Hydroboration highly stereoselective

This reaction occurs rapidly at room temperature using a small excess of alkyne and either pinacol- (HBpin) or catecholborane (HBcat). When an excess of borane was used, the Z/E ratio of the products was slowly eroded, eventually attaining a thermodynamic distribution of isomers. Equilibration presumably occurs via addition/elimination of excess Rh-H. Miyaura s method provides a useful synthetic complement to knovm cis-hydroboration methods. Under optimized conditions, good yields and high stereoselectivity (>90 10) were achieved for a variety of alkenylboronates (Table 9.8). The best selectivities were generally obtained with the use of catecholborane and Et3N as an additive. As in related reactions, the presence of base seems to suppress undesired reaction pathways. 

Straightforward is the stereoselective introduction of two oxygen functionalities onto carbon-carbon double bonds, as represented by the Sharpless epoxidation4 or osmium tetroxide oxidation of allyl alcohols.5 An alternative route may be envisioned by anti-Markovnikov hydration of enol ether derivatives, as in the present method, but such an approach has so far been rarely studied. The only other reported method is hydroboration which affords syn/anti ratios in the range of 83/17 to 3/97 in essentially the same systems as those examined in this procedure. The method described here is a highly stereoselective route to 2,3-syn isomers of 1,2,3-triol skeletons from a-hydroxy enol ethers. 

Control of acyclic stereochemistry. The five contiguous stereocenters of aldehyde 5, an intermediate in Kishi s total synthesis of monensin (6), were established by the hydroboration reactions of 1 and 3, which afforded 2 and 4, respectively, with high stereoselectivity (8 1 diastereomer ratio for 1 12 1 for 3). Kishi argues that the... 

Cross-coupling between 1-alkenyl- and arylboron compounds with organic electrophiles have found wide application (see Sect. 1.5.1.1) in organic synthesis [125, 132, 269-271]. The value of this methodology is further realized with the use of alkylboranes, such as B-alkyl-9-borabicyclo[3.3.1]nonanes (B-R-9-BBN), which can be conveniently prepared Ijy hydroboration of olefins [272]. The hydroboration proceeds with high stereoselectivity and chemoselectivity. The choice of phosphines in a catalytic system sometimes affects the chemo- and regioselectivity of the hydroboration. Hydroboration of l-(ethylthio)-l-propyne with catecholborane can be satisfactorily carried out with PdClj (dppf) but the regioselectivity is best for the dppe and dppp complexes of Ni [273]. Notably, PPhj complexes perform poorly. 

The hydroboration-coupling approach for the construction of carbon skeletons affords several advantages [139]. The high stereoselectivity of the hydroboration reaction provides a stereodefined alkyl center on boron. For instance, in the reaction shown in Scheme 2-49, the hydroboration occurs chemoselectively at the less hindered C(19)-C(20) double bond. In addition, the alkylboron group thus constructed can be readily cross-coupled with alkenyl or aryl halides under mild conditions. 

Similarly, 4-homoisotwist-2-ene (282) is hydroborated to give 283 and 284 with low regioselectivity (283 284 = 23 77) and high stereoselectivity (253a, 253b). These results may be compared to the reaction of l-terr-butyl-2,2-dimethylethene (257b), which upon hydroboration-oxidation yields 285 and 286 in a ratio of 98 2 (253c). 

Lower selectivity is observed for the hydroboration of 2-melhylenecyclohcxanol. even with sterically demanding 9-borabicyclo[3.3.1]nonane (9-BBN). In contrast, rhodium-catalyzed hydroboration is highly stereoselective, the addition proceeding from the opposite side to the substituent23. 

Scheme 1). Preparation of 1,3-diols by hydroboration of allylic alcohols with thexylborane has been shown to proceed with high stereoselectivity when the substrates are 1-(1-hydroxyalkyl)-cyclohexenes. Stereocontrol in the hydrosilation of allylic and homoaHylic alcohols has been studied jointly by the groups of Tamao... 

The basic idea of the following tetrahydroUpstatin synthesis is the combination oftwo methods with high stereoselectivity for the generation oftwo new stereo-genic centres in the 1,2- and 1,3-positions relative to the silyl substituent (1) alkylation of yS-silyl-enolates and (2) hydroboration of allylsUanes. [275, 276]... 

The synthesis of cyclopropanes from allyl chlorides, ejg. (17X via hydroboration is improved by the use of 9-borabicycIo[3,2,l]nonane (9-BBN) and is highly stereoselective. The elimination step (Scheme 1) involves inversion of configuration at the... 

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. 

BBN is a reagent of choice for high stereoselective hydroboration of 1,3-dimethlcycloalkenes, affording only 2,co-dimethylcycloalkyl-9-BBN, with both methyl groups trans to the 9-BBN moiety (Chart 6.6) [5]. 

Cyclohexenes. Gordon and Robinson have confirmed the high stereoselectivity in the hydroboration of certain cyclohexenes in which marked steric hindrance to one face is present. Thus compounds of this type (47) gave the corresponding all-equatorial cyclohexanol (48). Greater selectively is also possible using mono- or di-alkylboranes... 

Hydroboration and asymmetric reductions proceed with high stereoselectivity because of the small atom radius and the large electronegativity of boron [22]. Hence, organoboron compounds are used for asymmetric reactions, especially for asymmetric reduction [9,22,36]. Both the yield and the optical yield of asymmetric reactions with organoboron compounds are high, and the isolation of chiral products is easy. Therefore, these reactions are exploited for syntheses of terpenes [22]. 

Methods for the preparation of optically active compounds have been reviewed. Still and his co-workers recently developed the hydroboration of acyclic 1,4-dienes as a method for the controlled creation of stereocentres. Two groups have now exploited this method in a short synthesis of the Prelog-Djerassi lactone (111). The key step in both routes is the hydroboration-oxidation of (109) to (110) that is highly stereoselective with respect to three of the four stereocentres of (110). 

Hydroboration of dienes to cyclic boranes defines the stereochemistry of the diols obtained on treatment with sodium hydroperoxide in this way high stereoselectivity has been obtained in the conversion depicted in Scheme 12. ... 

It has become an important enrichment for this field and the clear advantages are its regioselectivity, the high stereoselectivity and the numerous options to manipulate the space demand of mono- and dialkylboranes. In addition, there are various efficient and highly selective catalysts. Taking all this under consideration, hydroboration presents itself as an ideal candidate for directed selectivity [43]. 

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