3,634,277 United States Patent fice Patented Jan. 11, 1972 2 reactive towards air and moisture. Consequently, it is dif 3,634,277 ficult to handle. Although attempts have been made to STABLIZED TETRAHYDROFURAN SOLUTIONS OF DABORANE compress it in tanks for shipment, this practice involves Herbert C. Brown, 1840 Garden St., major difficulty. Diborane in contact with the metal of the West Lafayette, End. 47906 cylinders or tanks decomposes spontaneously into hydro No Drawing. Eiled Mar. 20, 1969, Ser. No. 809,015 gen and higher hydrides of diborane. Indeed, it is recom int, C. CO 35/00 mended that Such cylinders be refrigerated and shipped U.S. C. 252-188 14 Claims or stored at low temperatures. A. desirable solution to these difficulties would be to O dissolve diborane into a suitable solvent and to ship ABSTRACT OF THE DISCLOSURE the material in that form. Unfortunately, the gas does not Solutions of diborane in tetrahydrofuran can be pre possess adequate solubility in hydrocarbon solvents. It pared without handling the gas by treating Suspensions of cannot be stored in Such solvents as esters, dimethyl sulf sodium borohydride in tetrahydrofuran with boron tri Oxide, or dimethylformamide, since these undergo rela fluoride, followed by decanting, filtering, or centrifuging 5 tively rapid reduction. Diborane reacts with amines to the tetrahydrofuran solution of diborane from the precip form relative stable amine-boranes, which fail to exhibit itated sodium fluoborate. Such solutions are normally the desirable properties of diborane itself. unstable at ordinary temperatures, undergoing reductive This leaves ethers as possible solvents. cleavage of the solvent. Consequently, they cannot be Unfortunately, the solubility of diborane in representa stored for any appreciable time or shipped any appreci 20 tive ethers, such as diethyl ether, di-n-butyl ether, mono able distance. However, these solutions are stabilized to glyme (dimethyl ether of ethylene glycol), and diglyme ward such reductive cleavage by utilizing in the Synthesis (dimethyl ether of diethylene glycol), is far too low to a slight excess of sodium borohydride or other ionic boro permit preparation of solutions of high enough concentra hydrides. These discoveries now make practical the manu tions of diborane to be useful. facture, shipping, storing and application of such solutions There is one exception known. Diborane is highly solu of diborane in tetrahydrofuran, highly useful for hydro ble in the cyclic ether, tetrahydrofuran (J. R. Elliott, W. L. Roth, G. F. Roedel and E. M. Boldebuck, J. Am. Chem. borations and hydrogenations. Soc., 74, 5211 (1952)). In this solvent it is possible to prepare Solutions which are as concentrated as 4 molar 30 in borane (BHs), without exceeding one atmosphere of BACKGROUND preSSure. Moreover, we have observed that such solutions (I) Field of invention are relatively safe to use. They have been exposed to the atmosphere without observable change. They have been This invention relates to a novel process for preparing poured through the air without catching fire. They have solutions of diborane in tetrahydrofuran without the in 35 been poured into evaporating dishes and allowed to evap dependent preparation of handling of gaseous diborane. orate, without catching fire. It also relates to a completely novel means of Stabilizing Such solutions of diborane are ideal for hydroborations such solutions toward reductive cleavage by utilizing a and Selective hydrogenations. Consequently, it would be small excess of sodium borohydride or other ionic boro highly desirable to manufacture and ship such solutions hydrides. It also describes as a novel composition of matter 40 of diborane in tetrahydrofuran. solutions of diborane in tetrahydrofuran containing dis Unfortunately, two major difficulties interfere. solved sodium borohydride or other ionic borohydride. In the past it has been customary to generate diborane These discoveries make it practical for the first time to by treating sodium borohydride in diglyme solution with manufacture solutions of diborane in tetrahydrofuran ca boron trifluoride-etherate. pable of being stored, shipped and used without Special 45 refrigeration or other special precautions. (II) Description of the prior art The diborane gas is then passed into the tetrahydrofuran Diborane, BH, is a chemical with remarkable prop Solvent to make the solution. erties. It reacts instantly with olefins in the presence of 50 This process requires the preparation and handling of ethers, such as tetrahydrofuran, to form organoboranes. large amounts of diborane, an exceedingly reactive and ether hazardous gas. This is the first of the major difficulties. 