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DOI: 10.1055/s-0034-1378531
Stereoselectivity of Metal-Catalyzed Cyclizations of 1,6-Dienes
Publication History
Received: 29 May 2014
Accepted after revision: 24 June 2014
Publication Date:
25 August 2014 (online)
Abstract
In this article, the stereoselectivity of metal-catalyzed cyclizations of 1,6-dienes is examined according to the elementary steps which can be envisaged as triggers of the rearrangements. Crucial results have been obtained which demonstrate that enantioselective and diastereoselective cyclizations are possible. However, a lack of stereoselectivity is often observed for 1,6-dienes having a prochiral tether, which appears to originate in the lack of differentiation between the two olefins of the substrates.
1 Introduction
2 Oxidative Cyclometalation
3 Copper-Catalyzed Photocycloaddition
4 Intermolecular Hydrometalation
5 Carbometalation and Sila-Metalation
6 π-Allyl Formation in Metallo-Ene Cyclization
7 Olefin Electrophilic Activation
8 C–H Activation
9 Conclusion
# 1
Introduction
Metal-catalyzed cyclization reactions of 1,6-dienes 1 have been developed into an arsenal of attractive methods which often deliver one of compounds 2–5 (Scheme [1, ]A).[1] However, the applications of these methods in synthesis are not as widespread as compared to those of the metal-catalyzed cyclizations of related 1,6-enynes.[2] One notably problematic aspect of metal-catalyzed cyclizations of 1,6-dienes is often the stereoselectivity of the reaction, particularly when the tether linking the two olefins of the substrate is prochiral. It is also noteworthy that 1,6-dienes have been recently folded into molecular scaffolds other than 2–5 with excellent stereoselectivity upon treatment with metal catalysts. For example, platinum catalysts have been developed for the enantioselective rearrangement of 6 into [4.1.0]bicycloheptane 7 (Scheme [1, ]B),[3] whilst a rhodium catalyst was recently reported to transform 8 into [2.2.1]bicycloheptanes 9–11, which were isolated in excellent yields as single diastereomers in this rearrangement triggered by C–H activation (Scheme [1, ]C).[4] Besides C–H activation, other triggers have been proposed for the reactions depicted in Scheme [1]. Oxidative cyclometalation and intermolecular hydrometalation are likely the most prominent, but others can also be envisaged. Herein, we do not intend to present a comprehensive review of the metal-catalyzed cyclization of 1,6-dienes and will instead discuss briefly how the stereoselectivity of the rearrangements is influenced by the elementary steps which triggers them. Cycloisomerization reactions of relative substrates, that is, 1,6,7-trienes,[5] 1,6,8-trienes,[6] and 1-en-6-alkylidenecyclopropanes,[7] will not be discussed.
# 2
Oxidative Cyclometalation
The zirconocene-catalyzed cyclomagnesiation of 1,6-dienes proceeds via zirconacyclopentane 12 (Scheme [2]).[8] Stoichiometric studies have demonstrated that this intermediate is formed predominantly as a trans isomer,[9] and a similar behavior has been observed with ansa-titanocenes.[10] However, the titanium-catalyzed cycloisomerization of 1,6-diene 13 leads to 14 as an almost equimolar mixture of diastereomers (Scheme [3, ]A).[11] Hence, although the stereochemistry of the ring junction in 15 might be very well controlled, its relation to the third stereogenic center is not. Similarly, the ruthenium-catalyzed cycloisomerization of prochiral 1,6-dienes often lacks stereoselectivity (Scheme [3, ]B).[12] Significantly, deuterium-labeling experiments suggest that the reaction of the substrate with a ruthenium-hydride species leads to 16 by oxidative cycloaddition, which is followed by reductive C–H elimination toward 17 and final β-H elimination.[12a]
Moreover, a cis-selective oxidative cyclometalation accounts for the stereoselectivity of the intramolecular [2+2] cycloaddition of 1,6-dienes catalyzed by iron complexes bearing a redox active bis(imino)pyridine ligand (Scheme [4]).[13] Hence, the oxidation state of the iron catalyst remains constant in the catalytic cycle whilst the oxidation state of the ligand alternates between minus two and zero.
