Alkene-Pinacolborane Hydroborations Catalyzed by Lanthanum Tris[bis(trimethylsilyl)amide]

Abstract

Tris[bis(trimethylsilyl)amide] has been shown to be an effective catalyst for the hydroboration of representative alkenes and styrenes by pinacolborane.

Key words

hydroboration - lanthanides - catalysis

References

• 1 Pelter A. Smith K. Brown HC. Borane Reagents   Academic Press; New York: 1988. and references cited therein
• 2a Evans DA. Fu GC. Hoveyda AH. J. Am. Chem. Soc.  1992,  114:  6671 ; and references cited therein
• 2b Hayashi T. Matsumoto Y. Ito Y. J. Am. Chem. Soc.  1989,  111:  3426 
• 2c Burgess K. Ohlmeyer MJ. J. Org. Chem.  1988,  53:  5179 
• 3a Harrison KN. Marks TJ. J. Am. Chem. Soc.  1992,  114:  9220 
• 3b Bijpost EA. Duchateau R. Teubin JH. J. Mol. Catal. A  1995,  95:  121 
• 4 Molander GA. Pfeiffer D. Org. Lett.  2001,  3:  361 
• 5a Kim YK. Livinghouse T. Bercaw JE. Tetrahedron Lett.  2001,  42:  2933 
• 5b For the use of novel group 3 chelates to accelerate intramolecular aminoalkene hydroaminations, see: Kim YK. Livinghouse T. Angew. Chem. Int. Ed.  2002,  41:  3645 
• 5c Kim YK. Livinghouse T. Horino Y. J. Am. Chem. Soc.  2003,  125:  9560 
• 6 Horino Y. Livinghouse T. Organometallics  2004,  23:  12 
• 7 Garrett CE. Fu GC. J. Org. Chem.  1996,  61:  3224 
• 8 Srebnik has reported the use of (Ph₃P)₃RhCl as a catalyst for the hydroboration of representative alkenes by 2: Pereira S. Srebnik M. Tetrahedron Lett.  1996,  37:  3283 
• 9 By way of contrast, Knochel has reported that crude 2 (2 equiv, prepared in situ from BH₃·SMe₂ and pinacol) is capable of hydroborating alkenes (ClCH₂CH₂Cl, 50 °C, 48 h): Tucker CE. Davidson J. Knochel P. J. Org. Chem.  1992,  57:  3482 
• 11 Evans has reported that SmI₃ is a catalyst for the hydroboration of alkenes with catecholborane: Evans DA. Muci AR. Stürmer R. J. Org. Chem.  1993,  58:  5307 
• 12 Temperatures of 125-200 °C are required for effecting the hydroboration of alkenes using BH₃·NEt₃: Hawthorne F. J. Org. Chem.  1958,  23:  1788 

10

¹H NMR revealed >99% conversion of 3a to the corresponding boronic pinacolate.

13

A General procedure for alkene hydroborations is as follows: In an argon-filled glove box, La[N(TMS)₂]₃ (1a, 18.6 mg, 0.03 mmol) and C₆D₆ (1.0 mL) were introduced into a J. Young NMR tube equipped with teflon screw cap and then pinacolborane (2, 256 mg, 2.0 mmol) and the appropriate alkene (1.0 mmol) were added by micro syringe. The homogeneous reaction mixture was maintained at 90 °C or 120 °C, respectively, using an oil bath until the hydroboration reaction was judged complete by the disappearance of the appropriate olefinic resonances in the ¹H NMR spectrum. The resulting solution was transferred to a 25 mL round-bottomed flask and was cooled to 0 °C with stirring whereupon 1:1 = THF:EtOH (6 mL), 2 N NaOH (3 mL) and 30% H₂O₂ (3 mL) were added. The reaction mixture was allowed to warm to r.t. and was stirred for 9 h. It was then extracted with Et₂O (3 × 10 mL) and the combined organic phases were dried over MgSO₄. After removal of the solvents in vacuo, the crude product was purified by column chromatography on silica gel (5% EtOAc-hexane for elution) to provide the corresponding alcohol as a colorless oil.

14

An analogous catalytic cycle has been proposed in connection with the hydroboration of alkenes by catecholborane (4) catalyzed by permethyllanthanocene complexes. [3a]

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