Hydrocarbyl complexes 5-elimination

Hydrocarbyl Complexes. Stable homoleptic and heteroleptic uranium hydrocarbyl complexes have been synthesized. Unlike the thorium analogues, uranium alkyl complexes are generally thermally unstable due to P-hydride elimination or reductive elimination processes. A rare example of a homoleptic uranium complex is U(CH(Si(CH2)3)2)3, the first stable U(I11) homoleptic complex to have been isolated. A stmctural study indicated a triganol... 

N-heterocyclic carbenes (NHC) are considered extremely effective hgands for homogeneous catalysis (Figure 6.2). These specific carbenes often lead to high efficiencies in metal-catalyzed reactions compared to traditional phosphines [49, 50). NHC complexes are usually considered to be very stable, due to their electronic properties and the unusually high bond dissociation energies (BDE) associated with NHCs [51]. Previous work has shown that Ni-hydrocarbyl complexes of NHCs readily decompose by reductive elimination to yield the 2-substituted imidazohum salts [52, 53). Later studies have shown that the reverse reaction, oxidative addition of imidazohum salts to zerovalent Group 10 metals, is feasible [53]. 

Simpler p-halide eliminations occur from late transition metal catalysts for olefin polymerization (Equations 10.25 and 10.26). Reactions of the cationic palladium-alkyl complexes occur in a similar fashion to the reactions of the cationic group 4 complexes, despite the softer nature of these species. In this case, propylene and the metal chloride are formed. Even a neutral nickel-hydrocarbyl complex (the salicaldimine complex in Equation 10.26) undergoes reactions with vinyl chloride that involve insertion followed by P-chloride elimination. 

The formation of a Th(C5H5)3 complex was suggested because the subsequent thermal reaction of the photogenerated thbrium monohydride with unreacted starting hydrocarbyl complexes, gives the binuclear reductive elimination of alkane ... 

Oxidative additions involving C-H bond breaking have recently been the topic of an extensive study, usually referred to as C-H activation the idea is that the M-H and M-hydrocarbyl bonds formed will be much more prone to functionalization than the unreactive C-H bond. Intramolecular oxidative additions of C-H bonds have been known for quite some time see Figure 2.15. This process is named orthometallation or cyclometallation. It occurs frequently in metal complexes, and is not restricted to "ortho" protons. It is referred to as cyclometallation and is often followed by elimination of HX, while the metal returns to its initial (lower) oxidation state. 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

The transient zirconocene butene complex, 105, has proved to be useful in a number of organic transformations. For example, butene substitution of zirconocene alkene complexes with alkoxy-substituted olefins results in /3-alkoxide elimination to furnish the zirconocene alkoxy compounds (R = Me, 123 R = Bnz, 124) (Scheme 16).50,51 Addition of propargyl alcohols to the zirconocene butene complex, 105, affords homoallylic alcohols. These reactions are of limited utility owing to the lack of stereoselectivity or formation of multiple products. Positioning the alkoxide functional group further down the hydrocarbyl chain allows synthesis of cyclopropanes, though mixtures of the carbocycle and alkene products are obtained in some cases (Scheme 16).52... 

A hydrocarbyl elimination approach is used to produce the Zr(rv) dibenzyl complex incorporating a tridentate bis(amido) silylether [iV, 0,(V ] complex 18261 (Equation (13)). The molecular structure of 182 features a distorted tbp geometry with an approximately linear ZrN20 unit and the two amido nitrogen atoms occupying approximately axial positions.One benzyl group is -coordinated. When activated with MAO, complex 182 shows moderate activity for ethylene polymerization. 

Amine or hydrocarbyl elimination was also employed to prepare the following t -mono-Cp-silylamido derivatives (Scheme 106), including zirconium bis(diethylamido) complexes 469 with variations on the ring and amido substitutions,330 zirconium dibenzyl complex 4 70,331 t -mono-Ind-silylamido zirconium complex 471, 0 isodicy-clopentadienyl zirconium complexes 472,332 and enantiomerically pure zirconium bis(dimethylamido) and dichloro complexes 473333 with the R or S -CH(Me)Ph group attached to the amido nitrogen the last two complexes of this... 

