Sulfur-Based Ylides in Transition-Metal-Catalysed Processes
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Traditionally employed in the synthesis of small ring systems and rearrangement chemistry, sulfur-based ylides occupy a unique position in the toolbox of the synthetic organic chemist. In recent years a number of pioneering researchers have looked to expand the application of these unorthodox reagents through the use of transition metal catalysis. The strength and flexibility of such a combination have been shown to be of key importance in developing powerful novel methodologies. This chapter summarises recent developments in transition metal-catalysed sulfonium/sulfoxonium ylide reactions, as well as providing a historical perspective. In overviewing the successes in this area, the authors hope to encourage others into this growing field.
KeywordsSulfonium ylides Sulfoxonium ylides Transition metal catalysis Recent developments Historical perspective One-carbon synthon Asymmetric catalysis
Density functional theory
Gas Chromatography–Mass Spectrometry
Light emitting diode
Nuclear magnetic resonance
Single electron transfer
Sulfur-based ylides are zwitterionic compounds in which each formal charge is stabilised by its proximity to the other. They have long been used as one-carbon synthons in a number of classical transformations, most notably for the synthesis of small rings such as epoxides, aziridines and cyclopropanes through ylide addition to electron-poor π-systems. While reactions with unstabilised sulfonium and sulfoxonium ylides are known, most modern processes utilise stabilised versions, in which the negative charge is delocalised into one or more electron withdrawing groups. This added stability means that sulfonium/sulfoxonium ylides can be used as practical, bench-stable reagents and enables the development of reactions of increased complexity. However, this extra stability is accompanied by a drop in the reactivity of the processes mentioned above. By combining sulfur ylides with transition metal catalysis, the utility of these unusual reagents can be greatly enhanced. In this chapter we seek to summarise the recent developments in this field. We have subdivided the reactions into three sections: (1) carbene-free processes; (2) reactions that employ sulfonium or sulfoxonium ylides as carbene precursors; (3) reactions that form sulfonium ylides in situ from metal carbenoids for cascade reactions. These distinctions were recently used in a related review, currently in press, in which the authors of the current review described such reactions in the context of structure and bonding .
2 Metal–Ylide Complexes
Most successful transition metal-catalysed reactions involving sulfonium ylides rely on activation of either the sulfonium ylide or the coupling partner by the transition metal complex. In order to study and develop these potential processes, it is often instructive to investigate the coordination chemistry that would be invoked. To this end, a number of publications detailing metal–ylide complexes have been published over recent decades.
Initial studies in 1974 began with investigations on Pd(II) salts  and Pt(II) salts [2, 3]. In these studies, X-ray crystallographic analysis showed that for carbonyl-stabilised ylides, σ-coordination through the ylide carbon-atom was the preferred mode of bonding, rather than the alternative oxygen-bound possibility. A similar ambidentate character has been observed for a number of ylides and is well summarised in a 1983 review by Weber .
In the intervening years, carbon-based coordination chemistry of sulfonium ylides with alternative metal complexes has been reported for Pd(II) , Pt(II) [2, 3], Hg(II) , Cd(II) , and Ag(I) , while for hard, oxophilic metals such as W(0) co-ordination through oxygen has been observed .
3 Non-Carbene-Based Transition Metal-Mediated Reactions of Sulfur Ylides
3.1 Sulfur Ylides as Single-Carbon Synthons in Formal (n + 1) Cycloadditions
3.2 Transition Metals as π-Acid Catalysts
During the development of the furanone formation, a side product was detected in which the alkyne coupling partner was not incorporated. By removing the alkyne from the reaction system the allyl-ester derived sulfonium ylides were directly converted into cyclopropane derivatives. This intramolecular gold-catalysed cyclopropanation proved to be remarkably efficient, resulting in good yields and excellent diastereoselectivity and tolerating a broad range of functional groups.
3.3 Sulfoxonium Ylides in C–H Functionalisation Processes
3.4 Sulfonium Ylides as C–S Activation Precursors
3.5 Photocatalysis Involving Sulfonium Ylides
4 Sulfonium and Sulfoxonium Ylides as Metal-Carbene Precursors
We have seen in the previous section how sulfur-based ylides have been used as carbenoid/one-carbon synthons in a number of transition metal-mediated processes. One of the most ubiquitous methods of introducing such synthons is through the action of metal carbenoids (also known as alkylidenes), generated from diazo compounds and metal complexes. Typical reactions include cyclopropanation and insertion into C–H and X–H (X = N, O, S, P, Se) bonds [50, 51, 52, 53, 54]. While a highly popular and efficient reaction choice, the use of diazo compounds on a large scale does have significant drawbacks for both safety and operational simplicity. A number of sulfonium and sulfoxonium ylides have therefore been investigated as alternative metal-carbenoid precursors . Although such approaches do offer notable improvements, the ease with which the reverse reaction occurs, i.e. the formation of sulfonium ylides from the combination of metal carbenoid species with sulfides (see Sect. 5), is a major hurdle to overcome.