6RCEI-CE -- B2H6 - 2CR CHCH2)3B The Second of the major difficulties arises from the This process is known as “hydroboration,” and is fully 55 observation that solutions of diborane in tetrahydrofuran described in my patent (H. C. Brown, U.S. 3,078,311, have a highly limited shelf-life at ordinary temperatures. Feb. 19, 1963) and in my book (H. C. Brown, Hydro Such Solutions undergo reductive cleavage of the tetra boration, W. A. Benjamin Co., New York, 1962). hydrofuran by the diborane (J. Kollonitsch, J. Am. Chem. Diborane is also an exceedingly powerful, but selective Soc., 83, 1515 (1961)). hydrogenating agent for functional groups. Whereas sodi 60 C-C, um borohydride is a basic-type reducing agent (H. I. 6 - BEds - 2CCHCHCH2CH2O)3B Schlesinger and H. C. Brown, U.S. 2,683,721, July 13, ce CH, 1954), diborane is an acidic-type reducing agent (H. C. O Brown and B. C. Subba Rao, J. Am. Chem. Soc., 82, Such solutions lose 1 to 3% of the available diborane 681 (1960)). The availability of both an acidic and a 65 per day at ordinary temperatures (25 to 30). Conse basic-type reducing agent makes possible numerous Selec quently, it is impractical to manufacture, store, and ship tive reductions or hydrogenations of functional groups Such solutions. (H. C. Brown, Hydroboration, W. A. Benjamin Co., The present invention solves these problems. New York, 1962, Chapter 17). Sodium borohydride is crystalline solid, a stable re 70 SUMMARY agent, easily manufactured, stored, shipped and used. Treatment of a suspension of sodium borohydride in However, diborane is a gas, B.P.-92.5 C. It is highly tetrahydrofuran with boron trifluoride converts the so

3,634,277 3 4. dium borohydride into diborane, which remains in solu However, sodium borohydride is the preferred embodi tion, and sodium fluoborate, which precipitates. The re ment of this invention. Sulting solution of diborane in tetrahydrofuran can be The boron trifluoride can be utilized in the form of decanted from the sodium floroborate. Alternatively, this gaseous boron trifluoride. Alternatively, boron trifluoride salt, which is essentially insoluble in the solvent, can be 5 can be added in the form of ethyl ether-boron trifluoride. removed by filtration of centrifugation. This causes some diethyl ether to be present in the di HF borane solution. However, this has a serious disadvantage. 6NaBE (solid) -- 8BF3 - 4B2H6 (in THF) -- 6Na3f4 The presence of the ethyl ether decreases the solubility This reaction can be carried out on a large scale and of the diborane. It is more desirable to introduce the does not involve any handling of gaseous diborane. O boron trifluoride by displacement. We discovered that the use of a slight excess of sodium borohydride in this process gives solutions which exhibit far higher stability toward reductive cleavage of the sol The volatile ethyl ether can be distilled out of the solution. vent than solutions prepared by dissolving diborane in the Sodium borohydride is our preferred stabilizer. It can solvent. This was a completely unexpected development. 5 be used in small quantities, from approximately 0.01 mole percent (based on borane) to approximately 10 mole per Research into this discovery revealed that the addition of cent, although the use of larger amounts of sodium boro small amounts of sodium borohydride to Solutions of di hydride has no deleterious effects. However, other sta borane in tetrahydrofuran greatly diminishes the reduc bilizers are also effective, such as potassium borohydride, tive cleavage of the solvent, as shown by the data lithium borohydride, rubidium borohydride, caesium in Table I. 20 borohydride, magnesium borohydride, calcium borohy TABLE. I.-STABILITY OF SOLUTIONS OF DIBORANE, IN TETRAHYDROFURAN IN PRESENCE AND ABSENCE OF dride, and tetraethylammonium borohydride. It appears SOIUM BOROHYDRIDE AT 23° that the presence of ionic borohydride in small amounts Concentration of Concentration of is the effective stabilizer, so that any of these materials in borane (no added borane (in pres Small amounts is effective. sodium boro ence of 5% sodium hydride) borohydride) Although we prefer to introduce the sodium borohy dride by using a small excess during the preparation of the Time, weeks Percent M. Percent solution, or by adding solid sodium borohydride to the 1.6 100 55 00 solution, it is possible to generate the Sodium borohydride 1.45 90 55 100 1.29 SO 1,52 98 in situ by adding appropriate reagents which form sodium ... 