Palladium-Catalyzed Carbonyl Allylation: Synthesis of Enantiomerically Pure α-Substituted Allylboronic Esters

Abstract

Palladium-catalyzed carbonyl allylation of stable alkenylboronic ester with SnCl₂ proceeded diastereoselectively to afford α-substituted allylboronic esters; the assignment of their configuration as well as allyl additions are presented.

boron - allylation - asymmetric synthesis - allyl additions

References and Notes

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<A NAME="RD06209ST-8">8</A>

General Procedure for the Palladium-Catalyzed Carbonyl Allylation of Aldehydes with SnCl ₂ - Synthesis of 7 To a solution of 1b (1.0 mmol) in DMF (3 mL) was added SnCl₂ (3.0 mmol), PdCl₂ (PhCN)₂ (5 mol%), H₂O (25 mmol), and the appropriate aldehyde (1.0 mmol). The solution was stirred at r.t. until the reaction was completed (monitored by TLC, 2 h). The reaction mixture was diluted with Et₂O (120 mL) and washed successively with aq 10% HCl soln (10 mL), sat. NaHCO₃ (10 mL), H₂O (10 mL), and brine (10 mL). The extracts were dried over anhyd MgSO₄, the solvent was removed under reduced pressure and the crude product subjected to flash column chromatography on SiO₂ (PE-EtOAc, 90:10) and MPLC (PE-EtOAc, 98:2) affording α-substituted allylboronic esters 7, 8, and 9 as colorless foams. Selected Data for 7b Prepared according to the general procedure: 79% yield of 7b after flash column chromatography. [α]_D ^²0 -93.2 (c 1.02, CHCl₃). ^¹H NMR (600 MHz, CDCl₃): δ = 1.85 (dd, ^³ J _2,1 = 6.0 Hz, ^³ J _2,3 = 9.7 Hz, 1 H, 2-H), 2.02 (d, ^³ J _OH,1 = 2.3 Hz, 1 H, OH), 2.97 (s, 6 H, OCH₃), 4.57 (dd, ^³ J _1,OH = 2.3 Hz, ^³ J _1,2 = 6.0 Hz, 1 H, 1-H), 4.73 (ddd, ⁴ J _{4- E ,2} = 0.7 Hz, ^² J _{4- E ,4- Z}  = 1.9 Hz, ^³ J _{4- E ,3} = 17.1 Hz, 1 H, 4-H _E ), 4.89 (dd, ^² J _{4- Z ,4- E}  = 1.9 Hz, ^³ J _{4- Z ,3} = 10.2 Hz, 1 H, 4-H _Z ), 5.29 (s, 2 H, 4′-H, 5′-H), 5.53 (ddd, ^³ J _3,2 = 9.9 Hz, ^³ J _{3,4- Z}  = 9.9 Hz, ^³ J _{3,4- E}  = 17.1 Hz, 1 H, 3-H), 7.01-7.40 (m, 25 H, arom. CH). ^¹³C NMR (151 MHz, CDCl₃): δ = 40.10 (C-2), 51.98 (OCH₃), 72.65 (C-1), 78.22 (C-4′, C-5′), 83.60 (CPh₂OMe), 117.78 (C-4), 126.58, 127.03, 127.62, 127.67, 127.79, 127.86, 128.05, 128.84, 129.89 (arom. CH), 134.25 (C-3), 141.20, 141.31, 143.18 (arom. C _ipso ). Anal. Calcd (%) for C₄₀H₃₉BO₅ (610.29): C, 78.69; H, 6.44. Found: C, 78.26; H, 6.59.

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<A NAME="RD06209ST-11">11</A> Freire F. Seco JM. Quiñoa E. Riguera R. J. Org. Chem.  2005,  70:  3778 ; attempts to separate the enantiomers by chromatographic methods failed; however, since starting from diastereomerically pure reagents with no racemization expected during the addition, the Mosher method lends additional support to the assignment