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Cite this: Chem. Commun., 2013, 49, 11173 Received 26th August 2013, Accepted 9th October 2013 DOI: 10.1039/c3cc46501b

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Chiral silicon Lewis acids having a pentacoordinate stereogenic silicon center: 29Si NMR studies and application to asymmetric Diels–Alder reactions† Yuhsuke Sakaguchi, Yuhki Iwade, Tohru Sekikawa, Tatsuya Minami and Yasuo Hatanaka*

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The

29

Si NMR studies of chiral pentacoordinate silyl triflimides

having a stereogenic center at silicon have revealed that a chiral silicon center is highly configurationally unstable. Such configurational instability has an enormously beneficial effect on the diastereo- and enantioselectivity of the catalytic asymmetric Diels–Alder reaction.

Chiral organosilicon compounds with a stereogenic silicon center have attracted a great deal of attention, owing to the effective asymmetric synthesis by chiral silicon auxiliaries as well as chiral silicon intermediates which have a stereogenic silicon center.1 The chiral silicon centers in tetraalkylsilanes and functionalized organosilanes exhibit high configurational stability under neutral conditions. Thus, high stereoselectivities of the reactions promoted by silicon-stereogenic chiral reagents and auxiliaries are attributable to the high configurational stability of the chiral silicon atom as well as the efficient silicon-tocarbon chirality transfer. Although synthetic applications of the tetracoordinate and pentacoordinate silicon reagents bearing a chiral silicon atom have been widely studied,1c the catalytic chemistry of pentacoordinate silicon Lewis acids bearing a chiral silicon center has remained unexplored.2 We have found that strongly Lewis acidic dialkoxysilyl triflimides, (RO)2(R 0 )SiNTf2 (R, R 0 = alkyl, aryl), have a pentacoordinate structure, while the structure of silyl triflimides is usually tetracoordinate.3 In this communication, we report the unusual structural change of chiral silyl triflimides bearing a stereogenic pentacoordinate silicon center, demonstrating the highly beneficial effect of the configurational instability of the chiral silicon on the diastereoand enantioselectivity of the catalytic Diels–Alder (DA) reaction. First 29Si NMR studies of dialkoxysilyl triflimides were undertaken in CDCl3 to shed light on the structures. In general, Department of Applied Chemistry, Osaka City University, Sumiyoshiku, Sugimoto, Osaka 558-8585, Japan. E-mail: [email protected]; Tel: +81-66605 2979 † Electronic supplementary information (ESI) available: Experimental procedures; characterization data for new compounds; and the detailed discussion about the 29Si NMR studies, the reaction mechanism, and the comparison with chiral copper catalysis in asymmetric DA reaction. See DOI: 10.1039/c3cc46501b

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Scheme 1

Formation of cationic pentacoordinate silyl triflimides.

replacement of a Cl atom of a chlorosilane with an NTf2 group effects a large downfield shift of the d29Si (e.g., Me3SiCl: 32.5 ppm; Me3SiNTf2: 55.9 ppm),3d indicating the formation of tetracoordinate silyl triflimides having sSi–N bonds.3 In sharp contrast, the 29 Si NMR spectra of dialkoxysilyl triflimide B derived from dialkoxy(chloro)silane A4 indicated that the chemical shift of B is slightly downfield by 5.6 ppm from that of A (Scheme 1). The observed unusually small downfield shift of the d29Si strongly suggests that B is a pentacoordinate silicon compound, since the 29 Si chemical shifts of cationic pentacoordinate silicon compounds, whose structures are unequivocally determined by spectral methods or X-ray crystallography analysis, appear in the range from +7 to 120 ppm.5 This result gave an idea that the introduction of a chiral diol ligand into the pentacoordinate silyl triflimides will form a chiral silicon Lewis acid possessing a chiral center at silicon (Scheme 1). A series of chiral silyl triflimides (R)-1NTf2 bearing (R)-BINOL were prepared in situ by protodesilylation of chiral allylsilanes (R)-1 with triflic acid, HNTf2 (Scheme 2). The 29Si NMR spectra of (R)-1a-NTf2 generated from the protodesilylation of (R)-1a have two peaks at 36.5 and 35.7 ppm

Scheme 2

Preparation of chiral silyl triflimides.

