Vinyl bond, hydrogenation

Hydrogenation of polybutadiene converts both cis and trans isomers to the same linear structure and vinyl groups to ethyl branches. A polybutadiene sample of molecular weight 168,000 was found by infrared spectroscopy to contain double bonds consisting of 47.2% cis, 44.9% trans, and 7.9% vinyl. After hydrogenation, what is the average number of backbone carbon atoms between ethyl side chains ... 

In marked contrast to the above results, double nitrile insertion into both the titanium-alkyl and titanium—vinyl bonds occurs to form the diazatitanacycles 74. Treatment of these titanacycles with dry hydrogen chloride affords the tetrasubstituted pyridine derivatives 75 (Scheme 14.32) [74], On the other hand, 2,3-diphenyltitanacyclobutene reacts with various nitriles to afford the products of mono-insertion, which afford the corresponding unsaturated ketones upon hydrolysis [73,74]. 

The relatively acidic nature of the vinylic anomeric hydrogen in glycals is at the origin of most of the protocols for C-1-C-1 bond formation in C-1... 

Enhanced acidity also characterizes the sp -carbon bonded hydrogen compared to the sp one. As a consequence, alkyl vinyl ethers are more acidic than dialkyl ether and thus can be deprotonated more easily. 1-Ethoxy-1-lithioethene 54252-254 gener-... 

In perfluorinated trialkylethenes the single vinylically bonded fluorine is hydrogenolyzed with hydrogen over palladium though only partially, but with complete regioselectivity.35 In contrast, in perfluorinated tetraalkylethenes allylic fluorine is eliminated while the hydrogen atom attacks the double-bonded carbon.36... 

Equilibrium Constants for Phenol-Vinyl Acetate Hydrogen Bonding ... 

The oxidative addition to palladium reagents proceeds readily with 1-haloalkenes at room temperature. However, the oxidative addition reactions of halides other than vinyl or aryl usually are very sluggish. Moreover, alkyl-Pd(II)-X complexes in which the alkyl moiety contains an sp -bonded hydrogen at the [3-position may undergo rapid dehydropalladation by. syn- 3-hydrogen elimination, generating the hydridopalladium complex and a double bond. Thus, the substrates used for the oxidative addition reaction are usually restricted to vinyl and aryl halides and triflates. 

In this case, the strongest C-H bond is the vinyl carbon/hydrogen bond, and this bond is practically never broken. It is the allylic hydrogen that is abstracted, because this carbon/hydrogen bond is relatively weak. Furthermore, cleavage of this bond results in a radical that is stabilised by delocalisation. 

Several mechanisms for curing gels are possible however, most have limitations involving either processing or the final gel properties. Condensation type cures form water or alcohol by-products which cause outgassing and voids. Free radical peroxide-activated addition cures make it difficult to control gel consistency from batch to batch. These problems are not evident in the most prevalent cure mechanism used today, the addition of silicon-bonded hydrogen atoms to silicon-bonded olefinic radicals, usually vinyl, in the presence of a few parts per million of a platinum catalyst (I). This system creates no by-products and is easily controlled. 

The ideal mixture would have an equal molar amount of silicon-bonded vinyl and silicon-bonded hydrogen so that the desired consistency is obtained at cure without excess vinyl or hydrogen remaining. Unfortunately, this idealized situation is very difficult to achieve. Without exception, some of one or both will remain unreacted. Even in the idealized case, not all vinyl and hydrogen groups would be in a position to react with each other once cross-linking began to restrict mobility of the polymer chains. 

The pure rranj-olefin can be obtained from a cis trans mixture, since a cis-disub-stituted double bond hydrogenates more rapidly than the frans-isomer over Pd , and to a lesser extent over Pt. Rhodium-alumina catalyzes the hydrogenation of vinylic and allylic halides with minimal hydrogenolysis, e.g. ... 

In the synthesis of ( )-D-homoestrone (Scheme 7), Michael addition of the monoketal 42 of the Wieland-Miescher enedione to 6-vinyl-2-methylpyridine 41 led to tricyclic adduct 43 in good yield. Reduction to the 17a-P-alcohol, double bond hydrogenation, and ketalization produced 44 with the requisite 8 J,14a-stereochemistry in 58% yield. However, modification of the hydrogenation conditions from ethyl acetate-triethylamine to ethanol-perchloric acid raised the yield to 82%.Birch reduction, hydrolysis, cyclization, and ketal reversal... 

The secondary vinylic carbon-hydrogen bond in 2-propenal-2-yl radical, C CJC 0, resulting from the dissociation of CH2=C(CH=0)—H has a bond strength of 112.72 kcal moT. This BDE is similar to the primary vinyl bond energy of ethylene C=C—H, given by Lineberger et al. [135] to be 111.2 kcal moT, and 7 kcal mol stronger than the secondary vinyl C—bond... 

The first diastereoselective synthesis of methyl (3R,7R)-jasmonate is attributed to Gerhard Quinkert. [94] In the initial step, starting from bis-(8-phenyhnen-thyl) malonate, a vinylcydopropane is constructed. After removal of the chiral auxiliary, the cyclopentanone is built up by means of a domino homo-Michael reaction / Dieckmann cydisation. Attack of dimethyl pent-2-ynylmalonate leads to inversion at the stereogenic centre at the vinyl cyclopropane. After decarboxylation, the vinyl residue is transformed into an ester group and the triple bond hydrogenated with a Lindlar catalyst. 

The spectra shown in Figure 9 indicate that there was a noticeable reduction of the absorbance of 1,2-vinyl, then 1,4-trans followed by 1,4-cis and the absorbance corresponding to styrene remains constant. In previous work (Escobar et al, 2000) it was shown that kinetics of hydrogenation is higher for 1,2-vinyl bonds than for 1,4-trans and 1,4-cis bonds. 

The 1,2-vinyl bonds are pendant from the backbone and, therefore, they are more accessible to hydrogen. The FTIR is a useful characterization technique in this case, since it was possible to establish the hydrogenation selectivity toward the 1,2-vinyl bonds and showed that polystyrene remained unsaturated during hydrogenation. 

An iodine-catalyzed intramolecular oxidative thiolation of vinylic carbon-hydrogen bonds via tandem iodocyclization and dehydroiodination has also been developed for the construction of 2-methylene-3-thiophe-nones (14ASC743).The synthetic strategy was extended to the preparation of 2-methylene-3-benzothiophenone via the cyclization of o-methylthio-phenyl vinyl ketones. 

We see that an allylic carbon—hydrogen bond of propene is broken with greater ease than even the tertiary carbon-hydrogen bond of isobutane and with far greater ease than a vinylic carbon-hydrogen bond ... 

Organolead hydrides are more reactive towards C=C and C=C bonds than are the corresponding tin hydrides. Therefore, it is very easy to add lead hydrides completely to unsaturated compounds by placing the latter into the mixture of RsPbX and organotin hydrides. Compare Section III, D,2. An analogous exchange of vinyl and hydrogen... 

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