Chemistry and catalysis advances in organometallic chemistry and catalysis
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7.10 Zr/Ni CO-MEDIATED [2 + 2 + 2] CYCLOADDITION OF CARBORYNE WITH TWO DIFFERENT ALKYNES Previous work shows that nickel–carboryne reacts with 2 equiv of alkynes to afford 1,2-benzo-o-caboranes, and no selectivity is observed if two different kinds of alkynes are used in the reaction system. It is noted that in the intermolecular
92 TRANSITION-METAL-PROMOTED FUNCTIONALIZATION OF CARBORANES [2 + 2 + 2] cycloaddition of alkynes, selectivity among three different alkynes can be achieved by using unsymmetrical zirconacyclopentadienes, prepared from oxidative coupling of two different alkynes with Cp 2 Zr(II), as intermediates to react with the third alkyne in the presence of NiBr 2 (PPh 3 ) 2 [39]. Zirconacyclopentenes incorporating a carboranyl unit are thermally very stable and chemically inert toward unsaturated organic molecules such as alkenes, alkynes, nitriles, CO, and CO 2 , which is significantly different from zirconacyclopentenes without the carboranyl group [40]. However, the corresponding nickelacyclopentenes incorporating the carboranyl unit are very reactive toward alkynes. These results clearly indicate that the nature of transition metals dominates the reactivity of the corresponding metallacycles. Transmetalation from zirconacycles to nickel should allow the insertion of the second alkyne, making chemoselective [2 + 2 + 2] cycloaddition of o-carboryne with two different alkynes possible. Reaction of zirconacyclopentenes with alkynes in the presence of a stoichiometric amount of NiCl 2 (PMe 3 ) 2 in hot toluene affords the expected 1,2-benzo-o-caborane 26 (Scheme 7.14) [41]. Both alkyl and aryl alkynes are compatible with this reaction except for those containing ester, amino, and very bulky TMS groups. A very reactive alkyne such as MeO
2 CC ≡CCO 2 Me (DMAD) is homo-cyclotrimerized in the presence of Ni(0) prior to the insertion. Unsymmetrical alkynes produce two regioisomers and their ratios are affected by both steric and electronic factors. Only one isomer is generated for highly polar alkynes [15]. It is noted that these benzocarboranes can be also prepared in similar yields from one-pot reaction of Cp 2 Zr( μ-Cl)(μ-C 2 B 10 H 10 )Li(OEt 2 ) 2 with alkyne, followed by treatment with another type of alkyne in the presence of NiCl 2 (PMe 3 ) 2 . This approach represents an equivalent of a three-component [2 + 2 + 2] cycloaddition of carboryne with two different alkynes. On the other hand, using a catalytic amount of nickel in the presence of 3 equiv of FeCl 3 in the above reaction system can dramatically reduce the formation of homo-trimerization products of alkynes, allowing the insertion of activated alkynes such as DMAD. This catalytic reaction represents an important advance in the development of zirconacycle-based synthetic methodologies. Transmetalation of zirconacycle to nickel is supported by the isolation and structural characterization of the nickelacyclopentene 1,2-[(dppe)NiC(Ph) =C(Ph)]-1,2-C 2 B
H 10 (27) from the reaction of 1,2-[Cp 2 ZrC(Ph)
=C(Ph)]-1,2- C 2 B 10 H 10 (19b) with 1 equiv of NiCl 2 (dppe) (Scheme 7.15). The use of diphenylacetylene can avoid the β-H elimination of the resultant nickelacycle. Furthermore, the presence of the dppe ligand makes complex 27 thermodynamically stable. Treatment of 27 with 3-hexyne yields 1,2-benzo-o-carborane 1,2-[C(Ph) =C(Ph)-C(Et)=C(Et)]-1,2-C 2 B
H 10 (26b). With the full characterization of this key intermediate, a proposed reaction mechanism is shown in Scheme 7.16. This reaction serves as an efficient protocol for the preparation of a new class of highly substituted benzocarboranes in a one-pot or a two-step manner via transmetalation of zirconacyclopentenes incorporating a carboranyl unit to nickel.
