Boronic preparation from trialkylboranes

Mercuration. Mercury(II) salts react with alkyl-, alkenyl-, and arylboranes to yield organomercurials, which are usehil synthetic intermediates (263). For example, dialkyhnercury and alkyhnercury acetates can be prepared from primary trialkylboranes by treatment with mercury(II) chloride in the presence of sodium hydroxide or with mercury(II) acetate in tetrahydrofuran (3,264). Mercuration of 3 -alkylboranes is sluggish and requires prolonged heating. Alkenyl groups are transferred from boron to mercury with retention of configuration (243,265). 

Besides the methyl compounds a number of other alkyldiboranes have been prepared from diborane and trialkylboranes (111, 128). The alkylated diboranes can also be obtained by treating at elevated temperatures NaBH4, LiBH4, or LiAlH4 with R3B, and either u hydrogen halide or a boron trihalide (83b). 

Dialkyl(alkyl-A/-1H-tetrazol-5-ylalkan(or benzenecarbox)imidato)-boron compounds of type 29 have been prepared from 5-aminotetrazole, nitriles, R CN, and trialkylboranes, R3B, by heating the stirred mixture (containing an excess of R CN) to 120°C [20, 21]. The derivatives are characterized by mass spectral, IR, and UV data. An X-ray structure analysis was performed on 29f. The bicyclic skeleton of this compound is almost planar with some distortion about the boron atom [20]. 

Two improved procedures for the preparation of borinic and boronic esters from "ate" complexes have been developed and a practical synthesis of dichloromethyl and 1,1-dichloroethyl boronic esters reported. The effectiveness of representative homologating agents for trialkyIboranes, borinic and boronic esters have been explored and LICHCI2, LiCHClSlMe3 and LiCH(OMe)SPh shown to work well with all three substrates. Other reagents are effective with trialkylboranes but not with boronic esters. Available procedures for homologation of boronic esters have been critically examined and a more convenient route reported. ... 

Allenylboranes can be prepared by treatment of propargylic acetates with butyl-lithium and a trialkylborane [19]. The reaction proceeds by initial formation of an alkynylboronate followed by migration of an alkyl group from boron to carbon (Eq. 9.17). 

By hydroboration of natural products such as a-pmene, H. C. Brown and coworkers have prepared mono- 2.15 (R = H) and diisopinocampheylboranes 2.16 (R = H). These reagents promote highly enantioselective hydroborations [580, 583], The two a-pinene enantiomers are available, so both enantiomers of these reagents can be used. The intermediate di- or trialkylboranes formed in these hydroborations are treated with MeCHO. This forms a chiral boronate 2.17, and the a-pinene is freed for recovery and recycling. From 2.17, it is possible to obtain many functionalized compounds. Additionally, new chiral boranes 2.18 are available, and these are precursors of many chiral compounds bearing the R group [169, 580, 583, 585-588] (Figure 2.2). 

Although symmetrical dialkylborinic acids can be obtained from boron alkoxides or boronic esters and organolithium or RMgX reagents " , these compounds usually are more conveniently synthesized, either by partial hydrolysis of trialkylboranes , or by hydroboration of alkenes with monohalogenoboranes followed by hydrolysis (see also refs. 1-8, 5.3.2.4 and ref. 2, 5.3.2.4.1). Diarylborinic acids are prepared by the reaction of 2 mol of ArMgX with trialkoxyborane at low The products are... 

The dialkylchloroboranes are stronger Lewis acids and consequently reduce ketones under milder conditions than those required for the trialkylboranes. They are prepared by hydrobo-ration of an alkene to produce a dialkylborane which is then treated with hydrochloric acid. In the case of diisopinocampheylchloroborane, the intermediate diisopinocampheylborane crystallizes from solution, and the major enantiomer of a-pinene is accumulated on boron. The final borane is thus enriched to 99% ee. The boranc reagent derived from (+ )-a-pinene has a negative rotation. 

Alkyl iodides (3, 160). The preparation of alkyl iodides by reaction of trialkylboranes with iodine and a base is improved by use of sodium methoxide in place of sodium hydroxide. Two groups on boron react rapidly in the case of trialkylboranes derived from terminal olefins, the third group also reacts, but more slowly (24 hr.). ... 

An excellent in situ preparation of diborane results from the reaction of boron trichloride (BCI3) or boron trifluoride (BF3) with sodium borohydride in the presence of alkenes, as shown above. This reagent gave virtually instantaneous conversion of an alkene to a trialkylborane at ambient temperatures, when done in a ether solvent. When diborane was isolated and purified it also reacted with alkenes, rapidly and quantitatively in ether solvents. 

The dialkyl(alkyl-A/-1H-1,2,4-triazol-3-ylbenzenecarboximidamidato)-boron compounds of type 30 are prepared in analogy to 29 from the corresponding trialkylboranes by reaction with 5-amino-1,2,4-triazole in refluxing xylene and treating the intermediate with a 3- to 5-fold excess of the corresponding aryl nitrile under reflux. The mass spectrum, the NMR spectrum, and IR data are given for the various species. The crystal and molecular structure of 30a has been determined by X-ray diffraction it resembles that of 29f [22]. 

The increasing importance of boronic acids as synthetic intermediates has justified the development of new, mild and efficient methods to provide access to a large pool. Of particular interest is the synthesis of arylboronic acids substituted with a wide range of other functional groups. As a consequence of their growing popularity and improvements in methods available for their preparation, many functionahzed boronic acids have become available from several commercial sources. Although several methods, like the oxidation or hydrolysis of trialkylboranes, have significant historical and fundamental relevance, this section is devoted mainly to modem methods of practical value to synthetic chemists. 

Preparation.—Variations continue to appear on the theme of alcohol production by hydroboration-oxidation of olefins. 5-Methoxydialkylboranes react with olefins in the presence of lithium aluminium hydride to afford a new route to trialkylboranes and thence, by carbonylation-oxidation, to trialkylcarbinols. Carbonylation with carbon monoxide is avoided in a new procedure in the presence of an excess of trifluoroacetic anhydride, trialkyl cyanoborates undergo a triple alkyl migration from boron to carbon to give, on oxidation, high yields of trialkylcarbinols (Scheme 126). Tri-... 

The reaction of 2-bromo-6-lithiopyridine (13) with trialkylboranes gives intermediate boron compounds which are versatile intermediates for the preparation of unsaturated nitriles (Scheme 9). A stereospecific synthesis of dehydronerol utilizes the dianion of 3-methylbut-2-enoic acid as an isoprene functionality (Scheme 10). Lithium dianions from aj8-unsaturated acids generally undergo alkylation reactions at the a-carbon atom. In contrast the dicopper dianions undergo more selective y-alkylation (62—99%) and this ratio is generally higher than with the corresponding esters. A study of various acids and their alkylation with allyl electrophiles showed that allylic electrophiles unsubstituted at the y-carbon react... 

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