Radicals, 1,2-shifts

As we shall see, nucleophilic 1,2 shifts are much more common than electrophilic or free-radical 1,2 shifts. The reason for this can be seen by a consideration of the transition states (or in some cases intermediates) involved. We represent the transition state or intermediate for all three cases by 1, in which the two-electron A—W bond overlaps with the... 

Unsaturated groups, however, can shift by a two-step addition-fragmentation mechanism via a discrete cyclopropane radical intermediate. Phenyl groups are especially good at undergoing radical 1,2-shifts 1,2-shifts of acyl, alkenyl, and, to a lesser extent, alkynyl and cyano groups are also seen. 

Heavy atoms such as Br and Se can also undergo a radical 1,2-shift by an addition-fragmentation mechanism. An intermediate that features a heavy atom with a nonet of electrons intervenes. First-row elements cannot accommodate a nonet of electrons like heavy elements can, so the addition-fragmentation mechanism is disallowed for them. 

Under similar profiles of raising in temperature, it was shown that the selectivity favoring 1,2 Stevens rearrangement is exemplified under the action of microwaves. A tentative explanation can be to consider that, under the action of radiation, the more polar mechanism (1,2 ionic shift) is favored when compared to less polar one (2,3 radical shift). Maybe this result is indicative of a competition between ionic and radical pathways. 

Cleavage of a bond without immediate dissociation of the precursor radical ion is one way for the generating distonic ions. Isomerization of molecular ions by hydrogen radical shift frequently leads to distonic ions prior to fragmentation and. 

The intermediacy of ion-neutral complexes is neither restricted to even-electron fragmentations nor to complexes that consist of a neutral molecule and an ion. hi addition, radical-ion complexes and radical ion-neutral complexes occur that may dissociate to yield the respective fragments or can even reversibly interconvert by hydride, proton or hydrogen radical shifts. Many examples are known from aliphatic alcohols, [180-183] alkylphenylethers, [184-187] and thioethers. [188]... 

May 17 In a letter to NRA chief lobbyist James Jay Baker, U.S. Attorney General John Ashcroft announces a radical shift in Justice Department policy in which it will accept the individual rights theory of the Second Amendment. However, subsequent statements imply that the Justice Department will not seek to overturn any existing federal firearms laws on Second Amendment grounds. 

This requires a radical shift of emphasis in the writing of computer programs. Knuth suggests that... 

By personal change is meant making some change to the way you behave, think, feel, see yourself or your external world. It does not mean some radical shift in personality which makes you virtually a different person. On the other hand, a shift in some aspect of your behaviour or in how you see yourself or the world can be experienced by you - and others - as profoundly transforming. You see your world differently. You may literally feel that you are living in a different world. You may feel that you have a new self. 

When C3 is unsubstituted, the di-w-methane rearrangement may still take place but, because of the greater difficulty in breaking the C2-C3 bond under these circumstances, a different stepwise mechanism involving hydrogen-radical shifts is followed.114... 

A psychosis is a pressured withdrawal with—in many cases—an incomplete return. A mystical state is a controlled withdrawal and return a death and rebirth, often a rebirth into a world with a radical shift in its iconography—a death and transfiguration (p. 74). 

The concept of d-SoCs comes to us in commonsense form, as well as in terms of my initial research interests, from people s experiences of radically altered states of consciousness-states like drunkenness, dreaming, marijuana intoxication, certain meditative states. These represent such radical shifts in the patterning, the system properties of consciousness, that most people experiencing them are forced to notice that the state of their consciousness is quite different, even... 

Thiopyrans are produced in high yield by the microwave-induced one-pot reaction between a,/ -unsaturated ketones, dienophilic alkynes, and Lawesson s reagent. The equivalent thermal reaction is unsuccessful <2006TL4925>. Calculations at the G3MP2B3 level show that the introduction of a 5-methyl substituent into n -sul I cnyl-. a-sulfinyl-, and ct-sulfonyl- hex-5-enyl radicals shifts the regioselectivity of cyclization to the 6-endo products, tetrahydrothiopyran derivatives <2006JOC9595>. 

Analogies between free radicals and carbocations have been drawn several times, but one of the typical reactions of carbocations, the concerted 1,2-shift, does not occur in free radicals. A concerted 1,2-radical shift is allowed only when one of the components can react antarafacially. The 1,2-hydrogen atom shift is geometrically impossible, and the geometric requirements of the 1,2-alkyl radical shift are so stringent that it is not observed. 

Fig. 5. (A) AA at 694 nm in TSF-I particles poised at 200 mV and excited by 50-ps flashes at 694.3 nm [(trace (a)] and AA measured in TSF-I particles with P700 pre-oxidized by background illumination [trace (b)] (B) AA measured in TSF-I particles poised at -625 mV (note the different scales for the time axes) (C) AA at 694 nm measured in TSF-I particles (a) poised at 200 mV, (b) P700 pre-oxidized by continuous illumination, and (c) heat treated to inactivate the bound iron-sulfur centers (D) AA (red-region) measured in TSF-I particles 150-ps and 800-ps after the flash 30-ps flashes at either 708- or 689-nm were used [see data-point code in the inset) Note the different absorbance scales used. (E) AA (450-600 nm) induced by 30-ps, 689-nm flashes (F) solid trace is the difference between AA measured at 150-ps and 800 ps the dashed trace is the in vitro difference spectrum for Chl-a anion radical, shifted toward the red by -25 nm. Figure source (A) and (B) from Shuvalov, Ke and Dolan (1979) Kinetic and spectral properties of the intermediary eiectron acceptor A, in photosystem I. Subnanosecond spectroscopy. FEBS Lett 100 6 (C)-(F) from Shuvalov, Klevanik, Sharkov, Kryukov and Ke (1979) Picosecond spectroscopy of photosystem I reaction centers. FEBS Lett 107 314, 315.

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