Zr Cp 2 R 1 R 2 R 3 R 4 R 2 R 3 R 4 R 1 19 26 R 1 , R 2 , R 3 , R
4 = alkyl, aryl NiCl 2
3 ) 2 Scheme 7.14 Reaction of zirconacycle with alkyne in the presence of NiCl 2 (PMe
3 ) 2 . NiCl
2 (dppe)
Et Et Ph Et Et Ph 26b 27 Zr Cp 2 Ph Ph 19b Ni Ph Ph P P Ph Ph Ph Ph Scheme 7.15 Isolation and reaction of nickelacyclopentene intermediate. ZR/NI CO-MEDIATED [2 + 2 + 2] CYCLOADDITION OF CARBORYNE WITH UNACTIVATED ALKENES AND ALKYNES 93 ZrCp
2 R 1 R 2 R 3 R 4 R 2 R 1 26 [Ni]
R 1 R 2 [Ni(II)]
+ Li Cl ZrCp 2 OEt 2 OEt
2 O [Ni]
R 1 R 2 P R 3 R 4 R 1 R 2 R 3 R 4 [Ni(0)]
Fe(III) Fe(II)
19 9 δ + δ − δ + δ − Scheme 7.16 Proposed reaction mechanism for Ni-catalyzed cycloaddition. 7.11 Zr/Ni CO-MEDIATED [2 + 2 + 2] CYCLOADDITION OF CARBORYNE WITH UNACTIVATED ALKENES AND ALKYNES Transition-metal-mediated cycloaddition of alkenes and alkynes meets a major challenge to achieve both high reactivity and predictable selectivity between different unsaturated substrates in the formation of complex molecules. The employment of zirconacyclopentanes incorporating a carboranyl unit to reaction with alkynes after transmetalation to nickel can realize the three-component [2 + 2 + 2] cycloaddition of carboryne, unactivated alkenes, and alkynes. Zirconacyclopentanes 20, prepared by the treatment of Cp 2 Zr( μ-Cl)(μ-C 2 B 10 H 10 )Li(OEt 2 ) 2 with 1 equiv of 1-hexene or styrene, are used to react with a variety of alkynes in the presence of 1 equiv of NiCl 2 , affording dihydrobenzocarboranes 28 and 28 (Scheme 7.17) [42]. Symmetrical alkynes give the single products in very good isolated yields. The regioselectivity in the reaction of unsymmetrical alkynes is dependent on both the polarity of the alkynes and relative bulkiness of two substituents. Similar to zirconacyclopentenes incorporating a carboranyl unit, the transmetalation species nickelacyclopentane 1,2- [Ni(dppe)CH 2 CH(Bu n )]-1,2-C
2 B 10 H 10 (29) can also be isolated from the reaction of 1,2-Cp 2 Zr[CH
2 CH(Bu
n )]-1,2-C
2 B 10 H 10 with 1 equiv of NiCl 2 (dppe). Its reaction with 5-decyne in THF at 110 ◦ C gives the corresponding dihydrobenzocarborane 28b in greater than 90% yield (Scheme 7.18). Accordingly, the formation of the products can be rationalized by the steps of transmetalation of Zr to Ni, insertion of alkyne into the nickel– C alkyl bond, and reductive elimination reaction. This Zr Cp 2 R 1 R 2 R 3 R 4 R 2 R 3 R 4 R 1 20 28 R 1 = H, R 2 = Ph; R 1 =
n Bu, R
2 = H
R 3 , R 4 = alkyl, aryl NiCl 2
2 R 4 R 3 R 1 +
Scheme 7.17 Reaction of zirconacycle with alkyne in the presence of NiCl 2 .
94 TRANSITION-METAL-PROMOTED FUNCTIONALIZATION OF CARBORANES Bu
Bu
Bu
[NiCl
2 (dppe)]
n Bu
Bu THF, 110 °C Toluene, 110 °C Zr
2 Bu
20b Ni Bu n 29 P P Ph Ph Ph Ph Scheme 7.18 Isolation and reaction of nickelacyclopentane intermediate. offers an example to control the chemoselectivity among different alkenes and alkynes for assembling complex molecular architectures. 7.12 CONCLUSIONS AND PERSPECTIVES The above results demonstrate that the metal–carbon bonds in metal–carboryne complexes are reactive toward electrophiles, whereas those in metal–carboranyl complexes are inert, which leads to very high chemoselectivity in the respective reactions. The chemical properties of metal–carboryne complexes are dependent on the electronic configurations of the metal center, and they show a diverse array of reactions toward unsaturated molecules. Ni–carboryne can react with 2 equiv of alkynes to afford benzo-o-carboranes, and with 1 equiv of alkenes to generate alkenylcarborane coupling products. However, it does not show any activity toward polar unsaturated molecules. In contrast, the Zr–carboryne can undergo monoinsertion reaction with alkynes, alkenes, and polar unsaturated molecules to afford zirconacycles. Such zirconacycles are very useful intermediates for the synthesis of various kinds of functionalized carboranes, while transmetalation to other metals creates further synthetic opportunities. In fact, after transmetalation of zirconacyclopentanes or zirconacyclopentenes incorporating a carboranyl unit to nickel, the resultant nickelacycles can react further with another kind of alkyne to give cycloaddition products. This represents an equivalent of a three-component [2 + 2 + 2] cycloaddition of carboryne with two different alkynes or with an alkene and an alkyne in one single operation. These techniques offer new methodologies for the functionalization of carboranes that cannot be achieved by other means. Compared to the very rich literature concerning the chemistry of metal–benzyne complexes, studies of metal–carboryne complexes remain a very young research area. It is anticipated that their