1 -alkenylboron reagents

Since a variety of 1-alkenylboron reagents including (E)- and (Z)-isomers are now available, their cross-coupling with 1-halo-l-alkenes affords various stereodefined alkadienes and trienes [64-66]. Many syntheses of alkadienes and trienes such as unsaturated fatty acid amides [76], alkenylsilanes [77], gem-difluoroalkenes [69, 78], cyclic alkenes [79], rran5-(C,o)-alIofamesene [80], trisporol B [81], and vinyl sulfides [82] are reported, by application of the reported Pd-catalyzed cross-coupling. The representative syntheses and reaction conditions ai e summarized in Table 2-2. 

The Pd(PhCN)2Cl2-catalyzed cross-coupling reaction of 1-alkenylboronic reagents with 3,4-epoxy- 1-butene afforded a mix-toe of 2-(l-vinyl)-3-alkenols and alka-2,5-dienols (eq 21). ... 

Complementary to the use of zinc reagents for the introduction of (functionalized) alkyl groups is the rhodium-catalyzed conjugate addition of aryl- and alkenylboron reagents. This method rapidly became popular, also because arylboron reagents are air and moisture stable and a large variety of them is commercially available . 

The addition of diboron compounds to alkynes is an excellent method for the synthesis of c -diboryl alkenes (Scheme 2-11) [34], The reaction is catalyzed by Pt(PPh3)4 at 80 and works well not only with terminal but also with internal alkynes. The addition of the Si-B [35] or Sn-B [36] bonds to alkynes gives mixed-metal alkenylboron reagents which have potential ability for use in the stepwise double cross-coupling reaction at both metallated carbons. 

The Pd(0)-catalyzed addition of the B-S bond to alkynes (thioboration) regio- and stereoselectively produces (Z)-2-(organothio)-l-alkenylboron reagents (Scheme 2-25) [63]. Tlie addition of (tetraalkoxy)diboron to alkynes to give cw-bis(boryl)alkenes (diboration) is catalyzed by a platinum(O) catalyst [34]. 

The organoborate intermediates can also be generated from alkenylboronic esters and alkyllithium or Grignard reagents, or from ttialkylboranes and alkenyllithium compounds. Conjugated symmetrical and unsymmetrical diynes (289—291), stereochemically pure 1,3-dienes (292,293), and 1,3-enynes (294) including functionali2ed systems can be prepared (289,295). 

It is worth noting that conjugate addition can also be effected by the addition of aryl- and alkenylboronic acid reagents to enones in the presence of rhodium catalysts. This new catalyst system has the following advantages ... 

A formal trans-hydroboration of terminal alkynes with catecholborane and pinacolborane to yield cis-l-alkenylboronates has also been carried out in the presence of Rh(I) and Ir(I). The dominant factors for reversing the conventional cis-hydroboration to the trans-hydroboration were the use of NEts and the use of bulk phosphines such as P Pr3 and PCy3, and an excess of alkyne in front of the boron reagent [46]. 

Addition of Organometallic Reagents to Enones in Aqueous Media Rhodium-catalyzed 1,4-addition of organometallic reagents to a,p-unsaturated compounds was first developed by Miyaura in 1997. Thus, Rh(acac)(CO)2/dppb was found to catalyze the 1,4-addition of aryl- and alkenylboronic acids to several ot,(3-unsaturated ketones in water-containing solvents at 50°C. The reaction conditions were successfully modified for the development of an asymmetric variant of this process by Hayashi and Miyaura in 1998. The important points of modification are (1) the use of Rh(acac)(C2H4)2/(5)-binap as a catalyst and... 

One limitation of this preparative method is the concurrent formation of an alkenylboronate, which is formed presumably via a-elimination pathways involving the intermediate ate adduct of addition between the organometaUic reagent and the a-haloalkylboronate (Eq. 25). ... 

To access anti-l,2-diols, indirect methods are required for the preparation of geometrically pure, chiral E-3-alkoxy reagents. To this end, the isomerization of alkenylboronic esters described above (Eq. 41), provides a reliable route to tartrate-derived E-3-siloxy allylboronate 99 (Fig. 7). The latter shows variable enantioselectivities in additions to aldehydes, with cyclohexanecarboxaldehyde affording the highest selectivity (Eq. 70). ... 

The procedure described here Is an example of a general method for preparing conjugated alkadienes by the palladium-catalyzed reaction of 1-alkenylboranes or boronates with vinylic halides. Hydroboratlon of l-a1kynes with catecholborane Is a standard method for obtaining (E)-l-alkenylboronates (1). Several different types of alkenylboranes and boronates (2-4) are now available as reagents for the cross-coupling reaction with vinyl halides. 

In addition to alkenylboron compounds, alkenylalane, alkenylzirconium, alkenyltin, alkenyl copper, and alkenylmagneslura reagents are reported to undergo a related alkenyl-alkenyl coupling reaction to give l,3-a1kad1enes. 

Third, chromium-based reagents prepared by reduction of 1,1-dihalo compounds with CrCl2 are suitable for the transformation of aldehydes to (i4)-iodoalkenes, (it)-alkenylsilanes, " and ( )-alkenylboronic esters with one-carbon homologation (Equation (8)). The amount of CrCl2 can be reduced to a catalytic amount by using manganese (or zinc) and MesSiCl. " ... 

Alkenylboron compounds cyclopropanations, 9, 181 haloetherification, 9, 182 hydrogenation and epoxidation, 9, 182 metal-catalyzed reactions, 9, 183 metallic reagent additions, 9, 182 via radical addition reactions, 9, 183 5-Alkenylboron compounds, cross-coupling reactions, 9, 208 Alkenyl complexes with cobalt, 7, 51 with copper, 2, 160, 2, 174 with Cp Re(CO) (alkene)3 , 5, 915-916 with dicarbonyl(cyclopentadienyl)hydridoirons, 6, 175 with gold, 2, 255... 

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). 

Chiral a-carbamoyloxy-2-alkenylboronates react with Grignard reagents with complete a- and diastereoselectivity through rearrangement of an intermediate ate -complex, yielding chiral allylboronates, which were oxidized to... 

The boron-Wittig reaction has been carried out by bis(boryl)methyllithium 309 generated in situ from tris(boryl)-methane 308 (Equation (91 )).469 The geminal dichromium reagent 3 12470 was found to be an excellent alternative yielding 1-alkenylboronic esters 313470-472 with high //w/ -selectivity (Equation (92)). 

A vinyl halide with an alkenylboronate ester, preserving the stereochemistry of the reagents. 

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