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Recent Advances in Transition Metal-Mediated Functionalization of O-Carboranes ⇑ Zaozao Qiu

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Recent Advances in Transition Metal-Mediated Functionalization of O-Carboranes ⇑ Zaozao Qiu Tetrahedron Letters 56 (2015) 963–971 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet Digest Paper Recent advances in transition metal-mediated functionalization of o-carboranes ⇑ Zaozao Qiu Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China article info abstract Article history: Icosahedral carboranes constitute a class of structurally unique molecules with exceptionally thermal Received 10 November 2014 and chemical stabilities, which limits the derivatization of these clusters. In view of the spectacular role Revised 22 December 2014 of transition metals in synthetic chemistry, novel o-carborane functionalization methods for both cage Accepted 2 January 2015 carbon and boron vertices have been developed by using transition metal-promoted reactions. These Available online 12 January 2015 methods offer a series of exceptionally efficient synthetic routes to a wide range of functionalized carbor- anes from readily available starting materials. An overview of recent advances in this field is presented in Keywords: this digest. Carborane Ó 2015 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecom- Carboryne Cycloaddition mons.org/licenses/by-nc-nd/4.0/). Coupling reaction Transition metal Contents Introduction. ...................................................................................................... 963 Cage carbon functionalization of o-carborane . ................................................................ 964 Copper-mediated reaction of lithiocarborane . ......................................................................... 964 Transition metal-promoted reaction of carboryne . ...................................................... 965 Reaction with alkynes . .............................................................................. 965 Reaction with alkenes . .............................................................................. 967 Cage boron functionalization of o-carboranes . ................................................................ 968 Cage B–I bond activation . ......................................................................................... 968 Cage B–H bond activation and functionalization ......................................................................... 969 Conclusions and outlook. ................................................................................... 970 Acknowledgments . ................................................................................... 970 References and notes . ................................................................................... 970 Introduction investigated, of all known carboranes, during the last 50 years. o-Carborane was obtained by the reaction of acetylene with decab- Carboranes are a class of boron hydride clusters in which one or orane.5 Recently, it was reported that the addition of a catalytic more BH vertices are replaced by CH units. They have many char- amount of silver salt to the above reaction significantly enhances acteristics such as spherical geometry, remarkable thermal and the yield of carborane formation.6 Unlike boron hydrides, chemical stability, and a hydrophobic molecular surface, leading o-carborane is stable in the presence of oxidizing agents, alcohols, to many applications in medicinal,1 materials,2 and coordination and strong acids. It exhibits phenomenally thermal stability up to chemistry.3 The synthesis and properties of icosahedral carboranes 400 °C. Under an inert atmosphere, it rearranges to m-carborane were first reported in 1963,4 which has been the most extensively between 400 and 500 °C and to p-carborane between 600 and 700 °C(Scheme 1).7 ⇑ Tel.: +86 21 54925569; fax: +86 21 54925383. One of the most important features of carborane is its ability to E-mail address: [email protected] enter into substitution reactions at both the cage carbon and boron http://dx.doi.org/10.1016/j.tetlet.2015.01.038 0040-4039/Ó 2015 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 964 Z. Qiu / Tetrahedron Letters 56 (2015) 963–971 H H H HH Lewis base I + HC CH F H o-Carborane NO C 2 o unmarked BH 400 ~ 500 C H NaH n BuLi NO2 H 600 ~ 700 oC H H H CuCl I H H p-Carborane m-Carborane H concentration time yield Scheme 1. Synthesis and thermal isomerization of o-carborane. 