Low-valent transition metals

Stoichiometric closure of furan and pyrrole cycles on McMurry reaction induced by low-valent transition metals 98PAC1071. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

Because low-valent transition metals such Ni(0) and Pd(0) are air and/or moisture sensitive,34 the exclusion of oxygen and/or moisture is also crucial for the polymerization. Failure to exclude oxygen will deactivate the catalysts, thus causing the termination of the polymerization and influencing the polymerization degree. 

Few quantitative data are available on the relative nucleophilicities of L toward various alkyl carbonyls. The rates of the reaction of CpMo(CO)3Me with L in toluene (Table II) decrease as a function of the latter reactant P( -Bu)3 > P( -OBu)j > PPhj > P(OPh)j, but the spread is relatively small (<8). The above order is that customarily observed for 8 2 reactions of low-valent transition metal complexes (J, 214). Interestingly, neither CpMo(CO)3Me nor CpFe(CO)2Me reacts with 1 or N, S, and As donor ligands 28, 79). This is in direct contrast to the insertion reactions of MeMn(CO)5 which manifest much less selectivity toward various L (see Section VI,B,C,D for details). 

Castro CE, WC Kray (1963) The cleavage of bonds by low-valent transition metal ions. The homogeneous reduction of alkyl halides by chromous sulfate. J Am Chem Soc 85 2768-2773. 

Radical attack yields nucleobase radical adducts that must undergo either oxidation or rednction to yield a stable final prodnct. The cellular oxidant in these reactions may be molecnlar oxygen or high-valent transition metal ions (e.g., Fe ), while the reduc-tant may be either thiols, snperoxide radical, or low-valent transition metal ions (e.g., Fe ). In many cases, the base remains largely intact and the seqnence of chemical events can be readily inferred. In some other cases, more extensive base decomposition occurs. Here, we will consider a set of representative examples that provide a framework for understanding virtnally all radical-mediated base damage reactions. 

Figure 27 Oxidative addition of the organochalcogen compounds to low-valent transition metal centers most often resulting in the cleavage of the chalcogen-chalcogen bond and the formation of mono- or dinuclear complexes with anionic bridging or terminal RE- (E = Se, Te) ligands.

Low valent transition metal centers preferentially coordinate to the phosphorus in diazaphospholes. Accordingly, P-coordinated complexes of [l,2,3]diazapho-spholes with Cr, W, Fe, and Mn carbonyls were obtained as early as 1980 [1, 2,4], Later, Kraaijkamp et al. observed [108] both P- or -coordination modes in complexes of [l,2,3]diazaphospholes with MX2(PEt3) (M = Pt, Pd X = C1, Br). Methanolysis of these complexes led to the diazaphosphole ring opening and formation of five membered metallacyclic P,/V-chelates (103), incorporating P-bonded phosphonite and /V-coordinated hydrazone functionalities (Scheme 32) [109],... 

Not included in the present review is the fascinating new chemistry which results from reaction between diazo compounds and low-valent transition-metal complexes bearing easily displaceable two-electron ligands as well as with metal-metal multiple bonds and metal hydrides whereby a variety of novel organometallic molecules could be obtained. This field has been covered, in accord with its rapid development, by successive reviews of Hermann 19 22) and Atbini23). 

The fixation of carbon dioxide into organics may involve its activation by coordination to low-valent transition metal complexes. Carbon-carbon bonds can be thus formed by simultaneous activation at a metal center of both carbon dioxide and an organic substrate. 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

Complexes of this type cannot be prepared from the parent ligand, the bivalent anion, [(CF3)2C2S2]2, which has not yet been isolated as a simple salt. Attempts to prepare it from the novel heterocycle bis(trifluoromethyl)-l,2-dithiete19 (CF3)2C2S2 cause dimerization or decomposition. The preparations are achieved by treating the heterocycle with a low-valent transition metal... 

The mechanism of the reaction of secondary alkyl halides with low-valent transition metal complexes is considerably more complex, and radical processes have been clearly identified in some cases (13, 14). 

The product distribution observed in the dimerization of polyene-substituted ketyl radicals is also remarkable in that only products involving dimerization at the carbonyl carbon atom are observed (equation 23)82,83. This finding is quite independent of the reducing agent, since ketyl radicals formed by reduction with low-valent transition metal complexes behave analogously84-86. 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

Scheme 2. Reaction proceeding by a low-valent transition metal complex...

