1. Introduction
1.4 Transition Metal-Catalyzed C–H Functionalizations Assisted by Bidentate Directing Groups 23
Scheme 34 Activation of remote meta C–H bonds assisted by an “end-on” template.
1.4 Transition Metal-Catalyzed C–H Functionalizations Assisted by Bidentate Directing Groups
As discussed above, chelation-assisted transition metal-catalyzed direct C–H functionalizations were considered to be an effective protocol for the formation of C–X (X = C, O, S, N, halides) bonds though a C–H cleavage. So far, a variety of heteroatom-containing directing groups such as pyridine, pyrimidine, oxazoline, amide, ester and ketone, were employed in these transformations.
In this context, a number of catalytic systems aimed at C(sp2)–H bonds functionalizations of arenes and heteroarenes have been developed. Additionally, in several cases the benzylic C(sp3)–H bonds were also viable in these reactions.76 However, the number of more challenging functionalizations of unactivated C(sp3)–H bonds under these catalytic conditions still remains greatly limited.
In 1993, van Koten77 and coworkers found that bidentate coordination limits the degree of freedom of the ligand around the coordination sphere of the metal thus allowing one to govern the cyclo-palladation selectively toward C(sp2)–H or C(sp3)–H bond activation via five- or
Introduction
six-membered ring formation.
a)
b)
Scheme 35 Palladium-catalyzed direct C(sp3)–H bonds arylation.
Inspired by this study, in 2005, Daugulis78 realized and reported palladium-catalyzed direct C(sp3)–H bond arylations assisted by 8-aminoquinoline-derived bidentate directing group, which surmounts the limitations of monodentates. This new process based on C(sp3)–H activation allows for the β-arylation of carboxamides 101 (Scheme 35a) and γ-arylation of amine derivatives 104 (Scheme 35b) to afford the corresponding products 103 and 106, respectively, in good yields.
Remarkably, this palladium catalytic system was not only restricted to the C(sp2)–H or C(sp3)–H bond arylations, but also allowed for alkylations 79 alkynylations, 80 acetoxylations, 81 aminations,82 iodinations83 and selenations.84 Importantly, ruthenium,85 copper,86 nickel,87 a,b
78 V. G. Zaitsev, D. Shabashov, O. Daugulis, J. Am. Chem. Soc. 2005, 127, 13154–13155.
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83 H. Kodama, T. Katutira, T. Nishida, T. Hino, K. Tsubata, 2001, Patent WO 2001083421A
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rhodium88c and iron88 catalysts were also found to be applicable in this bidentate-assisted catalytic system, meanwhile, variety of bidentate directing groups were introduced for this C–H functionalization (Scheme 36).
Scheme 36 Representative bidentate directing groups developed in recent years.
In 2014, Ackermann and coworkers89 found easily accessible 1,2,3-triazoles 107 which are available in a modular fashion can also serve as the bidentate directing group for the iron-catalyzed arylation reactions. With this catalytic system, benzamides substrates bearing differently substituted groups direct transformed efficiently under a considerably mild reaction conditions. Importantly, alkenylic substrate was also compatiable and delivering the thermodynamically less-stable Z-olefin as the sole product. It is noteworthy that the user-friendly iron catalyst was not limited to C(sp2)–H arylations of arenes, but also enabled more challenging C(sp3)–H functionalizations (Scheme 37).
85 (a) G. Rouquet, N. Chatani, Chem. Sci. 2013, 4, 2201–2208; (b) S. Inoue, H. Shiota, Y. Fukumoto, N. Chatani, J. Am. Chem. Soc. 2009, 131, 6898–6899.
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5151.
Introduction
Scheme 37 Iron-catalyzed C–H arylations directed by TAM.
Although the iron-catalyzed C(sp2)–H and C(sp3)–H alkylations achieved in high yields with broad substrate scope, these transformations inolve using of expensive diphosphine ligand, stoichiometric amounts of sacrificial oxidants which make this reaction not in a economically fashion. What’s more, the using of highly reactive Grignard reagents as the arylating reagent led a lower functional group tolerance. Therefore, Ackermann’s group 90 developed ruthenium-catalyzed alkylations with the TAM(triazolyldimethylmethyl) directing groups. The ruthenium(II) catalyzed C–H functionalization protocol was applicable to user-friendly aryl bromides as the arylating reagents under mild reaction conditions which allowed chemoselective C–H arylations of TAM amides bearing variety of functional groups (Scheme 38).
Scheme 38 Ruthenium-catalyzed arylations directed by TAM.
Very recently, Daugulis and coworkers reported on the cobalt(II)-catalyzed alkyne annulations assisted by bidentate directing group in the presence of Mn(OAc)2 as the oxidant (Scheme 39).91 Electron-rich or electron-poor, amides 110 were efficiently annulated, and a large variety of alkynes could be employed. Additionally, heteroarene-substituted amides 11 were also suitable in this cobalt catalyzed system. It is noteworthy that terminal alkynes 11 were reactive and gave the product in good yields with excellent chemo- and regioselectivity.
