1
Introduction
Sulfur ylides can be classified into four categories according to their chemical structure (Figure [1 ]): sulfur ylides (I ), sulfoxonium ylides (II ), sulfonyl ylides (III ), and sulfenyl ylide (IV ). Alkylation or acylation of sulfur ylides (I ) or sulfoxonium ylide (II ) increases their stability and practicability, permitting them to be widely used in epoxidations, aziridinations, cyclopropanations, rearrangements, and olefinations. A number of elegant reports and reviews by Aggarwal,[1 ] Tang,[2 ] Ye,[3 ] Xiao,[4 ] and their respective co-workers summarize the impressive successes achieved during different periods.
Figure 1 Four types of sulfur ylide
The diversification of sulfoxonium ylides II has continually broadened the domain of ylide chemistry. In particular, α-carbonyl sulfoxonium ylides, derived from the reaction of sulfoxonium ylides II with acid anhydrides, acyl chlorides, isocyanates, ethyl formate, or methyl chloroformate, has opened new fields in transition-metal-catalyzed C–H functionalization (Scheme [1 ]).
Scheme 1 Synthesis of α-carbonyl sulfoxonium ylides
Recent decades have witnessed great achievements in transition-metal-catalyzed ortho- C–H functionalization reactions[5 ] leading to the efficient and straightforward construction of C–C bonds, a process increasingly viewed as one of the most significant tools available to organic chemists. Meanwhile, numerous synthons have been used in C–H activation reactions; typically, these include benzenes, olefin derivatives, diazo compounds, halides, and boric acid reagents, as discussed in reviews by Song and Li[5c ] and by Glorius and co-workers.[5d ] However, sulfoxonium ylides, which were developed to access carbo- and heterocyclic compounds via metal carbene species, have not been thoroughly investigated and reviewed to date.
Here, we provide an overview of recent achievements in rhodium- and iridium-catalyzed cross-coupling reactions of α-carbonyl sulfoxonium ylides, including ortho -C–H acylmethylation reactions and sequential ortho- C–H activation/cyclization reactions in one pot.
2
Ortho -C–H Acylmethylation Reactions
Since 2017, the chemistry of α-carbonyl sulfoxonium ylides has boomed, following the pioneering work of Aïssa and co-workers on rhodium-catalyzed cross-coupling reactions of sulfoxonium ylides with C(sp2 )–H bonds of arenes or heteroarenes.[6 ] The [Cp*RhCl2 ]2 -catalyzed ortho- C–H acylmethylation of 2-phenylpyridine proceeds in moderate to excellent yields. Notably, either 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) or NaOAc can play an essential role in efficient conversion into products.
The mechanism of this reaction has been elucidated by means of deuterium-labelling experiments and X-ray crystallography. The reaction is triggered by insertion of rhodium(III) into 2-phenylpyridine (1 ), leading to the metallized intermediate 4 , which then produces a carbon–metal bond through insertion of the ylide 2 (Scheme [2 ]). After α-elimination of DMSO, intermediate 5 is transformed into a carbene species 6 , which then generates a six-membered-ring intermediate 7 by a 1,1-aryl shift process. Finally, protodemetallation of intermediate 7 delivers the desired cross-coupling product 3 .
Scheme 2 Tentative mechanism for C–H cross-coupling with α-carbonyl sulfoxonium ylides
Scheme 3 Aïssa’s ortho -C–H acylmethylation
The practicability of the reaction was increased by its application to heterocyclic substrates such as indole, furan, or pyrrole with pyridinyl, pyrazolyl, or pyrazinyl directing groups (Scheme [3 ]). Notably, alkyl, aryl, and heterocyclic α-carbonyl sulfoxonium ylides all performed well. Undoubtedly, this provides a new and efficient methodology for ortho -C–H acylmethylation.
At almost the same time, similar work on C–H activation by sulfoxonium ylide was reported by Li’s group.[7 ] In contrast with Aïssa’s work, this reaction was carried out under acidic conditions in which [Cp*Rh(MeCN)3 ](SbF6 )2 exhibited a high reactivity in the presence of 0.6 equivalents of Zn(OAc)2 as an additive in 1,2-dichloroethane (Scheme [4 ]). Gratifyingly, this ortho -C-H acylmethylation strategy is not only applicable to the benzene ring, but also to the C(2)–H bond of indole and the C(7)–H bond of indoline. Furthermore, α-carbonyl sulfoxonium ylides containing alkyl, aryl, heterocyclic, or even complex substituent groups, universally worked well in this transformation.
