Alkenylboronic acid esters

Fortunately, though, there is one borane, catecholborane (A in Figure 16.15), that adds to C=C triple bonds but not to C=C double bonds (in the absence of transition metal catalysts). This reagent adds to alkynes with cw-selectivity, so that the reaction stops at the stage of trans-alkenylboronic acid esters (B in Figure 16.15). 

These boronic esters are easily hydrolyzed to give /ra .v-alkenylboronic acids with complete retention of their stereochemistry (C in Figure 16.15). Alkenylboronic esters and alkenylboronic acids are organometallic compounds that can be alkenylated and arylated in Pd-catalyzed reactions (Section 16.4.2). Aside from this, the iraws-alkenylboronic acid esters as well as the /rau.v-alkenylboronic acids are valuable precursors of haloalkenes (Figure 16.15). 

Alkenylboronic acid esters B react with bromine to give the trans-addition product initially. This primary product is not isolated but is immediately reacted with a solution of NaOMe in MeOH. Addition of MeO ion to the B atom of the bromine adduct forms the borate complex D. This borate complex is converted into a cis-configured bromoalkene and a mixed boronic acid ester by way of a / elimination. 

Fig. 16.20. Alkenylation of isomeric alkenylboronic acid esters with isomeric iodo-alkenes stereoselective synthesis of isomeric 1,3-dienes.

Palladium-catalyzed methylene transfer from diazomethane has proved effective for the cyclopropanation of 1-alkenylboronic acid esters allylic alcohols and amines 1-oxy-l,3-butadienes and allenes " Readily accessible iron complex (CO)2FeCH2S Me2 BF4 35 undergoes direct reaction with a range of alkenes to give cyclopropanes (equation 67) The salt is sensitive to steric effects and the reaction proceeds... 

Interesting compounds, such as cyclopropylboronic acids and esters which are otherwise not easily accessible due to the apparent lack of convenient and versatile syntheses, can now be prepared by simple cyclopropanation with diazomethane. The carbene transfer to substituted and functionalized alkenes also takes place smoothly in the presence of palladium(II) acetate. Several 1-alkenylboronic acid esters 9 were converted to cyclopropylboronic acid esters 10 using this method. ... 

The carbenoid generated from diiodomethane/diethylzinc is often found to be more reactive than the conventional Simmons-Smith reagent. Thus, in the case of 1-alkenylboronic acid esters, where diazomethane failed to cyclopropanate trisubstituted derivatives (Section 1.2.1.1.1.) diiodomethane/diethylzinc gave good yields of the required products 16. ... 

Under N2, CH2N2 in Et2018 (30 mL, ca. 14 mmol) was added dropwise over a period of 10 min to a magnetically stirred mixture of 1-alkenylboronic acid ester (2 mmol) and Pd(OAc)2 (10 mg) in anhyd Et20 (5 mL). When half the addition had been made, more catalyst (10 mg) was added. After removal of the catalyst by filtration through activated neutral alumina (3 g), the solution was concentrated at 15 Torr, and the residue was purified by Kugelrohr distillation yield 63-92%. 

We developed several simple routes for the syntheses of pure isomers of CLA. The synthesis via an enyne-substructure (Fig. 3.6) required r-l-bromo-l-heptene [5] and l-(2 -tetrahydropyranyloxy-y)-10-undecyne [2] as building blocks. Cft-l-bromo-1-heptene was prepared by reaction of commercially available 1,3,2-benzodioxaborole with 1-heptyne [3], yielding the 1-alkenylboronic acid ester [4] after heating for 2 h. No solvent was required. The f 2 j-configurated double bond formed during the... 

Figure1.21 Common methods for the synthesis of alkenylboronic acids (esters).

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