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Insertion Chemistry Into Metal–Carbon Bonds

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Insertion Chemistry Into Metal–Carbon Bonds DALTON PERSPECTIVE Insertion chemistry into metal–carbon bonds Akio Yamamoto Department of Applied Chemistry, Graduate School of Science and Engineering, Waseda University, Shinjuku, Tokyo, 169-8555, Japan Received 26th October 1998, Accepted 14th December 1998 Transition metal alkyls (hydrocarbyls, including aryls) For me, however, it seemed to be extremely diYcult to establish occupy a central position in organometallic chemistry. the nature of the active species in the Ziegler catalyst systems Particularly, their reactions with unsaturated compounds by examining the mixed catalyst systems themselves. It was such as olefins and carbon monoxide are relevant to Cossee 5 who Wrst proposed a simple concept to elucidate the various important catalytic processes. The quest for the oleWn polymerization mechanism by assuming the formation of synthesis of various alkyltransition metal complexes and titanium alkyls as the catalyst center generated by interaction examination of their reactions with unsaturated com- of titanium chlorides with aluminium alkyls (Scheme 1). The pounds has led, often serendipitously, to findings of novel reactions. The present Perspective is an account of the personal inquiries of the author related to these transition metal alkyls and the results, including the polymerization of vinyl monomers and carbonylation of various sub- strates. Among various transition metal complexes studied, those of palladium were found most versatile, leading to findings of various novel catalytic processes. Alkyl compounds of transition metals are now known for most of the transition metals in the Periodic Table.1 They were a rarity, however, before the nineteen Wfties, when I started my research work as a graduate student, except for platinum alkyls that were regarded as exceptions.2 Only in the nineteen sixties Scheme 1 Cossee’s mechanism for polymerization of ethylene. some of the thermally stable transition metal methyl and ethyl complexes were isolated as stable complexes. As a young initiation of polymerization was explained by subsequent π student I was fascinated at that time by the exciting discovery of co-ordination of an oleWn monomer with the titanium alkyl the Ziegler catalyst 3,4 and by the subsequent development led leading to insertion of the oleWn into the metal alkyl bond by Natta in polymerization of various unsaturated compounds. followed by consecutive insertion of monomers, constituting I recall now the excitement Wlling the lecture halls in polymer the propagation steps in the oleWn polymerization. However, chemistry divisions of the Chemical Society of Japan and of veriWcation of the mechanism was much delayed because of the the Polymer Society, Japan, where people were heatedly dis- paucity of transition metal alkyls suitable as models of the cussing the mechanism of oleWn polymerization by the mixed active site. It is now hard to imagine the diYculties involved in systems of titanium chlorides and alkylaluminium compounds. establishing the active site of the catalyst systems at that time if we view the situation from the standpoint where the synthesis and polymerization abilities of various single site polymeriz- 6 Akio Yamamoto was born ation catalysts having transition metal alkyls are established. in Tokyo in 1930. He Some mystery was still attached then to the catalysis by tran- received his bachelor’s sition metals and their compounds. Many people did not degree from Waseda Uni- believe that pure compounds could serve as the catalysts and W versity and the doctor’s some speci c functions associated with the surface of transition degree from Tokyo Institute metals or their compounds were believed to promote the 7 of Technology. He was catalysis. It was Wilke who removed the myth by isolating pure promoted to Full professor low valent transition metal complexes that showed excellent of the Research Laboratory catalytic activities for butadiene oligomerization and polymeriz- of Resources Utilization at ation. Tokyo Institute of Technol- During my stay in his group at the Max Planck Institute for ogy in 1971 and has served Coal Research in Mülheim, Germany in 1962 to 1963, I was X as director since 1988. In very much in uenced by the elegance of his work and by the 1990 he was invited by power of the methodology of isolating low-valent transition Waseda University and has metal complexes and examine the behavior of the complexes worked as Research Profes- toward various unsaturated complexes. Some time after my sor since then. Recipient return from Germany we initiated a joint work with Professor of the Chemical Society Uchida’s group at Tokyo University attempting to isolate low of Japan Award, Society valent, catalytically active transition metal compounds from of Polymer Science Award, mixed systems of nickel, cobalt and iron acetylacetonates and aluminium alkyls containing various ligands such as 2,29- Akio Yamamoto and Violet Ribbon. bipyridine (bpy) and tertiary phosphines (PR3). The joint work J. Chem. Soc., Dalton Trans., 1999, 1027–1037 1027 quite unexpectedly led us to isolation of transition metal alkyls The bipyridine-co-ordinated nickel alkyls provided us with a having the supporting bipyridine ligand(s) [eqns. (1)]. These good model to demonstrate how the transition metal–alkyl alkyl complexes of late transition metals 8–10 proved to be the bonds are activated on interaction with oleWns as was proposed earliest examples of thermally stable, isolated transition metal in Cossee’s original concept. It was revealed that the more back alkyls except for platinum alkyls. bonding is provided from the transition metal to the co- ordinated oleWn, the stronger becomes the π bond between the Ni(acac)2 + AlEt2(OEt) + bpy NiEt2(bpy) metal and the co-ordinated oleWn, and that the stronger the 1 back bonding from the metal to the oleWn, the stronger becomes the activation of the nickel–alkyl bond to enhance the Fe(acac)3 + AlEt2(OEt) + bpy FeEt2(bpy)2 (1) reductive elimination. 2 On the other hand, the bipyridine-co-ordinated iron alkyl 2 was found to initiate the polymerization of electron-deWcient Co(acac)3 + AlEt2(OEt) + bpy CoEt(bpy)2 vinyl monomers such as acrylonitrile and methyl methacrylate. At that time NMR apparatus was not readily available in This work was the Wrst example of polymerization of polar our group and characterization of the metal alkyls was made monomers initiated by isolated alkyl complexes of late transi- mostly on the basis of chemical reactions and elemental analy- tion metals.14 The results we obtained contained important ses. When we could convince ourselves that the nickel and iron conclusions in the light of our present knowledge regarding the complexes contain alkyl groups attached to the metals on mechanism of vinyl polymerization by transition metal alkyls the basis of characterization by their thermolysis and treat- as reiterated below.15–20 (1) The polymerization is initiated by ment with protic reagents releasing the alkanes, the Wrst reac- co-ordination of an oleWn to a vacant site created by partial tions we attempted were with butadiene followed by reactions dissociation of the bipyridine ligands. (2) Competition reac- with various vinyl compounds. tions of various monomers indicate that oleWn binding more The reactions of these complexes with butadiene proved that strongly with the iron alkyl center by back donation is preferen- these complexes actually serve as the catalysts for oligomeriz- tially introduced into the copolymer and the feature of the ation of butadiene in the same way as with the mixed catalyst copolymerization is similar to that of anionic polymerization. systems. On the other hand, when we subjected these transition (3) The initiation by insertion of the co-ordinated oleWn into the metal alkyls to vinyl compounds we observed that the ensuing iron–alkyl bond is followed by rapid successive insertions of the processes varied depending on the transition metals employed. monomers constituting the propagation steps. (4) Termination Coupling of the alkyl groups from nickel dialkyls was observed involves β-hydrogen elimination from the polymer attached to on treatment of the nickel alkyls with electron-deWcient oleWns, the iron propagation site or proceeds by reductive elimination such as acrylonitrile and acrolein, whereas polymerization of of the iron-bound growing alkyl chain with the other remaining vinyl monomers occurred with the iron alkyls. The reactions of alkyl group (Scheme 2).21 W W NiEt2(bpy) 1 with electron de cient ole ns such as acrylonitrile However, the iron alkyls we prepared showed a diVerent at low temperatures were revealed to involve the co-ordination behavior from the Ziegler Natta type catalysts composed of of the oleWn to the nickel alkyls.11 Raising the temperature early transition metal compounds that showed high polymeriz- led to activation of the nickel–alkyl bonds in 1 and to coupling ation activity for non-polar α-oleWns. Thus a question has of the two alkyl groups by a process now termed reductive remained if there is an intrinsic diVerence between the reactivi- elimination 12,13 [eqn. (2)]. ties of early and late transition metal alkyls in their ability to R polymerize oleWns. Recent development of the chemistry of pal- ladium and nickel alkyls shows that α-oleWns such as ethylene Et and propylene also can be polymerized when a suitable ligand is NiEt2(bpy) + olefin (bpy)Ni 22,23 Et employed. Very recently, attainment of quite high activity for ethylene polymerization was reported by two groups using 3 iron- and cobalt-based initiators.24,25 The results indicate that – C4H10 high molecular weight polymers of α-oleWns are available even with late transition metal complexes such as iron and cobalt by (bpy)Ni(olefin) (2) n controlling chain transfer processes. 4 In the mean time, a study related to examination of the sol- n = 1, 2 vent eVect in propylene dimerization with nickel complexes in W Scheme 2 Mechanism of ole n polymerization initiated by FeEt2(bpy)2.
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