Boron compounds trialkylboranes

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]. 

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]. 

Apart from the boronic acids/boronates, and trialkylboranes, another important class of boron compounds that have been utilized for Suzuki-Miyaura cross-coupling reaction include the alkyl/... 

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... 

Another route to carbon-carbon bond formation is via iodine-induced rearrangement of adducts of trisubstituted boron compounds and organolithium reagents. The adducts of trialkylboranes with lithium acetylides give an akylated acetylene. The mechanism involves electrophilic attack by iodine on the acetylenic... 

In addition to trialkylboranes, various alkoxyboron compounds have prominent roles in synthesis. Some of these, such as catecholboranes (see. p. 340) can be made by hydroboration. Others are made by organometallic or related substitution reactions. Alkoxyboron compounds are usually named as esters. Compounds with one alkoxy group are esters of borinic acids and are called borinates. Compounds with two alkoxy groups are called boronates. Trialkoxyboron compounds are borates. 

Organoboranes can also be used to construct carbon-carbon bonds by several other types of reactions that involve migration of a boron substituent to carbon. One such reaction involves a-halo carbonyl compounds.20 For example, ethyl bromoac-etate reacts with trialkylboranes in the presence of base to give alkylated acetic acid derivatives in excellent yield. The reaction is most efficiently carried out with a 9-BBN derivative. These reactions can also be effected with (3-alkenyl derivatives of 9-BBN to give (3,y-unsaturated esters.21... 

However, these compounds proved to be unstable and difficult to characterize. The authors reasoned that the source of the instability was likely to be the trialkylboron moiety. Boronates are more stable than trialkylboranes since the lone-pairs of electrons on an oxygen atom can donate to the empty orbital of a boron atom. The corresponding gem-boriozirconocenes should also be more stable. Thus, hydrozirconation of the alkenyl-... 

Extension of this reaction to electrophiles other than aldehydes was unsuccessful [22, 23], However, propargylic boronates were found to react with allylic halides and various carbonyl compounds [23], The boronates were prepared by lithiation of a methyl-substituted alkyne with t-butyllithium followed by treatment with a trialkylborane. The propargylic boronate preferentially reacts with the electrophile at the y-position to yield propargylic products (Eq. 9.20). The methodology has also been applied to alanates with comparable results. 

Some other important aspects of boric acid chemistry are summarized in Fig. 5-26. Among these is the formation of borate esters [B(OR)3, R = alkyl or aryl], usually obtained as colorless liquids, on treatment with alcohols and H2S04. The vast literature on these compounds falls generally within the purview of organic chemistry, and will not be developed here however, it will be noted that a well-known qualitative test for boron involves treatment of the sample with methanol to form B(OMe)3, which produces a bright green color in a Bunsen burner flame. A major early discovery in this area was the synthesis of boronic acids by E. Frankland in 1862, via partial oxidation of trialkylboranes, with subsequent hydrolysis of the ester ... 

Niedenzu initiated studies of B-substituted analogs of (47) and (48) and various X = chloro, fluoro, alkyl, aryl, alkoxy, and amido derivatives of (49). These compounds should be very useful for further synthetic transformations. Koster and coworkers have examined the reactions of sterically congested pyrazoles with (9H-9-BBN)2 and unique neutral, monomeric products with tricoordinate boron centers (50) are obtained. The compounds are stable toward dimerization. When bulky pyrazoles are allowed to react with activated trialkylboranes, for example, BEts at 170 °C,... 

We often draw hydroboration as if addition stopped after one equivalent of alkene reacts with BH3. Instead, all three B-H bonds actually react with three equivalents of an alkene to form a trialkylborane. The term organoborane is used for any compound with a carbon-boron bond. 

The products (6) are very prone to hydrolytic cleavage by base this is presumably due to the ready production of an intermediate carbanion followed by its rapid protonation. This possibility was tested by treating a series of compounds (6 R = cyclohexyl) with Bu"Li at -78 C in THF. The products had properties consistent with the production of (7 equation 2). The by-product, a simple trialkylborane, also reacts with butyllithium, and so 2 equiv. of the latter must be add. Thus, the process is wasteful in that one atom of boron and 1 equiv. of base are consumed in an unproductive fashion (equation 3). 

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). 

The treatment of di- or trialkylborane products with MeCHO regenerates a-pinene and delivers chiral boronates ( 2.3), which are precursors of many enantioenriched, branched alkyl-substituted compounds including olefins, ketones, aldehydes, acids, nitriles or diamines [580, 583, 585, 587, 595],... 

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... 

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