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. 2017 Feb 20:13:323-328.
doi: 10.3762/bjoc.13.35. eCollection 2017.

(Z)-Selective Takai olefination of salicylaldehydes

Stephen M Geddis #  1 , Caroline E Hagerman #  1 , Warren R J D Galloway  1 , Hannah F Sore  1 , Jonathan M Goodman  1 , David R Spring  1
Affiliations

(Z)-Selective Takai olefination of salicylaldehydes

Stephen M Geddis et al. Beilstein J Org Chem. .

Abstract

The Takai olefination (or Takai reaction) is a method for the conversion of aldehydes to vinyl iodides, and has seen widespread implementation in organic synthesis. The reaction is usually noted for its high (E)-selectivity; however, herein we report the highly (Z)-selective Takai olefination of salicylaldehyde derivatives. Systematic screening of related substrates led to the identification of key factors responsible for this surprising inversion of selectivity, and enabled the development of a modified mechanistic model to rationalise these observations.

Keywords: Takai olefination; alkenyl iodides; salicylaldehydes; stereoselectivity; transition state.

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Figures

Scheme 1
Scheme 1
A) General overview of the Takai olefination for the formation of alkenyl halides 2 from aldehydes 1 and haloforms in the presence of Cr(II)Cl2. B) Proposed course of the Takai olefination. Possible ancillary ligands omitted for clarity. X = Cl, Br or I.
Scheme 2
Scheme 2
Proposed model for the chromium(II)-mediated homologation of aldehydes to form (E)-alkenes. Hodsgon et al. [7] and Takai et al. [5] have hypothesised that the addition of the gem-dichromium species 3 to aldehyde 1 proceeds via a six-membered pseudo-chair transition state 5. X = Cl, Br or I. Other ligands on chromium omitted for clarity.
Scheme 3
Scheme 3
An unusually high level of (Z)-stereoselectivity was observed in the Takai olefination of 6. (E):(Z)-ratio determined by 1H NMR analysis of crude material obtained after reaction work-up. See Supporting Information File 1 for more information.
Scheme 4
Scheme 4
Takai olefination of meta-hydroxybenzaldehyde.
Scheme 5
Scheme 5
Yield for both products and residual starting material following a scaled up Takai olefination of salicylaldehyde (12) using optimised work-up conditions. (Yields determined from crude NMR using 1,3,5-trimethoxybenzene as an internal standard. See Supporting Information File 1 for details).
Figure 1
Figure 1
Positive correlation between the amount (Z)-product and σm for the series of meta-halogenated salicylaldehydes.
Scheme 6
Scheme 6
Proposed mechanism for (Z)-selective Takai olefination, whereby coordination of the ortho-OH to the neighbouring Cr centre causes the aldehyde substituent to adopt a pseudo-axial orientation.
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References

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