Scheme 39 Cobalt-catalyzed oxidative alkyne annulations of amides 110.
90 H. H. Al Mamari, E. Diers, L. Ackermann, Chem. Eur. J. 2014, 20, 9739–9743;
91 L. Grigorjeva, O. Daugulis, Angew. Chem. Int. Ed. 2014, 53, 10209–10212.
Shortly thereafter, the same research group extended the scope of this reaction to alkenylations using an analogous method.92 These transformations proceeded efficiently at ambient temperature with good functional groups tolerance. Importantly, unactivated alkenes such as ethylene and cyclopentene were also reactive in this reaction (Scheme 40).
Scheme 40 Cobalt-catalyzed oxidative alkenylations of amides 110.
1.5 Transition Metal-Catalyzed Benzophosphole Syntheses
Phosphorus-containing heterocycles represent important structural building blocks in organic synthesis, medicinal chemistry, and material science.93 They have been found widespread applications ranging from ligands in transition metal complexes94 to organic semiconductor devices in material science.95 Particularly, benzophosphole derivatives have been extensively studied because of their unique optical and electronic properties. Representative examples of useful benzophospholes include n-type molecular material di(benzo[b]phosphole oxide)benzene (DBPOB, 113), electron-transporting material (ETM) di(benzo[b]phosphole) sulfide(DBPSB, 114)96 and highly luminescent π-conjugated materials 115.97 Therefore, there is a continued strong demand for chemo-and site-selective syntheses of this heteroaromatic scaffold.
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93 For a recent review, see: (a) T. Baumgartner, Acc. Chem. Res. 2014, 47, 1613−1622; (b). M. Stolar, T.
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94
Introduction
Scheme 41 Selected functional material structures of benzophosphole derivatives.
In 1971, Mislow and coworkers98 reported the first route for the synthesis of benzophospholes 117. However, this multistep method not only involved metalation of a P–X bond with a stoichiometric amount of organolithium or organomagnesium species, but also delivered the desired product in low yield. Subsequently, Winter,99 Berr,100 Nakamura101 and Tanaka102 developed several similar protocols for the synthesis of decorated benzophospholes 117 consisting of the cyclization of diphenylphosphinoxides 116 or diphenylphosphines 118 with alkynyl groups preinstalled in the ortho position (Scheme 42). However, these cyclization reactions were performed under strongly basic reaction conditions which reduced the functional group tolerance.
Scheme 42 Preparation of benzophospholes 117 under strong basic conditions.
In order to address these drawbacks, Tanaka’s group103 developed a rhodium-catalyzed asymmetric synthesis of benzopyrano- or naphthopyrano-fused helical phosphafluorenes via double [2+2+2] cycloaddition of dialkynyl phosphorus compounds 119 with phenol- or naphthol-linked tetraynes 120 (Scheme 43). These reactions proceeded in CH2Cl2 at ambient temperature to give the product in acceptable yield with good enantioselectivities. Importantly, the phosphafluorene compounds possess special photophysical properties which can be potentially applied to organic semiconducting material.
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Scheme 43 Rhodium-catalyzed enantioselective synthesis of fused helical phosphafluorenes 117.
Shortly thereafter, Takai104 developed a new protocol for the synthesis of dibenzophosphole oxides catalyzed by palladium (Scheme 44). This reaction successfully transformed P–H and C–H bond via intramolecular dehydrogenative cyclization under mild reaction conditions, which tolerated a wide range of functional groups. Especially, the chloride- and thiophene-substituted substrates were also compatible under this catalytic system and afforded the desired products in high yields. Additionally, more complicated molecules such as ladder-type dibenzophosphole oxides 122 were synthesized using this method.
Scheme 44 Palladium-catalyzed intramolecular dehydrogenative cyclization reactions.
Encouraged by Takai’s study, Nozaki105 reported on palladium-catalyzed intramolecular arylation reaction of phosphine triflate 123. After the final oxidation with H2O2 at ambient temperature, the desired dibenzophosphole products were obtained in high yields (Scheme 45). It is noteworthy that the λ5-phospha[7]helicenes 117 obtained by this method exhibited unique packing structure.
Scheme 45 Synthesis of λ5-phospha[7]helicenes 117.
Introduction
However, the instability of the hydrophosphine group in 123 limited the applicability of this method towards the synthesis of more complicateds compound. Thus, this method needs considerable improvement. In this context, in 2013, Chatani106 reported the palladium-catalyzed direct synthesis of phosphole derivatives from stable triarylphosphines 125 through cleavage of C–H and C–P bonds (Scheme 46). A wide range of substrates bearing ether, amine, ketone, ester, nitrile, and fluoride substituents worked very well in this reaction. Particularly, the chlorides and bromides were also well tolerated, thus can serve as handles for further structural modification of the useful phosphole skeleton 117.
Scheme 46 Palladium-catalyzed direct synthesis of phosphole derivatives from triarylphosphines 125.
.
106 K. Baba, M. Tobisu, N. Chatani, Angew. Chem. Int. Ed. 2013, 52, 11892–11895.