Metal-carbon bonds, stepwise

Reaction of Organometallic Complexes with Particles of Transition Elements The Stepwise Hydrogenolysis of Metal-Carbon Bonds... 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

The metal-n-cyclopenladienyl bond is somewhat stronger in n-cyclo-pentadienyl derivatives than the metal-carbon bond in metal carbonyl derivatives. However, stepwise loss of 7t-cyclopentadienyl ligands occurs in the mass spectra of 7t-cyclopentadienyl derivatives 24>. The following fragmentation of ferrocene is particularly important because of the occurrence of ferrocene as a pyrolysis product in numerous mass spectra ... 

Although CS2 insertion into the metal-carbon bond is rarely used to synthesize dithiocarboxylate salts of Group 14 elements, divalent tin salts of the dithiocar-boxylates 70 and 71 are synthesized by this method (Fig. 18). They are obtained by the stepwise insertion of CS2 into two Sn-C bonds of a diarylstannylene, Sn-[2,4,6- t-Bu) C L2 2 [128]. Reaction of another stannylene Sn 2,4,6-[(Me3Si)2-CH]3C6H2 [2,4,6-(z-Pr)3C6H2] with CS2 produces the stannylene-CS2 ylide 72, which is trapped by methyl acrylate to afford cyclic stannyl dithiocarboxylate 73 (Scheme 17) [129]. 

The mechanisms of the epoxidations of olefins have been studied intensively.3 3 -376-378 In general, the reactions of metal-oxo complexes with olefins to form epoxides do not involve intermediates containing metal-carbon bonds. The 0x0 group tends to act as an electrophile and interact with the HOMO of the olefin during the transfer of the 0x0 to the olefin. After this initial interaction, the epoxide may form by a non-radical concerted process or by a stepwise process involving radical or cationic intermediates. ... 

Many anionic nucleophiles add to the carbyne carbon in cationic Fischer complexes, and these reactions can be used to synthesize carbene ligands unavailable by other routes. The stepwise reduction of a metal-carbon triple bond has been demonstrated (32) ... 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

Catalytic hydrogenolysis of light alkanes (propane, butanes, pentanes) with the exception of ethane has been accomplished under very mild conditions over silica-supported hydride complexes.502 The hydrogenolysis proceeds over (=SiO)3 ZrH,503 (=SiO)3HfH,504 and (=SiO)3TiH505 by stepwise cleavage of carbon-carbon bonds by P-alkyl elimination from surface metal-alkyl intermediates. 

The mass spectra of certain metal carbonyl complexes of triphenyl-phosphine and l,2-bis(diphenylphosphino)ethane (Pf-Pf) have been investigated 48>. Besides the usual stepwise loss of carbonyl groups, cleavage of the phosphorus-carbon bond occurs. Thus triphenylphosphine complexes exhibit cleavage of the phenyl-phosphorus bond after all carbonyl groups are lost. The 1,2- bis(diphenylphosphino)ethane complexes (e.g. (Pf-Pf)[W(CO)5]2 and (Pf-Pf)M(CO)4) exhibit elimination of the ethylene bridge between two phosphorus atoms. 

Before the modem era of organotransition metal reactivity, it was observed that nickel carbonyl reacted with 2-methallyl chloride in methanol to give methyl 3-methyl-3-butenoate and 2,5-dimethyl-1,5-hexadiene as a by-product. In THF at 25 °C, the diene was the exclusive product. This mild formation of a carbon carbon bond, and the interest in the synthesis of terpene-based natural products led to efforts to test the scope and limitations of the process. An obvious pathway involves stepwise oxidative addition of each aUyl unit followed by Reductive Elimination. As discussed below, the key intermediates (left vague in Scheme 48) are likely to involve Ni -Ni couples. 

The addition of diboron compounds to alkynes is an excellent method for the synthesis of c -diboryl alkenes (Scheme 2-11) [34], The reaction is catalyzed by Pt(PPh3)4 at 80 and works well not only with terminal but also with internal alkynes. The addition of the Si-B [35] or Sn-B [36] bonds to alkynes gives mixed-metal alkenylboron reagents which have potential ability for use in the stepwise double cross-coupling reaction at both metallated carbons. 