In addition, excellent product selectivity and enantioselectivity were observed during the cycloaddition of 1,6-diene-ynes 18 into 19 and 20 (Scheme [5]).[14] The chiral ligand sphere enables the control of the absolute configuration of the sp3-hybridized stereogenic center during the oxidative cyclometalation affording intermediate 21. Once established, this stereogenic center controls the second stereogenic center in 19. Both 2,1-migratory insertion of the second terminal olefin of the 1,6-diene moiety in bond a and 1,2-migratory insertion in bond b of 21, leading to 22 and 23, respectively, can eventually lead to the formation of 19 after reductive elimination. Conversely, a 2,1-migratory insertion in bond b would afford 24, which after β-H elimination, followed by reductive C–H elimination, would lead to the formation of 20. In this case, the differentiation between the two olefins is obviously biased by the presence of the alkyne, which has a greater reactivity toward the metal than the olefins.
# 3
Copper-Catalyzed Photocycloaddition
The bidentate coordination of the two olefins of 1,6-dienes to a metal is arguably a requisite for all oxidative additions discussed in the previous section. Similarly, complexes 25 and 26 have been proposed to account for the diastereoselective copper-catalyzed photocycloadditions delivering 27 and 28, respectively, under UV irradiation (Scheme [6]).[15] [16] Poor diastereoselectivity was obtained with substrates having an internal olefin, as illustrated with 29.
# 4
Intermolecular Hydrometalation
Metal hydride species are likely the active catalysts of many cycloisomerization reactions of 1,6-dienes.[1] In contrast to the reaction depicted in Scheme [3] (B), one olefin of the substrate typically undergoes hydrometalation and the regioselectivity of this elementary step dictates the product selectivity. For example, the yttrium-catalyzed hydrogenative cyclization of 30 and 31 likely proceeds via 32 (Scheme [7]),[17] but the diastereoselectivity of the subsequent carbometalation is very dependent on the protective group of the secondary alcohol. The key yttrium hydride can be generated with a stoichiometric silane additive in silylative cyclization reactions.[18]
Whilst 1,2-hydrometalation prevails when 1,6-dienes are treated with yttrium hydrides,[17] [18] or scandium hydrides,[19] 2,1-hydrometalation of the substrates has been proposed to initiate the palladium-catalyzed cycloisomerization of 1,6-dienes into cyclopentenes 2.[20] [21] Hence, detailed mechanistic studies suggest that the intramolecular carbometalation of intermediate 33 is actually very diastereoselective in favor of the trans isomer of 34 (Scheme [8]).[20c] [22] Then, syn β-H elimination toward 35 and olefin rotation followed by reinsertion into the palladium–hydrogen bond can lead to 36, which affords 2 with excellent product selectivity after final β-H elimination. However, the reaction of a substrate having the two olefins linked by a prochiral tether typically leads to a balanced mixture of diastereomers.[20c] The intermolecular addition of a metal hydride also likely initiates the asymmetric cycloisomerization of the prototypical diethyl diallylmalonate substrate in the presence of a palladium[23] or nickel chiral catalyst (Scheme [9]).[24] Finally, it is noteworthy that although the postulated addition of a nickel hydride catalyst to 37 presumably initiates the catalytic cycle leading to 38 with excellent regioselectivity, the stereoselectivity of the reaction remains poor (Scheme [10]).[25]
# 5
Carbometalation and Sila-Metalation
A 1,2-carbometalation is proposed to trigger the cyclizations of 1,6-dienes in the presence of a titanium catalyst (Scheme [11]).[26] Although the stereoselectivity of the cyclization can be excellent with simple substrate 39, substrate 40 having a prochiral tether is more problematic, and its reactions leads to a mixture of diastereomers. These results indicate that the 6-exo-trig intramolecular carbometalation of 41a and of one of the two diastereomers of 41b is highly diastereoselective, whereas the intermolecular carbometalation leading to 41a and 41b is not. A similar behavior has been observed in the case of zirconocene-catalyzed carbometalation of 1,6-dienes.[27]
Moreover, 2,1-addition of a carbon–silicon bond across one of the two terminal olefins of the substrates has been proposed to trigger the palladium-catalyzed enantioselective silylative cyclization of 1,6-dienes (Scheme [12]).[28] Although good stereoselectivities can be obtained in favor of the trans isomers, substrates having a prochiral tether remain again more problematic. Hence, even with chiral ligands, the catalyst cannot very efficiently differentiate the two terminal olefins of the substrate.
