Conjugate Addition of Organometallic Reagents

Tomioka, Kiyoshi; Nagaoka, Yasuo

doi:10.1007/978-3-642-58571-5_5

Cited by 84 publications

(56 citation statements)

References 72 publications

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“…In a similar manner, the reaction of 2-cyclopentenone (1b) with the phenyltitanate 2m gave 91% yield of (S)-3-phenyl-1-(trimethylsilyloxy)cyclopentene (3bm), which is 99% enantiomerically pure (entry 2). Use of the phenyltitanate 2m generated from bromobenzene by lithiation with butyllithium followed by treatment with Ti(OPr-i) 4 or from phenyltitanium triisopropoxide [(PhTi(OPr-i) 3 ] and lithium isopropoxide (LiOPr-i) in THF gave essentially the same results (entries 3 and 4). Aryltitanate reagents 2n and 2o, which contain trifluoromethyl and methoxy groups, respectively, at the 4-position of the phenyl gave high yields of the corresponding silyl enol ethers 3bn and 3cn (entries 5 and 6).…”

Section: Resultsmentioning

confidence: 73%

“…It is remarkable that the yield of silyl enol ether 3bm obtained here in the addition to 2-cyclopentenone (1b) is much higher than that with phenyltitanium triisopropoxide [PhTi(OPr-i) 3 ], where the 1,4-addition product is titanium enolate (see Fig. 2) (35).…”

Section: Resultsmentioning

confidence: 76%

“…31 P NMR studies (Fig. 5) showed that addition of the phenyltitanium reagent PhTi(OPr-i) 3 to the (oxa--allyl)rhodium complex 6, which includes one equivalent of PPh 3 , gives back the phenylrhodium complex 5. Protonolysis of the resulting THF solution with methanol gave a high yield of (S)-3-phenylcyclohexanone (4am), indicating that the titanium enolate was formed at the transmetalation step (Fig.…”

Section: Resultsmentioning

confidence: 99%

“…C atalytic asymmetric 1,4-addition of organometallic reagents to electron-deficient olefins has been a subject of extensive investigations (1)(2)(3)(4). Recently, many reports appeared on the use of chiral phosphine-rhodium catalysts for the addition of organoboron reagents represented by arylboronic acids .…”

mentioning

confidence: 99%

“…A catalytic asymmetric 1,4-addition giving metal enolates as the products is more useful, because the enolates are versatile intermediates for further transformation by the reaction with electrophiles. We have recently reported that the use of aryltitanium triisopropoxides [ArTi(OPr-i) 3 ] as arylating reagents for the rhodiumcatalyzed asymmetric 1,4-addition in nonprotic solvent gives titanium enolates as the 1,4-addition products (Fig. 2) (35).…”

mentioning

confidence: 99%

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Mechanistic studies on the catalytic cycle of rhodium-catalyzed asymmetric 1,4-addition of aryltitanate reagents to α,β-unsaturated ketones

Tokunaga

Yoshida

Hayashi

2004

Proc. Natl. Acad. Sci. U.S.A.

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Addition of lithium aryl(tetraisopropoxy)titanates [ArTi(OPri) 4 ؊ Li ؉ ] to ␣,␤-unsaturated ketones proceeded with high enantioselectivity (up to 99% ee) in the presence of an excess amount of chlorotrimethylsilane and a rhodium catalyst (3 mol % Rh), generated from [RhCl(C 2H4)2]2 and (S)-binap, in tetrahydrofuran at 20°C to give high yields of the corresponding silyl enolates as 1,4-addition products. The presence of chlorotrimethylsilane is essential for the 1,4-addition to take place. 31 P NMR spectroscopic studies revealed that the catalytic cycle consists of three transformations, that is, (i) insertion of an enone into arylrhodium species forming (oxa--allyl)rhodium intermediate, (ii) silylation of the (oxa--allyl)rhodium with chlorotrimethylsilane giving silyl enolate and a chloro-rhodium complex, and (iii) transmetalation of aryl group from aryltitanate to the chloro-rhodium regenerating the aryl-rhodium.C atalytic asymmetric 1,4-addition of organometallic reagents to electron-deficient olefins has been a subject of extensive investigations (1-4). Recently, many reports appeared on the use of chiral phosphine-rhodium catalysts for the addition of organoboron reagents represented by arylboronic acids (5-28). ␣,␤-Unsaturated ketones, esters, amides, phosphonates, and nitroalkenes are all good substrates giving the corresponding 1,4-addition products with high enantioselectivity. The reactions are usually carried out in a solvent containing 5-20% water. We have established the catalytic cycle of the rhodium-catalyzed 1,4-addition to ␣,␤-unsaturated ketones in the aqueous solvent (29). The catalytic cycle for the addition of phenylboronic acid involves three intermediates: phenylrhodium A, (oxa--allyl)rhodium B, and hydroxorhodium C complexes (Fig. 1). Each intermediate is converted into the subsequent intermediate by insertion (A 3 B), hydrolysis (B 3 C), and transmetalation (C 3 A). The 1,4-addition product is formed at the hydrolysis step. The addition of organosilane reagents is considered to proceed through a similar catalytic cycle (30-34). Although the use of water as a cosolvent is one of the advantages of this reaction over others, one major drawback is that the 1,4-addition product is obtained as the hydrolyzed product. A catalytic asymmetric 1,4-addition giving metal enolates as the products is more useful, because the enolates are versatile intermediates for further transformation by the reaction with electrophiles. We have recently reported that the use of aryltitanium triisopropoxides [ArTi(OPr-i) 3 ] as arylating reagents for the rhodiumcatalyzed asymmetric 1,4-addition in nonprotic solvent gives titanium enolates as the 1,4-addition products (Fig. 2) (35). Although the enantioselectivity is very high for most of ␣,␤-unsaturated ketones examined, the yields of the enolates, especially for cyclopentenone, are not high. It was found that the combination of lithium aryltitanates and chlorotrimethylsilane constitutes effective arylating reagents giving high yields of silyl enol ethers as 1,4-add...

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Section: Resultsmentioning

confidence: 73%

Section: Resultsmentioning

confidence: 76%