Phosphine mimics

The nucleophilic carbcnes are phosphine-mimics but they are much more. They reside at the upper end of the Tolman electronic and steric parameter scales. Much remains to be explored with these ligands. With a rudimentary understanding of ligand stereoelectronic properties, we feel confident much exciting chemistry remains to be explored. 

Abstract N-Heterocyclic carbene (NHC) has become a major ligand class and has proven to be more than just a phosphine mimic . Some important features like electronic and steric properties are discussed and typical examples of NHC are given herein. 

The chemistry of NHC and of complexes bearing NHC continues to fascinate and surprise. The synthesis of abnormal coordinated carbenes (C5 coordination) in a process explored for more than 8 years is just one example that encourages chemists to examine closely the exact nature of active species involved in catalytic cycles. There is no doubt that recent and future developments in this field of chemistry will separate NHC ligands from the phosphine mimic label placed on them at the early development stages and will confer on them a distinct role in modern catalytic chemistry [102]. 

Cyclic /neso-epoxides, such as cyclopentene oxide (64), can be induced to undergo alkylative ring-opening in the presence of trialkylaluminum reagents under catalysis using N-heterocyclic carbenes (62). These unusual catalysts are derived from the deprotonation of imidazolinium salts (61), and serve as phosphine mimics in terms of electron-pair donation behavior to form aluminum complexes of the type 63 <01 OL2229>. 

In addition, electron-rich and bulky heterocyclic carbenes are attracting attention as effective phosphine mimics [18], Using carbene ligand XVI-6, HR of aryl bromides proceeds at 120 °C [19] and that of diazonium salts 31 at room temperature [20], A new phosphine-imidazolium salt (XVI-14) was found to catalyze HR efficiently [21]. 

N-Heterocychc carbene (NHC) ligands have recently received considerable attention as phosphine mimics in the Sonogashira and numerous other metal-catalyzed... 

Heterodinuclear Ni-Fe complexes, which are not stabilized by the phosphine and NO ligands, were synthesized by Tatsumi and coworkers as [NiFe] hydrogenase mimics [208-210]. Several examples are shown in Fig. 8. However, the catalytic activities of these complexes are not ascertained. 

Andrew Regan was born in Rawtenstall, Lancashire and studied at the University of Cambridge, where he obtained his BA in 1981 (MA 1985), and his PhD in 1984, under the supervision of Professor Jim Staunton. From 1984-1985 he held an SERC-NATO Research Fellowship at Columbia University in the laboratories of Professor Gilbert Stork. He returned to the UK in 1985 to a lectureship in organic chemistry at the University of Kent at Canterbury, and since 1990 has been a lecturer in the Department of Chemistry at the University of Manchester. His research interests include the synthesis of phosphinic-acid hormone mimics, simplified macrolide antibiotics and anti-tumour compounds, stereoselective methodology, and the use of enzymes in synthesis. 

Because of the sheer volume of work published over the last 5 years, only certain specialist topics will be discussed in any depth (Sections III-V). Whereas this survey covers many areas in a cursory fashion, it amply illustrates the rapid growth of knowledge in Os chemistry. It also serves to indicate where future advances are likely to occur, especially in the organometallic chemistry of Os(NH3) Lm]JC+ complexes, which often mimics or surpasses the extensive chemistry normally associated with phosphine ligands. Examples wherein novel organometallic chemistry has occurred are the Tj2-ketone and rj2-arene complexes of penta-ammineosmium. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

Mechanistic aspects of the stoichiometric palladium-catalyzed portion of the reaction have been studied by Rivetti and Romano 109-112) using phosphine model complexes of the form PdX2(P)2 (where X = Cl or OAc and P = tertiary phosphine, generally PPh3). Reaction conditions in these systems are much milder, typically less than 80°C. These systems effectively mimic the features observed in the catalytic cycle and appear to be good models for them. The overall reaction is... 

An alternate complementary approach toward catalytic antibody generation that could overcome the entropic disadvantage of preorganization of the amine cofactor with the Ab catalyst was investigated. A phosphinate (Scheme 5.64) was designed to mimic the expected transition state for the addition of a pheny-lacetone derived enolate to the carbonyl of benzaldehyde. An Ab raised against this hapten was expected to catalyze the aldol condensation of benzylacetone and benzaldehyde by both proximity and electrostatic effects. However, while... 

The generation of oxo(hydroxo)-Mo(V) species, for example, Tp MoO(OH)X, in the reactions of Tp Mo02X with half an equivalent of PPhs in wet solvents is interpreted in terms of combined OAT/CEPT reactions. Thus, water is an effective ligand for the binding site of putative Tp MoOX, formed upon OAT from Mo to phosphine, and unreacted Tp Mo02X is an effective oxidant for Tp MoOX(OH2). Hence, compropor-tionation (equation 17) mimics oxidation of reduced enzymes by external oxidants. " ... 

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