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Table 1

Fig. 1 29Si NMR spectra of silyl triflimides and complexes of silyl triflimide and N-alkenoyl oxazolidinone 2c in CD2Cl2.

at 50 1C, which are substantially upfield -shifted from the d29Si of (R)-1a (9.35 ppm) (Fig. 1A). The observed large upfield shift of the 29Si signals suggests the pentacoordinate structure of (R)-1a-NTf2.3d,6 The 29Si chemical shifts are significantly influenced by the coordination number as well as the kind of ligands coordinated to Si.7 Assuming that (R)-1a-NTf2 has a tetracoordinate structure, the d29Si of (R)-1a-NTf2 should appear around +14 ppm, since the d29Si are expected to be downfield shifted by about 50 ppm from those of the corresponding tetracoordinate (dialkoxy)(dialkylamino)methylsilanes, (RO)2Me–Si–NR 0 2 (R, R 0 = alkyl; d29Si: 34 to 36 ppm).7,8 This assumption completely disagrees with our result, indicating that (R)-1a-NTf2 has a pentacoordinate structure. Furthermore, the appearance of two 29 Si signals also supports the pentacoordinate geometry of the silicon center of (R)-1a-NTf2, because the pentacoordinate silicon is a stereogenic center. Therefore, it is reasonable to

Scheme 3 with 2c.

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Formation of chiral pentacoordinate silyl triflimides and complex

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Diels–Alder reaction of 2 catalyzed by (R)-1-NTf2a

Entry

Diene

2

Isolated yield (%)

endo : exo

eeb (%) [configuration]

1c 2d 3e 4f 5 6 7 8 9 10 f 11 f 12 f 13 f

CP CP CP CP CP CP CP CP CH 3a 3b 3c 3a

2a 2a 2a 2a 2a 2b 2c 2d 2a 2a 2a 2b 2b

4a, 85 4a, 85 4a, 70 4a, 87 4a, 96 4b, 93 4c, 92 4d, 98 4e, 85 5, 85 6, 78 7, 88 8, 85

>97 : 3 >97 : 3 >97 : 3 >97 : 3 >97 : 3 98 : 2 96 : 4 72 : 28 98 : 2 — 84 : 16g 5 : 95 —

69 56 70 61 94 97 94 94 88 75 88 87 81

[2S] [2S] [2S] [2R] [2R] [2R,3S] [2S,3S] [2S,3S] [2R] [R] [1R,2S] [1R,2R,6S] [1S,6S]

a Reaction was conducted with (R)-1a-NTf2 (2.5 mol%) unless otherwise noted. b Obtained from chiral HPLC analysis. c Reaction with (R)-1bNTf2 (5 mol%). d Reaction with (R)-1e-NTf2 (5 mol%). e Reaction with (R)-1c-NTf2 (5 mol%). f Reaction with (R)-1d-NTf2 (5 mol%). g Ratio of regioisomers.

suppose that 1a-NTf2 has a pentacoordinate structure, existing as a mixture of diastereomers, which would be in equilibrium (Scheme 3, C and D). Of particular interest is the structural change of 1a-NTf2 caused by addition of carbonyl compounds. Upon addition of N-cinnamoyl oxazolidinones 2c (10 eq.) to the CD2Cl2 solution, two 29Si peaks of (R)-1a-NTf2 disappeared, followed by appearance of a single peak at 32.8 ppm (Fig. 1B), implying that the complex of (R)-1a-NTf2 and 2c exists as a single diastereomer like E (Scheme 3). As shown in Table 1, (R)-1a-NTf2 exhibited high diastereo- and enantioselectivity in the catalytic asymmetric DA reaction. All these results make it clear that silicon Lewis acid (R)-1a-NTf2 is configurationally unstable, and the configurational instability of the pentacoordinate chiral silicon plays a critical role in achieving the high diastereo- and enantioselectivity in asymmetric DA reaction (Table 1, entries 5–9). It exhibits a striking contrast to chiral bis(oxazoline)Cu(II) catalysts, which are widely used in asymmetric DA reactions, having a configurationally stable square-planar structure.9 In contrast, the 29Si NMR spectra of (R)-1b-NTf2 derived from (R)-1b and HNTf2 have many peaks ranging from 34 ppm to 60 ppm (Fig. 1C). The appearance of many scattered peaks at a high field indicates the formation of a number of hypervalent silicon species.10 To our surprise, upon addition of N-cinnamoyl oxazolidinone 2c (10 eq.), these peaks disappeared and concomitantly a single 29Si chemical shift appeared at 37.3 ppm (Fig. 1D). This result affords abundant evidence that the resulting many hypervalent silicon species, which would be in equilibrium in solution, irreversibly rearrange to the most thermodynamically This journal is

