range of only 59-75%. Moreover, they appeared to vary somewhat with the sample of KIPBH used.7 llB NMR examination of commercial KIPBH solution in T H F showed two peaks, one at 6 6.1 and one at 6 2.7 (Figure 1). The relative areas of the two peaks varied from sample to sample from 9O:lO (1)(a) Postdoctoral research associate on Grant ARO-DAAG-29-79C-0027 supported by the US.Army Research Office. (b) Graduate research assistant on temporary academic leave from Monsanto Agricultural Products Co. (2) Brown, H. C.; Mead, E. J.; Shoaf, C. J. J. Am. Chem. SOC.1956, 78, 3616. (3) Brown, C. A. J. Chem. SOC.,Chem. Commun. 1974, 680. (4) (a) Brown, C. A. J. Am. Chem. SOC. 1973,95,4100. (b) Brown, C. A. J. Org. Chem. 1974, 39, 3913. (c) Brown, C. A.; Krishnamurthy, S.; Kim, S. C. J. Chem. SOC.,Chem. Commun. 1973, 391. (5) Brown, H. C.; Krishnamurthy, S. J.Am. Chem. SOC.1972,94,7159. (6) Kulkarni, S. U.; Lee, H. D.; Brown, H. C. J. Org. Chem. 1980,45, 4542. (7) Sikorski, J. A.; Brown, H. C., unpublished results.
0276-7333/S3/2302-0634$01.50/0
K(:-Pr0)3BF
-
H-(HzcHzR +
(1)
The discovery by C. A. Brown that potassium hydride, KH, is far more reactive than lithium hydride or sodium reacting readily at room temperature with such hindered Lewis acids of boron as tri-sec-butylboranek and triisopropoxyborane,4b" made these borohydrides readily available (eq 2 and 3). K-sec-Bu3BH, like its lithium
+
Ll
150 'C, DG
(i-PrO),B
R'CWCH,
i/-PrCi39
+
KCI
(5)
CH,CY,R
to 65:35, respectively.* Moreover, the yields of the products in the thexyldialkylborane synthesis varied roughly inversely with the magnitude of the minor peak. Investigation revealed that this minor peak was present not only in the commercial products but also in material prepared by the literature procedure.4bc Accordingly, we undertook a study to establish the nature of this impurity and to see if we could devise a means either of removing it from the commercial product or of synthesizing the reagent free of the impurity.
Results and Discussion It appeared clear that the major peak at 6 6.1 (d, JB+, = 117 Hz) must be due to K(i-Pr0)3BH. The question remaining was the source of the minor peak a t 6 2.7 (s). We suspected that this peak might be due to the presence of a tetraalkoxyborohydride, K(RO)4B. Indeed, the controlled addition of acetone resulted in a decrease in the major peak at 6 6.1 and a sharp increase in the minor peak (eq 6). Addition of 1 equiv of acetone resulted in the complete disappearance of the peak at 6 6.1 and its replacement by an equivalent large peak at 6 2.7. (CH3)*C0+ K(i-Pr0)3BH K(i-Pr0)4B (6) Similarly, addition of pure potassium tetraisopropoxyborate, readily prepared from the combination of equivalent quantities of potassium isopropoxide and triisopropoxyborane (eq 7) to the KIPBH sample, resulted in a sharp increase only in the minor peak.
-
i-PrOK
-
+ (i-PrO)3B THF
K(i-PrO),B
(7)
(8) Since concentrations of K(i-PrO),BH and K(i-PrO),B are not directly related to the observed areas of 'lB NMR spectra, we shall use only ares in discussing the relative magnitudes of these two peaks.
0 1983 American Chemical Society
Organometallics, Vol. 2, No. 5, 1983 635
Addition Compounds of Alkali-Metal Hydrides
Table I. Preparation of K(i-PrO),BH Using Different KH:(i-PrO),B Ratiosa expt no.
KH :(i-PrO),B
solvent
1 2 3 4 5 6 7
1;l:l.Ob 1.3:1.0b 1.5:l.O 2.0:l.O 2.0:l.O 2.0:l.O 1O.O:l.O
THF THF THF THF Et,O gbme THF
results formation of formation of formation of formation of formation of formation of formation of
1 5 - 2 0 s of the minor component 10-15% of the minor component 10-12% of the minor component 8-10% of the minor component 4 - 5 s of the minor component 8-10% of the minor component 2%of the minor component
a All preparations were done at room temperature. Using 10%and 30%excess potassium hydride, a third peak at 6-11.3 (9,J B - =~ 87.4) was observed. The peak is assigned to a K-i-PrOBH, moiety.