# 6
π-Allyl Formation in Metallo-Ene Cyclization
In contrast to the nickel-catalyzed cycloisomerization of 37 into 38 (Scheme [10]), the metallo-ene cyclization of 42 via putative (E)-σ-allyl intermediate 43 (or its syn-π-allyl equivalent) is highly diastereoselective and leads to 44 (Scheme [13]).[29] The diastereoselectivity of the palladium-catalyzed reaction of the same substrates is generally eroded, or quasi inexistent. In this context, it is noteworthy that mechanistic studies indicate that the palladium-ene reaction of 1,6-dienes proceeds by intramolecular insertion of the terminal olefin into a π-allyl palladium(II) complex.[30] This insertion is significantly slower than the anti-to-syn isomerization of the π-allyl complex.[31] [32] The stereoselectivity of rhodium-catalyzed metallo-ene cyclizations can be explained using the same rationale.[33]
# 7
Olefin Electrophilic Activation
The rearrangement of diene 6 into [4.1.0]bicycloheptane 7 (Scheme [1, ]B) is also catalyzed by other racemic platinum complexes, leading to the product in good to excellent diastereoselectivity (Scheme [14]).[3] In accordance with the general behavior of electrophilic platinum complexes, the dicationic catalyst presumably coordinates to the terminal olefin of 6, and the conformation of intermediate 45 would then dictate the course of the entire cascade leading to 7 and consisting in cation–olefin cyclization, 1,2-hydride shift, and cyclopropanation. In addition, cis-thujane (47) can be obtained from β-citronellene (46),[3b] [34] also with excellent stereospecificity, which can be accounted for by the positioning of the methyl group in pseudo-equatorial position in activated complex 48.
The transient formation of a tertiary carbocation dictates the regioselectivity of the cascades depicted in Scheme [14], and a similar rationale could perhaps explain the Sn(NTf2)4-catalyzed cycloisomerization of 1,6-dienes (Scheme [15]).[35] However, it is difficult to explain the diastereoselectivity observed for compounds 49–53. Hence, each was obtained as a single diastereomer except for 52. Interestingly, Tf2NH can catalyze the same rearrangement to 53 in 80% yield, whereas no conversion was observed toward compound 49, even at 101 °C. Other substrates underwent partial or complete polymerization in the presence of Tf2NH.
# 8
C–H Activation
Allylic C–H activation has been proposed in early reports to account for the observed palladium-catalyzed cycloisomerization of 1,6-dienes (Scheme [16]).[36] Hence, the authors postulated that 54 would be formed and lead to 55 after intramolecular hydrometalation of the second olefin and nucleophilic attack to the central carbon atom of the π-allyl complex. Then, either selective β-elimination toward 56 followed by reductive C–H elimination, or formation of 57 followed by isomerization, could both explain the formation of the cyclopentene. However, the diastereoselectivity of the reaction is poor.
As discussed previously, in most of the cyclizations of 1,6-dienes triggered by the intermolecular addition of a metal hydride, the two terminal olefins linked by prochiral tethers are not well differentiated in the hydrometalation step, which explains the low to moderate diastereoselectivity of these reactions. The rhodium-catalyzed rearrangement depicted in Scheme [1] (C) is triggered by a pyridine-directed activation of a proximal olefinic C–H bond (Scheme [17]),[4] and deuterium-labeling experiments indicate that the two terminal olefins of 58 are not differentiated in the rapid and reversible hydrometalation to 59. Importantly, the intramolecular hydrometalation of 58 to 59 is not diastereoselective and both syn and anti isomers are formed. Crucial for the exquisite stereoselectivity of the reaction is that the migratory insertion of the second olefin occurs from the syn isomer only, whereas the anti isomer either reverts back to intermediate 58 or undergoes reversible reductive elimination. This rationale is supported by the isolation and characterization of 60 [R = N(Me)Ts] as a transient intermediate in the formation of 11, both products displaying opposite relative configurations of the stereogenic centers highlighted by a red arrowhead.
