The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

Iron Catalysis in Organic Synthesis

Dehydrogenation as a substrate-activating strategy in homogeneous transition-metal catalysis.

http://alpha.chem.umb.edu/chemistry/ch611/documents/C-Htransitionmetalbonds.pdf

Carbonhydrogen-transition metal bonds

The anions of PdCl(2)L(2) and Pd(OAc)(2), precursors of palladium(0) used in cross-coupling and Heck reactions, play a crucial role in these reactions. Tricoordinated anionic complexes Pd(0)L(2)Cl(-) and Pd(0)L(2)(OAc)(-) are the effective catalysts instead of the usually postulated Pd(0)L(2) complex. The anion ligated to the palladium(0) affects the kinetics of the oxidative addition to ArI as well as the structure and reactivity of the arylpalladium(II) complexes produced in this reaction. Thus, pentacoordinated anionic complexes are formed, ArPdI(Cl)L(2)(-) or ArPdI(OAc)L(2)(-), the precursor of neutral trans-ArPd(OAc)L(2), instead of the usually postulated trans-ArPdIL(2) complex (L = PPh(3)).

/pdf/anionic-pd-0-and-pd-ii-intermediates-in-palladium-catalyzed-37e1zpb078.pdf

Anionic Pd(0) and Pd(II) intermediates in palladium-catalyzed Heck and cross-coupling reactions.

Series of trans- and cis-dialkylpalladium(II) complexes having tertiary phosphine ligands (L) of various basicities and bulkiness have been prepared and their thermolysis and isomerization mechanisms in solution have been studied. Examination of the cause of selective formation of cis-dialkyl isomers by using alkyllithium revealed a new type of trans to cis isomerization promoted by the alkyllithium. A process involving the formation of a trialkyl-palladate intermediate is proposed as a mechanism for the trans to cis isomerization. Evidence to support the mechanism has been obtained by experiments using LiCD3. Thermolysis of cis-PdR2L2 has been demonstrated to proceed through a unimolecular process initiated by a rate-determining dissociation of L to produce a three-coordinate “cis-PdR2L” which reductively eliminates the R groups. Addition of free ligand to the system containing cis-PdMe2L2 effectively blocks the reductive elimination pathway thus forcing the complex to be thermolyzed by a route involving...

Mechanisms of thermal decomposition of trans- and cis-dialkylbis-(tertiary phosphine) palladium(II). Reductive elimination and trans to cis isomerization.

trans-(PdR'/sub 2/(PR/sub 3/)/sub 2/) with a series of alkyl groups and tertiary phosphine ligands of various steric bulkiness (R' = Et, PR/sub 3/ = PMe/sub 2/Ph, PEt/sub 3/, PEt/sub 2/Ph, PMePh/sub 2/, PEtPh/sub 2/; R' = Pr, Bu, PR/sub 3/ = PMe/sub 2/Ph) are thermolyzed in solution by ..beta..-elimination processes liberating alkane and alkene in a 1:1 ratio. Kinetic studies on thermolysis of trans-(PdEt/sub 2/(PR/sub 3/)/sub 2/) revealed that they decompose obeying the first-order rate law with respect to the concentration of the palladium complexes. Thermolysis predominantly proceeds from the four-coordinated complex without dissociation of the tertiary phosphine ligand. The minor parallel thermolysis pathway involving the dissociation of the phosphine is completely blocked by addition of the phosphine. Activation enthalpies for thermolysis of the ethylpalladium complexes having various steric bulkiness were virtually constant in the range of 26.0 +- 1.0 kcal/mol whereas activation entropies showed considerable variation with increasing bulkiness of the phosphine. Thermolysis of trans-(Pd(CH/sub 2/CD/sub 3/)/sub 2/(PMePh/sub 2/)/sub 2/) cleanly liberated CH/sub 2/==CD/sub 2/ and CH/sub 2/DCD/sub 3/ with a small isotope effect (k/sub H//k/sub D/ = 1.4 +- 0.1). The kinetic results together with examination of the molecular model suggest that the interaction between the bulky phosphinemore » ligands and the ethyl groups causes the destabilization of the Pd-Et bonds. A thermolysis mechanism consistent with the kinetic results assuming an activation state distorted from the square-planar ground state is proposed. 6 figures, 5 tables.« less

Mechanism of thermal decomposition of trans-diethylbis(tertiary phosphine)palladium(II). Steric effects of tertiary phosphine ligands on the stability of diethylpalladium complexes

RuH2(PPh3)4 and other hydridoruthenium complexes catalyze selective conversion of aldehydes into esters in high yields. The method is applicable to most aliphatic aldehydes as well as to aromatic aldehydes. The purity of aldehydes is critical for achieving high conversions, since the presence of carboxylic acid completely inhibits the reaction and alcohol and triphenylphosphine reduce the yields of esters. RuH2(PPh3)4 is converted into Ru(CO)3(PPh3)2 through the reaction indicating the occurrence of decarbonylation of aldehyde. A mechanism involving the acyl-H cleavage of aldehyde is proposed to account for the catalysis and formation of compounds accompanying the reaction. The mechanism is compared with an alternative one which comprises of consecutive insertions of two aldehyde molecules into Ru–H bond followed by β-hydrogen abstraction from an alkoxo intermediate formed. Addition of water changes the reaction course to give carboxylato carbonyl complexes Ru(OCOR)2(CO)m(PPh3)2 (m=1 and 2). Cross esterif...

Selective Dimerization of Aldehydes to Esters Catalyzed by Hydridoruthenium Complexes

Carbon-hydrogen bond activation by transition metal complexes. Oxidative addition of alkyl methacrylate to ruthenium. The structure of hydrido(2-n-butoxycarbonylpropenyl-C1,O)tris(triphenylphosphine)ruthenium(II)

Takashi Ito

Papers

Mechanisms of thermal decomposition of trans- and cis-dialkylbis-(tertiary phosphine) palladium(II). Reductive elimination and trans to cis isomerization.

Mechanism of thermal decomposition of trans-diethylbis(tertiary phosphine)palladium(II). Steric effects of tertiary phosphine ligands on the stability of diethylpalladium complexes

Selective Dimerization of Aldehydes to Esters Catalyzed by Hydridoruthenium Complexes

Carbon-hydrogen bond activation by transition metal complexes. Oxidative addition of alkyl methacrylate to ruthenium. The structure of hydrido(2-n-butoxycarbonylpropenyl-C1,O)tris(triphenylphosphine)ruthenium(II)

Synthesis and reactivity of triethylborane adduct of N-heterocyclic carbene: versatile synthons for synthesis of N-heterocyclic carbene complexes.