Hydroboration of allylsilanes

Schama 5.1 Regiochemistry of hydroboration of allylsilanes using 9-BBN (in parentheses of BH, [21]) 

Consequently, the synthetic utility of both different metals, silicon and boron, which are in a 1,3-relationship, can be exploited as both can be replaced, independently of each other by a hydroxy group. The boron in the usual way, using alkaline hydrogen peroxide and the phenyldimethylsilyl group by electrophilic removal of the phenyl group, followed by treatment with peracid [22]. This pair of reactions can also be carried out in one pot [23], with the overall sequence to construct 1,3-diols from allylsilanes. 

---

Hydroboration of allylsilanes 1,2- or 1,3-diols.1 A dimethylphenylsilyl group can markedly affect the regiochemistry of hydroboration of an adjacent double... 

The /3-lactone was formed by the cyclization of a 3-hydroxycarboxylic acid with sulfonyl chloride. An alternative synthesis attempted to control all stereochemical relationships in the molecule using the properties of silyl moieties attached to substrates and reagents <20040BC1051>. Stereoselective reactions of this type included the use of silyl groups in enolate alkylations, hydroboration of allylsilanes, and an anti Se2 reaction of an allenyl silane with an aldehyde and ry -silylcupration of an acetylene. The /3-lactone was again formed by the standard sulfonyl chloride cyclization method. 

The silyl group is both larger and more electropositive than carbon, and these factors direct the hydroboration of allylsilanes to place exclusively the boron on C-3 of the allyl unit [20], in contrast to 1-butene, which gives 6% of hydroboration with diborane on C-2 [Ij. Fleming and Lawrence [20] have compared the placement of boron of diborane and 9-BBN on a variety of allylsilanes as shown in Scheme 5.1. 

The highly selective hydroboration of allylsilanes with 9-BBN affords the products having the silicon and boron in a 1,3-relationship (Scheme 5.1) [20]. As already evident [4] the addition of 9-BBN is affected significantly by steric factors. 

Fleming and Lawrence [24] have further demonstrated that hydroboration of allylsilane is highly stereoselective with 9-BBN and the selective conversion to 1,3-diols is shown in Scheme 5.2. 

Hydroboration of allylsilane with 9-BBN forms the corresponding borane, which is coupled [28] to bromostyrene resin with Pd(PPhj)4 and K COj in DMF to give bislactin. Acid hydrolysis of the bislactin affords chiral amino ester. It is noted that Boc-protected amino acids can be generated first, followed by hydroboration and Suzuki coupling of the allylsilane. 

Hydroboration of silylallenes with Ipc2BH, followed by addition to aldehydes, produced. sy/z-homoallylalcohols, which were converted into optically active cyclic allylsilanes via RGM (Equation (169)).644,645... 

The steric hindrance caused by the PhMe2Si group in allylsilanes 152 influences the orientation of hydroboration by 9-BBN, NaOH and H2O2 (equation 64)139. The corresponding alcohols 153 are produced stereoselectively in 79-96% yields. 

Diastereoselective Synthesis of (Z)- and (E)-Allylsilanes. DBBS has been utilized in the diastereoselective synthesis of (Z)- and ( )-allylsilanes following a hydroboration-substitution-displacement strategy starting from 1-alkynes (eq 17). ... 

In the intermolecular process, cross-metathesis offers a usefid and versatile alternative to alkyne hydroboration for the synthesis of functionalized alkenyl pinacol boronates with a moderate to high -selectivity. Styrenes, allylsilanes, protected alcohols and amines are good partners as 1,1-disubstituted olefins [105-107]. This reaction has been exploited recently as part of a macrocyclization strategy in the epothilone series (Scheme 9.52) [108]. 

---