Boron trifluoride-Ethanethiol

ETHERS Boron trifluoride-Ethanethiol. Boron trifluoride etherate. Boron triiodide. Lithium triethylborohydride-Aluminum r-butoxlde complex. Phase-transfer catalysts. 

Stereoselective synthesis of medium-sized rings was possible from compounds such as (77 Scheme 15),l0la which were readily derived from intramolecular cyclopropanations (see equation 18). Treatment of (77) with 1 equiv. of ethanethiol in the presence of a catalytic amount of zinc chloride generated the hemithioacetal (78), which in the presence of boron trifluoride etherate formed the cycloheptane system (79), in which the two carbonyls are differentiated. 

Thioacidolyis is an acidolytic degradation method used to analyze lignin subunit composition. The method relies on the use of boron trifluoride etherate ((C2H5)2-0-BF3) and ethanethiol (C2H5-SH) to cleave (3-0-4 bonds (1.82) in the lignin (4.19). The reaction mechanism is shown in Figure 4.4. 

One mechanistic possibility for this thioacetalation reaction invokes furanoside ring-opening of 17, initiated by complexation with boron trifluoride etherate (Scheme 12.9). Oxonium ion 28 could then get intercepted by the ethanethiol to produce 29. After further complexation with BF3, in the manner shown, thionium ion formation can again occur to give 30, which can then engage in yet another nucleophilic addition with the ethanethiol to produce 31 after protonation. 

As in acidolysis (Chap. 6.1), thioacidolysis proceeds mainly by cleavage of arylglycerol-/ -aryl ether linkages. In contrast to acidolysis, however, thioacidolysis is performed in anhydrous media. The ether-cleaving reagent combines a hard Lewis acid, boron trifluoride etherate, and a soft nucleophile, ethanethiol (Node et al. 1976, Fuji et al. 1979). 

Thiiranes can be cleaved by alcohols in the presence of boron trifluoride and glacial acetic acid, yielding / -alkoxy thiols and using thiols affords 2-(alkylthio)ethanethiols.631 The general procedure is as follows ... 

Cleavage of THP-ethers. The boron trifluoride etherate-ethanethiol mixture cleaves THP ethers at low temperatures without affecting the methylenedioxy group, acetonides, mesitaldehyde acetals, methoxymethyl ethers, and r-butyldimethylsilyl ethers. 

Dealkylation of Ethers. The combination of an aliphatic thiol such as ethanethiol and boron trifluoride etherate has been used to remove benzyl groups. However, although that method works well it does suffer from an incompatibility with certain functional groups. The presence of an a,/3-unsaturated ester in the same molecule can result in Michael addition as well as debenzylation (eq 1). Boron trifluoride in the presence of dimethyl sulfide, being a milder reagent, does not cause this type of complication, as shown in eq 2. ... 

Cleavage of Benzyl Carbamates. The removal of the benzy-loxycarbonyl (Cbz) group from nitrogen can be achieved successfully using boron trifluoride etherate in the presence of either a thiol or dimethyl sulfide. The carbamates derived from secondary amines are cleaved more rapidly than those from primary amines using the ethanethiol method, even when using BF3-OEt2 as the solvent. The procedure allows reasonable selectivity, as shown in eq 11. ... 

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