Tandem reactions using allylboronates

Many of the recent advances in synthetic applications of allylic boron reagents have focused on the use of these reagents as key components of tandem reactions and one-pot sequential processes, including multicomponent reactions. The following examples briefly illustrate the range of possibilities. Most cases involve masked allylboronates as substrates, and the tandem process is usually terminated by the allylboration step. 

Recent Advances in the Preparation of Allylboronates and Their Use in Tandem Reactions with Carbonyl Compounds... 

In a similar fashion, allylboronates can be used as allylation reagents under hydroformylation conditions. Thus condensed 1,5-oxazadecalin systems are achieved via tandem hydroformylation/allylboration/hydroformylation sequences starting from an N-allyl-y-amidoallylboronate (Scheme 23) [77,78]. The aldehyde obtained from a regioselective hydroformylation undergoes diastereoselective intramolecular allylboration to give an intermediate al-lylic alcohol derivative. The reaction does not stop at this stage, since this... 

This method has not yet found widespread use for the preparation of allylboronates. In fact, uncatalyzed hydroborations of dienes tend to provide the undesired regioiso-mer with the boron atom on a terminal carbon, i.e., homoallylic boranes. By making use of certain transition metal catalysts, however, Suzuki and co-workers found that (Z)-allylic catecholboronates such as 22 can be obtained in high yield from various substituted butadienes (e.g., isoprene. Equation 11) [44]. Whereas a palladium catalyst is the preferred choice for acyclic dienes, a rhodium catalyst (Rh4(CO)i2) was best for the hydroboration of cyclohexadiene. A suitable mechanism was proposed to explain the high regioselectivity of this process. In all cases, a reaction quench with benzaldehyde afforded the expected homoallylic alcohol product from a tandem hy-droboration/allylation (Section 6.4.1.4). 

In contrast to the preparative methods described above, a functionalized allyl-boronate can be created from a simpler allylboronate by olefin cross-metathesis [81, 82]. Here, treatment of pinacol allylboronate (2) with various olefin partners, exemplified with styrene in Equation (33), in the presence of ruthenium catalyst 58 smoothly furnishes a more elaborate 3-substituted allylboronate, the cross product 38 [81]. These reactions are noteworthy for their exceptional functional group tolerance allylboronates bearing primary halides can be directly synthesized using this method. Unfortunately, the E/Z selectivity in the formation of the 3-substituted allylboronates is variable. This metathesis approach to allylboronates was employed as the beginning of a tandem cross-metathesis/carbonyl allylation process [82] (discussed in more detail in Section 6.4.1.3). 

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