Boron trifluoride reactions with hydrides

Boron trifluoride is used for the preparation of boranes (see Boron compounds). Diborane is obtained from reaction with alkafl metal hydrides organoboranes are obtained with a suitable Grignard reagent. 

Enantiospecific syntheses of amino derivatives of benzo[ ]quinolizidine and indolo[2,3- ]quinolizidine compounds have also been achieved via A-acyliminium ion cyclization reactions, as an alternative to the more traditional Bischler-Napieralski chemistry (see Section 12.01.9.2.2). One interesting example involves the use of L-pyroglutamic acid as a chiral starting material to construct intermediates 240 via reaction with arylethylamine derivatives. Diisobutylaluminium hydride (DIBAL-H) reduction of the amide function in 240 and subsequent cyclization and further reduction afforded piperidine derivatives 241, which stereoselectively cyclized to benzo[ ]quinolizidine 242 upon treatment with boron trifluoride (Scheme 47) <1999JOC9729>. 

Benzyl Alcohols. Benzyl alcohols of nearly all kinds undergo reduction when treated with acid in the presence of organosilicon hydrides. The most obvious exception to this is the behavior of benzyl alcohol itself. It resists reduction by the action of trifluoroacetic acid and triethylsilane, even after extended reaction times.26 Reducing systems consisting of triethylsilane and sulfuric acid/acetic acid or p-toluenesullonic acid/acetic acid mixtures also fail to reduce benzyl alcohol to toluene.134 As previously mentioned, substitution of boron trifluoride for trifluoroacetic acid results in the formation of modest yields of toluene, but only when a very large excess of the silane is used in order to capture the benzyl cation intermediate and suppress Friedel-Crafts oligomerization processes.129,143... 

The value of methylation studies in structural determination of carbohydrates is well known. Methylation of sucrose has generally been achieved by the use of dimethyl sulfate-sodium hydroxide,34,35 methyl iodide-silver oxide-acetone,20 sodium hydride-methyl io-dide-N,N-dimethylformamide,35 or diazomethane-boron trifluoride etherate.36,37 The last method (already applied to monosaccharides38,39) has been found particularly useful for sucrose, because it proceeds without concomitant migration of acyl groups. The reaction of 2,3,6,T,3, 4, 6 -hepta-0-acetylsucrose (21) and 2,3,4,6,1, 3, 4 -hepta-O-acetylsucrose (22) with diazomethane in dichloromethane in the presence of a catalytic proportion of boron trifluoride etherate for 0.5 h at —5° gave the corresponding 4-methyl (23) and 6 -methyl (24)... 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

Borane, as a solution in tetrahydrofuran or generated in situ by the reaction of a metal hydride with boron trifluoride etherate, adds readily to alkenes to yield trialkylboranes. With a terminal alkene the reaction is highly (though not completely) regioselective and gives a primary trialkylborane, since the mode of addition results from the electrophilic character of the boron atom. 

Diborane can be prepared by a variety of methods, the most common being the reduction of boron trihalides with active metal hydrides - and the reaction of hydroborate salts with boron trifluoride, tin(II) chloride, sulfuric acid, methane-sulfonic acid, orthophosphoric acid, or polyphosphoric acid. Although diborane is commercially available in bulk quantities, we have found the reaction of potassium hydroborate, KBH4, with 85% orthophosphoric acid to be convenient for the rapid preparation in a vacuum line of small quantities of this material. ... 

Dihydroxylation of the stilbene double bond in the trans isomers of Combretastatin A-1 and A-4 produced diols which by treatment with boron trifluoride in ethyl ether [44] or with trifluoroacetic acid [17] resulted in pinacolic rearrangement to produce an aldehyde. The aldehyde was converted in a variety of derivatives, as illustrated in the Scheme 20, via the following reaction sequence reduction with sodium borohydride to primary alcohol which was derivatized to the corresponding mesylate or tosylate, substitution with sodium azide and final reduction to amine with lithium aluminum hydride. Alternatively the aldehyde was converted to oxime which was catalitically hydrogenated to amine [17]. 

Carboxylic Adds and Derivatives.—Esters may be reduced to ethers in low yield by lithium aluminium hydride-aluminium chloride.This reaction would not normally be used for preparative purposes, being more effectively carried out with sodium borohydride-boron trifluoride, but could complicate the use of LiAlH4-AICI3 for other purposes such as the reductive cleavage of the spiroacetal system in sapogenins. 

Danishefsky et al. (33) reported a method of one-carbon chain incorporation necessary for the elaboration to the five-membered D-ring of cephalo-taxine (Scheme 39). (See also Scheme 9, Section III, for the formal total synthesis of cephalotaxine by this method.) Reaction of dihydroisoquinoline 219 with acid chlorides 220 or 221 followed by the addition of aqueous sodium bicarbonate gave carbinolamides 222 or 223, respectively, which, on treatment with 1,3-propanedithiol and boron trifluoride etherate, yielded the ring-opened dithiane 224 or 225. These dithianes were converted by treatment with sodium hydride or potassium rert-butoxide to 226 or 227, respectively. Removal of the dithiane moiety in 226 or 227 by iV-bromosuc-cinimide gave the a-keto esters 228 or 229, which on treatment with Lawes-... 

A catalyst system159 consisting of a mixture of boron trifluoride, acetic acid and pyridine gives rapid and reproducible total hydroxyl analysis (silanol plus water) in a variety of silicone materials. This method avoids many of the interferences, empirical calibration and miscellaneous problems such as poor solubility, incomplete reaction and interfering siloxane cleavage associated with many earlier methods. Results obtained by this procedure compare well with results obtained by the lithium aluminium hydride procedure160. 

ALLYL PHENYL ETHER (1746-13-0) C ipO Combustible liquid (flash point 143°F/62°C). May form peroxides on contact with air. Violent reaction may occur with strong oxidizers, strong acids acetyl peroxide boron trifluoride aluminum hydride (possible explosion). Incompatible with nitrosyl perchlorate, ozone (may form explosive ethyl peroxide, aldehyde and acetic acid). On small fires, use dry chemical powder (such as Purple-K-Powder), foam, or CO2 extinguishers. 

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