Possible expansion mechanism

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Since the carboxonium ion has been eliminated as a possible propagating species, one is left with two alternatives which we may call the Keele and the Mainz theories. Plesch and Westermann [6, 8] have suggested that the cyclic formals polymerise by a ring-expansion mechanism, in which no free end is ever formed. This is illustrated in Reaction (B), where Y = H if the initiator is a protonic acid, and Y = Et if the initiator is a... 

Later, /J-lactams have been obtained by regiospecific metal-catalyzed ring expansion of aziridines. Treatment of A -t ri-butyl-2-phenylaziridine (252) with carbon monoxide in benzene, employing chlorodicarbonylrhodium(I) dimer as the catalyst at 90°C and 20 atm, produced the azetidinone 253 in quantitative yield. Several other )S-lactams were also synthesized from the corresponding aziridine derivatives by the application of this method, and it was noted that this reaction was completely regiospecific. The possible reaction mechanism for the formation of fi-btciams is outlined in Scheme 47. 

Miscellaneous Cranes.— Wall and jib cranes, whether stationary or of the traveling type, hoists of various kinds and all other varieties of cranes lend themselves to economic selection and analyses as to probable net cost of operation along lines very similar to those followed in the case of overhead electric cranes. A full knowledge of operating requirements is necessary, suitable provisions should be made for possible expansion, and the mistake avoided of assuming too great a mechanical efficiency for the equipment. An intimate knowledge of costs— labor and equipment—and depreciation expenses is also required for even such approximate estimates, if they are to be at all reliable. 

Since our knowledge of the reactions to be discussed is in its early stages, experimental verification of mechanisms is frequently lacking. A valency expansion mechanism is considered to be the most probable pathway for all of the examples included. Nevertheless, it is possible that in a specific case another mechanism may be operative, alternately or even exclusively. Thus it has been demonstrated that the reaction of trialkyl phosphite with a disulfide may proceed either through an ionic (124,125) or a free radical (300) mechanism. 

In contrast, the force of interaction between two ions is long-range and at large distances is proportional to 1/r, where r is the distance between the ions. Thus the solution cannot be considered to be composed of noninteracting clusters, and power series expansions in concentration are not possible. Statistical mechanical treatments of this problem demonstrate that the coefficients of the power series expansions diverge for coulomb forces and that another representation for the properties of the solution must be found. The rigorous molecular considerations confirm the results of the Debye-Huckel treatment for dilute solutions and demonstrate that the assumptions of the Arrhenius hypothesis are incorrect. 

These reamers have also more than one flute and have an incorporated expansion mechanism. This mechanism was developed to compensate the tool wear during the cutting process. The diameter of the tool can be expanded by an internal cone to readjust the requested tool diameter. The expansion mechanism works in the area of elastic deformation of the tool body material. The expanding mechanism is always of high precision. An adjustment of the tool diameter within 2 pm is possible. The targeted hole tolerance is IT6 and better. 

The mechanisms of these ring expansions are not certain. Both concerted" and diradical" pathways have been proposed, and it is possible that both pathways operate, in different systems. 

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