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stable pentacoordinate silicon-2c complex similar to E (Scheme 3, Ar = H). Moreover, (R)-1b-NTf2 exhibited moderate enantioselectivity as a catalyst in the DA reaction (Table 1, entry 1), reflecting that the [(R)-1b-2c]+ complex exists as a single diastereomer. Generally, asymmetric catalysts are designed to achieve the configurationally stable structures;9c however, our results clearly indicate that the equilibrium mixture of the configurationally unstable silicon Lewis acids exhibits the appreciable diastereo- and enantioselectivity in asymmetric reactions. Next, we examined the asymmetric DA reaction between alkenoyl oxazolidinones and cyclopentadiene (CP) catalyzed by a series of 1-NTf2 (5 mol%). This particular reaction has become a benchmark test for the evaluation of new chiral Lewis acid catalysts.9c As a catalyst, configurationally unstable pentacoordinate silyl triflimide (R)-1b-NTf2 was more effective than (R)-1e-NTf2, whose d29Si appeared at 49.0 ppm indicative of the tetracoordinate silicon center.2 Thus, catalyst (R)-1b-NTf2 gave the DA adduct (2S)-4a with a higher enantiomeric excess of 69% (Table 1, entries 1 and 2). This result strongly implies that a pentacoordinate chiral silicon catalyst is superior to a tetracoordinate silicon catalyst, which has no silicon-centered chirality.2 It must be noted that the introduction of 2-naphthyl substituents at the 3,30 -positions of the (R)-BINOL ligand (catalyst (R)-1d-NTf2) led to a dramatic reversal of the enantioselectivity, giving the DA adduct of opposite configuration, (2R)-4a, in 87% yield with 61% ee (entry 4). The reversal of the enantioselectivity is presumably due to the configurational inversion at the silicon chiral center promoted by sterically demanding naphthyl substituents.11 The best result was obtained by (R)-1a-NTf2 (2.5 mol%, 94% ee) (entry 5). These results strongly suggest that the configurational instability of the pentacoordinate silicon Lewis acids has a beneficial effect on improving the stereoselectivities. We have investigated the substrate scope of (R)-1a-NTf2 and (R)-1d-NTf2 catalysts. With a 2.5 mol% loading of (R)-1a-NTf2, the reactions of CP with b-substituted dienophiles 2b and 2c smoothly proceeded to furnish the DA adducts in good yield (92–93%) with high diastereo- and enantioselectivities (94–97% ee, endo) (entries 6 and 7).12 While fumarate derivative 2d led to moderate diastereoselectivity, a good level of enantioselectivity is still maintained (entry 8, 94% ee (endo), endo/exo = 72 : 28). Similarly, cyclohexadiene (CH) reacted with 2a to give (2R)-4e in 85% yield with 88% ee (entry 9). Catalyst (R)-1d-NTf2 (5 mol%) is effective for the DA reaction of less reactive acyclic dienes, giving the DA adducts with high enantioselectivities (entries 10–13). 1-Substituted acyclic dienes like 3b, which are often problematic substrates,9c readily underwent the reaction with 2a to give the corresponding adduct 6 with high enantioselectivity (88% ee, major isomer), while regioselectivity was moderate (entry 11). Despite the long research history, the enantioselective DA reaction of acyclic dienes with b-alkylsubstituted enoyl dienophiles are rather rare, due to the low reactivity of acyclic dienes.9c Catalyst (R)-1d-NTf2 (5 mol%) is highly effective for the DA reaction of acyclic dienes 3a and 3c with N-crotonoyl oxazolidinone 2b, affording exo-713 and 8 in good yields with high enantiomeric excess (entries 12 and 13).

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An observed sense of asymmetric induction implies that a catalyst–substrate complex like E would be involved in the transition-state assembly: the approach of dienes at the exposed back face (Si face) of s-cis N-alkenoyl oxazolidinones predicts the sense of absolute stereochemical induction (Scheme 3).11 In conclusion, we have revealed that the chiral petacoordinate silyl triflimides with a silicon stereogenic center are highly configurationally unstable: such a configurational instability plays a critical role in the effective asymmetric induction. The effectiveness of chiral pentacoordinate silicon catalysts is broadly comparable to that of the best catalysts for the asymmetric DA reactions reported so far, in terms of high stereoselectivity, low catalyst loading (2.5–5.0 mol%), and the wider substrate scope.14 This work was supported by a Grant-in-Aid for Science Research (C) (20550101) from JSPS.

