Carbene rhenium

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

Dihalocarbene complexes are useful precursors to new carbenes by nucleophilic displacement of the chlorine substituents. This has been nicely illustrated for Fe(TPP)(=CCl2) by its reaction with two equivalents of Re(CO)5J to give the unusual /t-carbido complex Fe(TPP)=C=Re(CO)4Re(CO)5 which also contains a rhenium-rhenium bond. " The carbido carbon resonance was observed at 211.7 ppm in the C NMR spectrum. An X-ray crystal structure showed a very short Fe=C bond (1.605(13) A, shorter than comparable carbyne complexes) and a relatively long Re=C bond (1.957( 12) A) (Fig. 4, Table III). " ... 

This facile approach to the carbene chemistry of rhenium has not yet been investigated with technetium. Further reactions with the technetium cluster 44a have been performed in C6H6/HC1 to yield the compound [( 6H6)Tc(CO)3]+ (66) which previously had only been described for manganese and rhenium [81]. "Tc-NMR of the latter compound exhibits a resonance at -1983 ppm (relative to [Tc04]- ), and it therefore fits very well into the range proposed for Tc(I) complexes. 

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... 

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 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). 

In addition to the metal-oxo complexes, several examples of NHC-stabilized rhenium(V)-nitrido complexes exist, e.g., 9 and 10 (Chart 1) [28]. These adducts, which feature triazole-based ligands, were prepared via displacement of phosphine ligands. The stability of both phosphine and triazole-based carbenes in these reactions suggest the nitrido ligand is relatively unreactive. 

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>. 

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