b Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
Metal carbenes,generated by metal-catalyzed dinitrogen removal from diazo compounds,are ubiquitous as reactive intermediates for useful chemical transformations [1]. In contrast to diazomethane and other diazoalkanes that can be toxic and explosive,the vast majority of the diazoesters is safely used under ordinary conditions and has proven to be widely applicable and versatile reagents for organic synthesis [2]. We have been major contributors to the chemistry of diazo compounds and their catalytic reactions,especially with dirhodium(II) compounds [3], and to highly enantioselective intramolecular metal carbene processes [4]. Recently,we have been involved in the development of highly selective catalytic metal carbene reactions with emphasis on [3 + 3]- and [3 + 2]-cycloaddition reactions with enoldiazoacetates [5]. Other research groups in areas related to investigations of diazo compounds and metal carbenes include the Davies group, which has provided major advances in [4 + 3]-cycloaddition reactions,mainly using styryldiazoacetates [6]; the Forkin group and others who have diverse interests in metal carbenes derived from triazoles [7]; ylide formation and subsequent transformations that are being actively pursued by the research groups of Zhou [8] and Hu [9]; Peter Zhang’s group,which is developing chiral porphyrin ligated cobalt(II) catalysts for highly stereoselective addition and insertion processes [10]; and cross-coupling reactions being developed by Wang [11].
Divergent outcomes,by which a reaction pathway can be redirected to different products by simply changing a reactant or reaction conditions,is well known and widely practiced [12]; and this term is broadly applied to methodology [13],synthesis [14],reactivity and selectivity [15]. Among these,those processes that form different products from same reactant(s), controlled solely by different catalysts,are especially important and meaningful [16]. We and others have reported exceptionally efficient catalyst-dependent processes that occur with the same β,γ-unsaturated diazo compounds to form structurally different compounds [17]. In this review,we report recent advances in catalyst-dependent divergent outcomes,which include dirhodium-catalyzed pathways,Lewis acid catalyzed pathways,and cooperative catalysis by these two catalysts in combination or in sequence (Scheme 1).
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| Scheme 1.Divergent outcomes initiated by dirhodium or Lewis acid catalysts. | |
Reactions catalyzed by dirhodium or Lewis acid catalysts occur by different reaction pathways. With dirhodium catalysts metal carbene intermediates are formed by dinitrogen extrusion from the diazo compound followed by nucleophilic attack by the substrate (Scheme 2,left side). With Lewis acid catalysis,the catalyst activates the substrate to react as an electrophile with the diazo compound followed by nucleophilic displacement of dinitrogen (Scheme 2,right side). Although both dirhodium and copper compounds are well-established catalysts for dinitrogen extrusion from diazo compounds,there can be striking differences between them in product outcomes from the same reactant(s). The different reaction pathways provide divergent outcomes controlled by the catalysts that are applied.
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| Scheme 2.Dirhodium catalysisvs.Lewis acid catalysis. | |
Dirhodium(II) and copper catalysts are remarkably effective for metal carbene reactions of diazo compounds [1]. However,some copper catalysts are Lewis acids that react with a substrate in preference to reaction with the diazo compound. The first example of this divergent behavior is the cyclization reaction of styryl diazoacetates with imines (Scheme 3) [18]. Reactions catalyzed by dirhodium acetate occur through metal carbene formation and subsequent electrophilic reaction by the metal carbene at the basic imine nitrogen (intermediate A in the case of rhodium) to give 4,5-dihydropyrroles A,while those catalyzed by copper(II) triflate involve initial iminium ion formation followed by electrophilic addition by the iminium ion at the diazo carbon (intermediate B in the case of copper) to give regioisomeric 2,5-dihydropyrroles A in moderate to high yield. Copper(II) triflate is a much stronger Lewis acid than is rhodium acetate and,consequently,its preferred association is with the more basic imine. Rhodium acetate may also associate with the imine,but its preferred reaction is at the diazo carbon of the vinyldiazoacetates that becomes an irreversible reaction following the loss of dinitrogen.
It is worthy of note that when two equivalents of styryl diazoacetate are used in this reaction catalyzed by dirhodium acetate,intermediate A' was intercepted by another molecule of the metal carbene intermediate to generate bicyclic pyrrolidines 3 in moderate to high yield (Scheme 4) [19]. Although this mechanism is speculative,the outcome of the reaction strongly suggests that the intermediate ‘‘free’’ ylide derived from A' has a sufficiently long lifetime to undergo a vinylogous reaction with a metal carbene intermediate.
