Scrambling process

Table III presents additional cyclopentene polymerization data with 1-pentene as a regulator at 0°C throughout a wide range of conversion. As long as the cis selectivity is maintained, the regulator remains inactive and does not participate in the scrambling process.

Note The pictorial term scrambling is used in mass spectrometry to describe rapid processes of (intramolecular) positional interchange of atoms. Scrambling may occur with hydrogens or may involve the complete carbon skeleton of an ion. Aryl radical ions and protonated aryl compounds are well known for their numerous scrambling processes. [54,55]... 

Recent experimental efforts by Yuan Lee s group have been focused on accessing the structure of CHj indirectly by attachment of a neutral molecule like H2 or One may assume that the attachment mainly restricts the scrambling process but does not significantly alter the structure of CH5. Using the well-characterized structures for Hj, CH4 and H2 as a reference for, e.g., the shifts in vibrational frequencies, one should be able to deduce the CHj structure. On the basis of the present study, we will discuss the effect of molecular hydrogen attachment to... 

Saunders and co-workers have recently reported results strongly supporting the protonated cyclopropane intermediate in the isotope scrambling process.114 They found no isomerization of the 2-butyl-1,2-13C2 cation (19a) at —78°C, whereas the isotopomer 2-butyl-2,3-13C2 cation (19b) showed rapid formation of all other iso-topomers except 19a (Scheme 3.2). These results are consistent with the involvement of protonated cyclopropane with the interchange of either C(l) and C(2) or C(3) and C(4) with the breaking of only the C(2)-C(3) bond. 

During the course of our studies (30-32) of the synthesis and structures of rhodium acetylide and hydrido-acetylide complexes, we developed (32) a step-wise route to rrans-bis(acetylides) of the general form mer-rra/tf-[Rh(PMe3)3(H)(C=CR) (OCR )]. Unfortunately, scrambling processes have thus far precluded the preparation of the unsymmetrically substituted complexes (R R ) in the absence of... 

Of course, the mechanisms of these enzymic processes are at present unknown. G. Lowe and B. S. Sproat (78) have suggested that the action of pyruvate kinase involves monomeric metaphosphate as an intermediate. His evidence concerns the effect of the enzyme in scrambling the oxygen atoms of ATP in the absence of pyruvate, Equation 15. This scrambling process is enzyme catalyzed, and is interpreted most easily, but not exclusively, in terms of the reversible formation of monomeric metaphosphate. 

Scheme 13. Proposed mechanisms for ligand scrambling processes in complexes Ni(ArNNNNAr)2- Adapted with permission from J. Chem. Soc., Dalton Trans., p. 1541 (1982).

It has been shown, however, that in the copolymerization much more reactive 1,3-dioxolane polymerizes first and is practically completely consumed at still low conversion of 1,3,5-trioxane [139]. Thus, by polymerization alone, it would not be possible to achieve the random distribution of 1,3-dioxolane units along the chain. Both analysis of the microstructure of the chain [140], as well as thermal behavior of the copolymer [141], indicate, however, that nearly random distribution of comonomer units is indeed attained by scrambling process similar to that described earlier for sequential polymerization of 1,3-dioxolane and 1,3-dioxepane. 

The acetate formed from [395] showed 26.1 3% D at C(9) while that produced from the mixture of [395] and [396] showed only 12.3 3%. The scrambling process under these conditions is clearly stereoselective if not entirely stereospecific. The amount of deuterium found at C(9) was concluded to indicate that about four Wagner-Meerwein shifts had occurred in the overall solvolysis. 

Similar results are observed in the reaction of a disilanyllithium with a chlorodisilane (28), as shown in Scheme 3. When a mixture of monosilyl and disilanyl anions is introduced to l-chloro-2-phenyltetramethyldisilane, the usual coupling products, 1,3-diphenyltrisilane and 1,4-diphenyltetra-silane, are formed. Alternatively, when the chlorosilane is added into a mixture of the anions, the scrambling processes via the Si-Si bond cleavage occur, to give a mixture of linear and cyclic oligosilanes. These results indicate that the scrambling processes are very fast, but much slower than the coupling of anions with chlorosilanes (see also Section VI-B). 

Figure 9. New pathway for fluxional interconversions of substituted icosahedra via tetracapped cuneane and tetracapped cubane intermediates (or transition states). This proposed scrambling process, which maximizes connectivity and obeys electron-counting rules, is illustrated for the interconversion of a 1,2-disubstituted ort/io-icosahedron into a 1,7-disubstituted meta-icosahedron.

Nelson, J. E., Bercaw, J. E., Labinger, J. A. An unexpected isotope scrambling process accompanies hydrozirconation of styrene. Organometallics 1989, 8, 2484-2486. 

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