Effect of chelating vs. bridging coordination of chiral short-bite P–X–P (X = C, N, O) ligands in enantioselective palladium-catalysed allylic substitution reactions
作者:GianPiero Calabrò、Dario Drommi、Giuseppe Bruno、Felice Faraone
DOI:10.1039/b309385a
日期:——
The chiral short-bite ligands (Ra,Ra)-bis(dinaphthylphosphonito)methane, (Ra,Ra)-1, (Ra,Ra)-bis-dinaphthylpyrophosphite, (Ra,Ra)-2, (Sc)-bis(diphenylphosphino)-sec-butylamine, (Sc)-3, (Ra,Ra)-bis(dinaphthylphosphonito)phenylamine, (Ra,Ra)-4a, (Ra,Ra,Sc)-bis(dinaphthylphosphonito)-sec-butylamine, (Ra,Ra,Sc)-4b, and (Ra,Sc)-(dinaphthylphosphonito)(diphenylphosphino)-sec-butylamine, (Ra,Sc)-5, have been synthesised. The cationic palladiumâallyl mononuclear chelate, [Pd(η3-PhCHCHCHPh)(μ-LâLshort-bite)]PF6 [LâLshort-bite
=
(Sc)-3, (Ra,Ra)-4a, (Ra,Ra,Sc)-4b and (Ra,Sc)-5 for complexes 8, 9, 10 and 11, respectively] and binuclear bridged [Pd(η3-PhCHCHCHPh)(μ-Ra,Ra-2)]2(PF6)2, 12, have been isolated. The short-bite chiral ligands synthesised have been tested in the palladiumâallyl catalysed substitution reaction of 1,3-diphenylallyl acetate with dimethyl malonate. The catalytic system was studied, in solution, by a multinuclear NMR technique. In the catalytically active species formed with (Ra,Ra)-2 ligand, [Pd(η3-PhCHCHCHPh)(Ra,Ra-2)]2(PF6)2, 12, the palladium(II) centres are bridged by two ligands which are forced to adopt a nearly cis-coordination to allow coordination of the allyl-moiety. Semiempirical calculations on a biphenyl-model molecule, similar to the species 12, indicate that this situation induces a strain and rigid conformation in the chiral ligands, which produce differences in the terminal allyl carbon atoms. As consequence, the catalytic product was obtained with an enantiomeric excess of 57.1% in the S form. A low e.e. value was obtained when the (Ra,Ra)-1, (Sc)-3, (Ra,Ra)-4a, (Ra,Ra,Sc)-4b and (Ra,Sc)-5 ligands have been tested in the same palladium-catalysed reaction.
手性短咬合配体(Ra,Ra)-双(二萘基膦)甲烷、(Ra,Ra)-1、(Ra,Ra)-双二萘基焦磷酸、(Ra,Ra)-2、(Sc)-双(二苯基膦)-仲丁胺、(Sc)-3、(Ra、(Ra,Ra)-4a、(Ra,Ra,Sc)-双(二萘基膦)-仲丁胺(Ra,Ra,Sc)-4b 和(Ra,Sc)-(二萘基膦)(二苯基膦)-仲丁胺(Ra,Sc)-5。阳离子钯烯丙基单核螯合物 [Pd(δ-3-PhCHCHPh)(δ-LâLshort-bite)]PF6[LâLshort-bite = (Sc)-3、(Ra,Ra)-4a、(Ra、(Ra,Ra)-4a、(Ra,Ra,Sc)-4b 和 (Ra,Sc)-5]以及双核桥联的[Pd(δ-3-PhCHCHPh)(δ-Ra,Ra-2)]2(PF6)2, 12。合成的短口手性配体在 1,3-二苯基烯丙基乙酸酯与丙二酸二甲酯的钯烯丙基催化取代反应中进行了测试。我们采用多核核磁共振技术对溶液中的催化体系进行了研究。在(Ra,Ra)-2 配体形成的催化活性物质 [Pd(δ-3-PhCHCHCHPh)(Ra,Ra-2)]2(PF6)2,12中,钯(II)中心被两个配体桥接,这两个配体被迫采用近似顺式配位,以便烯丙基配位。对类似于 12 物种的联苯模型分子进行的半经验计算表明,这种情况会导致手性配体的应变和刚性构象,从而产生末端烯丙基碳原子的差异。因此,S 型催化产物的对映体过量率为 57.1%。在相同的钯催化反应中,当测试 (Ra,Ra)-1、(Sc)-3、(Ra,Ra)-4a、(Ra,Ra,Sc)-4b 和 (Ra,Sc)-5 配体时,得到的对映体过量值较低。