Shift mechanisms for

Figure 11.16 Proposed NIH shift mechanism for hydroxyla-tion of the azinomycin naphthoate.

Scheme 3 n-allyl shift mechanism for double bond migration... 

Fig. 2.2.S.3 Structure of rhamnose isomerase, and proposed hydride-shift mechanism for the ketol isomerization.

Fig. 17. A possible transition state structure for the bridge-shift mechanism for cis-trans equilibria of XL3M(OH)2ML3Y + species shown for X = Y = H20.

The process occurring here is reminiscent of the N.I.H. shift, which is well known to occur in iron hydroxylases such as cytochrome P-450 and mammalian PAH [1,167], For example, action of PAH on [4-3H]phenylalanine produces >90% [3-3H]tyrosine. Here, a presumed electrophilic iron-oxy species produces a carbonium ion intermediate from which a 1,2-shift occurs, giving a resonance stabilized cation rearomatization through loss of H+ (or 3H+) gives the observed product as a result of a heavy atom isotope effect. Thus, it appears that the N.I.H. shift mechanism for copper has been discovered for a chemical model system prior to its observation in proteins. 

Scheme 12 Proposed hydride-shift mechanism for the Wacker oxidation of styrene catalyzed by (I Pr)Pd(OTf)2...

Schrauzer and Sibert [74] have proposed a 1,2-hydride shift mechanism for this reaction (Eqn. 38 for hydroxyethylcobalt)... 

KIRSCHNER ET AL. Equilibrium Shift Mechanism for the Pfeiffer Effect 305... 

For the isomerization of 1-butene and 2,3-dimethyl-l-butene catalyzed by cationic Pd complexes, such as Pd(MeCN)2(BF4)2, an alternate mechanism involving carbocationic intermediates shown in Scheme 8 has been proposed and experimentally supported. While the modified mechanism appears to be plausible for the reactions studied, it is not clear how generally observable this mechanism nught be. It seems reasonable to retain both 1,2- and 1,3-H shift mechanisms for other isomerization reactions. 

A mechanism for the formation of these three alkenes is shown m Figure 5 9 Dissociation of the primary alkyloxonmm ion is accompanied by a shift of hydride from C 2 to C 1 This avoids the formation of a primary carbocation leading instead to a sec ondary carbocation m which the positive charge is at C 2 Deprotonation of this carbo cation yields the observed products (Some 1 butene may also arise directly from the pri mary alkyloxonium ion)... 

A reasonable mechanism for this observation assumes rate determining ionization of the substrate as the first step followed by a hydride shift that converts the secondary carbocation to a more stable tertiary one... 

The time-temperature superpositioning principle was applied f to the maximum in dielectric loss factors measured on poly(vinyl acetate). Data collected at different temperatures were shifted to match at Tg = 28 C. The shift factors for the frequency (in hertz) at the maximum were found to obey the WLF equation in the following form log co + 6.9 = [ 19.6(T -28)]/[42 (T - 28)]. Estimate the fractional free volume at Tg and a. for the free volume from these data. Recalling from Chap. 3 that the loss factor for the mechanical properties occurs at cor = 1, estimate the relaxation time for poly(vinyl acetate) at 40 and 28.5 C. 

The many options available for lift trucks f l into two classes vehicle specialties, which include controls, transmissions, guards, etc. and accessories, which are devices that handle specific types of loads (Fig. 21-55). Included in this second category are high-lift masts, up to 7 m (24 ft) handhng attachments for circular products, such as drums and roll goods attachments such as carton clamps and the fork side-to-side shifting mechanism. 

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. 

Shuttle shifters that separate the plates one by one for cake discharge at a rate of 5-6 seconds per plate. A special design of the shifting mechanism ensures that two adjacent plates are not pulled together due to sticky cakes. 

In Problem 5.17 (Section 5.13) we saw that acid-catalyzed dehydration of 2,2-dimethyl-cyclohexanol afforded 1,2-dimethylcyclohexene. To explain this product we must write a mechanism for the reaction in which a methyl shift transforms a secondary carbocation to a tertiary one. Another product of the dehydration of 2,2-dimethylcyclohexanol is isopropylidenecyclopentane. Write a mechanism to rationalize its formation. 

We have proposed an initiation mechanism for the ROOH-amine system in which some H-bond complex may be formed [36]. Then Sun et al. [37,38] thoroughly investigated the initiation mechanism of ROOH-Amine through IR spectra of TBH-triethylamine, TBH-DMT, and CHP-DMT. From the wideness of the shift of OH absorption bands at 3120, 3336, and 3257 cm were... 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

The mechanism for aldehyde-derived enamines involves a Michael-type 1,4-addition of the enamine to the alkenylcarbene complex to generate a zwit-terionic intermediate which evolves to the final product by cyclisation. On the other hand, ketone-derived enamines react through an initial 1,2-addition to the carbene carbon to generate a different zwitterionic intermediate. Then, a [l,2]-W(CO)5 shift-promoted ring closure produces a new intermediate which, after elimination of the metal moiety, furnishes the corresponding cyclopen-tene derivatives (Scheme 30). 

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