4.1 Cyclopropanation Reactions
The synthesis of difluoromethyl cyclopropanes can also be achieved using the same catalyst, as reported in 2017 . A 20 mol% of Zn dust was added to assist the reduction of Fe(III) to Fe(II), believed to be the active cyclopropanation catalyst.
4.2 Insertion Reactions into X–H Bonds
The efficiency of these reactions drew interest to the extent that Merck & Company began their own research programme, and in 2009 Mangion and co-workers published a more general methodology, based on Baldwin and co-workers’ original conditions . By treating ylides of type 31 with [Ir(COD)Cl]2 Merck researchers were able to achieve intermolecular X–H insertion reactions with a wide range of coupling partners (Scheme 25b) . When Rh2(TFA)4 was used as the catalyst for the reaction of 31 and aniline, only 22% of the desired product was obtained. As a rationale for this observation the authors proposed that the DMSO released after formation of the metal-carbenoid poisoned the catalyst, an assertion that was reinforced by relevant control experiments. With [Ir(COD)Cl]2 it was possible to react the sulfoxonium ylides with a range of primary and secondary amines, alcohols and thiols to afford insertion products in yields ranging from 63 to 93%. The substrates previously investigated by Baldwin were also reactive under these conditions, generating various cyclic structures without the requirement for high reaction temperatures or for catalyst loadings of > 1% (Scheme 25a).
Further studies proved that Au and Pt salts were also competent catalysts for these transformations . AuCl(SMe2) proved to be the most effective catalyst (94% yield for the reaction of 31 with aniline), with Pt(COD)Cl2 and AuCl3 also proving satisfactory (yields of 81 and 78% respectively, compared with 91% for [Ir(COD)Cl]2). NMR spectroscopy was used to provide evidence for the existence of iridium carbenoids. However, such species were not observed in the gold-based systems, which the authors attribute to the short lifetimes of gold carbenoids, and therefore proposed analogous reaction mechanisms for the two processes.
The synthesis of MK-7655, a β-lactamase inhibitor, was similarly achieved within the Merck process group (Scheme 26b) . For the key N–H insertion step, iridium-based catalysts were again found to be the most suitable, and 34 was produced in 87% yield.
4.3 Insertion Reactions into C–H Bonds
That there only exists one successful case of formal C–H insertion is a clear indication that such processes can be regarded as the poor cousin of the N–H insertion and cyclopropanation reactions in cases when sulfur-based ylides are used as the precursor. The increased nucleophilicity of the ylides relative to diazo compounds is likely to be the key challenge, as nucleophilic attack of the ylide onto the electrophilic carbenoid can be seen to outcompete C–H insertion processes.
4.4 Miscellaneous Metal Carbenoid Reactions
A number of stable metal carbenes have long been used as catalysts for a range of reactions, not least the olefin metathesis reaction. As described so far in this section, sulfonium ylides have been used as metal carbenoid precursors for a number of onward reactions, but it was not until the early 2000s that this approach was used for the synthesis and isolation of such complexes [68, 69]. In studies by Milstein et al., this technique was applied on a number of metal complexes, including those of rhodium, iridium, ruthenium and osmium, as well as on a number of alkylidene units. Initially carried out using sulfonium ylides in solution, more recent investigations have centred on the use of polymer-supported sulfonium ylides, enabling the reuse of the sulfide carrier. Using this method, which relies on the reactivity of diaryl sulfides, the first-generation Grubbs’ catalyst and Werner’s carbene could both be prepared.
Sulfonium ylides have also been used in metal-catalysed polymerisation processes. De Bruin and co-workers found that when exposed to rhodium catalysis, dimethylsulfoxonium methylide polymerised to polymethylene in yields varying between 15 and 80% . The co-polymerisation of sulfonium ylides with diazocompounds was also reported using the same catalyst.
5 Cascade Reactions Involving Transition Metal-Catalysed Ylide Formation
Classical routes to sulfur-containing ylides mostly revolve around the deprotonation of the corresponding sulfonium/sulfoxonium salt, usually generated via alkylation. Although conceptually simple, this approach has, by its very nature, major limitations. Prominent among these is the incompatibility of base-sensitive functionalities. The choice of base is also an important consideration, as sulfonium salts have long found employment as alkylating reagents: a non-nucleophilic base is therefore essential. The final problem, albeit not limited to deprotonation-based approaches, is the potential for chemoselectivity issues if there are multiple protons with similar pKa values. In order to simplify matters, most sulfonium/sulfoxonium salt precursors either contain solely degenerate substituents (e.g. trimethylsulfoxonium) or direct the deprotonation through the use of one or more electron withdrawing groups. Such groups also help to stabilise the ylide thus generated, facilitating controllable onward reactions.
