Fischer carbene complexes deprotonation

A somewhat similar situation has been observed in the deprotonation of the Fischer carbene complexes 20-24 summarized in Table 7.72,73 There is an increase in acidity in the order 20 < 21 < 22 and 23 < 24 which has been... 

Table 7 Deprotonation of Fischer carbene complexes by HO in 50% MeCN-50% water (v/v) at 25°C...

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

It is known that a methyl group attached to the metal carbene C-atom can be deprotonated easily [3]. If the Fischer carbene complex 4 is treated with butyllithium and the resulting anion is silylated with 1, the diazomethylsilylmethyl substituted carbene complex 5 is obtained in 43 % yield (Eq. 3). 

The aminolysis of carboxylic esters to which the reaction of Fischer carbene complexes with amines has frequently been compared, typically proceeds by a similar mechanism with rate-limiting deprotonation of the corresponding zwitter-ionic tetrahedral intermediate (59) and so do many SnAt reactions where the deprotonation of the zwitterionic Meisenheimer complex (e.g., 60) is rate... 

As discussed in the section Alcohols and alkoxide ion nucleophiles, the most useful measure of reactivity of a reaction system is its intrinsic barrier or intrinsic rate constant because, in comparing different systems, these quantities correct for potential differences in the thermodynamics of the reactions. A comparison of intrinsic rate constants for the deprotonation of Fischer carbene complexes by buffer bases with those for the deprotonation of other carbon acids is revealing. A representative hst of such values is given in Table 26. They show the well-known... 

The hydrolysis of Fischer carbene complexes such as 11,13,42,87 and 112 has been discussed in the section Hydroxide ion and water as nucleophiles and proceeds by a standard nucleophilic addition-elimination mechanism (equation 74). On the other hand, carbene complexes with ionizable a-carbons are hydrolyzed by a different mechanism which involves the deprotonated carbene complex as the key intermediate. This conclusion is based on a detailed kinetic investigation of the hydrolysis of 66, 68, 144 and 8 in 50% MeCN-50%... 

The reaction of deprotonated 2-aminophenol 115 with an alkynyl-substituted Fischer carbene 277 gave a cyclic complex 279 in 40% yield, most probably via an intermediate adduct 278 (X = 0). On the other hand, complex 280, obtained from 115 and 277 without base, was heated in tetrahydrofuran to afford benzoxazepine derivative 281 (38%) and benzoxacinone 282 (36%) with reverse regiochemistry (Scheme 49) <2003CEJ4943>. 

The zirconoxycarbene complexes undergo a variety of typical Fischer-carbene reactions. Typically, unsaturated nine-membered zirconoxycarbene complexes such as 103 are readily deprotonated in the a-position to the carbene carbon atom. The stereochemistry of the subsequent alkylation reaction is very efficiently controlled by the remote stereogenic center at C2, resulting in an effective 1,5-asymmetric induction113 (Scheme 34, Fig. 10). 

From a methodological point of view, it should be pointed out the formation of 51, which is a result of the addition of acetone to an allenylidene ligand. Heteroatom-containing cyclic metal-carbene complexes [24] have been conveniently prepared via metal co-haloacyl, carbamoyl, alkoxycarbonyl, or imido intermediates [25], opening of epoxides by deprotonated Fischer-type carbene complexes [26], and activation of homopropargylic alcohols with low-valent d complexes [27], including ruthenium(II) derivatives [28]. In general, the preparation of unsaturated cyclic carbene complexes requires the previous preparation of functional carbenes to react with P-dicarbonyl derivatives, acrylates, and enol ethers [29]. 

Fischer carbene ligands can also be attacked by nucleophiles because of a minor, if any, contribution of dTt-pTt back-bonding to the metal-carbon bond. The attack may lead to formation of alkylmetal and new carbene complexes (Scheme 8.16). If there is a C-H bond next to the carbene carbon, deprotonation sometimes occurs upon attack of base to give vinylmetal complexes (Scheme 8.16)... 

The carbene-bound alkyl groups are acidic pX [(CO)5Cr=C(OMe)Me in H2O] 12.3 and can be easily deprotonated and alkylated [45,211,212] or acylated [213] (Figure 2.16). Stereoselective aldol-type additions can be realized with the aid of Fischer-type alkylcarbene complexes [214-216]. In these reactions the metallic fragment can either play the role of a bulky carbonyl group or stabilize a given conformation of the substrate by chelate formation [216,217]. 

The oldest route to carbyne (CR) complexes involves a Lewis acid (A+) attack on Fischer-type carbenes (equation 15). In this way, several Re carbyne derivatives under the general formula [[Re](CR)]° + have been prepared. The leading examples are [ReCp(CO)2(CR)]+ (R = Ph, SiPhs) because the analogous alkyl carbynes (alkylidynes) are much less stable due to facile deprotonation as elegantly shown by the spontaneous near-quantitative proton loss shown in equation (16). ... 

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