Tetrahedron 62 (2006) 5116–5125
Catalytic asymmetric allylation of aldehydes using the chiral (salen)chromium(III) complexes Piotr Kwiatkowski,a Wojciech Cha1adaja and Janusz Jurczaka,b,* a
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland b Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland Received 21 December 2005; revised 21 February 2006; accepted 9 March 2006 Available online 17 April 2006
Abstract—The enantioselective addition of allylstannanes to glyoxylates and glyoxals, as well as simple aromatic and aliphatic aldehydes, catalyzed by chiral (salen)Cr(III) complexes, has been studied. The reaction proceeded smoothly for the reactive 2-oxoaldehydes and allyltributyltin in the presence of small amounts (1–2 mol %) of (salen)Cr(III)BF4 (1b) under mild, undemanding conditions. However, in the case of other simple aldehydes, the use of high-pressure conditions is required to obtain good yields. Classic chromium catalyst 1b, easily prepared from the commercially available chloride complex 1a, affords homoallylic alcohols usually in good yield and with enantiomeric purity of 50–79% ee. The stereochemical results are rationalized on the basis of the proposed model. Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction The readily available chiral metallosalen complexes are potentially very attractive catalysts, e.g., for reactions catalyzed by Lewis acids. They have already been effectively applied in a variety of reactions,1 e.g., epoxidation2 and cyclopropanation3 of alkenes, epoxide ring opening,4 Diels–Alder,5 and Strecker6 reactions, as well as Michaeltype additions,7 alkylations of tin enolates,8 and hydrocyanation of aldehydes.9 One of the most promising and powerful salen-type Lewis acids is chromium(III) complex, well known as efficient enantioselective catalyst for several reactions.5,8,10 Salen–chromium complexes have also been employed in the allylation of aldehydes in the catalytic Nozaki–Hiyama–Kishi reaction with allylic halides,11 which is a redox process and requires anhydrous and oxygen-free conditions. Until now, many efficient methods of enantioselective allylation have been developed which, however, were almost exclusively applied to simple aromatic and aliphatic aldehydes.12 No efficient catalytic method for the enantioselective allylation of glyoxylates is currently known. This subject was investigated by Mikami et al.13 with the use of a BINOL–titanium complex (10 mol %) as a catalyst. However, the results obtained were unsatisfactory in terms of both the yield and enantiomeric excess. In the case of reacKeywords: Allylation; Asymmetric catalysis; Glyoxylates; High-pressure technique; Homoallylic alcohols; (Salen)chromiun complexes. * Corresponding author. Tel.: +48 22 6320578; fax: +48 22 6326681; e-mail: [email protected]
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2006.03.032
tions of glyoxylates with allyltrimethylsilane or allyltributyltin, the enantiomeric excess values were 30 and 10%, respectively, and the yield did not exceed 40%. Better results were obtained for crotyltin reagents. Of interest was the fact that the same catalytic system, independently used by Keck14 and Umani-Ronchi15 for the reaction of simple aliphatic and aromatic aldehydes with allyltributyltin, gave excellent results (the enantiomeric excess value was often above 90%). The allylation of glyoxylates leads to the corresponding 2-hydroxypent-4-enoates, compounds of significant synthetic interest.16 Recently, in order to synthesize these compounds and their derivatives in an enantiomerically pure form, diastereoselective methods, widely explored in our group,17 using chiral auxiliaries attached to the glyoxylate moiety18 or to the allylating reagents,19 have been applied. These facts prompted us to search for a catalytic system useful for the enantioselective allylation of glyoxylates using metallosalen complexes. For some allylation reactions carried out under normal conditions, metallosalen complexes cannot be useful due to their relatively low Lewis acidity. In such cases, the problem can be solved by the application of a high-pressure technique.20 Recently, we have published two communications concerning catalytic asymmetric allylation of glyoxylates21 and high-pressure methodology for the reaction with nonactivated aldehydes in the presence of a chromium–salen catalyst (Scheme 1).22 In this paper, we present in detail the studies on enantioselective addition of allylstannanes to various aldehydes, catalyzed by chiral (salen)Cr(III) complexes
P. Kwiatkowski et al. / Tetrahedron 62 (2006) 5116–5125
e.g., 1 (Scheme 1). Moreover, we decided to extend the investigation to other active aldehydes such as glyoxals, as well as to other allyltin reagents such as crotylstannanes.
