Methylidene formation

Leconte and Basset [161-166] proposed two other possible mechanisms (Scheme 39) the first one implies a 1,2 carbon-carbon activation which invokes the de-insertion of a methylidene fragment from a surface metal-alkyl species, and the second implies a 1,3 carbon-carbon bond activation in which the key steps are the formation of a dimetallacyle by y-H activation from a metal-alkyl followed by carbon-carbon bond cleavage via a concerted electron transfer. 

The next task was removal of the C3,C3 -esters. Although the palladium-catalyzed decarboxylation protocol performed well in previous systems, a competing C-H insertion reaction was discovered with the methylidene bridge needed for cercosporin (see below). Since reexamination of alternate decarboxylation methods [48] led to no success, a decarbonylation strategy was explored [49]. Formation of the requisite dialdehyde was best accomplished by overreduction using DIB AL and... 

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. 

In a formal sense, complexes 1 represent pre-catalysts that convert in the first turn of the catalytic cycle (vide infra) into ruthenium methylidene species of type 3 which are believed to be the actual propagating species in solution (Schemes 2,4). The ease of formation of 3 strongly depends on the electronic properties of the original carbene moiety in 1. In addition to complexes la-c with R1=CH=CPh2, ruthenium carbenes with Rx=aryl (e.g. Id, Scheme 3) constitute another class of excellent metathesis pre-catalysts, which afford the methylidene complex 3 after an even shorter induction period [5]. In contrast, any kind of electron-withdrawing (e.g. -COOR) or electron-donating substitu-... 

Formation of Titanocene-Methylidene and its Reaction with Olefins... 

In 1978, Tebbe and co-workers reported the formation of the metallacyde 4, commonly referred to as the Tebbe reagent, by the reaction of two equivalents of trimethylaluminum with titanocene dichloride. The expulsion of dimethylaluminum chloride by the action of a Lewis base affords the titanocene-methylidene 5 (Scheme 14.4) [8]. 

Scheme 14.4. Formation of titanocene-methylidene from the Tebbe reagent.

Despite the successful reactions mentioned above, olefin metathesis utilizing titanocene-methylidene is not necessarily regarded as a useful synthetic tool. Indeed, the steric interaction between the substituent at the carbon a to titanium and the bulky cyclopentadienyl ligand disfavors the formation of the titanocene-alkylidene 15. Hence, cleavage of the titanacycle affords only titanocene-methylidene and the starting olefin (Scheme 14.9). 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

Methylenative dimerization takes place when terminal alkynes are treated with the tita-nocene/methylidene/zinc halide complex generated from titanocene dichloride and CH2(ZnI)2. The process is believed to involve the formation of a titanacyclobutene intermediate [75],... 

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

Pyrrole is very reactive towards electrophiles charge distribution from the nitrogen makes either C-2 (or C-3) electron rich. Thus, a second porphobilinogen acts as the nucleophile towards the methylidene pyrrolium cation in a conjugate addition reaction. It is now possible to see that two further identical steps will give us the required linear tetrapyrrole, and that one more time will then achieve ring formation. 

When a CH2CI2 solution of 119a is stirred in the presence of 10 mol % of Ic under ethylene gas (1 atm) at room temperamre, pyrrolidine derivative 120a is obtained in high yield. Various cycloalkene-ynes 119b-c are examined, and pyrrolidine derivative 120b-c is formed in each case. Formally, the double bonds of cycloalkene and ethylene are cleaved, and each methylidene part of ethylene is combined with the cycloalkene and alkyne carbons, respectively, and bond formation between the double and triple bonds occurs to give pyrrolidine derivative 120 ... 

Application to methylidenation and epoxide formation will be described in Section 4.4.1. 

Olah et al.603 have observed the formation of cation 309 (protonated fluorometha-nol) upon treatment of formaldehyde in HF-SbF5 [Eq. (3.81)]. When Minkwitz et al.605 attempted to isolate salts of the ion, however, the hydroxymethyl(methylidene) oxonium ion 310 was obtained [Eq. (3.81)]. Crystal structure analysis of the hexafluoroarsenate salt shows that cations and anions are connected by short H -F distances, forming a three-dimensional network. The bond lengths of the C-0=C fragment (1.226 and 1.470 A) are longer than those in formaldehyde (1.208 A) and dimethyl ether (1.410 A). The C—O—C bond angle is 121.2°. 

Methylidenation of allylic thioethers. Methylidenation of an allylic phenyl-thioether with methylene iodide-diethylzinc is accompanied by a 2,3-sigmatropic rearrangement to a homologous allylic phenylthioether. The rearrangement is also initiated by ethylidene iodide. Cyclopropanation is not observed. The Simmons-Smith reaction with allylic sulfides results only in formation of an insoluble polymer. 

The [2+3] cycloaddition of methylidene borane 36 with alkyl azides furnishes the intermediate triazaboroles 37a, which further undergoes silicon migration from carbon to nitrogen resulting in the formation of 37b. The driving force for the reaction is the stabilization of the ring system due to aromatization (6rt electrons) (Scheme 4) <2004ZFA508>. 

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