Scrambling

In situ MAS NMR spectroscopy has also been applied to characterize the scrambling in n-butene conversion on zeolite H-ferrierite (97), -butane conversion on SZA (98), n-butane isomerization on Cs2,5Ho,5PWi204o (99), -pentane conversion on SZA (100), isopropylation of benzene by propene on HZSM-11 (101,102), and propane activation on HZSM-5 (103-105) and on Al203-promoted SZA 

H/D exchange and scrambling occurring during the activation of small alkanes on solid acid catalysts is discussed in detail in Section V. 

Isotopic labeling is a powerful tool being used to understand the nature of transition states as well as the mechanisms of surface-catalyzed reactions. By in situ MAS NMR spectroscopy, the scrambling of labels at room temperature was investigated upon adsorption of p C-1]- 1-octene on calcined zeolite HZSM-5 (wsi/ 

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Because of the indistingiiishability of the electrons, the antisynnnetric component of any such orbital product must be fonned to obtain the proper mean-field wavefunction. To do so, one applies the so-called antisynnnetrizer operator [24] A= Y.p -lf p, where the pemuitation operator mns over all A pemuitations of the N electrons. Application of 4 to a product fiinction does not alter the occupancy of the fiinctions ( ). ] in it simply scrambles the order which the electrons occupy the ( ). ] and it causes the resultant fiinction... 

Figure C3.3.1 A collision between a milk tmck and a bread tmck showing the well ordered tmck contents at the top, the scattering event in the middle and the post crash scrambling of the tmck contents at the bottom.

Since the reaction conditions are mild in step 2 (only 6% as much time allowed as in step 1 at a lower temperature) and no catalyst is present, it seems unlikely that any significant amount of ester scrambling occurs. Isomerization of maleate to fumarate is also known to be insignificant under these conditions. 

Recently, the use of Hpase enzymes to iateresterify oils has been described (23). In principle, if a 1,3-speciftc Hpase is used, the fatty acid ia the 2 position should remain unchanged and the randomization occur at the terminal positions. However, higher temperatures, needed to melt soHd fats, may cause a 1,2-acyl shift and fatty acids are scrambled over all positions. 

Like P—O—C linkages, P—O—P linkages are susceptible to hydrolytic degradation. Scrambling or interchange usually occurs for phosphoms oxyesters at temperatures and acidities lower than those required for the carbon esters but greater than those for the sulfur esters. 

Selectivities to various isomers are more difficult to predict when metal oxides are used as catalysts. ZnO preferentially produced 79% 1-butene and several percent of i7j -2-butene [624-64-6] (75). CdO catalyst produced 55% 1-butene and 45% i7j -2-butene. It was also reported that while interconversion between 1-butene and i7j -2-butene was quite facile on CdO, cis—trans isomeri2ation was slow. This was attributed to the presence of a TT-aHyl anion intermediate (76). High i7j -2-butene selectivities were obtained with molybdenum carbonyl encapsulated in 2eohtes (77). On the other hand, deuteration using H1O2 catalyst produced predominantly the 1,4-addition product, trans-2-huX.en.e-d2 with no isotope scrambling (78). 

Frozen Egg Products. Frozen egg products include egg white, plain whole egg, whole egg with yolk added (ie, fortified), plain egg yolk, fortified whole egg with com symp, sugared egg yolk, salted egg yolk, salted whole egg, and scrambled eggs and omelets. Egg products are frozen in a blast freezer at —40 C for up to 72 h, and then held for storage at —24 C (see Refrigeration and refrigerants). They are used by large and small bakeries and for other uses. 

Specialty Dried Egg Products. A dried scrambled egg mix purchased for the U.S. military by USDA is a product having 51% whole egg, 30% skim milk, 15% vegetable oil, 2.5% salt, and 2.5% moisture. 

Many examples are known of rearrangement of azoles involving scrambling of the ring atoms to give a new isomeric azole molecule. Different mechanisms are involved. 

The systems discussed here are aromatic systems which undergo a variety of isomerizations on irradiation. Irradiation of imidazoles led to a scrambling of substituents, whereas such scrambling has not been observed in the pyrazoles which undergo photoisomerization to imidazoles. 

A book (B-71MS) and a review by Nishiwaki (74H(2)473) contain much information about the behaviour of pyrazoles under electron impact. The Nishiwaki review covers mainly the hydrogen scramblings and the skeletal rearrangements which occur. One of the first conclusions reached was that pyrazoles, due to their aromatic character, are extremely stable under electron impact (67ZOR1540). In the dissociative ionization of pyrazole itself, the molecular ion contributes about 45% to the total ion current thus, the molecular ion is the most intense ion in the spectrum. 

