Alkenylboronic acids, addition with

In 1997, Miyaura and co-workers reported the nonasymmetric version of 1,4-addition of aryl- and alkenylboronic acids to a,/ -unsaturated ketones using rhodium-phosphine complex as the catalyst.97 Later, Hayashi and Miyaura realized the asymmetric 1,4-addition with high catalytic activity and enantioselectivity.98 In the presence of ( y)-BINAP, the reaction of 2-cyclohexenone with 2.5 equiv. of phenylboronic acid gave (A)-3-phenylcyclohexanone with 97% ee (BINAP = 2,2 -bis (diphenylphosphino)-l,l -binaphthyl Scheme 29).99... 

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). 

A highly stereoselective synthesis of retinol via a Cm + C6 route was depicted by De Lera et al. [52]. A Suzuki reaction of a C14 alkenyliodide with a Cg alkenylboronic acid afforded retinol in 83% yield, with retention of the geometries of the coupling partners. The alkenyliodide was obtained by a zirconium-mediated methylalumination and a subsequent Al/I exchange by slow addition of ICN. Coupling with the C6 boronic acid (12 hrs to reach completion), afforded retinol in 83% yield [53], Fig. (21). 

Alkenylboronic acid esters B react with bromine to give the trans-addition product initially. This primary product is not isolated but is immediately reacted with a solution of NaOMe in MeOH. Addition of MeO ion to the B atom of the bromine adduct forms the borate complex D. This borate complex is converted into a cis-configured bromoalkene and a mixed boronic acid ester by way of a / elimination. 

The addition of bromine or iodine to ( )-alkenylboronic acids or esters afforded intermediates, which were converted by base into (Z)-l-halo-l-alkenes.543 On the other hand, ( )-iodo-l-alkenes of high stereochemical purity were prepared by treatment of ( )-l-alkenylboronic acids or esters with sodium hydroxide followed by 1 molar equiv. of iodine.544 Iodine monochloride was used in place of iodine for such a base-induced iodination with retention or inversion of the stereochemistry (Equation (110)).545 Chloramine-T/Nal (Equation (111)),546,547 CuX2 (X = Cl, Br) (Equation (112)),548 and iV-halosuccinimides (Equation (113))549 halogenated the C-B bond with retention of stereochemistry. 

The reaction is catalyzed by transition metal complexes coordinated with phosphine ligands. Since chiral phosphine ligands are the chiral auxiliaries most extensively studied for transition metal catalyzed asymmetric reactions, one can use the accumulated knowledge of the chiral phosphine ligands for the asymmetric reaction. The asymmetric 1,4-addition of aryl- and 1-alkenylboronic acids to enones proceeded with high enantioselectivity in the presence of a chiral phosphine-rhodium catalyst (Table 2). 

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. 

Arylboronic and alkenylboronic acids undergo transition metal complex-catalyzed synthetic organic reactions such as cross-coupling with organic halides [58-60], 1,4-addition to a,/I-unsaturated ketones [61-63], and ring-opening addition to vinyl oxirane [64]. Scheme 5.8 depicts the mechanism proposed for the... 

Hara et al. found that, in the presence of cyanuric fluoride, alkenylboronic acids 310 reacted with a,/3-unsaturated ketones 311 through 1,4-addition to give 7,5-unsaturated ketones 312, selectively. The reaction proceeded under mild conditions, and a variety of functional groups could be introduced into the products (Equation 59) <1996SL993>. 

De la Herran, G., Murcia, C. and Csdky, A.G. (2005) Rhodium-catalyzed reaction of aryl- and alkenylboronic acids with 2,4-dienoate esters conjugate addition and Heck reaction products. Org. Lett., 1, 5629-32. 

Scheme 19.29 Enantioselective Michael addition of a, B-unsaturated ketones with alkenylboronic acids catalysed by thiourea 22.

Other approaches to trisubstituted aikenes via organoboranes involve alkynyltrialkyl borates, alkenyltrialkyl borates or the cross-coupling reaction of alkenylboronic acids with alkyl halides. Both ( )- and (2)-l,2-disubstituted alkenyl bromides can also be prepared by the methodology shown in eq 7. The boron trifiuoride etherate-mediated 1,4-addition of 1,2-disubstituted alkenylboronates affords y,5-unsaturated ketones (eq 9). The boronates can also be converted into chiral enoiborates for the enantioselective addition to aldehydes. ... 

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