Rhenium complexes carbene

Alkylation via approach of the electrophile from the least hindered side of an a-alkoxy vinyl -rhenium complex of antiperiplanar (alkoxy oxygen anti to the carbon monoxide oxygen) geometry has been proposed. The NMR spectroscopic data are consistent with the transient presence of rhenium-carbene complexes, such as 2 and 597. 

Rate-determining step, hydroformylation, 163 Reactivity, enantiomers, 286 Recognition, enantiomers, 278 Reduction and oxidation, 5 Reductive coupling, dissolving metal, 288 Reductive elimination, 5, 111 Resolution. See Kinetic resolution Rhenium-carbene complexes, 288 Rhodium-catalyzed hydrogenation, 17, 352 amino acid synthesis, 18, 352 BINAP, 20... 

The pArf values of the neutral (CO)sM type complexes that are summarized in Table 24 and span a range 8.4-14.5 the two cationic rhenium carbene complexes which are substantially more acidic (pA =4.18 and —0.03, respectively) will be dealt with below. 

The common alkyne trapping reagent 1,3-diphenylisobenzofuran was used as a precursor towards the synthesis of new analogs of famesyltransferase inhibitor RPR 130401 <04JOC7220>. A rhenium isobenzofuryl carbene complex was also synthesized recently <04OM4121>. As depicted below, thermal rearrangement of the ri -(o-ethynylbenzoyl)-rhenium complex produced the benzo[c]furyl rhenium carbene complex, presumably via a nucleophilic attack of the carbonyl oxygen on the rhenium-bound alkyne. The alkyne rhenium complex and the rhenium carbene complex were both observed at equilibrium. Like all other benzo[c]furans, the rhenium isobenzofuryl carbene species reacted smoothly with... 

Reaction of 3 with Ph3C+PF6" resulted in the formation of methylidene complex [(n-C5H5)Re(N0)(PPh3)(CH2)]+ PF6 (8) in 88-100% spectroscopic yields, as shown in Figure 11. Although 8 decomposes in solution slowly at -10 °C and rapidly at 25 °C (She decomposition is second order in 8), it can be isolated as an off-white powder (pure by H NMR) when the reaction is worked up at -23 °C. The methylidene H and 13C NMR chemical shifts are similar to those observed previously for carbene complexes [28]. However, the multiplicity of the H NMR spectrum indicates the two methylidene protons to be non-equivalent (Figure 11). Since no coalescence is.observed below the decomposition point of 8, a lower limit of AG >15 kcal/mol can be set for the rotational barrier about the rhenium-methylidene bond. 

Numerous examples have been reported of transition metal alkyl complexes which can be converted into carbene complexes by a-hydride abstraction [429-431], This process can also proceed intramolecularly by oxidative insertion of the metal into the a-C-H bond. Figure 3.8. shows some illustrative examples of iron, rhenium, and... 

The first such study involved the deprotonation of the cationic rhenium Fischer carbene complexes 48H+-X by primary aliphatic amines, secondary... 

Table 11 Acidities of rhenium Fischer carbene complexes and intrinsic rate constants for their deprotonation by amines and carboxylate ions in 50% MeCN-50% water (v/v) at 25°Ca,b...

The rhenium derivative (CH3)3GeRe(COb, can be converted into the carbene complex (CH3)2(jeRe(CO)4C< G )CH ), which in solution is in equilibrium with its dimer. The dimer crystallizes preferentially, and incorporates an eight-membered GeReCO]2 heterocycle (181). 

Molecular wires, 208 211 in which two metal centers are connected by an unsupported C3 chain, as in compound 191, have been generated by addition of BF3 to binuclear bridged (l-alkynyl)carbene complex lw (Scheme 80).27 29,212 Similar compounds were derived from tungsten, rhenium, and iron moieties. A binuclear complex 192 containing a C5 bridge has been also obtained 27... 

Acyloxy)alkenyl carbene complexes, synthesis, 223-224 Alkenes, bishydroxylation cycloreversion of rhenium diolates, 148-156... 

Vinyl-substituted benzo[c]furans can be prepared by reaction of n-alkynylbenzaldehydes with chromium Fischer carbene complexes. Initially a benzo[c]furan chromiumtricarbonyl complex is believed to be formed which is converted into an alkylidenephthalan derivative or can be trapped with electron-deficient dienophiles with excellent ( 3 ti-selectivity (Equation 128) <20000L1267>. More elaborate vinylidene Fischer carbene complexes yield dienyl benzo[c]furans that undergo [8-1-2] cycloaddition with DMAD to furnish furanophane derivatives <2003JA12720>. An equilibrium between 7] -(o-ethynylbenzoyl)rhenium complexes and rhenium benzo[f]furyl carbene complexes has been observed. These species behave like other benzo[< ]furans in the reaction with DMAD <20040M4121>. 

The electrophilicity of the alkylidyne carbon in cationic complexes of manganese and rhenium such as 154 or 157 is well established. A study by Chen et al. established that the carbonyl ligands are also potential sites for nucleophilic attack (157,158). Reaction of 154 with the bulky carborane anion LiCjBioH, results in the formation of two products the carbene complex 156, resulting from attack at the alkylidyne carbon, and the alkylidyne acyl complex 155, resulting from attack at a carbonyl ligand [Eq. (135)]. At room temperature complex 155 transforms into complex 156. 

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