Scheme 4 Li’s ortho- C–H acylmethylation
3
Ortho -C–H Activation/Cyclization Reactions
3.1
Ortho -C–H Activation/Cyclization of Anilines and Enamines
Indole derivatives are widely distributed in nature.[8 ] Moreover, the indole ring is of interest in fragrance chemistry, pesticide chemistry, and dyestuff chemistry.[9 ] As a result, numerous methods for the construction of indole scaffolds by C–H activation have been developed.[10 ]
[11 ] In 2017, Hopmann[12 ] reported an Ir(III)-catalyzed regioselectively C–H functionalization of free anilines by sulfoxonium ylides and subsequent cyclization to access 2-arylindoles 29 in moderate to good yields (Scheme [5 ]). The reaction was carried out by microwave irradiation of the aniline (2.0 equiv) with ylide 2 (1.0 equiv) in the presence of [{Ir(cod)Cl}2 ] (10 mol%) in toluene at 140 °C. Interestingly, substrates containing halogen atoms or electron-donating groups on the aniline ring afforded only the corresponding N-alkylated anilines instead of indoles.
Scheme 5 Reaction of sulfoxonium ylides with free aromatic amines to give indoles
Scheme 6 Carbene routes to α-carbonyl sulfoxonium ylides
The mechanism of this reaction is unlike that for Aïssa’s reaction described above (Scheme [6 ]). The iridium–carbene species 39 is produced by the reaction of iridium with the sulfoxonium ylide (Scheme [7 ]). The acylmethylation intermediate 41 transforms into 42 , which then give 43 through a simple Friedel–Crafts reaction. Intermediate 46 undergoes dehydration in the presence of 4-toluenesulfonic acid to complete the cyclization process.
Hopmann’s metal carbenoid C–H functionalization strategy has been successfully applied to access pyrroles 50 , including atorvastatin 55 , by the reaction of sulfoxonium ylides 2 with β-enamino esters 49 (Scheme [8 ]).
3.2
Ortho -C–H Activation/Cyclization of Azobenzenes
Scheme 7 Proposed mechanism involving an iridium carbene
Scheme 8 Reaction of ylides with enamines
In 2018, Kim[13 ] and Cheng[14 ] and their respective co-workers successively developed Rh(III)-catalyzed [4+1] annulation reactions of azobenzenes 56 with sulfoxonium ylides to give 3-acyl-(2H )-indazoles 57 (Scheme [9 ]). In these studies, a combination of [Cp*RhCl2 ]2 with AgSbF6 showed excellent performance in ortho -C–H acylmethylations of azobenzenes. Copper salts as oxidants made an important contribution to the cyclization of the intermediate to give the final product. In Kim’s work, the substrates were confined to symmetrical azobenzenes; in this reaction, substrates with electron-donating groups on the benzene ring showed obvious advantages over those containing electron-withdrawing groups. The reactions of asymmetric azobenzene substrates resulted in mixed products. Moreover, in addition to aryl-substituted sulfoxonium ylides, alkyl-substituted analogues also participated smoothly in the annulation process.
Scheme 9 Ortho -C–H activation/cyclization of azobenzenes
Scheme 10 Deuterium-labeling and KIE experiments with azobenzenes
Deuterium-labeling experiments showed deuterium incorporation (81%) at each of the ortho- positions of the azobenzene 73 , and kinetic-isotope-effect (KIE) studies were showed a KIE value of 2.6 for the azobenzene 74 (Scheme [10 ]), indicating that the C–H bond-cleavage process might be the rate-limiting step.
3.3
Ortho -C–H Activation/Cyclization of N-Methoxybenzamide
N -Methoxybenzamide, a common synthetic fragment, has been widely applied in syntheses of heterocyclic compounds through C–H functionalization.[15 ] In 2017, Aïssa and co-workers reported a convenient synthesis of valuable heterocycles, such as the 3-substituted 2-methoxyisoquinolin-1(2H )-one 79 (Scheme [11 ]), in their remarkable work on Rh(III)-catalyzed C–H acylmethylation of arenes.[6 ]
Scheme 11 Ortho -C–H activation/cyclization of N- methoxybenzamide by Aïssa
In 2018, Li[16 ] reported Rh(III)-catalyzed chemodivergent annulations of N -methoxybenzamide (77 ) with sulfoxonium ylides to give the isocoumarins 80 or the isoquinolinones 81 under acidic conditions (Scheme [12 ]).
