Organometallics 1995, 14, 3098-3100
3098
Boron Trihalide Mediated Cleavage of Diethyl Ether with [Tris(trimethylsily1)methyl]lithium Clifford L. Smith Department of Natural Sciences, Albany State College, Albany, Georgia 31 705 Received February 2, 1995@ Summary: Facile cleavage of Et20 occurs with [tris(trimethylsily1)methylllithium in the presence of BX3 (where X = F, Cl, or Br) yielding ethyl tris(trimethy1si1yl)methyl ether instead of the expected [tris(trimethylsily1)methyllboron dihalide; an analogous Et20 cleavage also occurs with AlC13. The sterically hindered ether formed was unreactive toward carbon-oxygen bond cleavage by HBr, BC13, and Me3SiI.
Introduction The very bulky tris(trimethylsily1)methyl ligand (hereafter referred t o as the “trisyl” moiety) imparts exceptional stability at metal and metalloid centers.’ The alkyl halide trisyl bromide is also surprisingly unreactive toward nucleophilic substitution reactions2 but is very reactive toward Li and Mg metals and organolithiu ~ s We . ~ now report on B&- and AlCl3-mediated cleavage of Et20 by trisyllithium in hexane/EtzO affording ethyl trisyl ether.4 I t is reported that 3 mol of [(trimethylsilyl)methyl]magnesium chloride react with BF3 yielding tris[(trimethylsilyl)methyl]b~rane:~
+
3Me3SiCH2MgC1 BF3/Et20
-
+
(Me3SiCH2),B 3MgClF
Only 2 mol of the bulkier [bis(trimethylsilyl)methylllithium reacted with BF3, even under forcing reaction conditions, yielding bis[bis(trimethylsilyl)methyllboron fluoride+
+
2(Me3SiI2CHLi BF3/Et20
-
+
[(Me3Si),CHI2BF 2LiF
The formation of tris[bis(trimethylsilyl)methyllborane is presumably prohibited by severe steric crowding that would be induced at the boron atom when bonded to three bulky bis(trimethylsilyllmethy1 groups. Reaction of the bulkiest trisyllithium with BF3 in THF/Et20 did not even afford the monoalkylated boron difluoride; (Me3Si13CBF2,but instead yielded an unexpected prodAbstract published in Advance ACS Abstracts, May 1, 1995. (1)Eaborn, C. In Organosilicon and Bioorganosilicon Chemistry; Sakurai, H., Ed.; Ellis Honvood: Chichester, U.K., 1985; pp 123-130, and references cited therein. (2) Cook, M. A,; Eaborn, C.; Walton, D. R. M. J . Organomet. Chem. 1971,29, 389. (3) (a) Smith, C. L.; James, L. M.; Sibley, K. L. Organometallics 1992,11, 2938. (b) Seyferth, D.; Lefferts, J . L.; Lambert, R. L., Jr. J . Organomet. Chem. 1977,142,39. (4) Smith, C. L. Abstracts of Papers, 209th National Meeting of the American Chemical Society, Anaheim, CA, April 2-6,1995; American Chemical Society: Washington, DC, 1995; ORGN 228. (5) Seyferth, D. J . Am. Chem. SOC. 1959,81, 1844. (6)Al-Hashimi, S.; Smith, J. D. J . Organomet. Chem. 1978,153, 253.
