Vinylidenes, formation

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

When the electrophile is an alkyl halide, a C—C a-bond is forged thus, alkenylidene formation is irreversible. Vinylidene formation by 1,2-migration, on the other hand, is generally reversible. Because of this contrast, alkenylidenes can offer access to new catalytic reaction manifolds, in addition to unique molecular architecture. 

Cp Ru [14] and TpRu [20] complexes have also been studied in depth. As represented in Scheme 2c, the catalytic alkyne dimerization proceeds via coordinatively unsaturated ruthenium alkynyl species. Either a direct alkyne insertion and/or previous vinylidene formation are feasible pathways that determine the selectivity. The head-to-tail dimer cannot be formed by the vinylidene mechanism, whereas the E or Z stereochemistry is controlled by the nature of the alkynyl-vinylidene coupling. 

Saa, C. (2009) Cydoisomerization of aromatic homo- and bis-homopropargylic alcohols via catalytic Ru vinylidenes formation of benzofurans and isochromenes. Organic Letters, 11, 5350-5353. 

Secondly, it should be noted that virtually all of the reactions involve disub-stituted alkynes only one example of the cleavage of a terminal alkyne, that of Me3SiC=CH at a tricobalt center, had been reported by Vollhardt before our work detailed below. This is clearly because in most cases other more favorable processes, such as oxidative addition or vinylidene formation, intervene. 

Abstract The cycloisomerization of enynes can be performed with a variety of catalytic systems which operate according to various types of activation process. Metal catalysts showing different activation abilities can promote as the first step either oxidative coupling, triple bond insertion into metal-hydride bond, allylic activation, electrophilic activation, metal-vinylidene formation, and [2+2]-cycloaddition (metathesis). Depending on the nature of the enyne and the catalytic system, all these processes lead to a variety of cyclic compounds. 

Scheme 6.20.2 Back-biting mechanism and vinylidene formation in the free-radical ethene polymerization process leading to the characteristic structural features of LDPE.

On using CpRuCl(PPh3)2 as catalyst in pyridine as solvent it was possible to synthesize not only indoles from aromatic homopropargylic amides but also 1,2-dihydroisoquinolines and 1,4-dihydroquinoline from bis-homopropargylic amides (Scheme 22) [121, 122]. Surprisingly, cyclization of the latter amides under other typical conditions used for vinylidene formation,... 

It has been shown52 that under similar conditions reduction of the nitrile groups in cellulose ethyl cyanate and of those in the copolymer of vinylidene cyanide with vinyl acetate, proceed simultaneously in two directions with the formation of aldehyde and amine groups. g+ g ... 

EPDM backbone, together with the carbonyl and ether groups generated on the rubber itself causes the carbonyl and ether absorptions to increase. The absorption at 1630 cm of the fraw -vinylene bonds of TMPTA is mainly due to the grafting of TMPTA onto EPDM and partly due to the formation of vinylidene bonds at the chain ends on irradiation. The absorptions at 1460 and 1379 cm due to >CH2 scissoring vibration and —CH3 stretching vibration, respectively, result from the rubber and the grafted TMPTA. 

In 1998, Wakatsuki et al. reported the first anti-Markonikov hydration of 1-alkynes to aldehydes by an Ru(II)/phosphine catalyst. Heating 1-alkynes in the presence of a catalytic amount of [RuCljlCgHs) (phosphine)] phosphine = PPh2(QF5) or P(3-C6H4S03Na)3 in 2-propanol at 60-100°C leads to predominantly anti-Markovnikov addition of water and yields aldehydes with only a small amount of methyl ketones (Eq. 6.47) [95]. They proposed the attack of water on an intermediate ruthenium vinylidene complex. The C-C bond cleavage or decarbonylation is expected to occur as a side reaction together with the main reaction leading to aldehyde formation. Indeed, olefins with one carbon atom less were always detected in the reaction mixtures (Scheme 6-21). 

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

Although the stoichiometric production of 9 and 10 was initially disappointing, it is consistent with the formation of a vinylidene ligand from 1-hexyne (see C, Scheme 3), and attack of a pendant heterocyclic nitrogen on C-l of the vinylidene. In this process, the heterocyclic nitrogen approaches C-l. Thus, we decided to slow the rate of heterocycle approach by adding a bulky substituent next to the nitrogen. 

V. Formation of Hydride-Vinylidene Complexes by Addition of Terminal... 

FORMATION OF HYDRIDE-VINYLIDENE COMPLEXES BY ADDITION OF TERMINAL ALKYNES TO OsHCI(CO)(P Pr3)2... 

This stretching, similar to the drawing of fibres, which promotes orientation and crystallite formation, is called biaxial orientation. It gives the film added strength and gas-barrier properties. In some processes, monoaxial (uniaxial) drawing is employed, e.g., polypropylene, which is then slit into thin strips and fabricated into heavy duty sacks, carpet backing, etc. The stenter process is used to make biaxial oriented poly(vinylidene dichloride) ("ding" film), polyester, polyamide and polypropylene films. 

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