Scheme 12 Ortho -C–H activation/cyclization of N- methoxybenzamide by Li
Both reactions involve the coordination of Rh(III) with N -methoxybenzamide (77 ) to form a carbene species 83 , which is then converted into a rhodacycle, which releases the rhodium catalyst to form an ortho -acylmethyl intermediate 84 (Scheme [13 ]). The additives promote a further transformation that is the key step in this process. In fact, PivOH activates the amide carbonyl group toward attack by oxygen, leading to the isocoumarin 80 with the elimination of MeONH3
+ . With the Lewis acid Zn(OTf)2 as additive and [RhCp*(MeCN)3 ](SbF6 )2 as the catalyst, the isoquinolin-1(2H )-ones 81 become the major products.
Scheme 13 Acid-controlled orth o-C–H acylmethyl/cyclization process
3.4
Ortho -C–H Activation/Cyclization of Imines
The imide group, a well-known directing group due to its strong ability to coordinate with transition metals, participates in many [4+1] and [4+2] C–H activation/cyclization reactions in the presence of common C2 synthons, such as alkynes,[17 ] diazo compounds,[18 ] 1,4,2-dioxazol-5-one,[19 ] or alkenes.[20 ] Sulfoxonium ylide have the potential to act as versatile and general-purpose C2 synthons in C–H functionalization and cyclization reactions to give heterocycles. Such a reaction was reported in 2018 by Li and co-workers,[21 ] who obtained a range of six-membered nitrogen heterocycles through the Rh(III)-catalyzed C–H activation of sulfoximines, N -aryl- or N -alkylbenzamidines, or benzophenone NH imines (Scheme [14 ]).
Scheme 14 Ortho -C–H activation/cyclization of imines
This strategy has been applied in syntheses of benzothiazines, isoquinolines, and isoquinoline N -oxides. Moreover, sulfoxonium ylides showed high reactivity with 2-arylimidazo[1,2-a ]pyridines to deliver fused heterocyclic products. Coincidentally, we recently reported a [Cp*RhCl2 ]2 -catalyzed reaction of 2-phenylbenzimidazole with sulfoxonium ylides to generate isoquinolines.[22 ]
We later reported a rhodium(III)-catalyzed C–H activation of ethyl benzimidates 97 with sulfoxonium ylides (Scheme [15 ]).[23 ] The dual ortho -C–H functionalization and cyclization of ethyl benzimidates with sulfoxonium ylides in one pot led to pyrano[4,3,2-ij ]isoquinoline derivatives 98 with interesting optoelectronic properties.
Scheme 15 Dual ortho -C–H activation/cyclization of ethyl benzimides
The reaction mechanism was elucidated by means of a series of control experiments. Initially, coordination of Rh(III) to the ethyl benzimide 97 after dedimerization of [Cp*RhCl2 ]2 produces a rhodacyclic intermediate 99 (Scheme [16 ]). Migratory insertion of the carbene species and protonolysis then affords the acylmethylated intermediate 100 . Unlike other reported reactions, the intermediate does not dehydrate immediately and, instead, is dehydrated after completion of a secondary catalytic cycle.
Scheme 16 Proposed mechanism for the dual ortho -C–H functionalization/annulation of ethyl benzimidates
3.5
Orth o-C–H Activation/Cyclization of N-Azoloimines
Ellman and co-workers reported the rhodium(III)-catalyzed C–H functionalization of alkenyl azoles with 1,4,2-dioxazolones, acetylenes, or diazones to give the corresponding [5,6]-bicyclic heterocycles.[24 ] A year later, this [4+2] reaction strategy was applied to annulations of N -azoloimines under redox-neutral conditions (Scheme [17 ]). Notably, the group of reaction partners contained not only acetylenes and diazo ketones, but also sulfoxonium ylides. This significant work pioneered the application of Rh(III) catalysts in direct imidoyl C–H activation, providing a range of polysubstituted imidazopyrimidines with good functional-group tolerance and high stereoselectivity (Scheme [17 ]). Both electron-rich and electron-deficient imines 101 reacted readily with phenyl sulfoxonium ylides to give the corresponding imidazopyrimidines 102 , generally in high yields. A furfural-derived imine and an aminopyrazole-derived imine showed the high reactivities, giving the corresponding products 106 and 107 in high yields. With regard to the ylide, the effects of various substituents on the reaction were investigated, and it was found that ylides containing electron-rich aryl groups showed greater efficiency than their electron-deficient analogues in the cross-coupling reaction.
