Chiral Amidophosphane-Rhodium(I)-Catalyzed Asymmetric Conjugate Arylation of Acyclic Enones with Arylboronic Acids

Abstract

The carbon-carbon bond forming catalytic asymmetric conjugate addition reaction of a,b-unsaturated carbonyl compounds with organometallic reagents has been the significant milestone of catalytic asymmetric reactions. [1][2][3][4][5] Tremendous efforts on the discovery of efficient chiral sources for the reaction with carbon-and hetero-nucleophiles provided a fruit of chiral sources like phosphorous compounds.6) A touchstone for the evaluation of chiral sources has been the asymmetric reaction of cyclohexenone 2 with diorganozinc-copper(I), 7,8) Grignard reagent-copper(I) 9,10) or arylboronic acid-rhodium(I) [11][12][13][14][15][16][17][18] reagent-catalyst combinations. We have also engaged in this fascinating challenge and fortunately succeeded in the development of chiral amidophosphane 1-based catalytic asymmetric conjugate addition reaction of 2 with organometallic reagents, especially, Grignard reagents [19][20][21] and diorganozincs. [22][23][24][25][26] Especially, 1-rhodium(I) catalyzed asymmetric conjugate arylation of 2 with arylboronic acids gave 3 with excellently high ee of up to 99% (Chart 1).27-30) However, there has been known the presence of a large gap in reaction efficiency between the arylation of cyclic and acyclic enones probably due to s-trans and s-cis conformation differences of enones. Now we describe a catalytic asymmetric arylation of acyclic enones 7 with arylboronic acids 5 yielding arylation products 8 with relatively high ee.Acyclic Enones as Good Acceptors for Conjugate Phenylation At the beginning of this arylation study 4-methylbenzylidenemalonate 4 31,32) was examined with its acceptor ability by treating with phenylboronic acid (5a) in a 10 : 1 mixture of 1,4-dioxane and water under the catalysis of 1-rhodium(I). Unfortunately, the starting 4 was recovered almost quantitatively without detectable production of adduct 6 (Chart 2). This disaster was overcome by the selection of enone 7 as an acceptor for the conjugate arylation.Non-3-ene-2-one (7a) was then selected as the second acceptor because of the simple methyl ketone structure ( Table 1). The phenylation of 7a with 5 eq of phenylboronic acid (5a) was catalyzed by 3 mol% of 1-rhodium(I) in a 10 : 1 mixture of 1,4-dioxane and water. 27) Although the chemical yield was not high under the catalysis of 1-acetylacetonatobis(diethylene)rhodium(I), the desired ketone 8aa 12,33) with 56% ee was obtained in 35% isolated yield (Table 1, entry 1). Enantioselectivity (ee%) was determined by HPLC with chiral stationary phase (Daicel Chiralcel OB-H, hexane/2-propanolϭ100/1, 0.5 ml/min, 254 nm) as described in Experimental. Other two N-Boc-valine-connected amidophosphanes 9 and 10 34) (Fig. 1) exhibited the same level of performance to give 8aa with 65% and 61% ees, respectively (entries 2, 3). The sense of asymmetric induction was controlled by the pyrrolidine part of the ligands and not by the stereochemistry of the valine moiety; 8aa of the same ab- A catalytic asymmetric conjugate arylation of acyclic a a,b b-unsaturated ketones wi...

Phosphino-carboxamides: the inconspicuous gems

“…290 The family of N-acyl-154 donors was further extended by compounds bearing N-Boc-protected amino acid and dipeptide units as the N-acyl chains. These donors were probed in Rh-catalysed arylations of N-sulfonylimines, 291 N-(diphenylphosphinoyl)imines 292 and a,b-unsaturated ketones 293 with arylboronic acids or boroxines (ArBO) 3 , and further in Cu-mediated conjugate additions of dialkylzinc reagents to nitroalkenes, 294 N-sulfonyl-a-styrylimines 295 and cycloalkenones. 296 Good results were achieved also in Cu-catalysed allylic alkylation of 3-substituted prop-2-en-1-yl bromides with Grignard reagents.…”