Development of a Hemilabile P-P=O Ligand Based on the Concept of ­Conformational Control

Abstract

Development of a new hemilabile P-P=O ligand based on the concept of conformational control is described. In Rh-catalyzed enantioselective phenylation of aromatic aldehydes with ­phenylboronic acid, MOD-DIOP monophosphine oxide is shown to function as a good ligand.

hemilabile chiral ligands - asymmetric arylation - optically active diarylmethanol - rhodium catalysts - boronic acid

References and Notes

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<A NAME="RU30805ST-2">2</A> For a review of hemilabile ligands, see: Grushin VV. Chem. Rev.  2004,  104:  1629 

Successful examples for asymmetric arylation of aromatic aldehyde with other metal catalysts:

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Miyaura has reported that DIOP and BINAP were not effective ligands (0% enantioselectivity) in this arylation. [4a]

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<A NAME="RU30805ST-10">10</A>

Other bases such as NaOH, KOH, NaOMe, CsOH, KOt-Bu, Na₂CO₃, Cs₂CO₃, LiOH and RbOH gave less-satisfactory results.

<A NAME="RU30805ST-11">11</A>

Other solvents such as toluene, PrOH and dioxane gave less-satisfactory results.

<A NAME="RU30805ST-12">12</A>

Only a fragment of mono-phosphineoxide Na⁺ 10 in the reaction mixture was observed by ESI-MS (Figure [3] ). HRMS: m/z calcd for C₄₃H₅₆O₇P₂Na: 769.3393; found: 769.3433.

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<A NAME="RU30805ST-15">15</A>

Representative Procedure for Asymmetric Phenylation.
To a stirred solution of RhCl₃·3H₂O (3.95 mg, 0.0150 mmol) in DME-H₂O (5:1, 2.4 mL) were added (R,R)-MOD-DIOP (4, 11.0 mg, 0.0150 mmol), NaOt-Bu (96.1 mg, 1.00 mmol), PhB(OH)₂ (122 mg, 1.00 mmol) and 1-naphthaldehyde (2) at r.t. The reaction mixture was stirred at 100 °C for 72 h under argon atmosphere, allowed to cool, diluted with H₂O and extracted with EtOAc. The combined extracts were washed with brine, dried (Na₂SO₄) and concentrated. The residue was purified by silica gel column (EtOAc-hexane = 19:1 to 9:1) to give (R)-(1-naphthyl)phenylmethanol (3, 114 mg, 97% yield, 75% ee) as a colorless oil. The spectral data of 3 were comparable to those reported. [4a] The ee of 3 was determined by HPLC analysis (Daicel chiralcel OD-H).

<A NAME="RU30805ST-16">16</A>

The reaction of 1-naphthaldehyde with 4-methoxy- and 4-chloro-phenylboronic acids did not proceed. The reaction of cyclohexylaldehyde with phenylboronic acid gave low enantioselectivity (44% yield, 25% ee).

<A NAME="RU30805ST-17">17</A>

As shown in Figure [4] , a linear correlation between the ee of the ligand and that of the product 3 was observed, indicating that a 1:1 ratio of ligand to Rh is present in the catalytic complex.

<A NAME="RU30805ST-18">18</A> For a bifunctional asymmetric catalysis, see: Shibasaki M. Kanai M. Funabashi K. Chem. Commun.  2002,  1989 ; and references cited therein