3 70 1.51 97 borohydride by reaction with diborane (H. C. Brown, 0.97 60 50 97 Hydroboration, W. A. Benjamin Co., New York, 1962, Chapter 4). Thus the addition of small amounts of finely There is an obvious disadvantage in this procedure. The divided sodium hydride serves satisfactorily. sodium borohydride is only slightly soluble in the tetra 35 THE hydrofuran. Thus the addition of sodium borohydride in Na -- BE3 - NaBEI amounts of approximately 5 mole percent gives a hetero Alternatively, sodium methoxide or other alkoxide can geneous mixture which can clog valves and constrictions be used. in equipment through which it is pumped. In shipping THE such solutions in tank cars the solid sodium borohydride 40 3NaO CH3 - 4 BH - 3 NaBH4 -- B (OCH3)3 settles out and creates difficulties in cleaning such cars. This problem can be solved by using the soluble lithium Similarly, sodium trimethoxyborohydride or sodium tetra borohydride. Unfortunately, the lithium borohydride is methoxyborohydride can be used. far more costly than the sodium borohydride, so that the NaBH(OCH3)3 -- BHis THE NaBH4 -- B (OCH3)3 use of the lithium salt adds to the cost. In part, this can be circumvented by adding small In the same way, any of the ionic borohydrides can be amounts of lithium hydride, lithium methoxide, or lithium synthesized in situ to provide the stabilizer. tetramethoxyborohydride. Lithium borohydride is then These relatively concentrated solutions of diborane in generated in situ. tetrahydrofuran containing small amounts of dissolved All of these additional operations add to the cost of the sodium borohydride or other ionic borohydrides as sta product. bilizers constitute a new composition of matter with un We have discovered that stabilized solutions of diborane expected properties of major importance-high stability in tetrahydrofuran can be prepared merely by using a toward reductive cleavage of the solvent. slight excess of sodium borohydride in the reaction with boron trifluoride. Filtration, centrifugation, or decanta DESCRIPTION OF THE PREFERRED tion removes the sodium fluoroborate with the slight 5 5 EMBODMENTS excess of sodium borohydride. Yet the clear solution of Example 1 diborane in tetrahydrofuran exhibits high stability, as shown by the data in Table II. In a two liter flask flushed with nitrogen was placed TABLE I-STABILITY OF SOLUTIONS OF DIBORANE IN 0.80 liter of dry tetrahydrofuran. Sodium borohydride, 57 TETRAEYDROFURAN BREPARED WITH THE USE OF SLIGHT EXCESS OF SODIUM BOROHYDRIDE g. (1.5 moles), was added and the mixture was vigorously stirred to keep the salt in solution. Two moles of ethyl Concentration of boane ether-boron trifluoride was added slowly to the well-stirred Time, weeks M Percera, Solution cooled in a water bath to maintain the tempera 0------.58 OO ture at 25 or lower. After all of the etherate had been ----- 58 100 2-- i.58 100 added, the solution was stirred for an additional hour. A 3-- 1.57 99 filter stick was then introduced and the solution pushed 4.-- 55 98 through the filter stick by nitrogen pressure into the stor age bottle. A Small amount of tetrahydrofuran was added These developments solve the two major problems pre to make the total volume 1.0 liter. The yield was almost viously outlined and make it practical to manufacture, quantitative, the Solution (1.0 liter) analyzing 1.96 M in ship, and store tetrahydrofuran solutions of diborane. borane. In the above process for manufacturing tetrahydrofuran This solution was not stable to reductive cleavage, Solutions of diborane, it is possible to substitute other but underwent a loss of approximately 2% of active hy alkali metal borohydrides, such as potassium borohydride. drogen per day.

3,634,277 5 6 The use of excess sodium borohydride gave a stabler as compared to a calculated concentration of 1.63 M. Solution, as shown in Example 2. Therefore the yield was approximately 97%. The solution Was allowed to stand at room temperature and aliquots Example 2 removed at regular intervals and analyzed for active hy The procedure of Example 1 was duplicated, but an 5 drogen. Over 4 weeks, the concentration had dropped to excess of Sodium borohydride was used, 60 grams, instead 1.55 M, a decrease of only 2% (Table II). This compares of the theoretical quantity of 57 grams used in Example to a decrease of 20% exhibited by a solution prepared 1. The solution contained nearly 2 moles of borane, 1.98 Without an excess of sodium borohydride. M. This solution was much more stable to reductive cleav Having thus described the general nature and specific age, undergoing no obervable change in two weeks at 25. O embodiments of the present invention, the true scope of the invention is now pointed out in the appended claims. Example 3 I claim: The procedure of Example 1 was repeated. The solution 1. A process for the preparation of stabilized solutions was divided into two equal portions. To one solution was of diborane in tetrahydrofuran which comprises contact added 5 mole percent of sodium borohydride. The boro 15 ing a Suspension in tetrahydrofuran containing from 0.01 hydride is only slightly soluble in the solution, but the sus mole percent to about 50 mole percent excess of a boro pension is entirely adequate. The two solutions were al hydride Selected from the group consisting of sodium bo lowed to stand at room temperature under nitrogen. At rohydride, potassium borohydride with boron trifiuoride regular intervals, aliquots were removed and analyzed for and separating the solution of diborane from the precip active hydrogen by reaction with water-glycerol mixtures. 