Elsevier

Tetrahedron: Asymmetry

Volume 14, Issue 7, 4 April 2003, Pages 823-836
Tetrahedron: Asymmetry

Asymmetric intramolecular cyclopropanation of diazo compounds with metallosalen complexes as catalyst: structural tuning of salen ligand

https://doi.org/10.1016/S0957-4166(03)00167-8Get rights and content

Abstract

Intramolecular cyclopropanation of alkenyl α-diazoacetates and alkenyl diazomethyl ketones was examined by using optically active (ON+)Ru(II)(salen) and Co(II)(salen) complexes as catalysts. For the cyclization of 2-alkenyl α-diazoacetates, Co(II)(salen) complexes 9 and 10 were found to be superior catalysts to the corresponding (ON+)Ru(II)(salen) complexes 4 and 5. On the other hand, (ON+)Ru(II)(salen) complex 2 was found to be the catalyst of choice for the cyclization of 3-alkenyl diazomethyl ketones, and complex 4 was found to be a good catalyst for the cyclization of (E)-4-alkenyl diazomethyl ketones. The present study demonstrates that metallosalen complexes, especially optically active (ON+)Ru(II)(salen) and Co(II)(salen) complexes, can serve as efficient catalysts for the cyclization of alkenyl diazocarbonyl compounds, if a suitable salen ligand is used as the chiral auxiliary.

Chiral (ON+)Ru(II)- and Co(II)-salen complexes are efficient catalysts for asymmetric intramolecular cyclopropanation of diazoacetates and diazoketones.

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Introduction

Molecules possessing a cyclopropane-ring substructure have received much attention because of their synthetic utility as building blocks and their occurrence as subunits of many natural products. Thus, a number of methodologies for the construction of cyclopropane units have already been reported. Among them, transition metal-catalyzed cyclopropanation of olefins with diazo compounds as carbenoid precursors is a highly useful method in terms of its simplicity and mild reaction conditions.1 Furthermore, use of transition metal complexes bearing appropriate chiral ligand(s) enables control of the stereochemistry of the cyclopropanation.1., 2., 2.(a), 2.(b) Therefore, many transition metal complexes with chiral ligands have been prepared and employed as catalysts for asymmetric cyclopropanation.1., 2., 2.(a), 2.(b) Consequently, highly diastereo- (trans- or cis-) and enantioselective intermolecular cyclopropanations have been achieved in the last three decades.1., 2., 2.(a), 2.(b), 3., 3.(a), 3.(b) In parallel with these studies, intramolecular cyclopropanation of diazocarbonyl compounds, which is an efficient method for construction of [n.1.0]bicyclic structures, has also been extensively studied using transition metal complexes as catalysts. Thus far, excellent catalysts such as Cu[bis(oxazoline)s],4., 4.(a), 4.(b), 4.(c) Cu(semicorrin),5 Cu(diamine),6 Ru(Pybox),7., 7.(a), 7.(b) Rh(MEPY) and its related complexes,8., 8.(a), 8.(b), 8.(c), 8.(d), 8.(e) Rh2(S-DOSP)2,9., 9.(a), 9.(b) (R)-β-cis-[Ru(II)(biaryldiamine)(CH3CN)2]10 have been introduced and high enantioselectivity has been achieved in the intramolecular cyclopropanation of various kinds of α-diazocarbonyl compounds. Attempts have also been made using chiral Cu(I) complexes of biferrocene-based bis(oxazoline),11 Ru(II)-diphenyl phosphino(oxazolinyl) quinoline,12 Ru-based porphyrin,13 though the enantioselectivity was moderate. However, the scope of each intramolecular cyclization reaction has been rather limited. For examples, chiral dirhodium(II)carboxamide complexes introduced by Doyle et al. are by far the best catalysts for cyclization of alkenyl diazoacetates and diazoacetamides,8., 8.(a), 8.(b), 8.(c), 8.(d), 8.(e) although they are not very efficient for the cyclization of alkenyl α-diazoketones.14 On the other hand, cyclization of alkenyl diazomethyl ketones with Cu(semicorrin) catalyst shows modest to high enantioselectivity (up to 95% ee), though it is less efficient for the cyclization of alkenyl α-diazoacetates.5 Recently, Pérez-Pı́etro et al. reported that a rhodium complex, bearing a unique o-metallated aryl phosphine ligand, catalyzed cyclization of 4-alkenyl and 3-alkenyl diazomethyl ketones with good to high enantioselectivity (up to 95% and 80% ees, respectively).15., 15.(a), 15.(b)

Despite the introduction of such excellent catalysts, no general catalyst for cyclization of alkenyl α-diazoesters and α-diazoketones is so far available. This might be mainly attributed to the fact that the transition state conformation of each cyclization varies with the substrate used.14 It was considered to be difficult to control a wide variety of transition state conformations with only one catalyst. In contrast, use of a series of catalysts that can be readily derived from partial modification of a parent catalyst was considered to be a more attractive strategy for developing the general cyclization of alkenyl α-diazoesters and α-diazoketones.1., 2., 2.(a), 2.(b) We have recently demonstrated that some metallosalen complexes [hereafter denoted as M(salen)s] such as chiral Co(III)(salen), Co(II)(salen), and (ON+)Ru(II)(salen) serve as efficient catalysts for intermolecular cyclopropanation.3., 3.(a), 3.(b), 16. As various types of salen ligands are easily derived from commercial or readily synthesized diamine and salicylaldehyde derivatives, the structure of M(salen)s was considered to be tunable in compliance with the substrate used and, therefore, M(salen)s were considered to be good candidates for the catalysts of various intramolecular cyclizations. Thus, we examined intramolecular cyclopropanation of alkenyl α-diazoesters17 and of alkenyl diazomethyl ketones18 using chiral M(salen)s as the catalysts.

Section snippets

Intramolecular cyclopropanation of 2-alkenyl α-diazoacetates using metallosalen complexes

We have already reported that chiral Co(III)(salen) 1 catalyzes cyclopropanation of styrene with t-butyl α-diazoacetate in a highly trans- and enantioselective manner,19., 19.(a), 19.(b) which chiral (ON+)Ru(II)(salen) 2 and Co(II)(salen) 3 do in a highly cis- and enantioselective manner (Scheme 1).3., 3.(a), 3.(b) It is also worth noting that complexes 2 and 3 show the opposite sense of enantioselection to each other, though they carry the same salen ligand. Based on this contrasting catalysis

Conclusion

It is well known that the catalyst of choice for asymmetric intramolecular cyclopropanation of alkenyl diazo compounds varies with the substrate used, probably because the transition state conformation for each cyclization is strongly affected by several factors: the substitution pattern of the alkenyl moiety of the substrate, the length and the nature of the linker connecting the alkenyl and diazomethyl moieties, and metal ion.1., 2., 2.(a), 2.(b), 14. This requires introduction of a

General

1H NMR spectra were recorded at 400 or 270 MHz on BRUKER DPX-400, JEOL GX-400, or JEOL EX-270 instruments. All signals were expressed as ppm down field from tetramethylsilane used as internal standard (δ value in CDCl3). IR spectra were obtained with a SHIMADZU FTIR-8600 instrument. Optical rotations were measured with a JASCO P-1020 polarimeter. High resolution EI mass spectra were obtained from JEOL JMX-SX/SX 102A spectrometer. Column chromatography was conducted on silica gel BW-820MH,

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