DBU-Mediated Mild and Chemoselective Deprotection of Aryl Silyl Ethers and Tandem Biaryl Ether Formation

Abstract

An efficient method for the selective cleavage of aryl ­silyl ethers is established using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). With either 1.0 or 0.10 equivalent of DBU, smooth de­silylation of various aryl silyl ethers was accomplished selectively in the presence of alkyl silyl ethers and other base-sensitive groups such as acetate and ester. In addition, direct transformation of aryl silyl ethers into biaryl ethers using a catalytic amount of DBU was possible through tandem desilylation and S_NAr reaction with ­activated aryl fluorides.

chemoselective - catalytic - desilylation - DBU - biaryl ether

References and Notes

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The nitro-substituted aryl fluorides were chosen as representative substrates, and we found that aromatic fluorides substituted with other electron-withdrawing groups, such as cyano or formyl, were also effective. The results will be described in a full account.

<A NAME="RU11206ST-14">14</A>

Cleavage of Aryl Silyl Ethers (Table 3, Entry 2); Typical Procedure: To a magnetically stirred solution of tert-butyldimethyl(2-naphthalenyloxy)silane (460 mg, 1.78 mmol) in anhyd MeCN (3.4 mL) and H₂O (0.18 mL) was added DBU (0.26 mL, 1.78 mmol). After the starting material disappeared (TLC), sat. aq NH₄Cl solution (5 mL) was poured into the reaction mixture. The mixture was extracted with CH₂Cl₂ (2 × 5 mL), and the organic layer was collected, dried over MgSO₄, filtered, and concentrated under reduced pressure. The resulting residue was purified further by passing through a short silica gel column (ca 5 cm) and after vacuum evaporation pure 2-naphthol was obtained (250 mg, 98% yield). Desilylated position of bissilyl ether was determined by the chemical shift difference of the free alcohol or alkyl substituents on silicon in NMR. Generally, the chemical shifts of aryl alcohols are higher than those of the aliphatic alcohols and alkyl substituent of aryl silyl ethers also exhibited more downfield signals in ¹H and ¹³C NMR than those of aliphatic ones.

<A NAME="RU11206ST-15">15</A>

Tandem, One-Pot Biaryl Ether Formation (Table 5, Entry 2); Typical Procedure: To a magnetically stirred solution of tert-butyldimethyl-(4-methoxyphenoxy)silane (410 mg, 1.73 mmol) in anhyd DMSO (3.5 mL) and H₂O (4 µL) were added p-fluoronitrobenzene (152 µL, 1.44 mmol), and DBU (13 µL, 0.173 mmol) sequentially at r.t. The mixture was heated to 80 °C, and the stirring was continued until the aryl fluoride disappeared on TLC. After completion of the reaction, the mixture was partitioned between Et₂O (5 mL) and brine (5 mL), and the organic layer was separated, dried over MgSO₄, filtered, and concentrated under reduced pressure. The residue was purified through silica gel column chromatography (n-hexane-EtOAc = 6:1) to afford the desired pure biaryl ether (320 mg, 91% yield).