Another interesting fact is that hydrogen scrambling, i.e. randomization of the ring hydrogens of pyrazole to lose positional identity on electron impact, has not been observed to any significant extent (see however 780MS575). 

For the oxygen migration test for oxirene participation to be valid, it must be shown (80ZN(B)1040) that intermolecular oxygen transfer does not occur, and that oxygen scrambling... 

Dewar thiophenes i.e. 22 and 23) are intermediates in the photoisomerization of cyanothiophenes. Their presence has been demonstrated by trapping and by direct NMR observation (79CC881, 79CC966). The rapid sulfur walk i.e. 22- 23) fully explains the substituent scrambling in the room temperature irradiations (i.e. 21 - 24). 

At low temperatures unstable adsorption products or reaction intermediates could be trapped. Thus, carbonite CO, ions arise on CO interaction with basic oxygen ions which account for catalytic reaction of isotopic scrambling of CO or thiophene on activated CaO. 

Racemization, however, does not alwiys accompany isotopic scrambling. In the case of 5ec-butyl 4-bromobenzenesulfonate, isotopic scrambling occurs in trifluoroethanol solution witiiout any racemization. Two mechanisms are possible. Scrambling may involve an intimate ion pair in which the sulfonate can rotate with respect to the caibocation without allowing migration to die other face of the caibocation. The alternative is a concerted mechanism, which avoids a caibocation intermediate but violates the prohibition of front-side displacement. ... 

Along with the minimal barrier for H shift, the 2-butyl to t-butyl rearrangement gives the energy surface shown in Fig. 5.9. This diagram indicates that the mechanism for C-3/C-4 scrambling in the 2-butyl cation involves the edge-protonated cyclopropane intermediate. 

Fig. 5.9. Energy profile for the scrambling and rearrangement of 4119 cation. A H-bridged B methyl-bridged C Edge protonated methycyclopropane D classical secondary E classical primary F tertiary. Adapted from refs 120 and 121.

A series of 0-labeIed sulfonate esters was prepared, and the extent of scrambling... 

A detailed study of the solvolysis of L has suggested the following mechanism, with the reactivity of the intermediate M being comparable to that of L. Evidence for the existence of steps ki and k 2 was obtained fiom isotopic scrambling in the sulfonate M when it was separately solvolyzed and by detailed kinetic analysis. Derive a rate expression which correctly describes the non-first-order kinetics for the solvolysis of L. 

Compound 1 undergoes rearrangement to 2 in SO2 at — 66°C. The deuterium label becomes imiformly scrambled among all the carbon atoms in 2. 

Two techniques, electrochemical reduction (section IIl-C) and Clem-mensen reduction (section ITI-D), have previously been recommended for the direct reduction of isolated ketones to hydrocarbons. Since the applicability of these methods is limited to compounds which can withstand strongly acidic reaction conditions or to cases where isotope scrambling is not a problem, it is desirable to provide milder alternative procedures. Two of the methods discussed in this section, desulfurization of mercaptal derivatives with deuterated Raney nickel (section IV-A) and metal deuteride reduction of tosylhydrazone derivatives (section IV-B), permit the replacement of a carbonyl oxygen by deuterium under neutral or alkaline conditions. 

Two serious drawbacks of this method are the extensive deuterium scrambling around the reaction site and the occasional formation of olefinic side products, which are hard to separate by conventional means. The extent of olefin formation may depend on the nature of the Raney nickel since it is known that desulfurization with deactivated Raney nickel can yield olefins. Best results are obtained when the deuterated Raney nickel is prepared very rapidly and used immediately after preparation. 

The successful preparation of Il,ll-d2-5l5-androstan-3o -oh and 11,11-d2-(25R)-5a-spirostane in 83% and 91% isotopic purity by Raney nickel treatment of the 11,1 l-d2-12-ethylene thioketal derivatives further confirms the low degree of isotope scrambling with C-12 keto steroids. 

Three different methods have been discussed previously (sections III-C,III-D and IV-A) for the replacement of a carbonyl oxygen by two deuteriums. However, in the conversion of a 3-keto steroid into the corresponding 3,3-d2 labeled analog, two of the three methods, electrochemical reduction (section ni-C) and Raney nickel desulfurization of mercaptal derivatives (section IV-A), lead to extensive deuterium scrambling and the third method, Clemmensen reduction (section III-D), yields a 2,2,3,3,4,4-dg derivative. 

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