Enones 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). 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

It is worth noting that conjugate addition can also be effected by the addition of aryl- and alkenylboronic acid reagents to enones in the presence of rhodium catalysts. This new catalyst system has the following advantages ... 

Addition of Organometallic Reagents to Enones in Aqueous Media Rhodium-catalyzed 1,4-addition of organometallic reagents to a,p-unsaturated compounds was first developed by Miyaura in 1997. Thus, Rh(acac)(CO)2/dppb was found to catalyze the 1,4-addition of aryl- and alkenylboronic acids to several ot,(3-unsaturated ketones in water-containing solvents at 50°C. The reaction conditions were successfully modified for the development of an asymmetric variant of this process by Hayashi and Miyaura in 1998. The important points of modification are (1) the use of Rh(acac)(C2H4)2/(5)-binap as a catalyst and... 

Figure 3.20. Scope of Rh/(5)-binap-catalyzed asymmetric 1,4-addition of alkenylboronic acid catechol esters to a,P-enones.

Alkenylboronic acids and esters underwent conjugated addition to ct,/ -unsaturated ketones in the presence of trifluoroborane etherate589 or cyanuric fluoride (Equation (129)).590,591 Alkadienyl trifluoromethyl ketones were stereoselectively prepared from (2-alkoxyvinyl) trifluoromethyl ketones (Equation (130)).592 Alkynyl boronates can transfer the alkynyl groups regioselectively and enantioselectively to enones (Equation (131 )).593... 

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 efficiency of transmetalation from boron to palladium in the catalytic 1,4-addition of aryl or 1-alkenylboronic acids to enones encouraged us to extend the protocol to the addition of aryl- and 1-alkenylboronic adds to aldehydes in an aqueous solution (Eq. 4). The insertion of carbonyl groups into transition metal-carbon bonds has not received much attention, but the catalytic use of transition metals may allow such addition of various organometallics which are inert without a catalyst, the asymmetric addition using a chiral phosphine complex, or the reaction in an aqueous phase. 

Y,S-Vnsaturated ketones. The fluoride is useful for promoting the transfer of the alkenyl group of an alkenylboronic acid to enones. 

Scheme 4.36 Enantioselective Michael addition of alkenylboronic acids and alke-nylboronates to enones.

The first Cu-catalyzed asymmetric borylative cyclization reaction between cyclohexadienone-containing 1,6-enynes and B2pin2 was established to afford an optically pure d5-hydrobenzo[l ]furan framework bearing alkenylboronate and enone substructures through a tandem process of selective P-borylation of the propargylic ether and subsequent enantioselective conjugate addition to cyclohexadienone (13JA11700). 

Takaya, T. Ogasawara, M. Hayashi, T. Sakai, M. Miyaura, N. Rhodium-catalyzed asymmetric 1,4-addition of aryl- and alkenylboronic acids to enones. /. Am. Chem. Soc. 1998,120,5579-5580. 

Sakai, M. Hayashi, H. Miyaina, N. Rhodimn-Catalyzed Conjugate Addition of Aryl- or 1-Alkenylboronic Acids to Enones. Organometallics 1 7,16,4229-4231. 

Miyaura and coworkers reported the Rh(l)-catalyzed conjugate addition of aryl- or 1-alkenylboronic acids, RB(OH)2,to enones in high yields at 50°C in an aqueous solvent. A... 

In 2009, Takemoto and coworkers reported an expansion of the scope of the above reaction, using alkenylboronic acids as nucleophiles and chiral thioureas as catalysts [100]. To activate the alkenylboronic acid a hydroxyl group at the P position was added to the enone. The thiourea activated the enone while the P-hydroxyl of the enone and the hydroxyl of the catalyst activate and direct the intramolecular... 

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