Application of a tridentate NHC containing pincer ligand to Pt catalyzed cascade cyclization reactions has allowed for the catalytic, diastereoselective cycloisomerization of biogenic alkene terminated substrates to the their polycyclic counterparts

Gagné, Michel R.

Geier, Michael J.

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Diastereoselective Pt Catalyzed Cycloisomerization of Polyenes toPolycyclesMichael J. Geier and Michel R. Gagne*́Caudill Laboratories, Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina27599-3290, United States*S Supporting InformationABSTRACT: Application of a tridentate NHC containingpincer ligand to Pt catalyzed cascade cyclization reactionshas allowed for the catalytic, diastereoselective cyclo-isomerization of biogenic alkene terminated substrates tothe their polycyclic counterparts.The construction of complex polycyclic hydrocarbonframeworks from simple polyene substrates by cyclazeenzymes (e.g., eq 1) has long been the envy of syntheticchemists.1 The remarkably selective fashion in which productsare formed has attracted a variety of biomimetic approaches.2Of note are recent reports in which cyclization is initiated byorganocatalysts,3 M+,4 H+,5 and X+ reagents.6The formation of polycycles via cascade cyclization reactionshas long benefitted from the installation of nucleophilicterminating groups, which enhance the efficiency of thecyclization.1,2 By contrast, the cyclization of strictly hydro-carbon containing substrates must overcome the lack of H-bond assistance in the terminating alkene, often leading to slowreactions and incomplete cyclization. While aryl terminatinggroups have been well tolerated as a result of cation−πstability,4a,e,5a,b,6a,b few examples of strictly polyene containingsubstrates are known.4d,7We have previously reported the synthesis of polycyclic Pt-alkyl compounds from simple and complex polyenes usingPt(triphos)2+ as the cyclization initiator (eq 2).8,9 Thetriphosphino pincer ligand renders the resulting Pt-alkylcompounds remarkably stable, but susceptible to Pt−C bondfunctionalization with strongly oxidizing agents (e.g., F+ orCu2+).10 We also considered the feasibility of an alternativeoxidant, H+, which oxidatively protonates the metal as the firstelementary step in the protodemetalation of a PtII−C bond.11By following a Pt(II) initiated electrophilic cascade cyclizationwith an oxidative protonolysis scheme, we sought a cyclization/protonation approach to sterol-like polycyclic compounds.Despite the desirability of such an atom economical cyclo-isomerization procedure, this goal required a solution to thehigh stability of Pt-alkyl cations to acid.12To facilitate such a scheme, we required a catalyst that wasnot only sufficiently electrophilic to initiate the cation olefincyclization but also electron rich enough to undergo rapidprotodemetalation. Our catalyst design features centered on thehypothesis that the electrophilic activation of a coordinatedalkene would be principally controlled by its trans ligand, whileits two cis ligands would primarily impact the facility with whichit could be oxidized at the Pt-alkyl stage. This led us to seek apincer ligand with a poorly donating trans ligand and stronglydonating cis ligands. Limbach et al. have recently reported thehydrovinylation13 and hydroamination14 of ethylene mediatedby a series of Pt catalysts engendering these target features, twocis NHC ligands and a trans pyridine.We initiated our studies by examining the ability of complex1 to induce cyclization of 2. Remarkably, after only 3 h at roomtemperature, complete conversion to the cyclization/proto-nolysis product 3 was observed. Surprisingly, the cyclizationproceeded in the absence of added base and provided none ofthe previously observed Brønsted acid catalyzed monocycliza-tion product 4,15 which reports on the buildup of acid; i.e., H+is rapidly consumed in situ.Our working mechanism has the reaction being initiated byselective coordination of the Pt electrophile to the leastsubstituted alkene,16 and subsequent ionic cyclization to B withliberation of H+ from the phenol terminating group (Scheme1). Protodemetalation of B generates 3 and regenerates the Ptcatalyst. The lack of 4 indicates that H+ never builds up and itreacts more quickly with B than it does with 2.Intrigued by the efficiency with which 1 induces cyclizationin phenol terminating substrate 2, we explored its applicabilitywith the significantly more challenging alkene terminatinggroups.8b Remarkably, 1 proved effective at initiating thediastereoselective cyclization of a variety of polyene substrates.Bi-, tri-, and tetracyclization reactions all proceeded in thepresence of 10% 1. Additionally tolerated were terminatingReceived: January 21, 2014Published: February 11, 2014Communicationpubs.acs.org/JACS© 2014 American Chemical Society 3032 dx.doi.org/10.1021/ja500656k | J. Am. Chem. Soc. 2014, 136, 3032−3035reactions to generate tertiary carbenium ions via 5-exo, 6-endo,and 6-exo cyclization geometries.Reaction times varied, from 5 h for 9 to up to 2 days for theslower bicyclization reactions (e.g., 5). While the bicyclizationsof 5 and 7 do not proceed to completion after 48 h, addition of20 