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| Scheme 3.Divergent pathways to isomeric dihydropyrroles catalyzed by Rh or Cu catalysts. | |
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| Scheme 4.Dirhodium-catalyzed bicyclic pyrrolidine synthesis. | |
Coordinative unsaturation at the metal center allows transition-metal complexes to react as electrophiles (Lewis acids),and this property is a core feature in determining the divergent pathways through which different catalysts can direct reactants to products. Copper catalysts are widely recognized Lewis acid catalysts [20],and these catalysts are also notable for the generation of metal carbene intermediates [21]. In contrast, dirhodium(II) catalysts are weaker Lewis acids [22],and they are often the preferred catalysts for metal carbene transformations [1]. Although Lewis basicity and hard and soft concepts are often used to explain differential selectivity in reactions,the difference between dirhodium catalysts and those of copper may lie in the structures of their associated complexes with rhodium(II) complexes acting as a surface upon which association is governed by stereoelectronic considerations [3a] 2.2. Formal [3 + 3]-cycloaddition reactions of enol diazoacetates with nitrones
Our recent research on enoldiazoacetates found that thein situ generated metal enolcarbene intermediate,formed by dirhodium catalysis,is an active 1,3-dipole equivalent; and a stepwise formal [3 + 3]-cycloaddition occurs with stable dipoles [5a]. The initial example of this cycloaddition reaction was found between enoldiazoacetate 4a and nitrones 5(Scheme 5,top part) [23]. 3,6-Dihydro-1,2-oxazines 6 are produced in high yields with high enantiocontrol when Hashimoto’s chiral dirhodium carboxylate catalyst Rh2(S-PTA)4 is used. The reaction is initiated by dirhodium-catalyzed dinitrogen extrusion to form a metal enolcarbene intermediate. Nucleophilic attack by the nitrone at the vinylogous position of the metal enolcarbene followed by intramolecular cyclization with elimination of the dirhodium catalyst,in a stepwise or concerted fashion completes the transformation. However,only intramolecular cyclopropenation is observed wheng-phenyl enoldiazoacetate 4b was used,probably due to steric inhibition to the intermolecular reaction with the dirhodium(II) catalysts. Further investigation showed that Lewis acid catalysts also promote the cycloaddition reaction,and that the reaction pathway with Cu(II) or Ag(I) as catalysts changed from that with dirhodium(II) catalysts,which is consistent with a mechanism involving Lewis acid activation of the nitrone followed by its iminium ion addition to the diazo carbon of the enoldiazoacetate and ring closure that occurred with the displacement of dinitrogen to form the corresponding formal [3 + 3]-cycloaddition products7in high yields and high diastereoselectivity (Scheme 5,bottom part) [24].
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| Scheme 5.Divergent pathways for formal [3 + 3]-cycloaddition with enoldiazoacetates. | |
With further investigation for the enantiocontrolled cycloaddition of γ-phenyl-enoldiazoacetate 4b with nitrones,AgSbF6/(S)-t BuBox catalyst was found to be superior to all other Lewis acid/ligand combinations used,giving product 7 in 92% yield but with only 61%ee. However,using the corresponding donor- acceptor cyclopropene generated in situ from 4c catalyzed by rhodium(II) acetate,formation of the [3 + 3]-cycloaddition product could be optimized to 93% yield with 90% ee when treated with the same silver catalyst (Scheme 6) [25]. In this case the silver(I)-catalyzed reaction may occur through either an cationic or silver enolcarbene intermediateviaelectrophilic addition of the catalyst to the carbon-carbon double bond of the donor-acceptor cyclopropene,and the actual mechanistic pathway remains unclear.
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| Scheme 6.Formal [3 + 3]-cycloaddition catalyzed by a chiral silver catalyst. | |
Lewis acid catalyzed dinitrogen extrusion in diazo compounds has turned out to be quite pervasive,especially in X-H insertion reactions [26]. Hu [27] and Davies [28] independently reported regioselective X-H insertion reactions of styryl diazoacetates using copper and silver catalysts,respectively (Scheme 7). In both of these cases,when the reaction was catalyzed by dirhodium catalysts,insertion occurs at the more electrophilic metal carbene position (right part); while in reactions catalyzed by Lewis acids, the nucleophilic attack occurs at the vinylogous position (left part). The contrasting behavior of styryldiazoacetates and enoldiazoacetates in their diverse metal carbene reactions may be due to conformational differences between the metal carbenes formed from styryldiazoacetate and enoldiazoacetates with enoldiazoacetatyes preferring thes-cis conformation and styryldiazoacetates reacting through thes-trans configured metal carbene [29].
