Adduct complexes, metathesis reaction

A whole series of (imidazol-2-ylidene)gold(i) complexes with the 1 1 stoichiometry has been prepared from the readily available silver chloride adducts, which upon reaction with (Me2S)AuCl, followed by chloride metathesis, give the complexes shown in Scheme 60. 

Adducts of M(CH-f-Bu)(NAr)(OR)2 complexes were prepared and studied as models for the initial olefin adduct [66] in an olefin metathesis reaction [67]. PMe3 was found to attack the CNO face of yy -M(CH-f-Bu)(NAr)(OR)2 rotamers to give TBP species in which the phosphine is bound in an axial position... 

One of the most thoroughly investigated Cp2 AnX systems is the chloride-bridged, trimeric complex [Cp 2U(/r-Cl)]3 The complex can be prepared by a number of routes, one of which is shown in equation (26). [Cp 2U(/r-Cl)]3 reacts with a variety of Lewis bases to generate mononuclear adducts, and will undergo metathesis reactions. 

Other methods for its preparation invariably result in the isolation of Lewis base adducts. The complexes U(BH4)3(THF) t and U(BH4)3(18-crown-6) were prepared by the metathesis reaction of LiBH4 with UCl3(THF) , or UCl3(18-crown-6) in TFIF. Although the stoichiometry of the THF adduct was not characterized in the initial report, the compound was later prepared from the reaction of UH3 and BFls in THF, and characterized to be U(BH4)3(THF)3. The molecular structure of the compound reveals that it adopts an octahedral geometry about the metal center with the borohydride and THF ligands mutually facial all borohydride ligands are tridentate. 

The first report of metathesis reactions with thorium involved the preparation of the complexes Th[HB(pz)3]4- X ( = 2, X = C1, Br n= 1, X = C1), ThpB(3,5-Me2-pz)3]Cl2, Th[B(pz)4]2Bt2, and adducts of the complexes Th[HB(pz)3]Cl3 and Th[HB(pz)3]4, although subsequent reports have appeared of other derivatives, including Th[HB(3,5-Me2-pz)3]Cl3. " The larger ionic radius of thorium enables higher coordination numbers unlike the uranium complexes, the thorium... 

Lew is base OH". The complex (or adduct) HSO is formed by the displacement of the proton from the hydroxide ion by the stronger Lewis acid SOj. In this way, the water molecule is thought of as a Lewis adduct formed from H and OH". Even though this fact is not explicitly shown in the reaction, the water molecule exhibits Bransted acidity (not only Lewis basicity). Note that it is easy to tell that this is a displacement reaction instead of just a complex formation reaction because, while there is only one base in the reaction, there are two acids. A complex formation reaction only occurs w ith a single acid and a single base. A double displacement, or metathesis, reaction only occurs with two acids and two bases. 

In this section we reviewed the latest theoretical studies on the metathesis reaction. In general, it has been shown that the metathesis reaction at first yields adduct complex then activation of the H-R bond takes place with a four-center transition state, which leads to the second adduct. It occurs more easily for scandium complexes than for lutetium analogues. The a-metathesis reactivity between R-H and M-R bonds decreases in the order of R C2H... 

It was first observed by Woon (1974) and Farona (1974) that acetylenes could be polymerized by catalysts of the type Mo(CO)3(toluene). This was followed by the discovery that conventional metathesis catalysts such as M0CI5 (Masuda 1974) and WCls (Navarro 1976 Masuda 1976), with or without a cocatalyst, could also bring about polymerization of acetylenes. At first there was some doubt as to whether these polymerizations were being propagated by the metathesis mechanism (Scheme 10.2) or whether a Ziegler-Natta mechanism was operating. However, the observation that metal carbene complexes could react with acetylenic molecules to form simple adducts as in reaction (20) (Fischer, H. 1980), and the fact that such complexes could initiate the polymerization of acetylenes, albeit somewhat slowly, but cleanly and in fair yield, soon allayed these doubts. 

The one-pot procedure will of course be more convenient, but in addition the adduct 5 is formed in higher yield (despite the lower temperature for the cycloaddition reaction). This is probably a result of the presence of the ruthenium complex, which presumably acts as a Lewis acid catalyst. The first step to form the diene is a metathesis reaction (see Section 2.10). See M. Rosillo, L. Casarmbios, G. Dominguez and L. J. P6res-Castells, Tetrahedron Lett., 42 (2001), 7029. 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

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