Vinylidenes precursors

This reaction is quite different from the other P-H addition reactions in that it involves external nucleophilic attack of HPPh2 on the vinylidene ligand as shown in Scheme 13. The ZIE ratio depends on the structures of the substrate and the catalyst. Ru-Cp" (Cp =77 -CsMes) species selectively forms the Z isomer while Ru-Cp (Cp r -CsHs) favors the E isomer. Since the key intermediate is the vinylidene species that has an electrophilic carbon, the reaction is applicable to other alkynes that are vinylidene precursors. Thus, phenylacetylene also reacts similarly to give Ph2PCH=CHPh ZIE=93I7), while internal alkynes are totally unreactive. 

P. Gonzalez-Herrero, B. Weberndorfer, K. Ilg, J. Wolf, and H. Werner, The Sensitive Balance Between Five-Coordinate Carbene Ruthenium Complexes and Six-Coordinate Carbyne Ruthenium Complexes Formed from Ruthenium Vinylidene Precursors, Organometallics 20, 3672-3685 (2001). 

Scheme 6. Routes to metal acetylides via direct deprotonation or isolation of vinylidene precursors and subsequent deprotonation.

The addition of a terminal alkyne to a Ru precursor with the objective of creating a metathesis catalyst is a known strategy. It has already been used by Grubbs to generate an efficient metathesis catalyst from [RuCl2(p-cymene)]2, a NHC and ieri-butylacetylene [68]. Indeed, when die reaction was performed with the catalytic system B, under the same conditions but with an acetylene atmosphere instead of an inert gas atmosphere, an active alkene metathesis catalyst was generated, and no cycloisomerization was observed. The metathesis products 75-78 were thus formed in 68-82% yield (Eq. 17) [66,67]. To understand this change of catalytic activity, the fast formation of a Ru vinylidene, precursor of Ru carbene species, is proposed [69]. 

Half-sandwich cr-enynyl complexes have also been synthesized by deprotonation of isolated monosubstituted alkenyl-vinylidene complexes. In this way, indenyl-Ru(ii) and indenyl-Os(ii) cr-enynyl derivatives 137 have been prepared in high yields by treatment of vinylidene precursors 136 with AI2O3 or KOBu under mild conditions (RT) (Equation (g)) 60,109,132,132a related CpRu(ii) complex [Ru(G=GC6H9)Cp(GO)(PPr 3)]... 

Scheme 4.16. The formation of titanocene-vinylidene precursors from alkynylalanes.

Most of the chemistry performed on orthometallated ylides has been carried out with Pd and Pt as metal centers. Few examples dealing with other metals (Co, Ru, and Os mainly, see 62 and 63) have been reported. Complex (74) has been prepared by reaction of a Ru-vinylidene with PPh3 [153] while Os derivative (75) has been obtained after treatment of the methylimido complex with PPh3 [154]. Orthoruthe-nated indenyl complexes [155] have been synthesized by reaction of the halomethyl precursors with PPh3, and the oxidative addition of the yUde Ph3P=CHC(0)H to clusters of Ru and/or Os also allows the synthesis of orthometallated complexes... 

The metal-ligand fragment L M, the number of carbon atoms x, and the substituents at the terminal sp -carbon may vary considerably and, correspondingly, the properties and reactivities. The early members of the series of cumulenylidene complexes (x=l, 2, 3 carbene, vinylidene and allenylidene complexes) have established themselves as invaluable building blocks in stoichiometric synthesis and as highly potent catalyst precursors. The higher members might potentially be very useful candidates for application as one-dimensional wires and in opto-electronic devices. 

None of the methods described in this chapter utilize a pre-formed metal vinylidene as an active catalyst precursor. The occurrence of metal vinylidene intermediates is instead inferred on the basis of product structure, isotopic labeling experiments, and computational studies. 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

Success was obtained with Ru3(CO)i2 as catalyst precursor [6], but the most efficient catalysts were found in the RuCl2(arene)(phosphine) series. These complexes are known to produce ruthenium vinylidene spedes upon reaction with terminal alkynes under stoichiometric conditions, and thus are able to generate potential catalysts active for anti-Markovnikov addition [7]. Similar results were obtained by using Ru(r]" -cyclooctadiene)(ri -cyclooctatriene)/PR3 as catalyst precursor [8]. (Z)-Dienylcarba-mates were also regio- and stereo-selectively prepared from conjugated enynes and secondary aliphatic amines (diethylamine, piperidine, morpholine, pyrrolidine) but, in this case, RuCl2(arene) (phosphine) complexes were not very efficient and the best catalyst precursor was Ru(methallyl)2(diphenylphosphinoethane) [9] (Scheme 10.1). 

Most of these catalytic systems are able to dimerize either aromatic alkynes, such as phenylacetylene derivatives, or aliphatic alkynes, such as trimethylsilylacetylene, tert-butylacetylene and benzylacetylene. The stereochemistry of the resulting enynes depends strongly on both the alkyne and the catalyst precursor. It is noteworthy that the vinylidene ruthenium complex RuCl(Cp )(PPh3)(=C=CHPh) catalyzes the dimerization of phenylacetylene and methylpropiolate with high stereoselectivity towards the ( )-enyne [65, 66], and that head-to-tail dimerization is scarcely favored with this catalyst. It was also shovm that the metathesis catalyst RuCl2(P-Cy3)2(=CHPh) reacted in refiuxing toluene with phenylacetylene to produce a... 

In parallel, since the first preparation of allenylidene-metal complexes in 1976, the formation of these carbon-rich complexes developed rapidly after the discovery, in 1982, that allenylidene-metal intermediates could be easily formed directly from terminal propargylic alcohols via vinylidene-metal intermediates. This decisive step has led to regioselective catalytic transformations of propargylic derivatives via carbon(l)-atom bond formation or alternately to propargylation. Due to their rearrangement into indenylidene complexes, metal-allenylidene complexes were also found to be catalyst precursors for olefin and enyne metathesis. 

Preparation of 18,18,18-trifluororetinal required synthesis of trifluorocyclocitral as precursor. For this purpose, the 1,4-addition of a cuprate (prepared from an a>-bromo-trifluoromethylcarbinol) onto a vinylidene phosphonate was performed. The alcohol moiety is then deprotected and oxidized into ketone. A further intramolecular Wittig-Horner reaction, followed by reduction, led to trifluorocyclocitral. 18,18,18-Trifluororetinal is then easily obtained from this compound (Figure 4.25). ... 

Dehydrochlorination of poly vinylidene chloride and chlorinated polyvinyl chloride was carried out. High chlorine content in the polymers (more than 60%) provides the formation of chlorinated conjugated polymers, polychlorovinylenes. The reactivity of chlorinated polyvinylenes contributes to the sp carbon material formation during heat treatment. Synthesis of porous carbon has been carried out in three stages low-temperature dehydrohalogenation of the polymer precursor by strong bases, carbonization in the inert atmosphere at 400-600°C and activation up to 950°C. 

Vinyl chloride has gained worldwide importance because of its industrial use as the precursor to PVC. It is also used in a wide variety of copolymers. The inherent flame-retardant properties, wide range of plasticized compounds, and low cost of polymers from vinyl chloride have made it a significant industrial chemical. About 95% of current vinyl chloride production worldwide ends up in polymer or copolymer applications. Vinyl chloride also serves as a starling material for Uie synthesis of a variety of industrial compounds. The primary nonpolymeric uses of vinyl chloride are in the manufacture of vinylidene chloride and tri- and tetrachloroethylene. 

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