B
Figure 1. 'H-Decoupled llB NMR spectra of K(i-Pr0)3BH reagents. (A) commercial reagent; (B)reagent prepared at 25 O C ; (C)reagent prepared at 0 O C ; (D)reagent refluxed over excess KH, 299% pure.
Addition of n-butyraldehyde to the KIPBH sample had the same effect. Consequently, while we can be confident that the peak a t 6 2.7 is attributable to a tetraalkoxyborohydride, we cannot assign the peak to (i-PrO)4Brather than to n - B ~ o ( i - P r 0 ) ~ B or- some other tetraalkoxyborohydride. A 1M solution of commercial KIPBH was analyzed for potassium (by titration as potassium hydroxide), isopropoxide (by GLC analysis as isopropyl alcohol), boron (by titration as boric acid in the presence of mannitol), and hydride (by measurement of hydrogen gas evolved). The composition agreed with the theoretical: Kl.,,,(iPrO)3.&l,,Hl.w Consequently, the minor peak cannot be due to the presence of an alkoxide impurity in the potassium hydride or an alcohol impurity in triisopropoxyborane used for the preparation. A sample of KIPBH, prepared by the published procedure,4b.cwas subjected to the same analysis. The llB NMR
spectrum showed the same two peaks (Figure 1)with the area of the minor peak at 6 2.7 being in the neighborhood of 8-10% that of the major peak a t 6 6.1. Here also, an almost theoretical analysis of 1.00:3.001.00:1.00 was obtained for the four components. Formation of n-butoxy through cleavage of the T H F solvent was ruled out since GC analysis of the hydrolyzed product revealed no l-butanol present in the 2-propanol. Preparation of KIPBH in monoglyme or diethyl ether yielded a solution that exhibited the same minor component in the llB NMR spectrum. Analysis of potassium hydride samples did not reveal the presence of any organic impurity that might affect the preparation of the reagent. The commercial sample of (i-PrO)3Bcontained ca. 0.5% free 2-propanol. Distillation from a small amount of potassium metal gave a pure sample of the ester, free of alcohol. Yet the product of this ester (i-Pr0)3Band potassium hydride revealed the same minor peak. We were therefore forced to the conclusion that the minor peak must arise not from any impurity in the solvent, inert atmosphere, or reactants. It must arise from a disproportionation of the product itself (eq 8). However, 2K(i-Pr0)3BH a K(i-PrO),BH2 + K(i-PrO),B (8) this conclusion left us with two puzzles. Why did different samples or reagent solution exhibit considerably different magnitude of the minor peak? Why did we not see a third peak, for K(i-Pr0),BH2, in the llB NMR spectrum? Careful llB NMR reexamination of commercial KIPBH samples did reveal three peaks: a major peak a t 6 6.1 (d, J B =~117 Hz), assigned to (i-PrO),BH-, a minor peak at 6 2.7 (s), assigned to (i-PrO),B-, and a new, very small peak at 6 -11.3 (q, JBH = 87.4). Since this peak is a quartet, it is attributed to the species i-PrOBH3-. Apparently, the equilibria in the spectrum are more complex than that shown in eq 8, involving also the formation of potassium monoisopropoxyborohydride(eq 9). 2K(i-Pr0)2BH2a K-i-PrOBH, +K(i-PrO),BH (9) There are two possible explanations for our failure to detect a fourth peak, a triplet, assignable to K(i-Pr0)2BH2. One is that the equilibrium favors the other three components, so that the concentration of K(i-Pr0)2BH2is too low to detect under these conditions. The second is that the exchange rate of K(i-Pr0I2BH2is such that it causes the peak to disappear into the backgroundas We attempted to prepare an authentic sample of K(iPr0)*BH2by treating diisopropoxyborane with potassium hydride a t 0 O C (eq 10). However, the reaction product KH
0 'C + (i-PrO),BH THFK(i-Pr0)2BH2
(10)
was a mixture of (i-PrO),B, K(i-PrO)4B,K(i-PrO),BH, and (9) Brown, C. A.; Krishnamurthy, S.J. Organomet. Chem. 1978,156,
111.