mol % Ph2NH as a proton shuttle allows for fullconsumption of starting material (e.g., 5) and shorter reactiontimes (e.g., 7, <24 h). Thus a positive role for a proton shuttleis noted, at least for some problematic cascade reactions.In addition to the 2→3 transformation and the fasterprotonolysis of B than 2, the cyclization of 9→10 also reportson a low steady-state concentration of H+, as 10 is prone toconversion to 23 under acidic conditions (Scheme 2).17 In thiscase, formation of 10 uncontaminated with 23 again suggeststhat Pt-alkyl protodemetalation occurs quickly, in this situationfaster than alkene isomerization. As Yamamoto has previouslyobserved using a Brønsted−Lewis acid (BLA) catalyst, the arylether substrate 11 cyclizes to rearranged 12.18Cyclization of 15 also proceeded rapidly to a mixture of twoproducts, the desired tricyclization product with 5-contiguousstereocenters and an unknown bicyclic product. Compound 16terminates the C-ring with a 3° cation-generating 6-exocyclization followed by elimination (23% yield, Table 1). In arare example of premature termination in Pt-initiated cascadecyclizations, an unidentified bicyclic product is competitivelygenerated (46% yield, see Supporting Information (SI)).For 19 to terminate in a 3° carbocation requires a C-ring 5-exo cyclization, the carbenium ion from which is particularlyprone to Wagner−Meerwein rearrangements.7a,8b In this waythe thermodynamically more stable tetrasubstituted alkeneproduct 20 forms from a combination of hydride/methyl shiftand subsequent elimination (39%). A compound consistentwith 24 is formed as a minor product (10%; see SI), likely theresult of a single hydride shift that instead eliminates to give thetrisubstituted alkene (Scheme 3).Finally, cyclization of 21 provided the tetracycle 22 in 44%yield, forming five stereocenters and providing the 6−6−6−5sterol framework in a single step. Initial cyclization againterminates the D-ring in a 5-exo manner to generate an exocylictertiary cation akin to the protosterol cation observed in theconversion of 2,3-oxidosqualene to lanosterol (Scheme 4).From this exocyclic cation, a methyl and a hydride shiftgenerates a tertiary cation at C13, with elimination to C14creating the unsaturation at the C/D ring junction.19 Incontrast, from the protosterol cation, a series of two hydrideScheme 1. Polyene Cycloisomerization MechanismScheme 2. Conversion of Kinetic Product 10 toThermodynamic Product 23Table 1. Polyene CycloisomerizationaProducts formed as a single diastereomer. bConditions: 1 (0.015mmol), substrate (0.15 mmol), solvent (1 mL). cPh2NH (0.030mmol).Scheme 3. Competing Wagner−Meerwein Rearrangement ofa Polycyclic Cation Intermediate Derived From 19Journal of the American Chemical Society Communicationdx.doi.org/10.1021/ja500656k | J. Am. Chem. Soc. 2014, 136, 3032−30353033and two methyl shifts generates a tertiary cation at C10 withelimination to C9 forming the unsaturation at the B/C ringjunction of lanosterol.20Nitromethane proved a suitable solvent in most cases;however, several slow reacting or incomplete cyclizing casesbenefitted from a change to dichloromethane. Although 1 has atendency to abstract chloride from dichloromethane,13 itprovided shorter reaction times in the case of substrates 17,19, and 21. It additionally enabled complete consumption of 19while leaving the product distribution unchanged. Previousstudies have shown that the equilibrium position ofbicyclizations can be significantly shifted to the product statein the more poorly solvating solvent.21 These data additionallysuggest that a lower dielectric solvent may also increase the rateof cyclization.In summary, we have reported an efficient, atom economicalprocess for the conversion of a variety of polyene substrates topolycycles. This method hinges upon the paradoxicalcombination of high electrophilicity at the (CNC)Pt2+ stageand ease of protonative oxidation at the (CNC)Pt−R+ stage.■ ASSOCIATED CONTENT*S Supporting InformationExperimental procedures as well as spectroscopic data areavailable in the Supporting Information. This material isavailable free of charge via the Internet at http://pubs.acs.org.■ AUTHOR INFORMATIONCorresponding Authormgagne@unc.eduNotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThe NIH (GM-60578) is thanked for their generous support.M.J.G. thanks NSERC of Canada for a Postgraduate Scholar-ship. The authors thank Laura L. Adduci for assistance withNMR experiments and Dr. Mee-Kyung Chung for assistancewith HRMS.■ REFERENCES(1) (a) Wendt, K. U.; Schulz, G. E.; Corey, E. J.; Liu, D. R. Angew.Chem., Int. Ed. 2000, 39, 2812. (b) Johnson, W. S. Acc. Chem. Res.1968, 1, 1. (c) Wendt, K. U. Angew. Chem., Int. Ed. 2005, 44, 3966.(2) (a) Kotora, M.; Hessler, F.; Eignerova,́ B. Eur. J. Org. Chem. 2012,29. (b) Yoder, R. A.; Johnston, J. N. Chem. Rev. 2005, 105, 4730.(3) (a) Knowles, R. R.; Lin, S.; Jacobsen, E. N. J. Am. Chem. Soc.2010, 132, 5030. (b) Rendler, S.; MacMillan, D. W. C. J. Am. Chem.Soc. 2010, 132, 5027.(4) (a) Schafroth, M. 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Diastereoselective Pt Catalyzed Cycloisomerization of Polyenes to Polycycles

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