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| Scheme 7.Regioselectivity controlled X-H insertion with styryl diazoacetates. | |
Reactions catalyzed with both dirhodium(II) and Lewis acid catalysts offer greater opportunities for the construction of diverse compounds,either by use of the two catalysts in combination or in sequence (Scheme 1). Functionalized diazocarbonyl compounds are generated by vinylogous addition to enoldiazo compounds catalyzed by Lewis acid catalysts followed by dirhodium-catalyzed metal carbene reactions of the diazocarbonyl compounds (Scheme 8,right side) [30] or,alternatively,the product formed through a metal carbene intermediate undergoes a further catalytic reaction involving the enol substituent (Scheme 8,left side) [31]. Although individual reactions in either sequence have been well studied, reported examples of their combination are rare for divergent synthesis,and we outline here examples that have been reported from our group.
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| Scheme 8.Cascade reactions catalyzed by dirhodium and Lewis acid catalysts in combination. | |
As stated earlier,compared to copper(I) calaysts,dirhodium(II) carboxylates can act as relatively weak Lewis acids,although both are well known catalysts for metal carbene transformations. In the reactions of cinnamaldehydes with enoldiazoacetates Cu(CH3CN)4PF6causes Lewis acid catalyzed reactions to give aldol addition products 12,whereas rhodium acetate catalyzes metal carbene reactions to give epoxides 13(Scheme 9) [32]. The product from the Lewis acid catalyzed reaction retains the diazo functionality and can,subsequently,undergo catalytic metal carbene reactions that occur as a relay when both catalysts are exposed to the reacting substrates at the same time in the same reaction flask to give bicyclic products 14 in high yields. On the other hand,the generated epoxide products undergo a subsequent copper promoted Cope rearrangement at a much higher temperature than the preceding epoxidation reaction to give oxepins 15 in greater than 80% yield with high selectivity; and this transformation can be carried out in one-pot,which constitutes a highly selective formal [4 + 3]-cycloaddition process.
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| Scheme 9.Divergent outcomes from copper- and rhodium-catalyzed reactions of enol diazoacetates with cinnamaldehydes. | |
With extension of these investigations to those with nitrones, when the reactions are catalyzed by Lewis acids,Mannich addition products 16 are observed with 100% conversion and above 90% isolated yields in 5 min [33]. When the Mannich addition process is followed by dirhodium-catalyzed dinitrogen extrusion,a novel N-OTBS insertion is observed with high diastereoselectivity. Acidpromoted aromatization of 17(elimination) gave 3-hydroxylpyrroles 18 in high yield (Scheme 10). This three-step process could be carried out in one-pot to provide general and efficient access to functionalized 3-hydroxypyrroles. In contrast,a high enantioselective formal [3 + 3]-cycloaddition was discovered when the reaction was catalyzed by chiral dirhodium carboxylate catalyst [23].
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| Scheme 10.Relay copper- and rhodium-catalyzed reactions of enol diazoacetates with nitrones. | |
In this paper divergent outcomes from β,γ-unsaturated a-diazocarbonyl compounds,including styryl diazoacetates and enol diazoacetates,are reported with reactions catalyzed by dirhodium and Lewis acid catalysts individually or in combination from the same reactants. Reactions of diazo compounds catalyzed by dirhodium catalysts or Lewis acid catalysts show different reaction pathways,which not only means the reaction selectivity is differentiated due to the diverse mechanisms,but also that catalyst-controlled divergent synthesis strategy could expand to other diazo compounds,for example,phenyl diazoacetates [34], some of which are summarized in a recent review [9] and not included in this here. Further systematic comparisons of catalyst activities and selectivities provide insights that we would not have encountered with a focus on only one set of catalysts,and these catalyst(s)-directed metal carbene reactions would not only be a strategy for synthesis of compounds with diverse structures,but also a new direction for exploring new transformations.
Acknowledgments
MPD is grateful to the National Institutes of Health (No. GM 46503) and the National Science Foundation (No. CHE-1212446). XFX is thankful to the starting funding from Soochow University and Key Laboratory of Organic Synthesis of Jiangsu Province.
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