# 9
Conclusion
Overall, the development of stereoselective metal-catalyzed cyclization of 1,6-dienes remains challenging. Currently, only few catalysts enable chemists to achieve this transformation in an enantioselective or diastereoselective manner. In this context, the differentiation of the two olefins in symmetrical 1,6-dienes having a prochiral tether is the most problematic aspect of these reactions.
#
#
Acknowledgment
The author is grateful to Professor Peter Vollhardt for the kind invitation to write this short article and to the other authors of the work cited in reference 4. Comments from the reviewers are also greatly appreciated.
-
References
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- 2b Ojima I, Tzamarioudaki M, Li Z, Donovan RJ. Chem. Rev. 1996; 96: 635
- 2c Trost BM, Krische MJ. Synlett 1998; 1
- 2d Aubert C, Buisine O, Malacria M. Chem. Rev. 2002; 102: 813
- 2e Jiménez-Núñez E, Echavarren AM. Chem. Rev. 2008; 108: 813
- 2f Aubert C, Fensterbank L, Garcia P, Malacria M, Simonneau A. Chem. Rev. 2011; 111: 1954
- 3a Kerber WD, Koh JH, Gagné MR. Org. Lett. 2004; 6: 3013
- 3b Feducia JA, Campbell AN, Doherty MQ, Gagné MR. J. Am. Chem. Soc. 2006; 128: 13290
- 4 Aïssa C, Ho KY. T, Tetlow DJ, Pin-Nó M. Angew. Chem. Int. Ed. 2014; 53: 4209
- 5a Trost BM, Tour JM. J. Am. Chem. Soc. 1988; 110: 5231
- 5b Kang S.-K, Ko B.-S, Lee DM. Tetrahedron Lett. 2002; 43: 6693
- 5c Makino T, Itoh K. J. Org. Chem. 2004; 69: 395
- 5d Brummond KM, Chen H, Mitasev B, Casarez AD. Org. Lett. 2004; 6: 2161
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- 6b Takacs JM, Anderson LG. J. Am. Chem. Soc. 1987; 109: 2200
- 7 Evans PA, Inglesby PA. J. Am. Chem. Soc. 2012; 134: 3635
- 8 Knight KS, Wang D, Waymouth RM, Ziller J. J. Am. Chem. Soc. 1994; 116: 1845
- 9a Rousset CJ, Swanson DR, Lamaty F, Negishi E.-i. Tetrahedron Lett. 1989; 30: 5105
- 9b Nugent WA, Taber DF. J. Am. Chem. Soc. 1989; 111: 6435
- 10 Pun D, Knobloch DJ, Lobkovsky E, Chirik PJ. Dalton Trans. 2001; 40: 7737
- 11 Okamoto S, Livinghouse T. J. Am. Chem. Soc. 2000; 122: 1223
- 12a Yamamoto Y, Nakagai Y.-i, Ohkoshi N, Itoh K. J. Am. Chem. Soc. 2001; 123: 6372
- 12b Michaut M, Santelli M, Parrain J.-L. Tetrahedron Lett. 2003; 44: 2157
- 12c Fairlamb IJ. S, McGlacken GP, Weissberger F. Chem. Commun. 2006; 98
- 13a Bouwcamp MW, Bowman AC, Lobkovsky E, Chirik PJ. J. Am. Chem. Soc. 2006; 128: 13340
- 13b Hoyt JM, Sylvester KT, Semproni SP, Chirik PJ. J. Am. Chem. Soc. 2013; 135: 4862