An alternative route, utilising metal carbenoid intermediates emerged shortly thereafter, in which Ando and co-workers employed copper sulfate as a precursor (Scheme 31) [73, 74]. As the metal carbenoids have lower energy than photolytically generated carbenes, the yields for this process were uniformly higher, many proceeding in excellent yields. The avoidance of UV-light, always an issue with scalability and potentially safety, was a further advantage.
The group of Porter showed in 1978 that rhodium acetate could serve as a replacement for copper sulfate in the synthesis of sulfonium ylides . As a result of the high efficiency of rhodium salts in the decomposition of diazo compounds, only 0.12 mol% of catalyst was needed to deliver the desired thiophenium ylide in 95% yield, compared to 35% when using copper catalysis at reflux for 8 days.
As a result of the mild reaction conditions and good chemoselectivity displayed, the transition metal-mediated generation of sulfonium and sulfoxonium ylides has rapidly been adopted by the synthetic chemistry community as one of the methods of choice for the generation of such compounds. The transformations are in fact so clean that a number of onward reactions, using in situ formed sulfonium ylides in domino processes, have been developed. Depending on the structure of the ylide, a number of transformations can occur such as [2,3]-sigmatropic rearrangements, 1,2-migrations and Corey–Chaykovsky-type reactions. These processes have been summarised in a number of review articles [76, 77, 78], and the purpose of this section of our review is to provide an overview of significant and recent examples.
5.1 [2,3]-Rearrangement of Sulfonium Ylides
Generally, the [2,3]-sigmatropic rearrangements of ylides result in the formation of chiral products. The development of an enantioselective variant would be highly appealing, as it would allow the stereocontrolled formation of a C–S and a C–C bond to the same carbon atom. Concerted efforts towards this goal have spanned the last two decades and predominantly focussed on the use of chiral metal complexes. However, only recently have good enantioselectivities in purely catalyst-controlled systems been achieved .
Diazo compounds are often excellent metal carbene precursors, as their decomposition in the presence of transition metal catalysts requires mild conditions and offers good chemoselectivity. However, they are associated with major drawbacks due to their hazardous and potentially explosive nature and are, consequently, of limited use for large-scale reactions. Furthermore, stable diazo compounds typically must carry at least one electron-withdrawing group. During recent decades, several alternative strategies have been laid forth that not only avoid the use of diazo compounds but also extend the scope and efficiency of the Doyle–Kirmse reaction. Promising results include the generation of diazo intermediates in situ and the direct generation of metal carbenoids from unsaturated C–C bonds in the presence of π-acid catalysts.
In this framework, tosylhydrazones are valuable precursors of unstabilised diazo compounds through the so-called Bamford–Stevens reaction. In the presence of catalytic Rh2(OAc)4 and allyl sulfides, tosylhydrazones were transformed into sulfonium ylides able to undergo [2,3]-rearrangement . Compared to previously reported methods, this approach allows for the formation of rearranged products without the requirement for an electron-withdrawing group usually necessary to stabilise the diazo starting material. Another method for the in situ generation of diazo compounds is the slow addition of sodium nitrite to a primary amine, and such an approach was employed by Koenigs and co-workers . Through the use of the appropriate amines, they were able to generate cyano-, ester- and trifluoromethyl-substituted diazo compounds which underwent Fe-catalysed Doyle–Kirmse reactions. While an improvement from the point of view of safety and scalability, this procedure does not yet offer as wide a scope as the Bamford–Stevens-based chemistry.
More recent methodologies concerning the intermolecular oxidation of alkynes using stoichiometric amounts of amine N-oxides have also been reported. These procedures have the advantage of not having to use highly designed substrates, but rather they can use simple starting materials. As the potential for the formation of regioisomeric mixtures is more of a problem in intermolecular processes, polarised alkynes (e.g. ynamides) were chosen for the oxidative synthesis of 2-(thio)amides bearing a α-quaternary centre . Only a narrow scope of ynamides was presented, which suggests difficulties in controlling the chemoselectivity in the presence of alternative external nucleophiles that can intercept the gold carbene intermediate. This competition was addressed in an independent publication by Zhang and co-workers describing the three-component synthesis of α-aryl(alkyl)thio-γ,δ-unsaturated ketones starting from terminal alkynes . This reaction could be carried out efficiently by the addition of the oxidising reagent via a syringe pump and, more importantly, P,S-bidentate ligands were used to decrease the electrophilicity of the α-oxo gold carbene complex.