636 Organometallics, Vol. 2, No. 5, 1983
Brown,Nazer, and Sikorski
K(i-PrOBH,), with no K(i-Pr0)2BH2detectable. Either the formation of this mixture confirms the suggested instability of K(i-Pr0)2BH2or it is an artifact of the synthesis as observed previously for the reactions of RLi and Rf2BH.I0 Potassium tetraisopropoxyborohydride, K(i-PrO),B, differs from its sodium analogue,’l Na(i-PrO),B, in beidg very soluble in a variety of solvents, such as THF, Et,O, monoglyme, and even pentane. Attempts to concentrate these solutions to precipitate selectively either K(i-PrO),B or K(i-PrO),BH failed. We next explored the preparation of KIPBH a t room temperature by using different ratios of KH and (i-PrO),B. Here we achieved our first success. A large ratio of KH to (i-PrO),B produced a relatively pure sample of KIPBH. For example, a tenfold excess of KH results in the formation of KIPBH with approximately 2 % (by area) of the minor component. Unfortunately, the need for this large excess of KH made this solution to the problem impractical. These results are summarized in Table I. We tested the preparation of KIPBH at 0 “C. This provided a material that contained only 4-6 % of the minor peak. Storage a t 0 OC also decreased the rate of growth of this minor peak. Finally, we observed an unexpectedly phenomenon, which ultimately provided a solution to the problem. When being refluxed a solution of K(i-PrO),B in T H F slowly liberated (i-PrO)3B(eq 11). When such a solution K(i-PrO),B
A
THF i-PrOK + (i-PrO),B
2724
-
21
-
18
-
m 3
9
4 .-
15-
y’ 129-
630I
(11)
was refluxed for a considerable time (18 h) and then cooled to room temperature, the IlB NMR spectrum revealed the presence of a peak a t 6 17 [(i-PrO),B] and one a t 6 3.7 [K(i-PrO),B]. The continued growth of (i-PrO),B in the solution and its failure to recombine with i-PrOK on cooling is a puzzle. I t must mean that there is a slow reaction that converts the i-PrOK to a form in which it does not react with (i-PrO),B. Yet the solution remains clear. Possibly the i-PrOK reacts slowly with the Pyrex glass to be converted into a soluble silicate ester. In any event, this experiment suggested the possibility of removing the K(i-PrO),B impurity from the reagent by refluxing it for 24 h over free KH. Indeed, this treatment reduced the minor component in the commercial product to a negligible amount (C1% by area). Presumably, the impurity of K(i-PrO)4B slowly dissociates to triisopropoxyborane, (i-PrO)3B(eq l l ) , and the liberated ester reacts with the KH present (eq 3). Alternatively, it proved possible to prepare pure KIPBH by adding (i-PrO),B to a modest excess of KH in THF, either at room temperature or in refluxing THF, followed by heating the reaction mixture under reflux for 24 h. Again llB NMR examination of the clear solution revealed essentially pure KIPBH (299%) with less than 1% (by area) of the impurity. The process evidently also removes the other minor components K(i-Pr0)2BH2and K-iPrOBH,. The KIPBH prepared under these conditions exhibited an additional stability toward the undesirable disproportionation if the material were stored over 10% excess KH. Under these conditions, no disproportionation of the KIPBH reagent has been observed after 8 months at room temperature. This excem KH is not soluble in the solution, and it readily settles to the bottom of the storage flask. (IO) Hubbard, J. L.; Kramer, G. W. J. Organomet. Chem. 1978, 156, 81. (11) Brown,H. C.; Mead, E. J. Am. Chem. SOC.1956, 78, 3614,
‘
0
3
6
9
12
15
18
Time (day) Figure 2. Stability of potassium triisopropoxyborohydride in tetrahydrofuran. Consequently, the clear solution of the reagent is readily removed, free of the KH. The clear solution, KIPBH, separated from excess KH, does undergo a slow disproportionation (Figure 2). Alternatively, potassium tetraisopropoxyborohydride, K(i-PrO),B, was added to the pure KIPBH reagent. This KIPBH with added impurity can be regenerated easily by refluxing over a moderate excess KH for 24 h. We then tested this improved reagent for the hydridation of thexyl-n-octylchloroborane and conversion into thexyl-n-octyl-n-decylborane, followed by carbonylation to the corresponding ketone (eq 12-14). It is clear from these results that our original problem has been solved. It is now possible to prepare and utilize potassium triisopropoxyborohydride in purities approaching 100%.