uct of ring-opening of THF:’ (Me3Si)3CLi
+r) 0
-
(M~~S~)~CB(FXO(CHZ)~C(S~M~~)~}
In contrast, the less reactive (PhMe2Si)aCLireacts with BF3 in the normal manner, yielding the monoalkylated boron difluoride product, (PhMe2Si)3CBF2.8 The lower reactivity of (PhMe2Si)sCLi is reported to be associated with its molecular structurega as opposed to the ionic nature of the more reactive t r i ~ y l l i t h i u m . ~ ~
Results and Discussion We recently discovered that cleavage of Et20 occurs in reactions of trisyllithium with B& in hexane/EtzO affording the sublimable ethyl trisyl ether. When X = fluoro, a 40% yield of ethyl trisyl ether was obtained, the chloro gave 45%, and the bromo afforded 64%: (Me,Si),CLi
Et,O + BX, (Me3Si),COCH2CH3
Contrary to BFs-induced ring-opening of THF, BX3mediated cleavage of Et20 by trisyllithium is quite astonishing because the product, ethyl trisyl ether, does not contain a boron atom. Eaborn et al. reasoned that complexes of THF with either BF3 or the monoalkylated product, (Me3Si)&BF2, might be responsible for ringopening of THFe7They also concluded that trisyllithium alone is unlikely to cause ring-opening in view of its exceptional stability in THF.IO It is apparent from the structure of the products that cleavage reactions in THF/Et20 must involve different intermediate boron complexes than those occurring in hexane/EtzO. The role of B& in cleavage of Et20 by trisyllithium is perplexing, especially in view of the absence of boron in the product. Consistent with Eaborn’s postulate, if complexes of Et20 with B& or the intermediate (Me,Si)3CBX2 are responsible for the cleavage of Et20, then the expected products would be (Me3Si)3CB(X)OCH2CH3 and CH3CH2X. For instance, facile cleavage of Et20 occurs in the presence of BC13 or BBr3, yielding the ethyl borate esters CH3CH20BX2 and the corresponding alkyl halides C H ~ C H Z X .Reaction ~~” of trisyllithium with the borate esters is thereby expected to give (Me3Si)&B-
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(7) Eaborn, C.; Retta, N.; Smith, J. D.; Hitchcock, P. B. J . Organomet. Chem. 1982,235,265. ( 8 ) Eaborn, C.; El-Kheli, M. N. A,; Hitchcock, P. B.; Smith, J . D. J . Chem. SOC.,Chem. Commun. 1984,1673. (9) (a) Eaborn, C.; Hitchcock, P. B.; Smith, J. D.; Sullivan, A. C. J . Chem. Soc., Chem. Commun. 1983,1390. (b) Eaborn, C.; Hitchcock, P. B.; Smith, J . D.; Sullivan, A. C. J . Chem. Soc., Chem. Commun. 1983,827. (10)Cook, M. A,; Eaborn, C.; Jukes, A. E.; Walton, D. R. M. J . Organomet. Chem. 1979,24,529. (11)(a) For a review on the cleavages of ethers see: Bhatt, M. V.; Kulkarni, S. U. Synthesis 1983,249. (b) Gerrard, W.; Lappert, M. F. J . Chem. SOC.1952,1486. ( c ) Benton, F. L.; Dillon, T. L. J . A m . Chem. SOC.1942,64, 1128.
0 1995 American Chemical Society
Notes (X)OCH2CH3or products of its hydrolysis formed during workup, (Me3Si)3CB(OH)OCH2CH3 and (Me3Si)3CB(0H)a. However, neither one of these compounds were isolated, the latter boronic acid having been synthesized by other methods.12J3 On the other hand, reaction of trisyllithium with CH3CH2X (a reaction expected t o be slower than attack at the boron center of CH3CH20BX2) would yield (Me3Si13CCH2CH3. This compound has been reported14 by Seyferth et al. from reaction of trisyllithium with CH3CH2I; however, the elemental analysis for (Me3Si)&CH2CH3 and the reported mp 208-211 "C were not in agreement with the mp 248251 "C of our product. If trisyllithium reacts with B& to form (Me3Si)sCBX2,which subsequently cleaves Et20, the product of the reaction also would be (Me3Si)3CB(X)OCH2CH3. The BF3-mediated cleavage of Et20 is even more remarkable because addition complexes of BF, with Et20 is perhaps its most readily accessible derivative15 (being distilled at bp 124 "(3760 Torr without decomposition); thus the cleavage fragments CH3CH20BF2 and CH3CH2F should not be significant intermediates available for reaction with trisyllithium. In view of these findings that BF3 is unreactive toward cleavage of Et20, then it seems reasonable that an intermediate (Me3Si)3CBF2 should also be even more unreactive toward cleavage of Et20 because nucleophilic attack upon boron would be sterically inhibited by the trisyl group.16 It is therefore apparent that trisyllithium complexed with BF3 is more reactive toward Et20 cleavage than complexes of Et20 with either BF3 or (Me3Si13CBF2. The monoalkylated dichloride compound (Me3Si)sCAlCl2 (15%)was obtained along with (Me3Si)3C(CH2)4OH (20%)and some (Me3Si)&H from reaction between trisyllithium and AlC13 in THF/Et20.13 In contrast, we obtained a 54% yield of ethyl trisyl ether from reaction of trisyllithium with AlC13 in hexaneEt20. In summary, the following results illustrate the diverse nature of reactions occurring between (RMe2Si)aCLi and B&: (a) trisyllithium cleaves Et20 in the presence of BF3 in hexane/EtzO, affording a trisylbearing ether without incorporation of a boron atom; (b) trisyllithium reacts with BF3 in THF/Et20 by ringopening of THF, yielding a trisyl-bearing ether with incorporation of a boron atom;7 (c) trisyllithium also reacts in THF/Et20 with B(OMe)3, giving the expected alkylated product, (Me3Si)3CB(0Me)2,l2and (d) the related alkyllithium, (PhMeaSi)aCLi,reacts with BF3 in THF/Et20, yielding the expected alkylated difluoride, (PhMe2Si)3CBF2.8 Ethyl trisyl ether is unreactive toward BCl3 in methylene chloride, toward Me3SiI in acetonitrile, and (12) Trisyllithium reacts with the borate ester B(OMe), in Et201 THF to give (Me,Si),CB(OMe)z,which was partially hydrolyzed during workup yielding a mixture of (Me,Si)&B(OMe)z, (Me3Si)aCB(OH)OMe, and (Me&,CB(OH)z: Lickiss, P. D. J . Organomet. Chem. 1980,308, 261. (13)Eaborn, C.; El-Kheli, M. N.; Retta, N.; Smith, J. D. J . Organomet. Chem. 1983,249, 23. (14) Seyferth, D.; Lefferts, J. L.; Lambert, R. L., J r . J . Organomet.Chem. 1977, 142, 39. (15) Brown, H. C.; Adams, R. M. J . Am. Chem. SOC.1942,64,2557. (16)The trichloride (Me3Si),CSiCl, is stable to boiling MeOH and PhLi, illustrating the very large steric hindrance toward nucleophilic substitution at silicon caused by the trisyl group: Dua, S. S.; Eaborn, C.; Happer, D. A. R.; Hopper, S. P.; Safa, K. D.; Walton, D. R. M. J . Organomet. Chem. 1979, 178, 75.
