P-Methyl transfer

Thermal treatment of (=SiO)Hf(CH2Bu )3 at increasing temperatures leads to the successive evoluhon of neopentane, isobutene and isobutane as well as several alkanes varying from Cj to C5. Polyisobutenes are also formed on the surface. The mechanism by which such decomposition occurs suggests a succession of y-H eliminations with formahon of neopentane followed by P-methyl transfer and formation of isobutene and [Hf]-Me (Scheme 2.14). This isobutene is reinserted into [Hf]-Me with formahon of isopentene and [Hf]-H. 

For examples of P-carbon elimination in late transition metal systems, Bergman et al. identified P-methyl transfer with four-membered ruthenacycles, which is driven by the formation of Ji-allyl and Ji-oxallyl complexes. Warming the solution of oxaruthenacycle 58 to 45°C led to formation of methane and cyclic enolate complex 60 [76]. ji-Oxallyl complex 59 initially arises from P-methyl... 

In the case of linear alkanes such as n-hexane, the he ion inserts into the central C-C bond followed by P-methyl transfer [Eq. (6.104)]. Again, these reactions presumably involve hypercarbon intermediates. 

P-Me = P-methyl transfer from a primary growing chain. 

A useful application in the manufacture of ion-exchange resins may well be possible which avoids the use of carcinogenic chloromethyl ether. Here, a polymer of p-methyl styrene is chlorinated on the side chain with aqueous NaOCl and a phase-transfer catalyst. Sasson et al. (1986) have shown how stubborn . substituted aromatics like nitro/chlorotoluenes can be oxidized to the corresponding acids by using aqueous NaOCl containing Ru based catalyst. 

The reaction of LTMP at 80° with 2,2,6,6-tetramethylcyclohexane (3) to give 4 evidently involves transfer of a 3-methyl of LTMP, since 6 is also formed during the reaction. The minor carbinol (5) of this reaction can be formed in quantitative yield when N-lithio-9-azabicyclo[3.3.1]nonane (7) is used as the base. This base lacks both p-hydrogens and p-methyl groups, but evidently can metalate a methyl group of 3 to form the anion b. The same paper also presents evidence for bridgehead metalation by LTMP. 

Workentin et al. (1994) described another interesting solvent effect on the competition between electron transfer and the addition reaction between organic cation-radicals and azides. TEE and AN were compared as solvents. In TEE, the cation-radicals of 4-methoxystyrene (R =R =H), P-methyl-4-methoxystyrene (R =Me, R =H), or p,p-dimethyl-4-methoxystyrene (R =R =Me) react with the azide ion according to the following equation ... 

An advantage of polymer-based reagents is that both the excess and the spent reagent are easily separated from the product. Bruno Linclau of the University of Southampton has reported J. Org. Client. 2004, 69,5897) the preparation of a polymer-bound carbodiimide. Exposure of the polymer to alcohol gives a family of O-alkylisoureas that smoothly convert carboxylic acids to the corresponding esters. Methyl, benzyl, ally and p-nitrobenzyl transfer smoothly. The polymeric (-butyl reagent could not be prepared. 

Indolmycin.—It has been shown that the C-methyl group in indolmycin (82) originates from the methyl group of methionine with inversion of configuration (cf. Vol. 8, p. 23). Previously published in preliminary form,71 the results are now available in a full paper.72 In addition, it has been shown that the A-methylation reaction which occurs in the course of the biosynthesis of indolmycin (82) also proceeds with inversion of configuration of the methyl group of methionine. Similar methyl-transfer with inversion has been recorded in the catechol-O-methyl-transferase reaction,73 and in this case it has been concluded that there is a tight SN2 transition state for the methyl-transfer.74... 

Shuikin (370) passed methyl and dimethyl cyclohexanes over nickel at 330-350°. In addition to the usual demethylation and dehydrogenation reactions, he found evidence of methyl transfer methylcyclohexane gave some p-xylene, while dimethylcyclohexane gave some trimethylbenzene. Platinum at these temperatures did not cause this methyl transfer. Plate and O. A. Golovina (306) reported that appreciable demethylation of 2,2,4-trimethylpentane took place over molybdena-alumina at 150-250°C. and was accompanied by the formation of small amounts of aromatics. 

Other processes also contribute to chain growth termination under special conditions. In particularly crowded catalysts, fi-methyl transfer to the metal centre can occur instead of p-H transfer. When other reaction paths are blocked, a-bond metathesis, i.e. transfer of an H atom from a monomer to the metal-bound alkyl C atom can release a polymer with a saturated chain end with formation of a new unsaturated metal-bound chain start. Saturated chain ends will also result when H2 gas is added to a catalyst system thus leading to the production of shortened polymer chains. Such an H2 addition will often also cause an increase in overall catalyst activity, since H2 will predominantly react with species - such as occasional 2,1-inserted units - which are rather... 

Cyclopolymerization of 2-methyl-l,5-hexadiene is catalyzed by a cationic zir-conocene complex [75]. Isolation of methylenecyclopentane derivatives 55-57 from the low molecular weight oligomeric products provides convincing evidence for chain transfer via P-methyl elimination. 

By means of C and H NMR, mainly propyl and 2-methyl-prop-1-enyl (vinylidene) end groups are detected in the polymers prepared by Cp2MtX2 (Mt = Zr, or Ti) catalyst. This indicates that the predominant monomer insertion is 1,2-insertion and that p-H elimination from the last inserted 1,2 unit is the major chain transfer pathway. " In PPs prepared with related catalyst systems (Cp )2ZrX2/MAO (Cp = ri -pentamethylcyclopentadienyl), wo-butyl and prop-1-enyl (vinyl) groups are found to be the major end groups, which suggests that chain transfer consists of predominant p-methyl elimination. ... 

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