Carbenes reversible

In dihalocarbene generation by phase-transfer catalysis the following steps seem to be involved (15) formation of CX anions dynamically anchored at the boundary reversible detachment with the help of the catalyst reversible carbene formation [Q+ CX3 ] [Q + X ] + CX2 addition to olefin. 

It was observed occasionally that the DTDAF 51a can revert back to the stable carbene so that the dimerization appears to be reversible (97LAR365). Tills splitting reaction could not always be clearly reproduced and seems to be influenced by trace contaminants that are as yet unidentified. 

The kinetic and thermodynamic properties of Fischer-type carbene complexes have also been addressed by Bernasconi, who relates the strength of the 7r-donor substituent to the thermodynamic acidity [95-101] and the kinetics and mechanism of hydrolysis and reversible cyclization to differences in the ligand X [96,102]. 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

Based on this work, it has been proposed that a specifically solvated carbene (Scheme 4.6, Reaction 2) nndergoes bimolecular reactions at slower rates than a free carbene (Scheme 4.6, Reaction 1). Other alternatives that mnst be considered are participation of rapid and reversible ylide formation with the ylide acting as a... 

The synthesis and X-ray structural determination of a stable Ir111 hydride/alkylidene complex, (165), has been reported, in which the tridentate N3 ligand is TpMe2. 9 The complex undergoes reversible hydride migration onto the electrophilic carbene atom, as shown in reaction Scheme 20. 

The osmium formimidoyl 16 is reversibly protonated to a secondary carbene complex (44) ... 

The driving force for this transformation is the fact that the less electronegative nitrogen atom is a better Tr-donor than oxygen and can form a stronger bond with the carbene carbon atom. Hence displacement of alkoxide by amines and thiols is commonly observed, but the reverse reactions are seldom seen. 

Bu2P(CH2)5P Bu2 to form IrHCl[ Bu2P(CH2)2CH(CH2)2P Bu2], (54) (59). Compound 54 reversibly eliminates hydrogen on thermolysis at reduced pressure, yielding the Ir(I) carbene complex 55 ... 

RG21 This is the formal reverse process of RG12, but here the valence of atom X is reduced. Obviously, this scheme should not be applied indiscriminately, (for instance to each tetravalent carbon, changing it to a carbene) since many reactions with no chemical significance would result. Care has therefore been taken in the evaluation phase to find the appropriate sites for its application. 

E.O. Fischer s discovery of (CO)sW[C(Ph)(OMe)D in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2), but It was not until about five years later that Chauvln proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now... 

Reactions of 1,2,4-thiadiazoles with radicals and carbenes are virtually unknown. Catalytic hydrogenations and dissolving metal reductions usually cleave the N-S bond in a reversal of the oxidative cyclization procedures used in synthesis of 1,2,4-thiadiazoles (see Section 5.08.9.4). 

Carbene generation is envisioned as being a reversible sequence as depicted in Eq. (24). 

Some notable reversals in the order of olefin reactivities were observed in contrast to the tantalum carbenes. Further, in comparing the reactivities of (CO)5W=CPh2 and (CO)5W=CHPh, Casey (70) noted additional striking contrasts The monophenyl carbene complex reacted rapidly at... 

Evaluation of kinetic data. Rate constants were determined for 2-H exchange from 3-R-4-methylthiazolium ions, catalyzed by D2O (pseudo first order) and DO- (second order).154 The observed rate constants for the pD-independent exchange reaction were corrected for the solvent isotope effect ( h2o/ d2o = 2.6), and the reverse protonation of the carbene by H30+ was assumed to be diffusion-controlled (k = 2 x 1010 M-1 s-1). A similar analysis was performed for the exchange catalysed by DO-. The results agreed nicely, giving pAfa = 18.9 for 213 and p/sfa = 18.0 for thiamine.154 The thiazolium ion 213 seems to be less acidic in water154 than in DMSO152 (Ap/fa = 2.4). Aside from the... 

Vinylcarbene is known to close to cyclopropene.59 The reverse reaction is also possible Triplet-propene-l,3-diyl (frans-T-33 ) can be generated from cyclopropene 32 by irradiation in a bromine-doped xenon matrix at 10 K 1-methylcyclopropene (34) yields triplet-2-butene-l,3-diyl (Iruns-T-SS ).60-62 The concentration of 35 under these conditions is high enough to be able to detect this diradical IR spectroscopically. The experiments suggest that even the parent vinyl carbene 33 is detectable.61,62 Calculations ((U)B3LYP/6-31G )61,62 not only allow the comparison of theoretical and experimental IR spectra but also... 

The correlation of the recorded IR spectrum with semiempirical calculations and the reversible cleavage into oxohexapentaenylidene (97) and carbon monoxide give a consistent picture for both, dione 96 and carbene 97. The spectrum of 97 correlates with scaled ab initio vibrational frequencies.123 C60 97 should be a triplet molecule. Indeed, Weltner et al.124 observed during the experiment concerning C4O T-88 another triplet ESR signal which they ascribed to T-97. 

Rearrangement of the 3,5-dimethylated carbene Id would yield the destabilized cyclopropene 3d with a methyl group in the bridgehead position 1, and consequently no detectable amount of cyclopropene 3d is formed during irradiation of Id. Indeed, whereas the 3,5-dimethyl substituted carbene Id is 3.4 kcal mol-1 more stable than the 2,6-dimethyl isomer lb, the stability is reversed for the cyclopropenes, as 3d is found to be 6.5 kcal mol-1 higher in energy than 3b at the B3LYP/6-31G(d) level of theory (Table 3). 

It was also found that the ring expansion could be accomplished photo-chemically, from either phenyldiazomethane or triplet phenylcarbene.7 Both the thermal and photochemical ring expansions were found to be reversible,5c, thus providing rare examples of carbene-to-carbene interconversions. One remarkable example of this reversibility is the interconversion of the isomeric tolylcarbenes upon pyrolysis — the ultimate products of which include styrene and benzocyclobutene (Scheme 3).6,8,9... 

A more general method for preparing carbenes often involves the a elimination of halides from carbanions.1-57 PAC can be used to examine the rates and energetics of the reverse reactions, the complexation of halides with carbenes (Fig. 5).58 Plots of A//com versus the proton affinities (PA) of the halides are linear for the two carbenes studied. Although the slopes of the plots are similar, complexation of the halides with phenylchlorocarbene is more exothermic than phenylfluorocarbene. This indicates that fluoro substitution stabilizes the carbene relative to the carbanion more than chloro substitution. The rate of complexation of carbenes with salts has also been examined by nanosecond absorption spectroscopy.59... 

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