Cyclic 1-alkenylboronates

Allylboration of 1-alkynes proceeds at room temperature to give cis addition products in high yields [32]. The Diels-Alder reaction between 2-(dialkoxyboryl)-1,3-butadiene and dienophiles at 50 °C provides cyclic 1-alkenylboronates (Scheme 2-10) [33]. 

Scheme9.38 Diels-Alder dimerization/allylboration sequence for the construction of cyclic alkenylboronates. ...

A broad substrate scope for the rhodium-catalyzed asymmetric 1,4-addition has been observed.98 Both arylboronic acids with either electron-donating or electron-withdrawing aryl substituents and alkenylboronic acids can be introduced into acyclic or cyclic enones with high enantioselectivities (Scheme 30). 

Selected 1-alkenylboronates synthesized by Equation (56) are shown in Scheme 17. The reaction afforded cyclic and acyclic vinylboronic esters possessing various functional groups, which are not available by conventional hydro-boration of alkynes or by a transmetallation method, for example 160-167. 

The reaction was efficiently catalyzed by a rhodium complex generated in situ by mixing Rh(acac)(CH2=CH2)2 with 1 equiv of (S)-binap in an aqueous solvent at 100 1C. It was interesting that the enantioselectivity was kept constant at the reaction temperature ranging between 40-120 1C in the addition to 2-cyclohexenone. High enantioselectivity exceeding 90%ee were readily achieved for both cyclic and acyclic a,3-unsaturated ketones with variety of aryl- and alkenylboronic acids. 

The vicinal diol cyclic sulfate from dimethyl tartrate undergoes nucleophilic opening to give substituted malate esters., However, for this application diethyl and diisopropyl L-tartrates give superior yields and selectivities. The asymmetric cyclopropana-tion of the 1 -alkenylboronic ester derived from dimethyl L-tartrate (eq 5) is another example where other tartaric acid derivatives surpass the performance of dimethyl tartrate. ... 

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. 

Cyclic allylboronate esters of diols can be obtained by reactions of appropriate diols or polyols (e.g. sugars) with activated triethylborane, or by photochemical combination of a trialkylborane with two equivalents of a secondary alcohol (Scheme 18). Conversion of alkenylboron compounds into alkenylmercury been extended to include allenyl and conjugated dienyl... 

Since the 1990s, the olefin metathesis reaction has become a major synthetic tool in organic chemistry. Organoboranes were first employed in the construction of carbo-cyclic and heterocyclic alkenylboronates by ring closure of the corresponding acyclic precursors [104], Ruthenium-catalyzed enyne metathesis of acetylenic boronates was later demonstrated as a concise route for the construction of cyclic 1,3-dienylboronic esters, which can be further engaged in [4-i-2] cycloadditions (Scheme 9.51) [86]. 

A novel type of coupling was introduced in 2006 when Pd(II) was directly used tmder an atmosphere of dioxygen to perform oxidative Pd(II) catalysis [63]. This methodology, which connects alkenylboron derivatives 71 to olefins 72 (even highly substituted or cyclic ones) in the absence of base, works at moderate temperatures and in short times, minimizing undesired side-reactions. The dienic esters are recovered stereoselectively in good to high yields (Scheme 38). 

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