Transition state double-bond rotation

To explain these results, Lee and colleagues " showed that in the absence of Lewis acids the rotational barrier of the C=N double bond is fairly high, but in the presence of catalysts the rotational barrier is lowered. The complex formation of tosylate and AICI3 makes the double bond rotation possible and the product distribution is determined by the relative stability of the oxime E-Z isomers (equation 89). A cychc transition state affords the corresponding quinolinone 281 and the isoquinolinone 282. 

In principle cis 2 butene and trans 2 butene may be mterconverted by rotation about the C 2=C 3 double bond However unlike rotation about the C 2—C 3 single bond in butane which is quite fast mterconversion of the stereoisomeric 2 butenes does not occur under normal circumstances It is sometimes said that rotation about a carbon-carbon double bond is restricted but this is an understatement Conventional lab oratory sources of heat do not provide enough energy for rotation about the double bond m alkenes As shown m Figure 5 2 rotation about a double bond requires the p orbitals of C 2 and C 3 to be twisted from their stable parallel alignment—m effect the tt com ponent of the double bond must be broken at the transition state... 

In view of the restrictions on the mode of approach of the radical to the double bond, significant strain develops at the transition state, and this requires rotation of the benzylic methylene group out of its preferred coplanar alignment. 

Some derivatives of triafulvene undergo rotation about the carbon-carbon double bond even at room temperature. Given that cis-trans isomerization about double bonds is normally very difficult (see Chapter 7, Problem 1), how would you rationalize this Examine the electrostatic potential map for perpendicular hexaphenyltriafulvene (the rotational transition state).Would polar solvents tend to lower or raise the rotation barrier Explain. 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

The apparent lowering of the rotational barrier in triafulvenes is open to interpretation either by substituent or solvent stabilization of ground-state polarity leading to a decrease of C3/C4 double bond character or by stabilization of a more polar - probably perpendicularly orientated184 — transition state by substituent or solvent effects. 

During the rotation about the N—0= bond, the interaction between the nitrogen lone pair and the rest of the molecule is destroyed, and the nitrogen atom adopts a pyramidal configuration. In the transition state for rotation about the N—C bond, the nitrogen lone pair and the C=C double bond system occupy orthogonal positions. 

In symmetric overcrowded or otherwise strained ethylenes, the strain may be partially released by rotation around the C=C bond or by other deformations, and the barrier to E-Z isomerization may be lowered compared to that of ethylene by ground state strain and by delocalization of the double-bond ir electrons into unsaturated substituents, forming a diradical transition state. The importance of the delocalization effect is illustrated by the low barrier (AC o = 23.2 kcal/ mol) in the diphenoquinone 125 (145), in which the ground state strain must be rather low. 

Stabilized allyl radical will be stabilized further if substituents are introduced. This stabilization occurs to different degrees in the ground state and the transition structure for rotation. In the ground state the substituent acts on a delocalized radical. Its influence on this state should be smaller than in the transition structure, where it acts on a localized radical. In the transition state the double bond and the atom with the unpaired electron are decoupled, i.e. in the simple Hiickel molecular orbital picture, the electron is localized in an orbital perpendicular to the jt(- c bond. 

Thermal ring opening to form the merocyanine form is less dependent upon the type of substituent keeping a AG of near to 101 kJmol for BIPS, 6-nitro-BIPS and 6,8-dinitro-BIPS. [37] The ring opening reaction s rate-determining step is probably a rotation cis to trans about the central p-methine bond and this is also consistent with the polar CCC or CCT transition state which have a rather double p-methine bond. 

Density functional theory studies of intramolecular retro-ene reactions of allyldiazenes have shown that the reaction is a concerted process involving a six-centre cyclic transition state.13 The cis- and tnms-allyldiazencs can interconvert by rotation around the double bond or by nitrogen inversion, it being predicted that nitrogen inversion is favoured. 

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