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DOI: 10.1055/s-2007-980376
Design versus Discovery in Synthetic Applications of Organoalanes
Publication History
Publication Date:
06 June 2007 (online)
Abstract
A discussion focusing on the benefits of considered forethought in proposing new asymmetric catalytic reactions is presented based on the author’s experiences in the areas of copper-promoted 1,4-addition and SN2′ chemistry and in the area of group 10 catalysed coupling reactions of organoaluminium reagents with aldehydes and allylic halides. While high-throughput methods have become the norm for attaining practical procedures in catalytic asymmetric chemistry appropriate strategic planning should not be abandoned in the conception of new catalytic processes.
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1 Introduction
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2 Design and Discovery in Copper-Based Catalysis
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2.1 Serendipity with Thio-Analogues of BINOL
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2.2 Design to the Fore in Conjugate Addition
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2.3 SN2′ Chemistry: You Can’t Always Get What You Want…
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3 Design and Discovery in DABAL-Based Catalysis
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3.1 Intelligent Approaches to Asymmetric 1,2-Aldehyde Methylation
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3.2 Scope of the DABAL-Me3 Reagent
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3.3 SN2′ Chemistry: You Can Get What You Want…
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4 Summary and Outlook
Key words
enones - aldehydes - asymmetric - catalysis - organometallics
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References and Notes
Calculations were carried out using Spartan (www.wavefun.com) for Mac ‘02 to generate equilibrium geometries at using the semi-empirical PM3; Chapron, A. unpublished results.
13The CSD database was accessed via: www.ccdc.cam.ac.uk.
18The transition states G/H are conjectures; they could not be attained by PM3 calculation.
19The enolates were prepared by the chemistry of ref. 15. Neat Ac2O (2.5 equiv) was added and the temperature raised from -45 to +6 °C over 6 h.
(
Z
)-(3
S
)-1,3-Dimethyloct-1-enyl Acetate
1H NMR (400.1 MHz, CDCl3): δ = 0.87 (3 H, t, J = 7.2 Hz, CH2
Me), 0.91 (3 H, d, J = 6.7 Hz, CHMe), 1.19-1.39 [8 H, m, -(CH2)4-], 1.87 (3 H, d, J = 0.9 Hz, =CMe), 2.17 (3 H, s, COMe), 2.34 (1 H, m, CHMe), 4.77 (1 H, dd, J = 9.8, 0.9 Hz, =CH). Irradiation of the olefinic =CH signal (δ = 4.77 ppm) produced a 3.6% NOE at the enol methyl (δ = 1.87 ppm) consistent with a Z double-bond geometry. 13C NMR (100.6 MHz, CDCl3): δ = 15.5 (Me), 19.5 (Me), 21.1 (Me), 22.7 (CH2), 27.0 (CH2), 30.6 (CH), 32.0 (CH2), 37.2 (CH2), 123.4 (=CH), 143.8 (=COAc), 169.1 (C=O). IR (CHCl3): νmax = 1741 (C=O) cm-1. HRMS (EI): m/z calcd for C12H22O2 [M]: 198.1620; found [M+]: 198.1620.
(
E
)-(3
S
)-1,3-Dimethyloct-1-enyl Acetate
1H NMR (400.1 MHz, CDCl3): δ = 0.88 (3 H, t, J = 7.2 Hz, CH2
Me), 0.98 (3 H, d, J = 6.7 Hz, CHMe), 1.19-1.39 [8 H, m, -(CH2)4-], 1.84 (3 H, d, J = 0.9, =CMe), 2.09 (3 H, s, COMe), 2.34 (1 H, m, CHMe), 4.88 (1 H, dd, J = 10.1, 0.9 Hz, =CH). Irradiation of the olefinic =CH signal (δ = 4.88 ppm) produced only a 1.7% NOE at the C(3) methyl (δ = 0.91 ppm) consistent with an E double-bond geometry. 13C NMR (100.6 MHz, CDCl3): δ = 14.1 (Me), 20.7 (Me), 21.3 (Me), 22.7 (CH2), 27.1 (CH2), 31.9 (CH), 32.0 (CH2), 37.7 (CH2), 123.9 (=CH), 144.4 (=COAc), 169.6 (C=O). IR (CHCl3): νmax = 1741 (C=O) cm-1. HRMS (EI): m/z calcd for C12H22O2 [M]: 198.1620; found [M+]: 198.1602.
I am indebted to Prof. D. Gillheany (UCD, Ireland) for this advice; he assures me it arose in the K. B. Sharpless group in the 1990s. It’s not always true, but is commonly observed.
47My sincere thanks to both Charles Davis (of Sigma-Aldrich) and John Blacker (at NPILPharma) for their advice and insights into the world of commercial chemistry. DABAL is now available from Sigma-Aldrich (catalogue no. 68210-1 DABAL-trimethylaluminium).
51El Hajjaji, S.; Woodward, S. unpublished results.
55Novak, A.; Woodward, S. unpublished results.