Organometallics, Vol. 14,No. 6, 1995 3099 toward HBr in acetic acid, presumably because of the steric effect of the bulky trisyl group.'
Experimental Section General Procedure. All glassware was oven-dried and purged with nitrogen while hot before reactants were introduced. Unless specified, the standard apparatus consisted of a 500 mL three-necked roundbottomed flask with ground glass joints which was fitted with a Trubore stirrer, a Friedrich condenser, and an addition funnel. Trisyllithium was synthesized from trisyl bromide and lithium metal in Et20 according to published directions3a and standardized by acid titration of hydrolyzed aliquots using phenolphthalein as the indicator. Diethyl ether was dried over sodium wire, and other solvents were dried over molecular sieves. All other reagents were used as obtained from commercial sources without further purification. Ethyl trisyl ether was routinely characterized by comparison of its physical properties, IR and NMR spectral data, and GC retention times with those of authentic samples. Elemental analyses and the molecular weights were performed by Galbraith Laboratories, Inc., Knoxville, TN. Melting points were determined with an electrothermal melting point apparatus in sealed capillary tubes and were uncorrected. IR spectra were obtained on a Perkin-Elmer Model 710B double-beam grating IR spectrophotometer. GC measurements were effected with a Perkin-Elmer Sigma 3B instrument using a 3% OV-101 on 80/100 mesh Chromosorb W packed column (4 ft x 'ISin.). lH NMR spectra were recorded on a Varian EM 3630L spectrometer using carbon tetrachloride as the solvent. Chemical shifts are reported in d units (ppm) from internal chloroform. Trisyllithium with BBr3. Trisyllithium (0.2 mol) in 250 mL of Et20 was added dropwise over a 30 min period to stirred BBr3 (50 g, 0.2 mol) in 250 mL of hexane which was cooled to ca. 5 "C with an external ice-water bath. Upon completed addition, the bath was removed and the mixture was refluxed for 2 h prior to hydrolysis with 250 mL of ice-cold water. The organic layer was separated, and the aqueous layer was extracted once with Et20 (100 mL). The combined organic layers were dried over molecular sieves and filtered, and the volatiles were removed from the filtrate via distillation leaving a solid. Recrystallization from ethyl acetate/95% ethanol prior to sublimation at 150 "C/O.1 mmHg afforded pure ethyl trisyl ether (35 g, 64% yield), mp 248-251 "C (sealed tube). Anal. Calcd for C12H32OSi3: C, 52.03; H, 11.65; Si, 30.42. Found: C, 52.21; H, 11.90; Si, 30.09. The molecular weight was determined by vapor pressure osmometry in benzene: found 272 (calcd 277). The lH NMR spectrum showed d a t 0.11 (s, 27H, SiMes), 1.18 (t, 3H, Me), and 1.77 (9, 2H, CH2). Trisyllithium with BCb. To a stirred solution of trisyllithium (0.15 mol) in 315 mL of EtzO, cooled to ca. 5 "C with a n external ice-water bath, was added dropwise BC13 (0.19 mol) in 180 mL of hexane over a 30 min period. The ice water bath was removed and the mixture was refluxed for 3 h prior t o hydrolysis with 250 mL of ice-cold water. Workup in the usual manner followed by removal of the volatiles by distillation
