Bond cleavage, oxidative addition

With C—C Bond Cleavage.—Oxidative addition of this sort only occurs with strained ring compounds, and metal insertions into cyclopropane rings have been reviewed. The oxidative additions of arylcyclopropanes to [ PtCl2-(C2H4))2], which exists as [PtCl2(C2H4)(THF)] in tetrahydrofuran, are first-order in both reactants [equation (18)]. In THF solution, in which the most detailed... 

It has, however, also been reported that the reaction of aryl ditellurides with Ni°, Pd°, and Pt° may result in the cleavage of the carbon-chalcogen bond. The oxidative addition of... 

The reductive elimination to form C-C and C-H bonds [45] is a crucial step in the cross-coupling processes, as well as many other transition metal-catalyzed reactions. Reductive elimination reactions comprise an early chapter in any organometallic text. Many examples of these reactions have been studied, and a great deal is known about the mechanisms of these processes. Similarly, the cleavage of C-H bonds by oxidative addition, including the C-H bond in methane, is now known [46]. Again, questions remain about how these reactions occur, but a variety of mechanistic studies have revealed key features of these reactions. Even some remarkably mild C-C cleavage reactions have now been observed with soluble transition metal complexes [47,48]. 

Keywords Activation, C-C bond, Transition metal, Cleavage, Oxidative addition, P-carbon elimination, Directionality, Homogeneous... 

The cleavage of sp C—H bonds by oxidative addition only occurs with fairly acidic molecules, and others have argued that this is an unpromising route to the activation of normal saturated alkanes. The reductive elimination of methane from [PtHMeP2] (P=tertiary phosphine) has the stoicheiometries shown in equations (15) and (16) in the absence or presence of added L e.g. PPhg or... 

Finally, C—H bond activations that proceed through organometallic intermediates with M—C bonds are common mechanistic pathways toward C—H functionalizations. Several distinct mechanisms for C—H cleavage (o-bond metathesis oxidative addition concerted metalation deprotonation through 4- or 6-membered transition states) have been elucidated for this step. The exact pathway of C—H bond activation typically depends on the identity of the metal, its oxidation state, and the ancillary ligands. 

Reaction of the ferrate [Fe3(CO)n] with CI2BNR2 results in double O-borylation to furnish the di(/t3-boroxy-methylidyne) cluster, Fe3(CO)9(p3-COBClNR2)2 57 (X, Y = OB(Cl)NR2). Thermal reaction of EtSC CMe with Fe2(GO)9 in the presence of trimethylamine oxide produces the (/X3-propylidyne)(/i3-dimethylaminomethylidyne) cluster compound 57 (X= Ft, Y = NMe2) via activation of the three G-H bonds in one of the three methyl groups in the amine oxide and S-G=bond cleavage, in addition to the dinuclear complex, Fe2(GO)6(/i-SEt)2 (cf. Scheme 2). ... 

Intramolecular nitrone cycloadditions often require higher temperatures as nitrones react more sluggishly with alkenes than do nitrile oxides and the products contain a substituent on nitrogen which may not be desirable. Conspicuously absent among various nitrones employed earlier have been NH nitrones, which are tautomers of the more stable oximes. However, Grigg et al. [58 a] and Padwa and Norman [58b] have demonstrated that under certain conditions oximes can undergo addition to electron deficient olefins as Michael acceptors, followed by cycloadditions to multiple bonds. We found that intramolecular oxime-olefin cycloaddition (lOOC) can occur thermally via an H-nitrone and lead to stereospecific introduction of two or more stereocenters. This is an excellent procedure for the stereoselective introduction of amino alcohol functionality via N-0 bond cleavage. 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

The Ir complexes 83 or [lr(lMes)Cl2Cp ], in the presence of NaOAc and excess of (Bcat), catalyse the diboration of styrene, at high conversions and selectivities for the diborated species, under mild conditions. Other terminal alkenes react similarly. The base is believed to assist the heterolytic cleavage of the (cat)B-B(cat) bond and the formation of Ir-B(cat) species, without the need of B-B oxidative addition [66]. 

In aromatic combustion flames, cyclopentadienyl radicals (c-CgHj ) can be precursors for PAH formation. " At high temperatures, benzene is oxidized by reaction with an oxygen molecule to yield phenylperoxy (C6H5O2 ) radical, via the initial formation of the phenyl radical (by C-H bond cleavage) and then the rapid addition of O2 (reaction 6.16). After expulsion of CO from phenylperoxy radical, a resonance-stabilized cyclopentadienyl radical (c-CgHg ) is formed (reaction 6.16). 

The catalytic process is also achieved in the Pd(0)/Cr(II)-mediated coupling of organic halides with aldehydes (Scheme 33) [74], Oxidative addition of a vinyl or aryl halide to a Pd(0) species, followed by transmetallation with a chromium salt and subsequent addition of the resulting organo chromate to an aldehyde, leads to the alcohol 54. The presence of an oxophile [Li(I) salts or MesSiCl] allows the cleavage of the Cr(III) - 0 bond to liberate Cr(III), which is reduced to active Cr(II) on the electrode surface. 

Reaction under solvolytic conditions such as in ethanol or aqueous tetrahydrofuran caused exclusive C-14—N bond cleavage and introduction of an ethoxyl or hydroxyl group at C-14, giving 30 in excellent yields (34). Addition of a base such as magnesium oxide to the reaction mixture was found to be useful to avoid the recovery of the starting material as the hydrobromide (35). The reaction was used for a synthesis of protopine alkaloids (Section V,E,5). 

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