In all of the above systems (1-4) reduction can be regarded to occur stepwise. Carbon monoxide may become coordinated as reduction proceeds with partial loss of the original anionic ligands and the oxidation state of the cation decreases. The oxidation state at which the metal-CO bond acquires some stability is a function of the metal. 

Another synthetically useful carbon bond-forming reaction involves reaction of diiron nonacarbonyl with halo-carbonyl compounds. Noyori found that a,a -dibromoketones (498) react with diiron nonacarbonyl [Fe2(CO)9] to give an iron stabilized alkoxy zwitterion (499). The intermediate Jt-allyl iron species reacts with alkenes in a stepwise manner (initially producing 500) to give cyclic ketones such as 501, 23 and the product is equivalent to the product of a [3-t2]-cycloaddition with an alkene (sec. 11.11). This cyclization method is now known as Noyori annulation. This reaction is related to the Nazarov cyclization previously discussed in Section 12.3.C. Enamines can react with 498, but the initially formed enamino ketone product eliminates the amino group to form cyclopentanone derivatives. Intermediates such as 499 may actually exist as cations hound to a metal rather than as the alkoxide-iron structures shown.323b-d noted that Zn/B(OEt)3 is... 

Stepwise Addition of SMe+ to the Metal Carbon Multiple Bond. 

The rich nucleophilic reactivity of square-planar platinum(II) and palladium(II) complexes is well established. One of the most documented examples is the stepwise oxidative addition of aUcyl halides to organoplatinum(II) [1] and organopalladium (II) [2,3] complexes via SN2-type substitution at the sp carbon center. Additionally, electron-rich Pt centers are subject to protonation at the metal to generate Pt hydrides as the first step in the protonolysis of many platinum-carbon bonds [4—7]. With a less reactive Lewis acid such as SO2, reversible adduct formation is observed [8], and this reaction has been used in the development of sensors [9-11],... 

The second mechanism proceeds by the stepwise addition of monomers to the metal followed by the formation of carbon-carbon bonds in a multicentered bond process ... 

I have also included in this book the insertion reactions of carbon cumulenes into polarized metal single bonds, which can be perceived as an initial [2+2] cycloaddition, which subsequently rearranges to give a linear adduct. The reactivity of the metal substituent appears to be NR2 > OR > SR. When the metal compound contains several reactive groups, stepwise insertion occurs. For example, Sn(OR)4 reacts with phenyl isocyanate to give the tetracarbamate Sn[N(Ph)COOR]4. Mixed insertion products are obtained using different isocyanates. In the insertion reactions of carbodiimides sometimes ionic cyclic amidinate complexes are formed. 

Metalorganic substrates derived from niobium and tantalum also undergo insertion reactions into metal-carbon and metal-hydrogen bonds. Also, stepwise insertion into Nb(OR)5 is observed . Similarly, chromium trialkoxides undergo insertion reactions with aryl isocyanates ". ... 

Reactions.— Nitriles can be reduced to aldehydes in aqueous media by photochemically generated hydrated electrons. The reduction of nitriles to amines by sodium borohydride is catalysed by Raney nickel. The transition metal promoted reductive decyanation of alkyl nitriles to homologous hydrocarbons with one less carbon atom is reported, and has been shown by van Tamelen to proceed by a different mechanism from that of a similar reduction by alkali metals in ammonia, where a process of stepwise two-electron transfer has been proposed. In the former reaction, where use is made of ferric acetylacetonate and sodium, the proton required for alkane production is derived exclusively from the acetylacetonate ligand, alkane being formed before a proton source is added this suggests a mechanism which involves initial co-ordination of the nitrile to iron, followed by reductive cleavage of the nitrile 1,2-carbon bond and proton transfer to this area, as pictured in Scheme 42. 

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