Scheme 17 Orth o-C–H activation/cyclization of N -azoloimines
Scheme 18 Mechanism of ortho -C–H functionalization/annulation of N -azoloimines
The mechanism was elucidate after the identification of the C–H activation intermediate rhodacycle 111 through X-ray analysis (Scheme [18 ]). C–H activation of the imine 101 produces the rhodacyclic intermediate 111 , which is then transformed into 112 through insertion of the sulfoxonium ylide. After sequential α-elimination of DMSO, migration, protonolysis, and dehydration, rhodacycle 102 is formed. Deuterium and kinetic-isotope experiments suggested that the formation of rhodacycle 111 through concerted metalation/deprotonation of imine 101 controls the reaction rate. Therefore, breaking of the C–H bond is the rate-determining step.
3.6
Ortho -C–H Activation/Cyclization of Benzoylacetonitriles
In the reactions discussed above, the nitrogen atom of the directing group plays a key role in metal coordination and C–H activation. However, as a result of Wang’ s pioneering work on Rh(III)-catalyzed oxidative annulation of benzoylacetonitriles to give naphthols and naphtho[1,8-bc ]pyrans,[25 ] interest was aroused in the use of benzoylacetonitrile derivatives in transition-metal-catalyzed C–H activations in which there are no NH directing groups.[26 ] Li and co-workers developed several syntheses of 1-naphthols, especially an excellent recent synthesis involving sulfoxonium ylides.[27 ] In that work, sulfoxonium ylides served as C2 synthons in reactions with benzoylacetonitriles to give 1-naphthols 115 or naphtho[1,8-bc ]pyrans 117 (Scheme [19 ]). This alternative transformation was catalyzed by Cp*Rh(OAc)2 ·H2 O under mild conditions in DCE and gave polysubstituted cyclization products. Notably, because of the high reaction efficiency of sulfoxonium ylides, the desired naphthols 115 could be generated only when the ortho- positions of the benzoylacetonitriles were blocked. Otherwise, dihydronaphtho[1,8-bc ]pyrans 116 were obtained, and these could undergo further dehydration to 117 in the presence of trifluoromethanesulfonic acid in a one-pot process.
Scheme 19 Ortho -C–H activation/cyclization of benzoylacetonitriles by Wang
Recently, Zhou et al. successfully developed a procedure for accessing 3-substituted naphthols through a Rh(III)-catalyzed relay functionalization with benzoylacetonitriles and sulfoxonium ylides (Scheme [20 ]).[28 ] The significant steps were concerted metalation/deprotonation of the benzoylacetonitrile and the formation of intermediate 119 . Unlike Li’s work, this transformation of benzoylacetonitrile can be controlled by replacing water with CsOAc and reducing the temperature of the reaction system to 50 °C, with catalysis by [Cp*Rh(Cl)2 ]2 , to give the 1-naphthol 120 as the sole product.
Scheme 20 Ortho -C–H activation/cyclization of benzoylacetonitriles by Zhou
3.7
Ortho -C–H Activation/Cyclization of Benzoyl Sulfoxonium Ylides
Sulfoxonium ylide can serve not only as C2 carbene synthons, but also as traceless bifunctional directing groups. Li and co-workers were the first to apply benzoyl sulfoxonium ylides 133 in C–H activation reactions with symmetrical alkynes 134 to give a range of 3,4-disubstituted naphthalen-1-ols 135 in moderate to excellent yields (Scheme [21 ]).[29 ]
Scheme 21 Ortho- C–H activation/cyclization of benzoyl sulfoxonium ylides
The following mechanism was proposed by the authors (Scheme [22 ]). Coordination of the oxygen atom in the benzoyl sulfoxonium ylide 133 to the rhodium catalyst gives a five-membered rhodacyclic intermediate 142 , which then transforms into the seven-membered rhodacyclic intermediate 143 through the coordination of the alkyne and migratory insertion of the aryl group. The key rhodium carbene species 145 is formed after release of DMSO. Subsequent protonolysis completes the rhodium catalytic cycle and releases the final product, the 1-naphthol 135 .
Scheme 22 Proposed mechanism for the C–H activation of benzoyl sulfoxonium ylides