reaction scope would be further explored. The synthesis of carborane heterocycles would be expected via metal–carborynes intermediates. ACKNOWLEDGMENT The authors are grateful to all coworkers involved in the studies described in this paper for their valuable contributions. Financial supports from The Research Grants Council of The Hong Kong Special Administration Region and The Chinese University of Hong Kong are also gratefully acknowledged. REFERENCES 1. (a) Hawthorne, M. F. Angew. Chem. Int. Ed Engl. 1993, 32 , 950; (b) Soloway, A. H.; Tjarks, W.; Barnum, J. G. Chem. Rev. 1998, 98 , 1515; (c) Valliant, J. F.; Guenther, K. J.; King, A. S.; Morel, P.; Schaffer, P.; Sogbein, O. O.; Stephenson, K. A. Coord. Chem. Rev. 2002, 232 , 173; (d) Armstrong, A. F.; Valliant, J. F. Dalton Trans. 2007, 4240; (e) Issa, F.; Kassiou, M.; Rendina, L. M. Chem. Rev. 2011, 111 , 5701. 2. (a) Yang, X.; Jiang, W.; Knobler, C. B., M. F. Hawthorne J. Am. Chem. Soc. 1992, 114 , 9719; (b) Colquhoun, H. M.; Herbertson, P. L.; Wade, K.; Baxter, I.; Williams, D. J. A. Macromolecules 1998, 31 , 1694; (c) Jude, H.; Disteldorf, H.; Fischer, S.; Wedge, T.; Hawkridge, A. M.; Arif, A. M.; Hawthorne, M. F.; Muddiman, D. C.; Stang, P. J. J. Am. Chem. Soc. 2005, 127 , 12131; (d) Dash, B. P.; Satapathy, R.; Gaillard, E. R.; Maguire, J. A.; Hosmane; N. S. J. Am. Chem. Soc. 2010, 132 , 6578.
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40. (a) Xi, Z.; Fischer, R.; Hara, R.; Sun, W.-H.; Obora, Y.; Suzuki, N.; Nakajima, K.; Takahashi, T. J. Am. Chem. Soc. 1997, 119 , 12842; (b) Liu, Y.; Shen, B.; Kotora, M.; Takahashi, T. Angew. Chem. Int. Ed. 1999, 38 , 949; (c) Sun, H.; Burlakov, V. V.; Spannenberg, A.; Baumann, W.; Arndt, P.; Rosenthal, U. Organometallics 2001, 20 , 5472. 41. Ren, S.; Qiu, Z.; Xie, Z. J. Am. Chem. Soc. 2012, 134 , 3242. 42. Ren, S.; Qiu, Z.; Xie, Z. Angew. Chem. Int. Ed. 2012, 51 , 1010. 8 WEAK INTERACTIONS AND M–H BOND ACTIVATION Elena Shubina * , Natalia Belkova, Oleg Filippov, and Lina Epstein A. N. Nesmeyanov Institute of Organoelement Compounds (INEOS), Russian Academy of Sciences, Moscow, Russia 8.1 INTRODUCTION The term “Y–Z bond activation” is traditionally understood as a reaction that cleaves the bond [1]. Often, the term is restricted to reactions involving organometallic complexes and proceeding by Y–Z coordination to the inner sphere of metal, either via an intermediate state or as a transition state. We are inclined to use the term “activation” for weaker (noncovalent) binding that results in the altered reactivity of a molecule through associated changes in the relative energies of its orbitals or in verified polarity. The ability of transition-metal hydrides to be a source of H + or H − is a well-known phenomenon. It serves as a basis for the use of hydride complexes in various catalytic processes as, for example, hydrogenation or reduction of H + to H 2 [2]. Transition-metal hydrides exhibit the same two modes of reactivity in proton transfer reactions, behaving as either acid or base. In spite of being a seemingly simple reaction, the proton transfer is a multistep process occurring via hydrogen- bonded intermediates of molecular and ionic types. The position of equilibrium depends on the relative acidity and basicity of interacting molecules as well as on the media properties and temperature. With a small interaction enthalpy, hydrogen bonding usually results only in a small perturbation of the electronic structure of the participating molecules. Nevertheless, it modifies their properties, giving the opportunity to fine-tune the properties of an organometallic complex [3–5] and to influence the reactions they are involved in [6, 7]. Data have begun to appear confirming the importance of this phenomenon in catalytic reduction [8–10] and hydrogen activation [11]. One major recent achievement in this area is the discovery of nonclassical hydrogen bonds between transition-metal hydrides and proton donors, M–H δ− · · ·H δ+ –X, now widely called a dihydrogen bond [12–14]. These bonds precede the proton transfer to hydrides, yielding in most cases nonclassical η 2 -H 2 complexes. More recently, we have shown that neutral hydrides can behave as proton donors in the hydrogen bond M–H δ+ · · ·Y and such hydrogen bonds precede the proton transfer from a transition-metal hydride to a base Y [15, 16]. The structural parameters and spectral properties of hydrogen bonds involving hydride ligands are similar to those of classical hydrogen bonds [13, 17, 18]. Also, hydrogen bonding activates the participating M–H bonds and induces them to take part in proton transfer reactions.
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