Non-stabilized transition metal carbenes as intermediates in intramolecular reactions of alkynes with alkenes

In this tutorial review we summarize the two major pathways followed in the reaction of alkenes with alkynes catalysed by electrophilic transition metals. If the metal coordinates simultaneously to the alkyne and the alkene, an oxidative cyclometallation can ensue to give a metallacyclopentene, which usually evolves by [small beta]-hydrogen elimination to give Alder-ene cycloisomerisation derivatives. On the other hand, coordination of the metal to the alkyne promotes the attack of the alkene to give metal cyclopropyl carbenes.     

Synthesis and reactions of organometallic carbodiimides and isocyanates

Conclusions A convenient laboratory method has been proposed for the preparation of organometallic tin and germanium carbodiimides in excellent yields and satisfactory purities.Organotin carbodiimides react with dicarboxylic acid anhydrides to give addition products, namely, organotin derivatives of N-substituted dicarboxylic acid monocyanamides. The corresponding reaction of tin and germanium isocyanates lead to the formation of N-tributylstannyl(germyl)imides of dicarboxylic acids, and CO₂.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 859–865, April, 1988.     

Allylic alcohols as synthetic enolate equivalents: isomerisation and tandem reactions catalysed by transition metal complexes

Allylic alcohols can be isomerised into carbonyl compounds by transition metal complexes. In the last few years, catalyst design and development have resulted in highly efficient isomerisations under mild reaction conditions, including enantioselective versions. In addition, the isomerisation of allylic alcohols has been combined with C-C bond forming reactions when electrophiles such as aldehydes or imines were present in the reaction mixture. Also, C-F bonds can be formed when electrophilic fluorinating reagents are used. Thus, allylic alcohols can be treated as latent enol(ate)s. In this article, we highlight the latest developments concerning the isomerisation of allylic alcohols into carbonyl compounds, focusing in particular on tandem isomerisation/C-C or C-heteroatom bond formation processes. Significant attention is given to the mechanistic aspects of the reactions.     

Further studies on reactions of organic halides with disilanes catalysed by transition metal complexes

The interaction of a range of organic halides with (Cl₃Si)₂ or (Me₃Si)₂ in the presence of a variety of transition metal catalysts (very predominantly Pd⁰ or Pd^II complexes) have been examined. PhSiMe₃ was formed from PhCl[m.y., 15%] (m.y. - maximum yield), PhBr (m.y., 92%, with [PdL₂Br₂] as catalyst (L - PPh₃)), and (contrary to earlier reports) PhI (m.y. 51%, with [PdL₂I₂]). MeSiCl₃ was formed from MeBr (m.y., 78% with [PdL₄]) and MeI (m.y., 91% with [PdL₄]), and EtSiCl₃ from EtBr (m.y., 49%, with [PdL₂“Br₂]; L” - P(C₆H₄OMe-p)₃) and EtI (m.y. 45%, with [PdL₄]). Me₄Si was satisfactorily formed from MeBr (m.y. 42%, with [PdL₄]). Evidence was obtained for the formation of Me₃SiCF₃ from CF₃I. Very poor yields of XC₆H₄CH₂SiMe₃ were obtained from XC₆H₄CH2Br (X - H orp-Me) (with X - H some PhSiMe₃ was formed), butp-O₂NC₆H₄CH₂SiMe₃ was formed in 48% yield fromp-O₂NC₆H₄CH₂Cl with [PdL“₄] as catalyst. PhCOSiMe₃ was formed from PhCOCl (m.y. 52% with [PdL₂I₂]. The nickel complex [NiL₄] was moderately effective as a catalyst for reactions between (Cl₃Si)₂ and MeBr, EtBr, or PhCH₂Br. The new complex [PdL₂(SiCl₃)₂] was prepared by treatment of [PdL₄] with (Cl₃Si)₂ or Cl₃SiH, and shown to catalyse the reaction between MeBr and (Cl₃Si)₂.     

