Unusually Stable Silyl Ketones with Bowl-shaped Tris(2,6-diphenylbenzyl)-silyl Group for Various Nucleophilic Attacks and α-Deprotonation

Abstract

A newly designed, bowl-shaped tris(2,6-diphenylbenzyl)silyl (TDS) group can be successfully utilized as a highly effective shield of ketone carbonyls for various nucleophilic attacks and α-deprotonation. The high shielding effect of the bowl-shaped TDS moiety toward carbonyl and even α-protons is verified by the ¹H NMR study of TDS ketone 1a in CDCl₃, and by its X-ray analysis. The TDS ketone 1b can be cleaved with Bu₄NF/BF₃·OEt₂.

Key words

ketone - silicon - nucleophilic attack - alkyllithium - Grignard reagent

References

• 1 Greene TW. Wuts PGM. Protective Groups in Organic Synthesis   3rd ed.:  John Wiley and Sons; New York: 1999.  p.293 
• 2 Iwasaki A. Kondo Y. Maruoka K. J. Am. Chem. Soc.  2000,  122:  10238 
• Reviews:
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• 3b Weber WP. Silicon Reagents for Organic Synthesis   Springer-Verlag; Berlin: 1983. 
• 7a Kawahara S. Wada T. Sekine M. Tetrahedron Lett.  1996,  37:  509 
• 7b Gevorgyan V. Yamamoto Y. Tetrahedron Lett.  1995,  36:  7765 
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4

Synthesis of α-TDS ketones 1a and 1b (Scheme [4] ):

Scheme 4 Conditions: (a) cat. Pd(OAc)₂ (20 mol%), 30% H₂O₂, solvent; (b) t-BuLi, BnBr, THF; (c) t-BuLi, HMPA, BnBr, THF.

The starting allylsilane 10 was prepared from allyltrichlorosilane in a similar manner as described in the synthesis of tris(2,6-diphenylbenzyl)silane. See ref. ²

5

Quenching with D₂O in place of benzaldehyde acceptor resulted in 99% recovery of the starting α-TDS ketone 1b.

6

The single-crystal of α-TDS ketone 1a was obtained by recrystallization from benzene/ether solvents. Crystal structure data for 1a: C₆₇H₅₆OSi·2 C₆H₆, M _w = 1061.47, monoclinic, space group P-2/a (#13), a = 20.629 Å, b = 14.155 Å, c = 22.437 Å, V = 6274.8 ų, Z = 4, D _calcd = 1.111 gcm^-3, R ₁ = 0.121.