Boronate esters from hydroboration

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

In the interim period, results have accumulated steadily, in endeavors to address and extend the chemistry beyond the initial perceived limitations. These limitations include the following (a) the effective catalytic syntheses are confined to the reactions utilizing catecholborane (b) the scope of alkenes for which efficient rate, regio- and enantio-selectivity can be achieved is limited, and (c) the standard transformation mandates the oxidation of the initially formed (secondary) boronate ester to a secondary alcohol, albeit with complete retention of configuration [8]. Nonetheless, for noncatalytic hydroboration reactions that lead to the formation of a trialkylborane, a wide range of stereo-specific transformations may be carried out directly from the initial product, and thereby facilitate direct C-N and C-C bond formation [9]. 

Sulfonylhydroxylamines and hydroxylamine O-sulfonic acid have found wide apph-cation in synthesis of amines from achiral or chiral organoboranes and boronate esters and the hydroboration-amination methodology is successfully used for direct amination of alkenes. 0-Sulfonyloximes were also found to be good reagents for synthesis of amines from organomagnesium, -copper and -zinc reagents. 

The boronate esters used for Suzuki reactions can be synthesized from commercially available alkyl-, vinyl-, and arylboronic acids. Alkyl and vinyl boronate esters are also synthesized by the hydroboration of double and triple bonds, similar to the hydrobora-tion of alkenes and alkynes in Chapters 8 and 9. Note that the boron atom generally adds to the less substituted end of a double or triple bond. Also, the B and H add to the same side of a triple bond (syn addition) to give a trans alkenylboronate ester. 

Terminal and internal (Z)-l-alkenylboronates are prepared from (Z)-(haloalkenyl)boronic esters (4) which can be readily obtained by hydroboration of 1-halo-1-alkynes [15, 17, 18]. 

Benzylic boronic esters [52]can be obtained from bromomethyl boronic esters and aryl or vinyl stannanes [53]. Alkyl and vinyl boronic esters are accessible through hydroboration of alkynes with dialkylboranes followed by oxidation with acetaldehyde [43]. Alternatively, vinylboronates can be obtained directly from alkynes in a reaction -with catecholborane without a subsequent oxidation step [53]. Further diversification in the boronic acid derivatization is achieved by binding the boronic acids bearing another derivatizable functional group (for instance an amine) to suitable polymeric supports. The modified boronic acid may then be... 

Some authors have described the generation of boronates and stannanes bound to a support starting from the corresponding aryl haHdes [372, 6]. Boranes, boronic esters, and stannanes can furthermore be readily obtained from vinyl halides or from alkenes or alkynes by means of hydroboration or hydrostannylation (see Section 4.2). Boronates and silanes or stannanes can act as carbanion equivalents. Thus, support-bound boronates can release aryl alkenyl groups upon transmetala-tion to Rh. The intermediately formed Rh species can act as nucleophiles, and react with aldehydes to give alcohols (618) or can perform Michael additions (621, 622) [437, 438, 439] (Scheme 129) (see also Sections 4.7.15 and 4.2). 

Removing the aromatic ring from the diene makes good sense once we know about Suzuki coupling as it allows us to build the two halves of the molecule separately. There is a free choice as to whether we put the boron atom on the diene or on the aromatic ring, but Marko put it on the diene because he planned to make the vinyl boronate 248 by hydroboration of an alkyne 249. Note the protection of the phenol and the carboxylic acid as methyl ether and ester respectively. 

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... 

Although group 5 organometallic systems have been found to be of relevance in transition-metal catalyzed hydroboration reactions, structurally authenticated group 5 boryl complexes remain relatively few in number. Smith and co-workers, for example, have probed the mechanisms for the formation of niobium and tantalum mono- and bis(boryls) from propylene complex precursors, with concomitant formation of propyl boronate esters [31,32]. Of particular interest from a structural viewpoint are the relative merits of alternative bonding descriptions for metal(V) boryl bis(hydrides) as borohydride complexes or as mono(hydride) a-borane systems [31-34]. 

A problem with this methodology is that only one of the three alkyl groups is transferred to the unsaturated carbonyl compound. A solution to this uses the radical generated from the boronic ester, itself derived from hydroboration with catecholb-orane 8. Treatment of the boronic ester with oxygen and 1,3-dimethyl-hexahydro-2-pyrimidinone (DMPU) in the presence of the a,3-unsaturated aldehyde or ketone gives the desired radical addition product, with transfer of the S-alkyl group. Thus, cyclohexene was converted to l-cyclohexyl-3-pentanone 24 using this chemistry (5.38). 

The hydroboration of 1,3-dienes has also been reported, and these reactions generate Z-allylic boronic ester products. These reactions have been reported with palladium catalysts and are thought to occur through Tr-allyl intermediates. Reactions with butadiene and isoprene, followed by addition of the product to benzaldehyde, are shown in Equation 16.47. This equation also shows the presumed mechanism that proceeds by generation of a palladium allyl from the combination of diene and palladium hydride formed by oxidative addition of the borane. 

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