Recent advances in the synthesis of N-containing heteroaromatics via heterogeneously transition metal catalysed cross-coupling reactions

N-containing heteroaromatics are important substructures found in numerous natural or synthetic alkaloids. The diversity of the structures encountered, as well as their biological and pharmaceutical relevance, have motivated research aimed at the development of new economical, efficient and selective synthetic strategies to access these compounds. Over more than 100 years of research, this hot topic has resulted in numerous so-called "classical synthetic methods" that have really contributed to this important area. However, when the selective synthesis of highly functional heteroaromatics like indoles, quinolones, indoxyls, etc. is considered these methods remain limited. Recently transition metal-catalysed (TM-catalysed) procedures for the synthesis of such compounds and further transformations, have been developed providing increased tolerance toward functional groups and leading generally to higher reaction yields. Many of these methods have proven to be the most powerful and are currently applied in target- or diversity-oriented syntheses. This review article aims at reporting the recent developments devoted to this important area, focusing on the use of heterogeneous catalysed procedures that include either the formation of the heterocyclic ring towards the nuclei or their transformations to highly substituted compounds.     

Synthesis of new ligands for transition metal complexes: menthyl- and neomenthyl-cyclopentadienes

Syntheses of (?)-menthylcyclopentadiene (MCp) and (+)-neomenthyl cyclopentadiene (NMCp) from (?)-menthol are described. These chiral ligands have been used to prepare (η⁵-MCp)₂TiCl₂, (η⁵-NMCp)₂, TiCl₂, (η⁵-MCp)₂ZrCl₂, (η⁵-NMCp)₂ZrCl₂, (η⁵-Cp)(η⁵-MCp)TiCl₂ and (η⁵-Cp)(η⁵-NMCp)TiCl₂. The structure and absolute configuration of (η⁵-Cp)(η⁵-MCp)TiCl₂ has been established by X-ray analysis.     

Synthesis of bis N-heterocyclic carbenes,derivatives and metal complexes

A method for the synthesis and isolation of 1,1′-methylene-bis-(3-aryl-imidazol-2-ylidene) ligands, aryl = 2,6-diisopropyl-phenyl (DiPP), L^DiPP, mesityl (mes), L^mes, is reported, which provides synthetically useful quantities of high purity. Derivatisation of L^DiPP with chalcogenides gave the adducts L^DiPPE₂, E = S, Se, Te. Reaction of L^DiPP with [Pd(tmeda)Me₂], [Pt(μ-SMe₂)Me₂]₂, [Ir(1,5-COD)(μ-Cl)]₂/KPF₆ and [NiBr₂(dme)] gave [Pd(L^DiPP)Me₂] (1), [Pt(L^DiPP)Me₂] (2), [Ir(L^DiPP)(1,5-COD)](PF₆) (3) and [Ni(L^DiPP)Br₂] (4), respectively. The latter was reduced in the presence of CO to [Ni(L^DiPP)(CO)₂] (5). The structures of L^mes, L^DiPPTe₂, and 1–5 are also reported.     

A new approach to the gas phase chemistry of ternary transition metal complexes: Ion-beam induced reactions

High Energy Cs⁺-ion bombardment of glycerol containing a suitable transition metal chloride, an amino acid (A.A.) and 1,10-phenanthroline (phen) induces the formation of singly charged ternary complexes of the type [Cat(A.A. - H) (phen)]⁺, where Cat = Cr, Mn, Fe, Co or Ni. The investigation of the complexes is performed by MS/MS experiments using a four-sector instrument. Metastable decompositions of iron containing ternary complexes [Fe(A.A. - H) (phen)]⁺ provide abundant fragment ions indicating the coordination of functionalized amino acid side chains. Fragmentation mechanisms and ion structures are discussed.     

Synthesis of new polyamidoximes and their crosslinking by transition metal ions

Three new polyamidoximes (PAO) having appropriate functionalities to bind transition metal ions were prepared. The polymers were obtained by the reaction of dichlorooximino ethane with the corresponding diamine. Characterization and crosslinking of PAOs via coordination with transition metal ions such as Ni(II), Co(II), Cu(II), and UO₂(II) are presented. The crosslinked polymer complexes exhibit good thermal stability. It was also found that both square planar and tetrahedral coordination structures are present in the crosslinked polymers.     