0.1 M 48 h 41% 0.3 M 5 h 84% atoms without degradation of the cage. The stability of the carbo- Scheme 3. Synthesis of mono- and diaryl-o-carboranes. rane cage is demonstrated under many reaction conditions used to prepare a wide range of C- and B-substituted carborane deriva- tives. In view of the important role of transition metals in chemical transformations, transition metal promoted carborane functionali- zation methodology has experienced tremendous growth to afford various novel carborane derivatives which cannot be generated by H other conventional methods. Despite the relatively weaker induc- I tive electron attraction in m- and p-carboranes, which leads to I H reduced C–H reactivity toward metallation at carbon atoms, many o-carborane derivatization methods are applicable to m- and H p-carboranes.7a As the icosahedral o-carborane has been most I extensively investigated and is commercially available, this digest Cu focuses on recent advances in transition metal-mediated/-cata- I lyzed functionalization of o-carboranes. Cu H I Cage carbon functionalization of o-carborane H Copper-mediated reaction of lithiocarborane I H During the past decades, investigations in the field of carbon- substituted carboranes were directed at improving the synthetic Scheme 4. Reaction of C,C0-dicopper(I) o-carborane with ortho-, meta-orpara- methods for the preparation of organic and organometallic diiodobenzene. H H nBuLi CuCl X R Ph H H H NN Zn Ph N N Fe Ph I H N I I N H H Scheme 2. Cu(I)-mediated coupling of o-carboranyl with arylhalides. Z. Qiu / Tetrahedron Letters 56 (2015) 963–971 965 Li Cu(Tol) C R B Li Cu(Tol) unmarked BH carboryne 1,3-dehydro- o-carborane H (Tol)Cu [M ] [M ] metal-carboryne H Cu(Tol) Chart 1. Carboryne, 1,3-dehydro-o-carborane, and metal–carboryne. Scheme 5. Synthesis of 1,10-bis(o-carborane) from Cu-mediated reaction. R1 carboranyl compounds used in materials as well as in biological and δ− δ+ medical applications. There are two conventional synthetic meth- Cp2Zr R2 ods leading to carbon-substituted carboranes: the reaction of substituted acetylenes with decaborane and electrophilic substitu- R1 7 Cl OEt tions of lithiocarborane. The latter is limited only to electrophiles 2 2 3 Cp2Zr Li Cp2Zr R with sp carbons. R1 2 OEt2 R Copper-mediated coupling reaction is developed for the synthe- toluene 8 sis of alkenyl, alkynyl, and aryl carboranes. Selected examples are (R 1,R2 =alkyl,aryl) illustrated bellow. Direct Cu(I)-mediated coupling of o-carborane is used to intro- Scheme 6. Reaction of zirconocene–carboryne precursor with alkynes. duce various aryl containing functional groups (Scheme 2). For examples, carborane–ferrocene conjugated dyads are synthesized Et to investigate their nonlinear optical (NLO) properties;9 direct cou- Et H N Et pling of o-carborane with a porphyrin via a carbon–carbon bond is H Et achieved;10 and Cu-mediated method is used to introduce a sty- rene conjugation bridge in the carborane-based tribranched com- pounds to study the influence of isomeric carboranyl units as well as their push and pull electronic effects on photophysical XylNC H +O properties in bioimaging.11 3 As the aforementioned reactions can afford only monoaryl-o- Et Et I carboranes, the aromatic nucleophilic substitution (SNAr) reaction Cp2Zr Et H Et of 1-aryl-o-carboranes with 4-nitrofluorobenzene in the presence CuCl of NaH or KOtBu is then developed to yield various 1,2-diaryl-o-car- I2 boranes that are useful precursors for macromolecular construction 19a and drug design (Scheme 3).12 On the other hand, Ullmann-type coupling reaction of 1-iodonaphthalene with CuCl2 I o-carboranyl copper gives efficiently 1-(o-carboranyl)-naphthalene Et (Scheme 3). It is found that the concentration is crucial for this reac- Et Et I CuCl Et tion. When the reaction is conducted at a high concentration, such as 0.3 M, it affords the desired compound in excellent yield (84%), which is much better than that of 41% at 0.1 M concentration.13 0 Reactions between C,C -dicopper(I) derivative of o-carborane Scheme 7. Transformations of zirconacyclopentene incorporating a carboranyl and ortho-, meta-, or para-diiodobenzene are investigated. The unit. 0 reactions with 1,3- or 1,4-C6H4I2 provide 1,3-bis(1 -o-carbora- nyl)benzene and 1,4-bis(10-o-carboranyl)benzene, respectively, 0 0 whereas reaction with 1,2-C6H4I2 afford unexpectedly 2,2 -bis(1 - controlled manner. It has been reported that the M–Ccage (Ccage: o-carboranyl)biphenyl, [HCB10H10CC6H4]2 (Scheme 4). Electronic hypervalent cage carbon) r bonds in metal–carboranyl complexes communication between the carborane cages via the para-pheny- (Chart 1) are generally inert toward various electrophiles due to lene bridge is found only in the para-substituted benzene steric reasons.19 To overcome this problem, metal–carboryne com- 14 compound. plexes with a metallacyclopropane structure are constructed to CuCl-mediated C–C coupling of dilithiocarborane in toluene is a reduce the steric hindrance around
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