Conclusion Potassium triisopropoxyborohydride,K(i-PrO),BH, can now be prepared in essentially pure form, 299%, by adding the ester, triisopropoxyborane, to excess potassium hydride in THF, refluxing the mixture for 24 h, and then storing the product over a small quantity of potassium hydride (- 10%). Under these conditions, no disproportionation was observed after 6 months a t room temperature. This reagent exhibited excellent behavior in the hybridation of thexylmonoalkylchloroborane in the synthetic route to “mixed” thexyldialkylboranes, providing essentially quantitative yields of the intermediate boranes and the ketones into which these intermediates can be transformed. Experimental Section Materials. Tetrahydrofuran was dried over a 4-A molecular sieve and distilled from a sodium benzophenone ketyl prior to use. Potassium hydride was purchased from Alfa and was freed from the mineral oil according to the published p r o c e d ~ r e . ~ , ~ Triisopropoxyboranewas either purchased from Aldrich or pre-
Organometallics, Vol. 2, No. 5, 1983 637
Addition Compounds of Alkali-Metal Hydrides H
---
-\AAAA/
,
C
=
O
I. GO 2 . 101
(14)
100%GC yield, 94% isolated pared from 2-propanol and the methyl sulfideborane complex.12 Triisopropoxyborane was distilled from a small piece of potassium metal prior to use. All glassware was dried thoroughly in a drying oven and cooled under a dry stream of nitrogen. Spectra. Spectra were obtained under an inert atmosphere by using apparatus and techniques described e1~ewhere.l~"B NMR spectra were recorded on a Varian FT-80A spectrometer equipped with a broad-band probe and a Hewlett-Packard 3335A frequency synthesizer. All IIB NMR chemical shifts are reported relative to BF3.0Et2 (6 0), with chemical shifts downfield from BF3.0Et2assigned as positive. Potassium Triisopropoxyborohydride,K(i-Pr0)3BH,in THF (Room Temperature)pb An oven-dried, 100-mL,roundbottom flask with side arm and an adaptor was attached to a mercury bubbler. The flask was charged with 2 g of KH (50.0 mmol) as an oil dispersion; the mineral oil was removed with pentane. The KH was suspended in 30.0 mL of THF, and 5.8 mL (25 "01) of freshly distilled triisopropoxyboranewas added. After the mixture was stirred for 2 h at room temperature, the formation of borohydride was completed. [The IIB NMR spectrum did not show any signal corresponding to (i-PrO)3Bat 6 17.1 The clear solution was analyzed for potassium (as potassium hydroxide), isopropoxide (as isopropyl alcohol), boron (as boric acid), and hydride (as hydrogen gas liberated upon hydr~lysis).'~ Potassium Triisopropoxyborohydride, K(i-Pr0)3BH,in Monoglyme and Diethyl Ether. The usual procedureqbwas followed, except two other solvents, monoglyme or diethyl ether, were used in place of THF. Potassium Triisopropoxyborohydride, K(i-Pr0)3BH,in THF (at 0 "C). A clean, dry, tared 1-1, round-bottom flask equipped with a nitrogen inlet and magnetic stirring bar was charged with 50 g of potassium hydride (1.25 mol) as an oil dispersion via double-ended needle. The system was maintained under nitrogen. The suspension was allowed to settle, and then the excess mineral oil was decanted off through a double-ended needle. The remaining material was then washed several times with 250-mL portions of dry pentane under nitrogen. The remaining cake of potassium hydride was then dried in vacuo to give a light brown powder, 45.3 g (1.12 mol). This powder was slurried in 500 mL of THF at 0 "C. Then 250 mL of a 3.0 M solution of triisopropyl borate (0.75 mol) in THF was added dropwise over a 1-h period. When the addition was complete, the resulting mixture was stirred at 0 "C for 20 h. At this time the llB NMR spectrum of the supernatant solution showed that all of the (i-PrO)3B,b 17.0, had even consumed with the con= 117 Hz) along comitant formation of KIPBH (6 6.1 (d, JB-H with 4-670 of K(i-PrO)4B. The supernatant solution was then (12) Brown, C.A.; Krishnamurthy, S. J. Org. Chem. 1978, 43, 2731. (13) Brown, H. C.;Kramer, G . W.; Levy, A. B.; Midland, M. M. "Organic Syntheses via Boranes";Wiley-Interscience: New York, 1975.