3100 Organometallics, Vol. 14, No. 6, 1995 yielded a viscous liquid which, upon further heating under reduced pressure (-2 mmHg), afforded 33 g of a solid. Recrystallization from ethyl acetate/methanol prior t o sublimation at 150 "C/O.l mmHg afforded pure ethyl trisyl ether (18.5 g, 44.5%),mp 246-248 "C (sealed tube). Trisyllithium with BF3. To stirred trisyllithium (0.15 mol) dissolved in 155 mL of Et20 was added BFyEt20 (45.4 g, 0.32 mol) dissolved in 150 mL of hexane over a 5 min period prior to continued stirring at room temperature for 24 h. The reaction mixture was thereafter refluxed for 8 h prior to hydrolysis with 250 mL of ice-cold water. Workup in the usual manner was followed by removal of the volatiles by distillation to a viscous liquid which, upon heating under reduced pressure (-2 mmHg), afforded a 34 g of a solid. Recrystallization of the solid from ethyl acetate/methanol prior to sublimation at 150 "C/O.lmmHg afforded pure ethyl trisyl ether (15.5 g, 40% yield), mp 246-248 "C (sealed tube). Trisyllithium with AlCl3. Trisyllithium (0.06 mol) in 105 mL of Et20 was added over a 10 min period to a stirred suspension of anhydrous AlCl3 (7.5 g, 0.06 mol) in 200 mL of hexane a t room temperature. The exothermic (ca. 48 "C) reaction mixture was maintained a t reflux for 1 h with external heating. Volatiles were removed by distillation under nitrogen until the temperature of the mixture reached 70-72 "C, and thereafter the mixture was refluxed for an additional 2 h. After this time, about 100 mL of the solvent was removed by distillation under nitrogen and the mixture was allowed to cool to room temperature, depositing crystals of salts which were collected by filtration under reduced pressure. Volatiles were removed from the filtrate by distillation yielding a viscous liquid which, upon heating under reduced pressure (ca. 140 "C/O5 mmHg), afforded 15 g of a solid. Recrystallization from methanol prior to sublimation at 150 "C/O.1 mmHg afforded pure ethyl trisyl ether (9 g, 54% yield), mp 247-248 "C (sealed tube). Anal. Calcd for C12H32OSi3: C, 52.03; H, 11.65. Found: C, 52.10; H, 11.56. The molecular weight was determined by vapor pressure osmometry in benzene: found 268 (calcd 277).
Notes
Ethyl Trisyl Ether with HBr in Acetic Acid (Attempted). A mixture of ethyl trisyl ether (10 g, 0.04 mol), 48% aqueous HBr (5.5 mL) and 60 mL of acetic acid was refluxed for 20 h in a 100 mL round-bottomed flask fitted with a water condenser. After this time, ca. 1g of a white solid was visible and an additional 1mL of 48% aqueous HBr was added prior to continued reflux for 56 h subsequent to removal of acetic acid by distillation, leaving a solid. Recrystallization from methanol prior to sublimation at 150 "C/O.l mmHg afforded a 71% recovery of ethyl trisyl ether, mp 244248 "C (sealed tube). Ethyl Trisyl Ether with BCl3 (Attempted). A solution of ethyl trisyl ether (11.1g, 0.04 mol) and BCl3 (0.06 mol) in 60 mL of methylene chloride was allowed to stand under nitrogen in a sealed vial (100 mL) at 35 "C for 18 h. Hydrolysis of the reaction mixture with 250 mL of ice-cold water was followed by the usual workup yielded a solid. Recrystallization from ethyl acetate/methanol prior t o sublimation at 135 "C/1 mmHg afforded pure ethyl trisyl ether (7.2 g, 65% recovery), mp 245-248 "C (sealed tube). Ethyl "risyl Ether with Iodotrimethylsilane (Attempted). A stirred mixture of ethyl trisyl ether (10 g, 0.03 mol), chlorotrimethylsilane (3.6 g, 0.03 mol), and sodium iodide (5 g, 0.03 mol) in 50 mL of acetonitile was refluxed under nitrogen for 12 h in a 100 mL roundbottomed flask fitted with a water condenser topped with a nitrogen inlet tube. The cooled reaction mixture was hydrolyzed with 150 mL of ice-cold water prior to extraction with 150 mL of ether. The organic layer was shaken with saturated aqueous Na2S203 (150 mL), washed twice with cold water (150 mL), and dried over molecular sieves, and the volatiles were removed by distillation. Recrystallization residue from 95% ethanol afforded an 80% recovery of ethyl trisyl ether, mp 242245 "C (sealed tube). Acknowledgment. The National Institutes of Health (Grant No. 2 SO6 GM08023) is gratefully acknowledged for financial support. OM950085Q