5.2 1,2-Migration of Sulfonium Ylides
While being powerful transformations, the Doyle–Kirmse and Sommelet–Hauser reactions by no means represent the only rearrangements available to sulfur-based ylides. Direct 1,2-migrations of alkyl substituents from the cationic to the anionic centre are known as Stevens rearrangements. These reactions have been thoroughly investigated in systems based on oxonium and ammonium ylides, but the sulfur-equivalent, i.e. thia-Stevens rearrangement, has not received nearly as much attention. Though underutilised, the thia-Stevens rearrangement has still proven to be a powerful tool for C–C (and C–N ) bond formation and the synthesis of quaternary centres.
When combined with the power of in situ, transition metal-mediated ylide formation, the synthetic potential of this synthetic strategy is fully unlocked. In particular, such processes have most commonly been used for the synthesis of sulfur-containing heterocycles through ring-expanding 1,2-shifts.
5.3 Synthesis of Small Rings
Beyond rearrangement reactions, any number of other ylide-mediated transformations can be achieved through metal-catalysed, in situ generation of the sulfonium ylide, rather than using a preformed reagent. The prototypical example of such chemistry is the synthesis of small rings through the Corey–Chaykovsky reaction. These processes have long been studied, with highly diastereo- and enantioselective reactions reported. The reader is referred to several in-depth reviews and book chapters on this topic for further information [123, 124].
The synthesis of epoxides as synthetic intermediates is a rich field in synthetic methodology. Whilst a wide array of catalytic alkene oxidation conditions have been developed, there are often advantages to using alternative disconnections in which enantioinduction can be more easily achieved. The synthesis of enantiopure epoxides is of key importance, as the stereoselective ring opening of such structures with suitable nucleophiles can create contiguous chiral centres. Chiral variants of the Corey–Chaykovsky reaction are well precedented, although the majority of these reactions are stoichiometric in the chiral sulfonium. Catalytic variants are possible, although the scope of these transformations remains limited.
By exchanging the aldehyde component for an imine, it is possible to access aziridines. These structures are synthetically useful building blocks for similar reasons to epoxides, and a number of asymmetric methods for their construction have been developed. Aggarwal demonstrated an extension to his above-mentioned epoxide synthesis in 1996 by utilising electron-poor imines, affording protected aziridines . The necessity of the electron-withdrawing functionality on the nitrogen was a result of the ability of metal carbenoids to react with imines directly, negating the effect of the chiral sulfide. The higher electrophilicity of the electron-poor imines renders them at once more reactive towards the sulfonium ylide and less prone to react with the metal carbenoid. In addition to diazo compounds, tosylhydrazone salts were again suitable carbenoid precursors [132, 133], and ketimines and alkyl aldimines were successfully employed in the process with no loss of reaction efficiency or yield.
Aggarwal and co-workers further demonstrated that the strategy of in situ generation of sulfonium ylides was applicable to the synthesis of cyclopropanes from electron-deficient olefins . While the enantioselectivities of the generated cyclopropanes were excellent, the yields remained moderate while using sulfide 63, previously employed in epoxidations and aziridinations. Higher yields of the cyclopropane products were obtained by switching to sulfide 64 (Fig. 15) . Finally, it was once again shown that the use of tosylhydrazone salts in lieu of diazo compounds could be achieved .
5.4 In Situ Generation of Thiocarbonyl Ylides
Further experiments by Padwa and co-workers on similar substrates found that if the cyclic thiocarbonyl ylide had additional stabilisation, such as in the mesoionic, aromatic thiaisomünchnone 68, then the compound is stable and isolable or, alternatively, it can react in situ with suitable dipolarophiles in a (3 + 2) fashion (Scheme 52b) . Ylide 66, even in the presence of an excess of dipolarophile was not observed to undergo this cycloaddition, with the (1,3) electrocyclisation occurring much more rapidly.
Throughout each of the sections in this review, we have demonstrated how the power and flexibility of transition metal catalysis can be harnessed for the development of innovative, novel transformations involving sulfonium and sulfoxonium ylides. Such ylides traditionally react as nucleophiles, which restricts their application as single carbon synthons to a handful of valuable, but limited reactions. By introducing transition metal catalysts into the equation, a considerable breadth of reactivity can be accessed. A number of enantioselective transformations have been demonstrated utilising chiral ligands, often through the enantiocontrolled, in situ generation of chiral sulfonium ylides for use in cascade processes. Whether through carbene transfer, oxidative addition pathways or Lewis acid activation, the use of transition metal catalysts in combination with sulfur ylides represents an underexplored and underutilised area of synthetic organic chemistry, where further exciting developments can be anticipated in coming years.
Open access funding provided by University of Vienna. Funding was provided by Deutsche Forschungsgemeinschaft (Grant no. MA 4861/4-2).
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