Asymmetric intramolecular cyclopropanation of diazo compounds with metallosalen complexes as catalyst: structural tuning of salen ligand
Chiral (ON+)Ru(II)- and Co(II)-salen complexes are efficient catalysts for asymmetric intramolecular cyclopropanation of diazoacetates and diazoketones.
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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2008, TetrahedronCitation Excerpt :The stereoselective epoxidation of suitable substituted cyclopentenes, followed by an intramolecular epoxide ring opening, has also been used in the synthesis of some fluorinated derivatives containing the desired skeleton.10 In the course of our studies on the influence of the sulfinyl group on the asymmetric 1,3-dipolar cycloadditions with diazoalkanes11 we have recently reported that reactions of these dipoles with (S)-3-p-tolylsulfinylfuran-2(5H)-ones proceed with high yields and excellent stereoselectivity.12 Moreover, the stereoselective extrusion of nitrogen from the resulting 1Δ-pyrazolines under Yb(OTf)3 catalysis provides an excellent method for the synthesis of enantiomerically pure 3-oxabicyclo[3.1.0]hexanones.13
Recent developments in asymmetric cyclopropanation
2008, TetrahedronCitation Excerpt :In recent years, only a few methods of synthesis of chiral cyclopropanes via other ring-closing reactions based on the use of a chiral catalyst have been developed. As an example, Hoppe et al. have reported an asymmetric synthesis of 1-methylene-2-vinylcyclopropanes by intramolecular S′E-cycloalkylation reaction, involving a 4-chloromethyl-2,4-dienyl carbamate, and using (−)-sparteine as a chiral inductor.298 Indeed, the α-deprotonation of the substrate performed in the presence of n-BuLi associated with (−)-sparteine, followed by the intramolecular cycloalkylation reaction, led to the formation of the corresponding diastereomerically pure cyclopropane in moderate enantioselectivity, as shown in Scheme 163.