Boron trifluoride alkynylation

The alkynyl reagent 9 was recently introduced for the dia stereoselective synthesis of tertiary propargylic alcohols144. 9 can be prepared as a solid 1 1 complex with tetrahydrofuran by treatment of 9-methoxy-9-borabicyclo[3.3.1]nonane with (trimethylsilylethynyl)lithium, followed by addition of boron trifluoride-diethyl ether complex. The nucleophilic addition of reagent 9 to (R)-2-methoxy-2-methylhexanal (10) afforded a mixture of the diastereomers 11 with a considerable preference to the nonchelation-controlled (3S,4R)-isomer144. 

Cumulenes have also been prepared by treating alkynyl epoxides with boron trifluoride. 1,4 Elimination of BrC C=C—CBr has been used to prepare conjugated dienes (C=C—C=C). ... 

To a suspension of iodosylbenzene (1.1 g, 5 mmol) and the appropriate 1-trimethylsilyl-l-alkyne (5 mmol, prepared from 1-alkyne and trimethylchlorosilane in nearly quantitative yield, or obtained commercially) in chloroform (10 ml) was slowly added boron trifluoride etherate (710 mg, 5 mmol) at 0°C. The mixture was stirred at room temperature for 3-4 h, then recooled at 0°C and a solution of p-toluenesulphonic acid hydrate (3.8 g, 20 mmol) in water (20 ml) was added the resulting mixture was stirred vigorously for a few minutes. The organic phase was separated and the aqueous phase was washed with additional chloroform. The combined organic phase was washed with water, dried and concentrated. The residual oil solidified upon addition of ether. The solid was filtered, washed with ether and air-dried to give alkynyl phenyliodonium tosylates (62-89%). 

Alkynyl dialkyl phosphates of the general formula RC=COPO(OR )2, were prepared by a similar spontaneous decomposition of the less stable salts RC=CI+ PhC OPO(OR)2, some of which are isolable alternatively, either alkynes and [hydroxy(dialkylphosphoryloxy)iodo]benzene or alkynysilanes and iodosylben-zene-boron trifluoride can be used [63]. 

Boron reagents add to conjugated carbonyl compounds. Alkynyl borate esters (p. 815) give conjugate addition in the presence of boron trifluoride ethe-... 

Reaction of 1-alkynyl thiocyanates (300) with alcohols, phenols, thiols, or secondary aliphatic amines in the presence of a Lewis acid (anhydrous zinc chloride or boron trifluoride diethyl etherate) affords 2,4-disubstituted thiazoles (301) in good yields (Equation (52)) <82RTC310>. 

A common method for preparing alkynyl(phenyl)iodonium tetrafluoroborates involves the reaction of iodosylbenzene in the presence of triethyloxonium tetrafluoroborate or boron trifluoride etherate with alkynyl-silanes. For example, the complex of iodosylbenzene with triethyloxonium tetrafluoroborate (362) reacts with alkynylsilanes in dichloromethane at room temperature to afford alkynyl(phenyl)iodonium tetrafluoroborates 363 in good yield [496]. A variation of this procedure employs the complex of iodosylbenzene with boron trifluoride etherate (364) followed by treatment with aqueous NaBp4 [493,497] or sodium arylsulfonates to furnish, respectively, the appropriate alkynyl(phenyl)iodonium tetrafluoroborates 363 [483,487] or organosul-fonates 365 [488,489] (Scheme2.102) [498,499],... 

B-l-Alkynyl-9-BBN, easily and quantitatively prepared by the reaction of boron trifluoride-diethyletherate with the corresponding lithium methyl alkynyldial-kylborinate [12], undergoes a smooth 1,4-addition in pentane at room temperature to methyl vinyl ketone (MVK) and ketones capable of adopting cisoid conformation [13]. The reaction proceeds through srx-membered cyclic transition state to afford the enol borinate intermediate, which on hydrolysis afford y,6-acetylenic ketones, in high yields (Scheme 7.8). 

The first synthesis of a cyclic hypervalent iodine reagent was reported by Ochiai and co-workers in 1991 by the reaction of 2-hydroxy-benziodoxolone 28 with alkynyl trimethylsilane 29 in the presence of boron trifluoride etherate (Scheme 12,... 

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