The first example of macrocycles containing butterfly transition metal cluster cores via novel tandem reactions

A novel type of double butterfly, two mu-CO-containing dianions {[(mu-CO)Fe2(CO)6]2[mu-SCH2(CH2OCH2)nCH2S-mu]}2- (m1, n = 2, 3), has been synthesized from dithiol HSCH2(CH2OCH2)nCH2SH (n = 2, 3), Fe3(CO)12, and Et3N in THF at room temperature. While dianions m1 react in situ with CS2 followed by treatment with dihalide 1,4-(BrCH2)2C6H4 or 1,4-I(CH2)4I to give macrocyclic clusters [mu-SCH2(CH2OCH2)nCH2S-mu](mu-CS2ZCS2-mu)[Fe2(CO)6]2 (1a, n = 2, Z = 1,4-(CH2)2C6H4; 1b, n = 3, Z = (CH2)4), reactions of dianions m1 with (mu-S2)Fe2(CO)6 followed by treatment with dihalide 1,4-I(CH2)4I afford macrocyclic clusters [mu-SCH2(CH2OCH2)nCH2S-mu]{[Fe2(CO)6]2(mu4-S)}2[mu-S(CH2)4S-mu] (2a, n = 2; 2b, n = 3). The crystal structures of 1a and 2b are described.     

Synthesis of isocyanates and carbodiimides

The reaction of 1,3-disubstituted ureas and 1-arylsulfonyl-3-alkylureas with phosgene permits the preparation of a number of isocyanates and sulfonyl isocyanates that had not previously been described, or that were difficult to prepare. 1,3-Disubstituted thioureas and 1-aryl-(or alkyl)sulfonyl-3-alkylthioureas give carbodiimides. Thionyl chloride and phosphorus pentachloride react with the urea derivatives in the same way as phosgene. Oxalyl chloride reacts with ureas to form parabanic acid derivatives, whereas with thiourea it gives 2-imino-1,3-thiazolidine-4,5-diones, which can isomerize to parabanic acid derivatives.     

Influence of anellation in N-heterocyclic carbenes: novel quinoxaline-anellated NHCs trapped as transition metal complexes

The synthesis, NMR-, and crystal structure data of novel electron-deficient quinoxaline anellated imidazol-2-ylidene precursors and complexes thereof are reported and compared with related less electron-withdrawing or non-anellated N-heterocyclic carbenes and complexes to illustrate anellation effects.     

Recent progress in transition metal catalysed hydrofunctionalisation of less activated olefins

The electrophilic activation(C-H activation) of alkenes by transition metal catalysts is a fundamental step in a rapidly growing number of catalytic processes since it would provide simple, clean, and economic methods for making controlled and selectively functionalized organic moieties directly from simple olefins. Also catalytic activation of C-H bonds leading to useful organic reactions such as new C-C, C-N and C-O bond formation is of considerable interest for the chemical and pharmaceutical industries and remained a long-term challenge to chemists. A substantial progress has made in the last decade in this area. Contrary to traditional belief, it is nowadays possible to control the regiochemistry of various additions of nucleophiles to alkenes by the choice of transition metal catalysts. Atom economy, an inevitable factor of current research also can be accomplished in these reactions. Developments in this area of selective hydrofunctionalisation of alkenes by taking into consideration of the mechanistic aspects and the role of organometallic catalyst or active species formed during the reaction on the outcome of the reactions are reviewed.     

Synthesis of 2,3-benzoxepins by sequential cyclopropanation/ring-enlargement reactions of benzopyrylium triflates with diazoesters

Functionalized 2,3-benzoxepins were prepared by cyclopropanation of benzopyrylium triflates with diazoesters and subsequent TFA-mediated ring enlargement.     

Reductive coupling of carbodiimides with metallic lithium: a new route to metal oxalamidino complexes

Carbodiimides (RN)2C were reductively coupled with Li powder into [{Li(py)2}2(mu-C2N4R4)] (R = p-tolyl) (1) and [{Li(THF)}2(mu-C2N4R4)] (R = Cy) (2), and 2 reacted with UCl4 to give [Li(py)4]2[{UCl4(py)}2(mu-C2N4R4)] (R = Cy) (3); the crystal structures of 1 and 3 x 2py, the first oxalamidino compounds of an alkali metal and of a 5f-element, have been determined.