20 itated alkali metal fluoroborate and undissolved borohy The unstabilized solution lost 20% of its active hydrogen dride. in 4 weeks. The sodium borohydride stabilized solution 2. The process as claimed by claim 1 wherein the boron showed only 2% loss over the same period of time. trifluoride is introduced as a gas. The detailed data are summarized in Table I. 3. The process as claimed by claim 1 wherein the boron Similar results are realized with 0.1 mole percent of 25 trifluoride is introduced as an etherate. Sodium borohydride, 1% of sodium borohydride, and 10% 4. The process as claimed by claim 3 wherein the ether of sodium borohydride. Consequently, the precise amount ate is ethyl ether-boron trifluoride. of sodium borohydride is not of major importance. 5. The process as claimed by claim 3 wherein the ether The addition of lithium borohydride, potassium boro ate is tetrahydrofuran-boron trifluoride. hydride, tetramethylammonium borohydride and other 30 6. The process of stabilizing clear solutions of diborane ionic borohydrides likewise stabilizes such solutions of in tetrahydrofuran which comprises adding to the solution diborane in tetrahydrofuran. an amount of an ionic borohydride slightly less than the amount required to saturate the solution, said ionic boro Example 4 hydride being selected from the group consisting of so dium, potassium, lithium, rubidium, caesium, magnesium, A 2.0 M solution of borane in tetrahydrofuran was pre calcium and tetraethylammonium borohydrides. pared as described in Example 1. To one liter of the 1.96 7. The process as claimed by claim 6 wherein the ionic M solution was added 0.06 moles of sodium tetrameth borohydride is selected from the group consisting of so oxyborohydride. There occurs an immediate formation dium borohydride and potassium borohydride. and precipitation of sodium borohydride. The resulting 40 8. A composition produced by the process as claimed solution exhibits no measurable change by claim 6. 9. A process for the preparation of solutions of diborane in tetrahydrofuran which comprises contacting a suspen in active hydride over four weeks. sion of a borohydride in tetrahydrofuran selected from Similarly, sodium hydride or sodium methoxide can be 45 the group consisting of sodium borohydride and potassium added to form sodium borohydride in situ. borohydride with slightly less than the stoichiometric Example 5 quantity of boron trifluoride, and separating the solution of diborane from the precipitated alkali metal fluoroborate The sodium borohydride was the commercial product and the excess of borohydride. (Ventron Corporation, D.98%) and was used as such. 10. The process as claimed by claim 9 wheerin the boron Tetrahydrofuran was dried and distilled over lithium alu trifluoride is introduced as a gas. minum hydride. Boron trifluoride-tetrahydrofuranate was 11. The process as claimed by claim 9 wherein the boron prepared as follows. trifluoride is introduced as an etherate. To a freshly distilled boron trifluoride-etherate (242 g., 12. The process as claimed by claim 11 wherein the 2 moles), 152 g. of tetrahydrofuran (2.1 moles) was add etherate is ethyl ether-boron trifluoride. ed in a 1-liter flask. The ether was removed under vacuum 13. The process as claimed by claim 11 wherein the at 25 until the weight of the contents of the flask had etherate is tetrahydrofuran-boron trifluoride. dropped to a constant value, 280 g. The boron trifluoride 14. A composition produced by the process as claimed tetrahydrofuranate was distilled, BP 79-80 at 10 mm., d52 1.25. (This distillation step is not necessary.) 60 by claim 1. A 2-liter flask, oven-dried and cooled under nitrogen, References (Cited equipped with a magnetic stirrer, inlet tube capped with UNITED STATES PATENTS a rubber septum, and a reflux condenser connected to a 3,078.309 2/1963 Brown ------260-462 R mercury bubbler, was placed in a cold-water bath. To 3,078,311 2/1963 Brown ------260-606.5 B the flask was added 60 g. of sodium borohydride (5% excess) and then 1 liter of tetrahydrofuran was introduced. RICHARD D. LOVERING, Primary Examiner To the stirred slurry was added 281 g. (2.0 moles) of boron trifiuoride-tetrahydrofuranate as the temperature I. GLUCK, Assistant Examiner was maintained at 10-15. The slurry was allowed to U.S. C. X.R. settle overnight, yielding a clear solution. The concentra 70 tion of the clear solution was 1.58 M in borane (BH3), 23-59, 149; 149-22; 252-105; 260-462 R, 606.5 B