CH3 Elimination

Poor molecular weights of polymer products is a major drawback of metaUo-cene-based polymerization catalysts, limiting them from being fully exploited for practical applications. However, technical breakthroughs have been achieved in that specially designed zirconocene catalysts (Spaleck et al., 1992) can produce polymers with molecular weights far above 100,000. 

A unique feature of polymer products obtained with homogeneous metallocene-based catalysts is their narrow molecular weight distribution (MWD). Thus the values of for polyethylenes and polypropylenes are typically near 2 which 

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Under field ionization kinetic conditions (FIK) the process of CH3 elimination from ionized 27 is not observed at t < 10-10 sec. This is a surprising result bearing in... 

Taylor et al. [104] investigated the reaction of hydroxyl radicals with acetaldehyde in a wide temperature range using a quantum RRK model to describe the competition between addition and abstraction. They conclude that different reaction mechanisms occur, depending on the temperature, and that OH addition followed by CH3 elimination is the dominant reaction pathway between 295 and 600 K. Moreover, they claimed that the H-atom elimination pathway is largely insignificant, except possibly at the lowest temperatures. 

Fluorination of carbanionsThese reagents selectively fluorinate a broad variety of carbanions in fair to good yield. Oxygen and nitrogen anions are not affected. The base can be NaH, KH, or n-BuLi, although lithium enolates are less reactive than potassium enolates. Nonpolar solvents are preferable to DMF, THF, or ether. Strongly basic anions can effect (3-elimination of HF from 1 when R = CH3. Elimination is suppressed when R = neopentyl or norbomyl. 

Turner and co-workers and Bochmann el al. have reported that propylene is polymerized by in situ generated base-free cations Cp2M(R) (49,51). In contrast, Cp 2M(CH3)(THT) (M = Zr, Hf) complexes catalyze propylene oligomerization under mild conditions (PhNMe2, 25°C, 1 atm) by an insertion/)J-CH3 elimination process (Scheme 8) (53). The conventional J3-H elimination chain transfer process is disfavored by severe steric interactions between the C)J-substituents and the bulky Cp ligands in the transition state (Scheme 8). The Hf system produces only Cs and C, products whereas the Zr system produces oligomers up to C24 under these conditions. 

The formation of the OH-ethene adduct is 32 kcal mole exothermic (76, 170) (formation of other OH-alkene adducts have similar exothermlcltles), and elimination of an H atom Is endothermic by <7 kcal mole l hence the only thermochemlcally favorable decomposition pathway for the adduct Is back to the reactants. For the larger alkenes, decomposition of the adduct Into fragments other than the reactants becomes possible. Thus, for propene It Is thermochemlcally possible for CH3 elimination to occur, although this will probably occur only after OH-radlcal addition at the central carbon atom (see the mechanistic discussion for the haloalkenes). 

Based on H NMR analysis of low molecular weight polypropylene formed by 40b/MAO at 25 C with a propylene concentration of 0.5 M (in toluene solution), the predominant chain termination pathway appears to be f)-H elimination (i.e., vinylidene end groups are present). The primary kinetic isotope effect observed for 35b/MAO (vide supra) is also indicative of P-H elimination operating as the predominant chain termination pathway. No fl-CH3 elimination was observed for these catalysts specifically, no vinylic end groups are present in the H NMR spectra. For 35a-c and 40c, chain transfer was found to be primarily unimolecular, derived from f)-H elimination. For 40b, a small bimolecular contribution (chain dansfer to monomer) was also suggested, owing to the less than first order dependence of molecular weight on propylene concentration. 

P-CH3 Elimination This chain transfer mechanism has been discovered in the polymerization of propylene. The metal center abstracts a CH3 group, instead of a H atom, at the S-carbon of the growing chain, thus forming a M-CH3 bond at the active center and leaving a polymer with an allyhc end group (Resconi et al., 1992) ... 

The remainder of the product alkoxy radicals are thermalized by collisions and then either decompose via CH3 elimination, or react with O2 ... 

Like the reaction of tert butyl alcohol with hydrogen chloride step 2 m which tert butyloxonium ion dissociates to (CH3)3C and water is rate determining Because the rate determining step is ummolecular the overall dehydration process is referred to as a ummolecular elimination and given the symbol El... 

Both CH3CH2CH2C=CH and CH3CH2C=CCH3 can be prepared by alkylation of acety lene The alkyne (CH3)2CHC=CH cannot be prepared by alkylation of acetylene because the required alkyl halide (CH3)2CHBr is secondary and will react with the strongly basic acetylide ion by elimination... 

CH3 1.83 9 other linkages are eliminated on the basis of the molecular formula. 

Similarly, von Braun and Schmitz found that A-benzoylconiine, on distillation with phosphorus pentachloride, underwent ring scission with elimination of the nitrogen atom and formation of the dichlorooctane, CH2CI. (CH2)3. CHCl. CHa. CH. CH3. 

Tetraphenylmolybdenocene dihydride Mo(r 5-C5HPh4)CpH2 (45) was formed by addition of diphenylacetylene to MoCpL(PhC CPh)CH3 (L = P(OMe)3) (Eq. 15), presumably via an ot-hydrogen abstraction to an intermediate methylidene hydrido complex, followed by addition of two equivalents of diphenylacetylene and C — H insertion with concomitant elimination of L [57 b],... 

Neutral Loss Only a limited number of neutral fragments of low mass which are eliminated in decompositions of molecular ions. Examples are H, H2, CH3 and OH. Therefore, the presence of a major ion below the molecular ion at an improbable interval (eg, loss of 4 to 14, 21 to 25 amu) will indicate that the latter is not the molecular ion Postulation of Molecular Structures The. postulation of the structure of an unknown molecule is based on several major kinds of general structural information available in the mass spectmm. McLafferty (Ref 63) suggests the following systematic approach ... 

Another helpful test is the so-called crossover experiment. Thus, a mixture of di-Pd(CH3) (PR3)2 and cA-Pd(CD3)2(PR.3)2 yields only C2D6 and C2H6 C2H3D3 is absent.20 This result shows that the elimination reaction is intramolecular. 

Reaction of sulfones, such as CH3S02(CH2) C00Et (n = 2-5), with sodium ethoxide were shown to result in elimination when n = 2 and in cyclization when n = 3-5. By this method, tetrahydrothiapyran-3-one-1-dioxide 265, 2-methylsulfonylcyclopentanone 266 and 2-methylsulfonylcyclohexanone 267 were obtained347. Cyclization of co-cyanosulfone, CH3S02(CH2) C(CH3)2CN, gave 4,4-dimethyltetrahydrothiophene-3-one dioxide 268 when n = 1,2,2-dimethyl-5-methylsulfonylcyclopentanone 269 when n = 3 and 2,2-dimethyl-6-methylsulfonylcyclohexanone 270 when n = 4348. 

The surface of glass contains many —OH groups that are bonded to the silicon atoms of Si02, which is the major component of glass. If the glass is treated with Si(CH3)3Cl (chlorotrimethylsilane), a reaction takes place to eliminate HC1 and attach the Si atom to the oxygen atom ... 

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