Imido alkylidyne complexes

Ostensibly minor variations of a synthetic procedure sometimes can have significant consequences. For example, substituting KOCMe(CF3)2 for LiOC-Me(CF3)2 is believed [85] to lead to formation of the alkylidyne complex shown in Eq. 16 instead of the known [83] Mo(CH-f-Bu)(NAd)[OCMe(CF3)2]2 (Ad=ad-amantyl). A proton is likely to be transferred before formation of the final product, since it has been known for some time that both W(CH-f-Bu)(NAr)[OC-Me(CF3)2]2 and W(C-f-Bu)(NHAr)[OCMe(CF3)2]2 are stable species that cannot be interconverted in the presence of triethylamine [41]. In such circumstances the nucleophilicity of the alkoxide ion and the nucleophilicity and acidity of the alcohol formed upon deprotonation of the alkylidene will be crucial determinants of whether the imido nitrogen atom is protonated at some stage during the reaction. At this stage few details are known about side reactions in which amido alkylidyne complexes are formed. 

Hydrolysis of the vinyl groups from the metal center affords olefins. Weiss, Schubert, and Schrock investigated the reaction of the alkylidyne complex W(CCMe3)Cl3(dme) with cyclohexyl isocyanate (208). Two isocyanate molecules are incorporated into the complex as shown in Eq. (212). The reaction was postulated to proceed via a cycloaddition adduct of isocyanate to the metal-carbon triple bond and cleavage of the four-mem-bered ring into metal imido and ketenyl species. Subsequent insertion of a second isocyanate into the metal-ketenyl bond would then give the observed product. 

In alkylidyne complexes that bear no ir-acid co-ligands, thermodynamic protonation occurs either on the alkylidyne face or at the metal, and steric considerations are invoked to explain the preference. Sub-stoichiometric quantities of H2O or HCl catalyse the tautomerisation of TpW( = C Bu)Cl(NHPh) to the corresponding imido alkylidene complex TpW( = CH Bu)Cl( = NR). The first step in the tautomerisation is, however, suggested to involve protonation of the amide group (Scheme 45) since weaker donors (i.e., those with electron-withdrawing... 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

For a general review of metal-ligand multiple bonds, see W. A. Nugent and J. M. Mayer, Metal-Ligand Multiple Bonds The Chemistry of Transition Metal Complexes Containing Oxo, Nitrido, Imido, Alkylidene, or Alkylidyne Ligands (New York, Wiley, 1988). 

The first well-defined, stable d°-alkylidene species having catalytic activity in alkene metathesis was Ta(=CHtBu)(OtBu)2Cl(PMe3) (5) [16]. However, research efforts moved rapidly to Mo and/or W alkylidenes, first in the form of 0X0 complexes (6) [17, 26], and then with the more easily accessible, imido bisalkoxy alkylidene complexes of general formula M(=NR )(=CHR )(OR )2 (7) [18, 19, 27], as well as the isoelectronic Re-alkylidyne-alkylidene complexes Re(=CR )(=CHR )(OR )2 (8) [28]. In 2001, it was shown that the silica-supported, Re-alkylidyne-alkylidene complex (=SiO)Re(=CtBu)(=CHtBu)(CH2tBu) (12) was unexpectedly very active [21,29]. According to computational studies, which will be described later in this chapter, the remarkable reactivity of this silica-supported catalyst arises from the different nature of the X and Y ligands (referred to as dissymmetric complexes) [7]. In addition, new molecular precursors based on bis-alkyl and bis-pyrrolyl M(=NR )(=CHR )(X)2 complexes became available [30]. Consequently, from 2005, considerable research effort has been devoted toward generating silica-supported (14 to 16 in Scheme 6.4) [23, 31, 32] and molecular (17 and 18 in Scheme 6.4) [22, 33, 34] Mo- and W-imido complexes... 

Schemes 6.3 and 6.4) [3-5, 7-9, 11, 35-44]. Other studies focused on the classical heterogeneous catalysts, such as M0O3 [6, 10, 45-54] and Re20y supported on silica or alumina [55-59]. We will first focus on the studies of well-defined complexes of general formula M(ER )(=CHR )(X)(Y) (M = Re, Mo, or W ER = alkylidyne, imido, or 0x0 X and/or Y = alkoxy, siloxy, alkyl, or pyrrolyl). Afterwards, we will summarize the contributions of classical heterogeneous catalysts with the aim of highlighting the differences and similarities with the well-defined catalysts. 

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