Methyl-transfer agent

Three papers have appeared in which tetramethyltin is used as a methyl-transfer agent one involves an aryl triflate [151], one a vinyl triflate [152], and the third a sextuple coupling [153] (conversion of three PCI2 units in hexachlorocyclotriphosphazene into PMej units). 

Biological methylation of arsenic is effected via the transfer of a carbocation, a process known as oxidative methylation. The methyl transfer agent can vary from simple sulphonium salts, for example, 5 -adenosyl-.S -methionine (67)... 

Cation-JT interactions are also prominent at the active sites of enzymes involving cationic substrates. Key examples include the blood coagulation serine proteases Factor Xa and thrombin, and a number of enzymes that use S-adenosyhnethionine. a sulfonium ion that serves as nature s ubiquitous methyl transfer agent. A spectacular series of examples is the array of enzymes that catalyze the cationic cyclizations of polyenes in a key step of terpene and steroid biosynthesis. It is now clear that... 

The molecular weight of a polymer can be controlled through the use of a chain-transfer agent, as well as by initiator concentration and type, monomer concentration, and solvent type and temperature. Chlorinated aUphatic compounds and thiols are particularly effective chain-transfer agents used for regulating the molecular weight of acryUc polymers (94). Chain-transfer constants (C at 60°C) for some typical agents for poly(methyl acrylate) are as follows (87) ... 

Chain transfer to solvent is an important factor in controlling the molecular weight of polymers prepared by this method. The chain-transfer constants for poly(methyl methacrylate) in various common solvents (C) and for various chain-transfer agents are Hsted in Table 10. 

Hexachloroethane has been suggested as a degasifter in the manufacture of aluminum and magnesium metals. Hexachloroethane has been used as a chain-transfer agent in the radiochemical emulsion preparation of propylene tetrafluoroethylene copolymer (152). It has also been used as a chlorinating agent in the production of methyl chloride from methane (153). 

The molecular weight of the continuous phase is an important parameter that affects the mechanics and the melt flow of the end product. It can be controlled by the use of a suitable chain transfer agent (e.g., /er/-dodecyl mercaptan Ct = 4.0) or their combinations (e.g., primary mercaptans Or = 26.0 and dimeric a-methyl styrene Ct = 0.1) [132]. 

Roland et al. obtained 23% ee in the addition of Et2Zn to cyclohexenone using the silver(I) complex 78 having a chiral backbone and methyl groups on the nitrogen atoms. This complex acts as an efficient carbene transfer agent towards Cu(OTf)2. The conjugate addition proceeds rapidly in toluene at 0 °C (Scheme 52). 

The above explanation of autoacceleration phenomena is supported by the manifold increase in the initial polymerization rate for methyl methacrylate which may be brought about by the addition of poly-(methyl methacrylate) or other polymers to the monomer.It finds further support in the suppression, or virtual elimination, of autoacceleration which has been observed when the molecular weight of the polymer is reduced by incorporating a chain transfer agent (see Sec. 2f), such as butyl mercaptan, with the monomer.Not only are the much shorter radical chains intrinsically more mobile, but the lower molecular weight of the polymer formed results in a viscosity at a given conversion which is lower by as much as several orders of magnitude. Both factors facilitate diffusion of the active centers and, hence, tend to eliminate the autoacceleration. Final and conclusive proof of the correctness of this explanation comes from measurements of the absolute values of individual rate constants (see p. 160), which show that the termination constant does indeed decrease a hundredfold or more in the autoacceleration phase of the polymerization, whereas kp remains constant within experimental error. 

RAFT polymerization of two anionic acrylamido monomers sodium 2-acrylamido-2-methylpropane-sulfonate, AMPS, and sodium 3-acrylamido-3-methyl-butanoate, AMBA, (Scheme 29) was conducted in water at 70 °C using 4,4/-azobis(4-cyanopentanoic acid) as the initiator and 4-cyanopentanoic acid dithiobenzoate as the RAFT chain transfer agent [80]. The synthesis was initiated either from one monomer or the other leading to narrow molecular weight distributions in both cases (Mw/Mn < 1.2). 

The experiment with CS2 showed up another extremely interesting effect. Over almost the whole range of compositions the DPs obtained were very significantly greater than those obtained without carbon bisulphide - with methyl chloride as sole diluent. This CS2 effect has been reported previously for the cationic polymerisation of a-methylstyrene [57] and of isobutene [50]. It seems likely that it is due (at least partly) to the fact that CS2 does not act as a transfer agent, whereas most alkyl halides do. 

From the radioactivity of the polymer and of the original methyl chloride it was found that if there was not more than one 14C atom per polymer molecule, 0.27 of the polymer molecules contained a methyl group derived from the solvent. The authors concluded that the formation of this fraction of molecules had been started by initiation according to equation 10, and that the remainder had been started by transfer with monomer. It has been admitted by the authors (private communication, and in press) that this conclusion is not warranted on this evidence, since alkyl halides such as methyl chloride, are known to act as transfer agents by a reaction which can be represented by Equation 7. At best, the evidence shows that methyl chloride was involved in starting - by initiation and/or transfer - about a quarter of the polymer molecules. The results of further studies with 14CH3C1 and CH336C1 are in process of publication [12]. 

Two molecules of carbon monoxide were successively incorporated into an epoxide in the presence of a cobalt catalyst and a phase transfer agent [29]. When styrene oxide was treated with carbon monoxide (0.1 MPa), excess methyl iodide, NaOH (0.50 M), and catalytic amounts of Co2(CO)8 and hexadecyltrimethylammonium bromide in benzene, 3-hydroxy-4-phenyl-2(5H)-furanone was produced in 65% yield (Scheme 7). A possible reaction mechanism was proposed as shown in Scheme 8 Addition of an in situ... 

---