removed under nitrogen with a double-ended needle and was stored at 0 "C. Standardization for active hydride13 indicated that the solution was 0.90 M. Potassium Triisopropoxyborohydride Using Different KH:(i-Pr0)3B Ratios. The usual procedureqb was followed, except that different KH(i-Pr0)3Bratios were examined. The results are summarized in Table I. Potassium Tetraisopropoxyborohydride, K(i-PrO)4B. An oven-dried, lOO-mL, round-bottom flask was charged with 10 mL of a 3.1 M potassium isopropoxide in THF. To this was added 7.15 mL (31.0 mmol) of triisoptopoxyborane in 20 mL of THF at room temperature. The llB NMR spectrum of the solution species. showed a single peak at 1 2.7 assigned to the (~-PI-O)~BThe potassium tetraisopropoxyborohydridewas analyzed13for its potassium, isopropoxide, and boron contents. A ratio of 1.00:4.00:1.00 for Ki-PrOB, within the experimental error, was observed. The solvent THF was pumped out under reduced pressure. The resulting solid K(i-Pr0)4Bwas soluble in monoglyme, diethyl ether, and pentane. Preparation of Potassium Triisopropoxyborohydride under Reflux Conditions in THF. An oven-dried, 2-L, round-bottom flask with side arm, condenser tube, and an adaptor was attached to a mercury bubbler. The flask was flushed with dry nitrogen and maintained under a static pressure of nitrogen. To this flask was added 50.0 g of KH (1.25 mol) as an oil dispersion with the aid of a double-ended needle. The mineral oil was removed with THF (3 X 50 mL). To this pure KH was added ca.500 mL of freshly distilled THF. The suspended KH was kept at room temperature by using a water bath. A total of 164.4 g (201.7 mL, 0.87 mol) of distilled triisopropoxyborane was added to the KH suspension via a double-ended needle while the mixture was stirred. After ca.4 h, the formation of KIPBH was completed. The llB NMR spectrum showed the formation of 10-15% of the minor compound K(i-PrO)4B. To purify the reagent, the THF solution of KIPBH was brought to gentle reflux over the excess KH. The llB NMR spectrum of the mixture after 24 h showed the formation of a 299% pure triisopropoxyborohydride (Figure 1). An aliquot of the above K'IPBH solution was quenched with water, and its potassium and boron contents were measured as potassium hydroxide and boric acid.13 Hydride measurement was done by calculatingthe number of moles of hydrogen gas evolved after the reagent was quenched with a mixture of THF, glycerine, and 2 N HCl. A 1.200 M concentration of boron and hydride contents was observed. Potassium content was measured as 1.205 M. Hence, a 1.00:1.001.00 ratio of K:B:H was established. Stability of 99% Pure KIPBH. The above KIPBH with purity of 299% was tested under different conditions. A sample was separated from KH and was kept in a storage bottle under Argon atmosphere. The llB NMR spectra of the sample during a time interval (ca. 6 months) were examined. An increase of ca. 2630% of the minor peak was observed (Figure 2). In another sample of KIPBH, maintained over 10% KH, no disproportionation was detectable after 6 months. Alternatively,the 99% pure reagent can be kept over the unused portion of potassium hydride. The reagent exhibited a much faster rate of disproportionation when refluxed without the presence of extra KH (Figure 2). Addition of K(i-PrO)( to the 99% Pure KIPBH. A 10.0-mL sample of a 1.0 M potassium tetraisopropoxyborohydride[K(iPrO),B] was added to a 50.0-mL sample of 1.0 M 99% KIPBH. An immediate increase in the minor compound at 6 2.7 was observed. The solution was transferred to a 4.1 g of KH (102 mmol), freed from the mineral oil, and it was refluxed for 24 h. After this period, the llB NMR of the clear solutionshowed a complete recovery of the KIPBH reagent. Acknowledgment. We thank Dr. C. A. Brown for helpful discussions during this study and gratefully acknowledge the financial assistance from the U S . Army Research Office (No. ARO-DAAG-29-79-C-0027). Registry No. K(~-PI-O)~BH, 42278-67-1; KH, 7693-26-7;K(i-Pr0)4B, 84581-08-8; triisopropoxyborane, 5419-55-6; THF, 109-99-9; thexylchloroborane, 75030-54-5; 1-octene, 111-66-0; thexyl-n-octylchloroborane, 75052-81-2; 1-decene, 872-05-9; thexyl-n-octyl-n-decylborane,84521-30-2; 9-nonadecanone, 75030-48-7.