- 14 Shibata T, Tahara Y.-k, Tamura K, Endo K. J. Am. Chem. Soc. 2008; 130: 3451
- 15a Salomon RG, Coughlin DJ, Ghosh S, Zagorski MG. J. Am. Chem. Soc. 1982; 104: 998
- 15b Ghosh S, Raychaudhuri SR, Salomon RG. J. Org. Chem. 1987; 52: 83
- 16 Salomon RG, Kochi JK. J. Am. Chem. Soc. 1974; 96: 1137
- 17 Molander GA, Hoberg JO. J. Am. Chem. Soc. 1992; 114: 3123
- 18a Molander GA, Nichols PJ. J. Am. Chem. Soc. 1995; 117: 4415
- 18b Molander GA, Nichols PJ. J. Org. Chem. 1996; 61: 6040
- 18c Molander GA, Nichols PJ, Noll BC. J. Org. Chem. 1998; 63: 2292
- 19 Piers WE, Shapiro PJ, Bunel EE, Bercaw JE. Synlett 1990; 74
- 20a Kisanga P, Goj LA, Widenhoefer RA. J. Org. Chem. 2001; 66: 635
- 20b Bray KL, Fairlamb IJ. S, Lloyd-Jones GC. Chem. Commun. 2001; 187
- 20c Bray KL, Fairlamb IJ. S, Kaiser J.-P, Lloyd-Jones GC, Slatford PA. Top. Catal. 2002; 19: 49
- 21 For a discussion on the effect of heteroatoms and ligand on the regioselectivity of the intermolecular addition of metal hydrides in the nickel-catalyzed cycloisomerization of 1,6- and 1,7-dienes, see: Ho C.-H, He L. J. Org. Chem. 2014; 79 in press; DOI: 10.1021/jo5008477
- 22a Goj LA, Widenhoefer RA. J. Am. Chem. Soc. 2001; 123: 11133
- 22b Bray KL, Lloyd-Jones GC, Muñoz MP, Slatford PA, Tan EH. P, Tyler-Mahon AR, Worthington PA. Chem. Eur. J. 2006; 12: 8650
- 23 Heumann A, Moukhliss M. Synlett 1998; 1211
- 24a Böing C, Franciò G, Leitner W. Chem. Commun. 2005; 1456
- 24b Böing C, Franciò G, Leitner W. Adv. Synth. Catal. 2005; 347: 1537
- 24c Böing C, Hane J, Franciò G, Leitner W. Adv. Synth. Catal. 2008; 350: 1073
- 25 Radetich B, RajanBabu TV. J. Am. Chem. Soc. 1998; 120: 8007
- 26 Negishi E.-i, Jensen MD, Kondakov DY, Wang S. J. Am. Chem. Soc. 1994; 116: 8404
- 27 Shaughnessy KH, Waymouth RM. J. Am. Chem. Soc. 1995; 117: 5873
- 28a Perch NS, Widenhoefer RA. J. Am. Chem. Soc. 1999; 121: 6960
- 28b Perch NS, Pei T, Widenhoefer RA. J. Org. Chem. 2000; 65: 3836
- 28c Pei T, Widenhoefer RA. Org. Lett. 2000; 2: 1469
- 28d Pei T, Widenhoefer RA. J. Org. Chem. 2001; 66: 7639
- 29 Oppolzer W, Keller TH, Kuo DL, Pachinger W. Tetrahedron Lett. 1990; 31: 1265
- 30 Gómez-Bengoa E, Cuerva JM, Echavarren AM, Martorell G. Angew. Chem., Int. Ed. Engl. 1997; 36: 767
- 31 Oppolzer W, Birkinshaw TN, Bernardinelli G. Tetrahedron Lett. 1990; 31: 6995
- 32a Oppolzer W. Angew. Chem., Int. Ed. Engl. 1989; 28: 38 ; and references cited therein
- 32b Oppolzer W, Stammen B. Tetrahedron 1997; 53: 3577
- 32c Hiroi K, Hirasawa K. Chem. Pharm. Bull. 1994; 42: 786
- 33 Oppolzer W, Fürstner A. Helv. Chim. Acta 1993; 76: 2329
