A mild method for the synthesis of 2-deoxysugars from the coupling of glycals with a range of nucleophiles is described. The method employs 1 mol % of an air- and moisture-tolerant rhenium-oxo complex [ReOCl3(SMe2)(Ph3PO)] as a catalyst for the formation of O-, N-, and S-alpha-glycosides. The catalytic system tolerates a number of commonly employed protecting groups, including isopropylidene acetals, alkyl and silyl ethers, acetates, and benzoates. Furthermore, the high-oxidation-state complex selectively catalyzes the coupling with the glycal acceptor in preference to oxidation of the glycals, alcohols, and even thiols.
Titanium tetrakis(amido) complexes catalyze the intramolecular hydroamination of alkynes and allenes more efficiently than Cp-based species. We report here that electron-withdrawing and sterically demanding bis(sulfonamido) ligands lead to enhanced catalytic activity. Zirconium analogues have also been prepared, and the tosyl-substituted complex 20 has been structurally characterized. As in the titanium series, bis(sulfonamido) zirconium catalysts are more efficient in the intramolecular hydroamination of allenes than bis(cyclopentadienyl) complex Cp(2)ZrMe(2) (23). Furthermore, these compounds transform 1,3-disubstituted aminoallenes with high stereoselectivity to the Z-allylamines and allow the hydroamination of a trisubstituted allene. Titanium bis(sulfonamido) imido complex 27 was synthesized. It converts aminoallene 10 to cylic imine 11 with a rate comparable to that of tetrakis(amide) 15, supporting the hypothesis of a catalytically active titanium imido intermediate.
The catalytic enantioselective intramolecular ring-opening of oxetanes with alcohols is catalyzed by (salen)Co(III) complexes. Either a monomeric or oligomeric catalyst can be used successfully in this transformation, providing 3-substituted tetrahydrofurans in both high yield and enantioselectivity. This methodology extends the range of electrophiles that can be activated toward highly enantioselective addition reactions by (salen)metal catalysts to an important new class.Oxetanes are receiving increased attention as intermediates in organic synthesis and drug discovery, thanks in part to the development of new methods for their preparation. 1,2 At this stage, few enantioselective reactions of oxetanes have been realized; these include ring expansions catalyzed by chiral copper complexes 3 and ring openings with organolithium reagents promoted by a chiral boron reagent. 4 We became intrigued by the possibility of E-mail: jacobsen@chemistry.harvard.edu. activating oxetanes with (salen)Co(III) complexes for enantioselective ring opening (e.g., eq 1), given the successful application of these catalysts in the asymmetric ring-opening of epoxides. 5,6 Herein, we describe intramolecular openings of oxetanes catalyzed by (salen)Co (III) complexes 1 and 2 to afford functionalized tetrahydrofurans in high yields and enantioselectivities. NIH Public Access(1)Lewis acid catalysis represents a viable approach to enantioselective ring opening of oxetanes, given that oxetanes possess lower ring strain 7 but superior Lewis basicity 8 relative to epoxides. Mechanistic studies of (salen)Co(III)-catalyzed reactions have established that epoxide ringopenings occur through cooperative bimetallic mechanisms involving simultaneous activation of nucleophile and Lewis acid activation of epoxide. 9 By enforcing cooperative interactions between (salen)Co units, oligomeric catalysts such as 2 have been shown to provide greatly enhanced reactivity compared with monomeric catalysts. 10 We chose to examine achiral 3-substituted oxetanes as potential reacting partners, as these substrates are readily accessed from malonate esters or 3-oxetanone, 11 and are susceptible, in principle, to enantioselective ring-opening with nucleophiles other than water. Intermolecular additions to 3-butyloxetane were studied using nucleophiles proven effective in (salen)Co(III)-catalyzed epoxide ring-opening reactions, such methanol, 10c,e 4-methoxyphenol 10c,e,12 and tert-butyl carbamate. 13,10e However, no desired ring-opened product was obtained in any case using either 10 mol% monomeric (salen)Co(III) complex 1 or 2 mol% of the oligomeric complex 2. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptEncouraged by the excellent reactivity and enantioselectivity obtained in intramolecular epoxide openings with alcohols using monomeric (salen)Co(III), 14 we examined intramolecular opening of oxetanes as a potential route to pharmacologically active and synthetically useful 3-substituted heterocycles (Table 1). [15][16][17] A ...
A new Pd-catalyzed reaction for the coupling between perfluoroalkyl iodides (R(F)I) and simple aromatic substrates is described. The perfluoroalkylated arene products are obtained in good to excellent yields in the presence of a phosphine-ligated Pd catalyst and Cs(2)CO(3) as a base. The development, optimization, scope, and preliminary mechanistic studies of these transformations are reported.
Use of Group 4 Bis(sulfonamido) Complexes in the Intramolecular Hydroamination of Alkynes andAllenes. -Titanium as well as zirconium tetrakis(amido) complexes (I) [except for (Id)] show an enhanced catalytic activity in the intramolecular cyclization of aminoallenes and aminoalkynes compared to the corresponding cyclopentadienyl-based series. The reactivity and regioselectivity of these cyclizations are strongly dependent on the transition metal as well as on the ligand set. The catalytic activity can be increased through the use of electron-withdrawing or sterically demanding bis(sulfonamido) ligands. Remarkably, in the presence of complex (Ie), aminoallenes are transformed with high regio-and stereoselectivity into (Z)-vinylpyrroles. The hypothesis of a catalytically active titanium imido intermediate is supported by the use of a titanium bis(sulfonamido) imido complex instead of complex (Ib) in the cyclization reaction of allene (Va). -(ACKERMANN, L.; BERGMAN*, R. G.; LOY, R. N.;
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