Enyne metathesis is caused by transition metals. There are two types of enyne metathesis one is caused by a carbene complex, as is olefin metathesis, via [2+2] cocyclization and the other type is a reaction that proceeds via oxidative cycli-zation by a low-valent transition metal complex (Scheme 2). 

Equations 1 to 3 show some of fixation reactions of carbon dioxide. Equations la and lb present coupling reactions of CO2 with diene, triene, and alkyne affording lactone and similar molecules [2], in a process catalyzed by low valent transition metal compounds such as nickel(O) and palladium(O) complexes. Another interesting CO2 fixation reaction is copolymerization of CO2 and epoxide yielding polycarbonate (equation 2). This reaction is catalyzed by aluminum porphyrin and zinc diphenoxide [3],... 

Silicon hydrides can also oxidatively add to low-valent transition metal complexes forming a metal hydride silyl complex which can undergo subsequent insertion reactions. This elementary step forms the basis for the hydrosilylation process for alkenes and ketones. 

Oxidative addition. The oxidative addition of a phosphine to a low valent transition metal can be most easily understood by comparing the Ph2P-fiagment with a Cl- or Br- substituent at the phenyl ring electronically they are very akin, cf Hammett parameters and the like. The phosphido anion formed during this reaction will usually lead to bridged structures, which are extremely stable. 

Carbon monoxide and low valent transition metals are known to give various quite well described complexes. However, due to the strong coordination to CO, these metal carbonyl compounds are not very reactive towards carbon-halogen bonds. Thus the carbonylation of organic halides remains a difficult reaction since the presence of CO leads to the deactivation of the catalytic system. Various attempts to overcome this drawback have however been reported. 

Alternatively, CO2 can be used as source of CO. Indeed, it is well known that low-valent transition metal complexes can catalyze the chemical or electrochemical reduction of CO2 into CO. This approach was used to generate the mixed nickel complex Ni°bpy(CO)2 by the electrochemical reduction of Nibpy in NMP or DMF in the presence of CO2. The reduced complex can react with alkyl, benzyl, and allylhalides to give the symmetrical ketone along with the regeneration of Nibpy ". A two-step method alternating electroreduction and chemical coupling leading to the ketone has thus been set up (Scheme 9) [126,127]. 

Because of n-electron donation by the heteroatom, these carbene complexes are generally less electrophilic at C than the corresponding non-heteroatom-substituted complexes (Chapter 3). This effect is even more pronounced in bis-heteroatom-substituted carbenes, which are very weak Tt-acceptors and towards low-valent transition metals show binding properties similar to those of phosphines or pyridine. Alkoxycarbenes, on the other hand, have electronic properties similar to those of carbon monoxide, and stable heteroatom-monosubstituted carbene complexes are also usually formed from metals which form stable carbonyl complexes. 

Diaminocarbene complexes were reported as early as 1968 [152], Preparation and applications of such complexes have been reviewed [153], Because of 7t-electron donation by both nitrogen atoms, diaminocarbenes are very weak tt-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [18,153]. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3). 

Fig. 19 Complexes of C(PPh3)2 with low valent transition metal carbonyls...

Low valent transition metal centers coordinate preferentially to the phosphorus atom of azaphos-pholes, P-Coordinated complexes of 2,5-dimethyl-1,2,3-diazaphosphole with Cr(CO)5 (molecular structure in Table 2), W(CO)5, Fe(CO)4, MnC5H4Me(CO)2 yoJOM(i85)53>, and Pt(PPh3)3 <83JOM(256)375>, of 2,5-dimethyl-4-(dimethylthiophosphoryl)- <80CB2278> and 1,5-dimethyl-1,2,3-... 

The catalytic homogeneous oxidation at low temperatures is therefore economically interesting, but also very difficult to achieve due to the high stability of CH-bonds. Partial oxidation is particularly hard in alkanes as classical oxidation procedures tend to over oxidize them. In the case of methane this would result in the formation of CH2O, CO and CO2. Low valent transition metals, however, are capable of activating the CH bond and rendering that problem less important as the difference in reactivity between the CH bond in methane and methanol is not that big. 

Alkenes are known to form d - complexes with low valent transition metal ions (or atoms), thus stabilizing their low valent complexes (152). Complexes of this type are key intermediates in a variety of catalytic processes, e.g., hydrogenations, polymerizations,... 

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