- 34 Kerber WD, Gagné MR. Org. Lett. 2005; 7: 3379
- 35 Grau F, Heumann A, Duñach E. Angew. Chem. Int. Ed. 2006; 45: 7285
For selected examples, see:
For selected examples, see:
For other relevant examples of stereoselective palladium- and nickel-catalyzed metallo-ene cyclizations, see:
-
References
- 2a Lautens M, Klute W, Tam W. Chem. Rev. 1996; 96: 49
- 2b Ojima I, Tzamarioudaki M, Li Z, Donovan RJ. Chem. Rev. 1996; 96: 635
- 2c Trost BM, Krische MJ. Synlett 1998; 1
- 2d Aubert C, Buisine O, Malacria M. Chem. Rev. 2002; 102: 813
- 2e Jiménez-Núñez E, Echavarren AM. Chem. Rev. 2008; 108: 813
- 2f Aubert C, Fensterbank L, Garcia P, Malacria M, Simonneau A. Chem. Rev. 2011; 111: 1954
- 3a Kerber WD, Koh JH, Gagné MR. Org. Lett. 2004; 6: 3013
- 3b Feducia JA, Campbell AN, Doherty MQ, Gagné MR. J. Am. Chem. Soc. 2006; 128: 13290
- 4 Aïssa C, Ho KY. T, Tetlow DJ, Pin-Nó M. Angew. Chem. Int. Ed. 2014; 53: 4209
- 5a Trost BM, Tour JM. J. Am. Chem. Soc. 1988; 110: 5231
- 5b Kang S.-K, Ko B.-S, Lee DM. Tetrahedron Lett. 2002; 43: 6693
- 5c Makino T, Itoh K. J. Org. Chem. 2004; 69: 395
- 5d Brummond KM, Chen H, Mitasev B, Casarez AD. Org. Lett. 2004; 6: 2161
- 6a Takacs JM, Myoung Y.-C, Anderson LG. J. Org. Chem. 1994; 59: 6928
- 6b Takacs JM, Anderson LG. J. Am. Chem. Soc. 1987; 109: 2200
- 7 Evans PA, Inglesby PA. J. Am. Chem. Soc. 2012; 134: 3635
- 8 Knight KS, Wang D, Waymouth RM, Ziller J. J. Am. Chem. Soc. 1994; 116: 1845
- 9a Rousset CJ, Swanson DR, Lamaty F, Negishi E.-i. Tetrahedron Lett. 1989; 30: 5105
- 9b Nugent WA, Taber DF. J. Am. Chem. Soc. 1989; 111: 6435
- 10 Pun D, Knobloch DJ, Lobkovsky E, Chirik PJ. Dalton Trans. 2001; 40: 7737
- 11 Okamoto S, Livinghouse T. J. Am. Chem. Soc. 2000; 122: 1223
- 12a Yamamoto Y, Nakagai Y.-i, Ohkoshi N, Itoh K. J. Am. Chem. Soc. 2001; 123: 6372
- 12b Michaut M, Santelli M, Parrain J.-L. Tetrahedron Lett. 2003; 44: 2157
- 12c Fairlamb IJ. S, McGlacken GP, Weissberger F. Chem. Commun. 2006; 98
- 13a Bouwcamp MW, Bowman AC, Lobkovsky E, Chirik PJ. J. Am. Chem. Soc. 2006; 128: 13340
- 13b Hoyt JM, Sylvester KT, Semproni SP, Chirik PJ. J. Am. Chem. Soc. 2013; 135: 4862
- 14 Shibata T, Tahara Y.-k, Tamura K, Endo K. J. Am. Chem. Soc. 2008; 130: 3451
- 15a Salomon RG, Coughlin DJ, Ghosh S, Zagorski MG. J. Am. Chem. Soc. 1982; 104: 998
- 15b Ghosh S, Raychaudhuri SR, Salomon RG. J. Org. Chem. 1987; 52: 83
- 16 Salomon RG, Kochi JK. J. Am. Chem. Soc. 1974; 96: 1137
- 17 Molander GA, Hoberg JO. J. Am. Chem. Soc. 1992; 114: 3123
