Alkylidene with olefin reactions

Scheme 14.11. Reaction of titanocene-alkylidenes with olefins.

Neutron diffraetion eonfirmation of alkylidene interaetion with metal [31] Reaction of Ta alkylidene with olefins and metallacycle rearrangement [38] Synthesis of a titanaeyelobutene [45]... 

The reaction of an alkylidene phosphorane 1 (i.e. a phosphorus ylide) with an aldehyde or ketone 2 to yield an alkene 3 (i.e. an olefin) and a phosphine oxide 4, is called the Wittig reaction or Wittig olefination reaction. ... 

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

The synthetic method leading to Nb-alkylidenes and Nb-alkylidynes was particularly successful, due to a quite remarkable difference in the reaction rate of 29 with ketones or aldehydes, vs the subsequent reaction of the alkylidene with ketones and aldehydes (see Scheme 37). The former reaction takes a few minutes at -40°C, while the latter one occurs in hours at room temperature.88 The reaction between 178 and benzaldehyde led to triphenylethylene and the niobyl derivative 184. Due to the difference in reaction rates between a and b in Scheme 37, it was found that the sequential addition of two different ketones or aldehydes to a THF solution of 29 produced a nonsymmetric olefin in a stepwise McMurry-type reaction.84 This is exemplified in the coupling shown in reaction c (Scheme 37). The proposed reaction pathway does not involve the intermediacy of a pinacolato ligand and therefore differs from the mechanism of the McMurry reaction and related reductive couplings at activated metal sites.89... 

Scheme 4. Ketone olefination reaction of a ketone with a metal alkylidene, which results in the destruction of the catalytic species...

The organotitanium compounds produced by desulfurization of the diphenyl thioacetals of aldehydes 28 with the titanocene(II) species Cp2Ti[P(OEt)3]2 29 react with carbon—carbon double bonds to form the olefin metathesis-type products. Thioacetals 28 may be transformed into terminal olefins by desulfurization with 29 under an ethene atmosphere (Scheme 14.15) [27]. This reaction is believed to proceed through a titanacyclobutane intermediate, formed by cycloaddition of the titanocene-alkylidene with ethene. 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

Scheme 8. Reactions of the titanium-alkylidene species 12, prepared from dithioacetals, with olefins and acetylenes.

Very recently, Grubbs and coworkers completed an analysis based on insight from mechanistic work on the relative rates of phosphine dissociation and olefin coordination (vide infra) in ruthenium alkylidene catalyzed olefin metathesis reactions. The study was based on numerous analogues of (4a), having different phosphine groups, for example, (4e), (4f), and (4g). Rates for ROMP of cyclooctadiene with the most potent of these new complexes were 340-fold greater than with (4a) (Scheme 1) ... 

Two of these are the cycloaddition of the methyhdene with ethylene (path E, non-productive), reaction of the methylidene with an internal olefin such that the alkyl substituent on the metallacyclobutane is in the j9-position (path H, non-productive). The other two pathways are the cycloaddition of the alkylidene with an internal olefin to give the trisubstituted metallacyclobutane (path G, frans-metath-esis, non-productive) and the reaction of the alkylidene with a terminal olefin to give the a,a -disubstituted metallacyclobutane (path F), which can be looked at as a chain transfer-type event, albeit not in the sense of a chain polymerization. In this case, the alkylidene is shifted from the end of one chain to the end of another chain. So, assuming that all pathways have somewhat similar rates, the elimination of ethylene will drive the reaction to high polymer. In the case of ADMET, these additional mechanistic pathways do not prevent the polymerization reaction, since these additional pathways are either degenerate or represent processes that do not affect the overall molecular weight distribution of the polymer. 

Formation of Titanocene-Methylidene and its Reaction with Olefins 476 Formation of Trtanocene-Alkylidenes and their Application to Olefin Metathesis 479... 

Alkylidene exchange 5.8.2.3.11 Reaction with olefins S.8.2.3.6 Reaction with vinyl ethers 5.8.2.3.11... 

Turning to carbene related reactive species, alkylidene carbenoids like 26 (X = halogen, OR, NR2) are particularly valuable for preparative purposes since they can undergo cycloaddition reactions with olefins (to methylenecyclopropanes), isomerizations (to alkynes by the so-called Fritsch-Buttenberg-Wiechell rearrangement), and dimerization (to [3]cumulenes). Although carbenoids have been studied extensively by NMR spectroscopy [23], the first X-ray structural analysis of a stable carbenoid, 27, as a TMEDA 2THF complex has been reported only recently [24]. 

Compounds of type 7 proved to be remarkably active catalysts for the metathesis of internal olefins. [44,68,69] The activity of such species for the metathesis of ordinary internal olefins (e.g., c 5 -2-pentene) appeared to maximize for the OCMe(CF3)2 species. New alkylidene complexes such as W(NAr)(CHPh)[OCMe(CF3)2]2 could be isolated, and in some cases trigonal bipyramidal (TBP) tungstacyclobutane intermediates were stable enough to be observed and isolated. On the basis of this work it was proposed that the rate of reaction of alkylidene complexes with olefins correlated directly with the electron-withdrawing ability of the alkoxide, as found in acetylene metathesis systems described earlier. In many circumstances trigonal bipyramidal or square pyramidal tungstacyclobutane intermediates could be observed. [44] In any system in which ethylene could be formed, unsubstituted metallacycles could... 

Reactions of metal alkylidene and alkylidyne complexes with olefins and alkynes. 

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