Chiral (salen)Cr(III) complexes
Table 1. Screening of the metallosalen complexes of type 1 with a classic salen ligand in the reaction of 2a with 3a
R= Alk CO2, AlkCO, ArCO - 1 bar R= Ar, Alk - 10 kbar
1a X= Cl 1b X= BF4
Classic Jacobsen's chromium salen complexes
1 2 3 4 5 6 7 8 9 10 11 12 13
1a 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1l 1m
CrCl CrBF4 CrClO4 TiCl2 VO MnCl FeCl Co CoCl Ni NiBF4 Cu AlCl
73 82 90 82 85 80 83 80 79 66 79 70 78
54 61 65 <5 0 <5 <5 6 10 <5 <5 <5 <5
2. Results and discussion b
2.1. Allylation of activated aldehydes
The metallosalen complexes were chosen as candidate chiral Lewis acids. In many cases, they are easily prepared and handled, and stable in the presence of moisture and oxygen. The model reaction was the allylation of n-butyl glyoxylate (2a) with allyltributyltin (3) leading to n-butyl 2-hydroxypent-4-enoate (4a) (Scheme 2). Subsequent to the preliminary screening of the chiral salen complexes of type 1 (Fig. 1) of the following metals: Ti(IV), VO(IV), Cr(III), Mn(III), Fe(III), Co(II) and (III), Ni(II) and (III), Cu(II) and Al(III), it transpired that the only enantioselectively efficient catalysts were the (salen)chromium(III) complexes 1a–c (Table 1, entries 1–3). Although the remaining complexes 1d–m (entries 4–13) did catalyze the allylation, the O H
Bu n O
The reactions were carried out using 1 mmol of n-butyl glyoxylate (2a), 2 mol % of complex 1, and 1.1–1.2 mmol of allyltributyltin, in 1 ml of CH2Cl2, at 20 C for 3–4 h. The yield was determined by GC. The enantiomeric excess was determined by GC on a capillary chiral b-dex 120 column.
enantiomeric excess was 10% at best. The commercially available (salen)CrCl complex 1a (2 mol %) provides the reaction at moderate enantioselectivity (54% ee) and in good yield. However, higher activity and slightly better enantioselectivity (over 60% ee) were observed for the chromium complexes with less coordinating counterions such as BF 4 (1b) and ClO 4 (1c) (entries 2 and 3, respectively) both easily prepared from 1a.6 We also tested the applicability of other chromium complexes with modified salen ligands. We studied the effect of the ligand structure with respect to the substituted
Chiral metallosalen complex 20 °C, 1 bar CH2Cl2
Bu n O
Bu n O
Scheme 2. The model reaction.
1h - Co
1b - CrBF4
1i - C oCl
1c - CrClO4 1d - TiCl2
1j - Ni 1k - NiBF4
1e - VO
1l - C u
1f - MnCl
1m - AlCl
1g - FeCl
Metalosalen complexes with classic ligand
Figure 1. The metallosalen complexes used in this work.
6a - R= Ph 6b - R= But
Cr O BF O 4
5a - R = Bu , R = Me 5b - R1= But, R2= OMe 5c - R1= Me, R2= But 1a - CrCl
Chromium complexes with modified salen ligand
P. Kwiatkowski et al. / Tetrahedron 62 (2006) 5116–5125
Table 2. The reaction of 2a with 3 catalyzed by chromium(III) complexes with modified salen ligandsa Catalyst
1 2 3 4 5 6 7 8 9
1a 5a 5b 5c 6a 6b 7 8 9
73 82 85 75 70 52 70 65 49
54 55 49 29 44 21d <5 <5 7
b c d
The reactions were carried out using 1 mmol of n-butyl glyoxylate (2a), 2 mol % of chromium complex, and 1.1–1.2 mmol of allyltributyltin, in 1 ml of CH2Cl2, at 20 C for 3–4 h. The yield was determined by GC. The enantiomeric excess was determined by GC on a capillary chiral b-dex 120 column. Opposite sense of asymmetric induction.
salicylidene and diamine (Table 2). When the R1 substituent being tert-butyl was conserved, and the R2 substituent was replaced by smaller groups such as methyl and methoxyl, the asymmetric induction remained similar (entries 1–3). A more significant decrease in induction was observed for replacing tert-butyl with methyl as an R1 substituent (entry 4). We investigated the chromium complexes with diamines other than 1,2-diaminocyclohexane, but the enantioselectivities obtained were lower (entries 5–9). Only the complex derived from 1,2-diphenylethylenediamine 6a produced results similar to 1a (entry 5). For the complex 6b, reversed and lower enantioselectivity was observed (entry 6). This is a consequence of the altered conformation of the complex, since two tert-butyl groups of the amine, for steric reasons, cannot occupy both the pseudodiequatorial positions. The complex with 1,10 -binaphthyl-2,20 -diamine (8) gave practically no induction (entry 8); it likely adopts the cis-b configuration,23 departing structurally from the complexes of type 1, which typically adopt the trans conformation. In the context of the works of Jacobsen concerning the tridentate chromium(III) complex 9,24 we tested its performance in the model reaction. The results however, were unsatisfactory, and the enantiomeric excess of product 4a was not greater than 10% (entry 9). As already mentioned, (salen)chromium complexes have been applied to the enantioselect...