- 18a Molander GA, Nichols PJ. J. Am. Chem. Soc. 1995; 117: 4415
- 18b Molander GA, Nichols PJ. J. Org. Chem. 1996; 61: 6040
- 18c Molander GA, Nichols PJ, Noll BC. J. Org. Chem. 1998; 63: 2292
- 19 Piers WE, Shapiro PJ, Bunel EE, Bercaw JE. Synlett 1990; 74
- 20a Kisanga P, Goj LA, Widenhoefer RA. J. Org. Chem. 2001; 66: 635
- 20b Bray KL, Fairlamb IJ. S, Lloyd-Jones GC. Chem. Commun. 2001; 187
- 20c Bray KL, Fairlamb IJ. S, Kaiser J.-P, Lloyd-Jones GC, Slatford PA. Top. Catal. 2002; 19: 49
- 21 For a discussion on the effect of heteroatoms and ligand on the regioselectivity of the intermolecular addition of metal hydrides in the nickel-catalyzed cycloisomerization of 1,6- and 1,7-dienes, see: Ho C.-H, He L. J. Org. Chem. 2014; 79 in press; DOI: 10.1021/jo5008477
- 22a Goj LA, Widenhoefer RA. J. Am. Chem. Soc. 2001; 123: 11133
- 22b Bray KL, Lloyd-Jones GC, Muñoz MP, Slatford PA, Tan EH. P, Tyler-Mahon AR, Worthington PA. Chem. Eur. J. 2006; 12: 8650
- 23 Heumann A, Moukhliss M. Synlett 1998; 1211
- 24a Böing C, Franciò G, Leitner W. Chem. Commun. 2005; 1456
- 24b Böing C, Franciò G, Leitner W. Adv. Synth. Catal. 2005; 347: 1537
- 24c Böing C, Hane J, Franciò G, Leitner W. Adv. Synth. Catal. 2008; 350: 1073
- 25 Radetich B, RajanBabu TV. J. Am. Chem. Soc. 1998; 120: 8007
- 26 Negishi E.-i, Jensen MD, Kondakov DY, Wang S. J. Am. Chem. Soc. 1994; 116: 8404
- 27 Shaughnessy KH, Waymouth RM. J. Am. Chem. Soc. 1995; 117: 5873
- 28a Perch NS, Widenhoefer RA. J. Am. Chem. Soc. 1999; 121: 6960
- 28b Perch NS, Pei T, Widenhoefer RA. J. Org. Chem. 2000; 65: 3836
- 28c Pei T, Widenhoefer RA. Org. Lett. 2000; 2: 1469
- 28d Pei T, Widenhoefer RA. J. Org. Chem. 2001; 66: 7639
- 29 Oppolzer W, Keller TH, Kuo DL, Pachinger W. Tetrahedron Lett. 1990; 31: 1265
- 30 Gómez-Bengoa E, Cuerva JM, Echavarren AM, Martorell G. Angew. Chem., Int. Ed. Engl. 1997; 36: 767
- 31 Oppolzer W, Birkinshaw TN, Bernardinelli G. Tetrahedron Lett. 1990; 31: 6995
- 32a Oppolzer W. Angew. Chem., Int. Ed. Engl. 1989; 28: 38 ; and references cited therein
- 32b Oppolzer W, Stammen B. Tetrahedron 1997; 53: 3577
- 32c Hiroi K, Hirasawa K. Chem. Pharm. Bull. 1994; 42: 786
- 33 Oppolzer W, Fürstner A. Helv. Chim. Acta 1993; 76: 2329
- 34 Kerber WD, Gagné MR. Org. Lett. 2005; 7: 3379
- 35 Grau F, Heumann A, Duñach E. Angew. Chem. Int. Ed. 2006; 45: 7285
For selected examples, see:
For selected examples, see:
For other relevant examples of stereoselective palladium- and nickel-catalyzed metallo-ene cyclizations, see: