Catalytic Asymmetric Synthesis. Группа авторов
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rel="nofollow" href="#ulink_4edcb3c3-6854-5af2-9c70-13f4012ec25d">Scheme 2.20b) [63]. Based on the density‐functional theory (DFT) studies, Goodman [64], and Houk‐Antilla [65] independently proposed a transition state model of the allylboration and the propargylation. Phosphoric acid forms a hydrogen bond with boronate pseudo axial oxygen and at the same time, phosphoryl oxygen forms a hydrogen bond with formyl hydrogen (Figure 2.6).
Scheme 2.20. Allylation of aldehyde by allylboronate (a) (
Source: Based on [62]
), and proparygylation with allenylboronate (b) (
Source: Based on [63]).
Figure 2.6. Transition state model of the allylation with allylboronate.
Scheme 2.21. Allylation of aldehyde with γ‐silyl boronate.
Barrio employed γ‐silyl boronate to form α‐silyl homoallylic alcohols with excellent diastereoselectivity and with 59–96% ee catalyzed by CPA 6h (Scheme 2.21). Subsequent treatment with Selectfluor furnished fluorinated allylic alcohols [66].
Chen reported an enantioselective addition of achiral α‐vinyl allylboronate to aldehyde to furnish dienyl homoallylic alcohols with high Z‐selectivity and with 90–97% ee (Scheme 2.22) [67, 68].
Scheme 2.22. Allylation of aldehydes with α‐vinyl allylboronate.
Source: Based on [67, 68].
As an extension of the allylation reaction, Murakami combined allylboration and a Pd‐catalyzed transposition reaction of homoallylic bisboronate. Treatment of 1,1‐di(boryl)alk‐2‐ene with Pd catalyst generated allylboronate, in situ, which underwent an allylboration reaction with an aldehyde using CPA 6e to afford anti‐homoallylic alcohols with high diastereo‐ and enantioselectivities (Scheme 2.23a) [69]. The same group subsequently reported an enantioselective synthesis of anti‐1,2‐oxaborinan‐3‐enes from aldehydes and 1,1‐di(boryl)alk‐3‐enes using Ru(II) complex and CPA 6e (Scheme 2.23b) [70].
Scheme 2.23. Reaction between di(boryl)butane and aldehyde, leading to the formatinof anti‐homoallylic alcohols (a) (
Source: Based on [69]
), and anti‐1,2‐oxaborinan‐3‐enes (b) (
Source: Based on [70]).
With regard to the allylation reaction using allylsilane, List reported the Hosomi‐Sakurai allylation reaction between allylsilane and aromatic aldehydes catalyzed by chiral DSI 9c (Scheme 2.24a) [71]. DSI 9c acted as a precatalyst, N‐trimethylsilyl sulfonimide was generated as a chiral Lewis acid catalyst, and sulfonimide anion controlled the enantioselectivity by acting as a chiral counteranion. Subsequently, List developed highly acidic IDPs 11b and 11c [14], and reported a catalytic addition reaction between allyltrimethylsilane and aldehydes, which is based on the silylium‐based Lewis acid organocatalysis (Scheme 2.24b) [72]. Both aromatic and aliphatic aldehydes were suitable substrates. It is noted that as low as 0.5 mol% of the catalyst promoted the allylation reaction with aromatic aldehyde, and as low as 0.05 mol% of the catalyst could be employed with aliphatic aldehydes.
Scheme 2.24. Allylation reaction using 9c (a) and 11b,c (b) (
Source: Based on [72]).
List reported a catalytic Mukaiyama aldol reaction that uses chiral IDPi 11d as the catalyst to furnish aldol products with 80–98% ee. The silylium ion is considered to be the actual catalyst. It is noted that parts per million (ppm) levels of catalyst were sufficient for the aldol reaction [73]. In general, aldehyde is more reactive than ketone. They subsequently developed a ketone‐selective Mukaiyama aldol reaction catalyzed by 11e to afford tetrahydrofuran derivatives with high chemoselectivity and with 84–97% ee (Scheme 2.25). The in situ generated silylium ion pair coordinated to sterically less hindered aldehydes and subsequent intramolecular cyclization gave a highly active cyclic oxocarbenium ion intermediate bearing a chiral counteranion, which appears to be responsible for the chemoselectivity (Figure 2.7) [74].
Scheme 2.25. Mukaiyama aldol reaction with ketones (a) and ketone selective Mukaiyama aldol reaction (b).
Figure 2.7. Reaction intermediate in the ketone‐selective reaction.
Source: Based on [74].
Seidel developed an enantioselective synthesis of isoindolinones through the condensation of 2‐acylbenzaldehydes with anilines in the presence of CPA 6e (Scheme 2.26) [75]. They proposed that the tautomerization of the hydroxy‐isoindoline intermediate to isoindoline is proposed to be the enantiodetermining step.
Yang developed an efficient method for the synthesis of