Functionalized -vinylboronates

The method provides an efficient and convenient route to access a wide variety of unsymmetrical 1,3-dienes, because the borylation proceeds in high yields under mild conditions and is applied to the preparation of 1-alken-2-ylboron, cyclic vinylboron, and functionalized vinylboron compounds which cannot be directly obtained by the conventional methods based on hydroboration and transmetalation. Table 4.3 shows generality of the process. 

A series of remarkable stoichiometric CH functionalization reactions have been employed, including transmetalla-tion with a vinylboronic acid moiety, to synthesize the core of teleocidin B-4 (Equation (200)).1... 

Selected 1-alkenylboronates synthesized by Equation (56) are shown in Scheme 17. The reaction afforded cyclic and acyclic vinylboronic esters possessing various functional groups, which are not available by conventional hydro-boration of alkynes or by a transmetallation method, for example 160-167. 

Based on our experience in using A1203 as base in the iodination15 and bromination16 of vinylboronic acids, we hoped that this solid would likewise function as base in the Suzuki reaction. To simplify the reaction, we hoped to use ligandless palladium black [Pd(0)] as catalyst, thus alleviating the need for expensive or custom-made phosphine ligands. 

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... 

An electronically and sterically diverse array of arylboronic acids serve as useful reaction partners (Table 2, entries 1-4). In addition, vinylboronic acids can be cross-coupled in good yield (entry 5), although reactions of alkylboronic acids proceed with somewhat lower efficiency (entry 6). The catalyst tolerates a range of functional groups, including esters, thioethers, and cyanides. 

The chemists have long believed that the trivalent boron atom with its empty p orbital should behave in a fashion to that of the traditional organic electron-withdrawing group [1]. This idea is attractive when applied to the activation of Diels-Alder dienophiles, because the intermediate boron compounds can be transformed to an array of different functional moieties, none of which can usually be obtained by a direct Diels-Alder reaction. Matteson [2] and Evans [3] have reported the reactions of vinylboronic esters with dienes under strenuous reaction conditions but with low yields of the products. Similarly, vinyldichlo-roborane gives low yield [4] on reaction with cyclopentadiene. 

Recently, McKinley and O Shea [43] introduced the use of a trivinylboroxine/pyri-dine complex in place of the imstable vinylboronic acid for 0-vinylation (Scheme 5.24). Excess pyridine (10 equiv.) was necessary to obtain high yields. Mechanistic studies revealed the possibility of the formation of intermediate 22. One postulated function of pyridine is involvement in the conversion of the trigonal boron into tetrahedral boron to facilitate the transmetallation of the vinyl group to the copper center (as in transition state 23). The excess pyridine could be replaced by 1 equiv, of CS2CO3 when the trivinylboroxine/pyridine complex is used. Thus, the second postulated function of pyridine is as a base (vide infra). 

Keywords Monosubstituted alkynes, methylpentanediolborane (MPBH), HZrCp2Cl, dichloromethane/toluene, room temperature, regio- and stereoselectivity, functionalized (E)-vinylboronates... 

A dramatic rate enhancement was realized by Kishi using TlOH as a base. The coupling can be achieved even at 0°C and a wide variety of sensitive functional groups can be readily incorporated into the synthetic protocols because of these mild conditions. Kishi utilized these conditions in the synthesis of Palytoxin via the coupling of vinylboronic acid 102 with the vinyl iodide 103 (Scheme 28.25). However, TlOH is air and moisture sensitive and has a limited shelf life. In this regard, Roush found TlOEt to be superior to TlOH due to its ready availability and greater stability. " ... 

While most of the hydroboration methods resulted in the formation of p-substituted species, a rare example of an addition reaction that formed the a-substituted vinylboronate has been reported (Example 6.6) [78], The overall reaction was a two-step process that proceeds through the initial formation of a vinyl aluminate that was quenched by the addition of methoxy(pinacolato)borane. The reaction was highly regioselective, and less than 2% of the p-substituted product was formed. In addition to the functionalization of styrene, other arylalkynes as well as alkylalkynes were converted into a-substituted vinylboronates in high yield with outstanding selectivity. 

The formal hydrochlorination of alkynylboronates can be achieved through the use of Cp ZrCHICl followed by the addition of NCS (Scheme 6.44) [84], The chemistry was regioselective for the formation of the -isomer, and the yields of the chlorinated vinylboronates were fair to moderate. The incorporation of the chlorine was valuable since it provided a synthetic handle for further functionalization. A variation of this approach was used in the stereoselective hydroboration of terminal alkynes (Scheme 6.45) [85]. 

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