The conversion of the Ir(III) cyclohexyl hydride complex to an Ir/cyclohexane system involves a change in the formal oxidation state of Ir from + 3 to +1 (i.e., a formal two-electron reduction). As a result, this elementary reaction step is generally called a reductive coupling (Chart 11.4). From a metal hydrocarbyl hydride complex (i.e., M(R)(H)), the overall process of C H bond formation and dissociation of free hydrocarbon (or related functionalized molecule) is called reductive elimination (Chart 11.4). The reverse process, metal coordination of a C—H bond and insertion into the C—H bond, is called oxidative addition. Note Oxidative addition and reductive elimination reactions are not limited to reactions involving C and H.)... 

As a result of the combined effect of the factors mentioned above, Pd monoaUcyl complexes are expected to be the least stable among hydrocarbyls (M = Pd, Pt). Such complexes are currently unknown, whereas monoaUcyl ft " complexes were among the first compounds characterized in C(sp )-0 reductive elimination reactions [16-19]. Among monoaryl Pd " derivatives only few were isolated to date aU of them are stabilized by fluoride ligands [26, 27]. No monohydrocarbyl Pd complexes supported by 0-donor ligand have yet been reported and characterized. The most studied O-ligated organopalladium(lV) complexes are diaryl dicarbox-ylates 13 shown in Fig. 7 [11, 12]. 

An early theoretical study suggested that better 0 donors are eliminated more easily from cA-MR(R )L2 type complexes [8]. However, this prospect is now known to be applicable only to a dialkyl complex series [e.g., Me > Et > Pr > Bu]. Thus the rate of reductive elimination is rather sensitive to the other factors, especially to the orbital hybridization of hydrocarbyl ligands. In general, the reactivity decreases in the order [hydrido (s)alkenyl (sp ) > aryl (sp ) 3> alkynyl (sp) > alkyl (sp )]. While a Jt-allyl ligand ranks middle, its behavior in reductive elimination will be described separately in Section 9.4. 

A vacant coordination site is created during the forward insertion reaction of Equation 9.1, and a vacant coordination site must be present for the reverse reaction to occur. This mechanism makes it necessary for coordinatively saturated (18-electron) metal-alkyl complexes to dissociate a ligand prior to 3-hydrogen and 3-hydrocarbyl elimination reactions. 

Insertion of one of the double bonds of a diene into a Pd-C or a Pd-H bond also leads to ff,rf -cnyh. If the diene is not coordinated in the precursor complex, a coordination site needs to be made available to the double bond that undergoes insertion, and this is reflected in the reaction conditions and additives used. The stereochemistry of the attack is as, Pd and the hydrocarbyl (or hydrido) group adding to the double bond on the same side. 1,5-COD is a frequently used olefin that inserts into Pd-R bonds and leads to 8-R-l,4,5- -cyclooctenyl frames. Pd(2-OH-C6H4)I(N-N) (N-N = bipy, phen, TMEDA) react with 1,5-COD in the presence of TlTf to give Pd 8-(2-OH-C5H4)-l,4,5-77 -cyclooctenyl (N-N), obtained by insertion of one of the double bonds into a Pd-(2-hydroxyphenyl) bond. " The derivative shown in Equation (58) is another example, formed by insertion of the coordinated COD into a transient Pd-H bond generated by /3-H elimination in the precursor ethyl complex.A multistcp reaction that implies insertion of an alkyne into the Pd-Me bond and subsequent insertion of a double bond of COD leads to the cyclooctenyl complex depicted in Equation (59). 

Addition of the conjugate acid of the desired ligand to a metal-carbene or -carbjme compound results in protonation of the hydrocarbyl ligand (see also Sections 2.6.1.4 and 2.6.4.4), transforming it into an allcyl or carbene ligand with formation of a new metal— heteroatom bond. Examples are the preparations of compound 19 (Scheme 7) and compound 65 (Scheme 27). The reaction of the carbene substrate 101 with tert-butyl alcohol produces product 102 (Scheme 46) which, upon heating, eliminates alkane and affords a new carbene compound in an overall process which resembles the protonolysis of an al-l 1 The same reaction of 101 with triphenylsilanol at low temperatures leads to the protonolysis product directly. 

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