Notes and references 1 Selected recent reports: (a) W. A. Charifoux, S. K. Reznik and ¨hlich, J. L. Leighton, Nature, 2012, 487, 86; (b) S. Rendler, R. Fro M. Keller and M. Oestreich, Eur. J. Org. Chem., 2008, 2582. For reviews, see: (c) Li.-W. Xu, L. Li, G.-Q. Lai and J.-X. Jiang, Chem. Soc. Rev., 2011, 40, 1777; (d) J. L. Leighton, Aldrichimica Acta, 2010, 43, 3. 2 Successful chiral silicon Lewis acid catalysts for asymmetric synthesis are exceedingly rare: (a) Z. Tang, B. Mathieu, B. Tinant, G. Dive and L. Ghosez, Tetrahedron, 2007, 63, 8449; (b) K. Kubota, C. L. Hamblett, X. Wang and J. L. Leighton, Tetrahedron, 2006, 62, 11397; (c) B. Mathieu, L. De Fays and L. Ghosez, Tetrahedron Lett., 2000, 41, 9561; (d) M. Johannsen, K. A. Jørgensen and G. Helmchen, J. Am. Chem. Soc., 1998, 120, 7637. 3 (a) B. Mathieu and L. Ghosez, Tetrahedron Lett., 1997, 38, 5497; (b) A. Ishii, O. Kotera, T. Saeki and K. Mikami, Synlett, 1997, 1145; (c) K. Ishihara, Y. Hiraiwa and H. Yamamoto, Synlett, 2001, 1851; (d) B. Mathieu and L. Ghosez, Tetrahedron, 2002, 58, 8219. 4 S. N. Greszler and J. S. Johnson, Org. Lett., 2009, 11, 827. 5 Hexacoordinate structure of B can be ruled out, because hexacoordinate silicon compounds have d29Si at lower frequency from 139 to 200 ppm: see ref. 7. 6 Contrarily, d29Si of tetracoordinate silyl triflimides is strongly downfieldshifted from that of the corresponding allylsilanes (e.g., Me3SiNTf2: 54.3 ppm; Me3SiCH2CHQCH2: 0.39 ppm; CDCl3): R. L. Shcoll and W. K. Mushker, J. Am. Chem. Soc., 1972, 94, 6376. 7 E. A. Williams, in The Chemistry of Organosilicon Compounds, ed. S. Patai and Z. Rapport, Wiley, Chichester, 1989, ch. 8, pp. 511–554. 8 For the detailed discussion of the 29Si NMR studies, see ESI.† (a) J. Pikies and W. Z. Wojnowsky, Z. Anorg. Allg. Chem., 1985, 621, 173; (b) B. Hienz, H. Marsmann and U. Z. Niemann, Z. Naturforsch, 1977, 32, 163. 9 The most extensively studied catalysts for the asymmetric DA reaction of alkenoyl oxazolidinones are chiral bis(oxazoline)/Cu(II); however, 10 mol% of the catalysts are often used. Selected recent reports, (a) A. Sakakura, R. Kondo, Y. Matsumoto, M. Akakura and K. Ishihara, J. Am. Chem. Soc., 2009, 131, 17762; (b) M. P. Sibi, S. Manyem and H. Palencia, J. Am. Chem. Soc., 2006, 128, 13660. For review, see; (c) S. Reymond and J. Cossy, Chem. Rev., 2008, 108, 5359. 10 (R)-1b-NTf2, which is less bulky than (R)-1b-NTf2, would undergo the oligomerization to give the many silicon species. See ESI†. 11 For more detailed discussion of the reaction mechanism, see ESI†. 12 The DA reaction of CP and dienophiles 2b and 2c often leads to low exo/endo ratio, see Table 3 in ref. 9c and ESI†. 13 Y.-H. Lam, C. Bobbio, I. R. Cooper and V. Gouverneur, Angew. Chem., Int. Ed., 2007, 46, 5106. The steric repulsion between 3-TMSCH2 of 3c and 3-naphthyl on BINOL of 2b seems to invert the diastereoselectivity, leading to exo-DA adduct. See ESI†. 14 For a detailed comparison between chiral silyl triflimide catalysts and representative chiral copper catalysts in the asymmetric DA reaction, see ESI†.

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Chiral silicon Lewis acids having a pentacoordinate stereogenic silicon center: 29Si NMR studies and application to asymmetric Diels-Alder reactions.

The (29)Si NMR studies of chiral pentacoordinate silyl triflimides having a stereogenic center at silicon have revealed that a chiral silicon center i...
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