Inversion vinylic carbon

Evidence for this mechanism comes from the observation that the rearrangement takes place with an inversion of the allyl group. That is, allyl phenyl ether containing a 14C label on the allyl ether carbon atom yields o-allylphenol in which the label is on the terminal vinylic carbon (green in Figure 18.1). It would be very difficult to explain this result by any mechanism other than a pericyclic one. We ll look at the reaction in more detail in Section 30.8. 

The Sn2 reaction involves the attack of a nucleophile from the side opposite the leaving group and proceeds with exclusive inversion of configuration in a concerted manner. In contrast to the popular bimolecular nucleophilic substitution at the aliphatic carbon atom, the SN2 reaction at the vinylic carbon atom has been considered to be a high-energy pathway. Textbooks of organic chemistry reject this mechanism on steric grounds [175]. 

When ( )-[(/i-arylethyl)vinyl]phenyliodonium tetrafluoroborates are heated in various solvents (at 40 °C or 60 °C), Friedel-Crafts cyclizations occur and dihydronaphthalenes are obtained (equation 26l)127. 6-Bromo-2i7-chromene can be prepared in the same way (equation 262). The alkenylation of benzene with 4-rm-butyl-1 -cyclohexenyl-(phenyl)iodonium tetrafiuoroborate, an intermolecular version of these reactions, has also been demonstrated (equation 263)127. From a stereochemical standpoint, the cyclization reactions are constrained to occur with inversion of configuration at vinyl carbon, while the cyclohexenylation of benzene must proceed with retention. 

Trialkylaluminums have shown the capability of entering into nickel-catalyzed reactions with allenic bromides, to afford direct alkylation at vinyl carbon. Inversion of configuration occurs at this carbon to a degree which is greater than with other alkylmetals (magnesium, zinc) (see equation 6O)90. 

In an earlier chapter, we dealt with SN2 reactions in aliphatic compounds. In that chapter it was stated that the normal nucleophilic pathway could not operate at carbon centres that were unsaturated, because of the impossibility of inversion of the configuration of the carbon involved. This does not mean that nucleophilic substitution at a vinyl carbon is impossible if it is to occur, however, then there must be a different pathway by which it must proceed. 

Nucleophilic addition of diethyl sodiomalonate to (270) may also be either at C-2 or at the terminal vinyl carbon but in this case, unlike in radical addition, attack at C-2 predominates. When the nucleophile is pyrrolidine, it has been shown that the initial attack at C-2 of optically active (270) occurs with inversion of configuration. ... 

T -dienyliron cations (Scheme 4-97) (c/ Section 2.5.2.b). Oxidatively induced reductive elimination of (pentenediyl)iron complexes gives vinylcyclopropanesJ 2-226] Depending on the substitution pattern and the conditions applied in some cases the reaction proceeds with retention of the configuration, whereas sometimes inversion of the configuration at the vinylcyclopropyl carbon is observed in combination with an inversion of the configuration at the double bond. The latter has been proven by monodeuteration of the terminal vinylic carbon atom. ... 

For trisubstituted olefins, the nucleophile attacks predominantly at the less substituted end of the allyl moiety, e.g. to afford a 78 22 mixture of 13 and 14 (equation 7). Both the oxidative addition of palladium(O) and the subsequent nucleophilic attack occur with inversion of configuration to give the product of net retention7. The synthesis of the sex pheromone 15 of the Monarch butterfly has been accomplished by using bis[bis(l,2-diphenylphosphinoethane)]palladium as a catalyst as outlined in equation 87. A substitution of an allyl sulfone 16 by a stabilized carbon nucleophile, such as an alkynyl or vinyl system, proceeds regioselectively in the presence of a Lewis acid (equation 9)8. The... 

From these results it is reasonable to conclude that no inversion in the order of the chemical shifts of the tertiary carbon atoms belonging to the different triads occurs from the starting PV0CC1 to the poly(vinyl phenyl carbonate). Moreover the chemical modification of PV0CC1 by phenol does not induce any degradation of the polymer. 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

Trisubstituted carbon-centred radicals chemically appear planar as depicted in the TT-type structure 1. However, spectroscopic studies have shown that planarity holds only for methyl, which has a very shallow well for inversion with a planar energy minimum, and for delocalized radical centres like allyl or benzyl. Ethyl, isopropyl, tert-butyl and all the like have double minima for inversion but the barrier is only about 300-500 cal, so that inversion is very fast even at low temperatures. Moreover, carbon-centred radicals with electronegative substituents like alkoxyl or fluorine reinforce the non-planarity, the effect being accumulative for multi-substitutions. This is ascribed to no bonds between n electrons on the heteroatom and the bond to another substituent. The degree of bending is also increased by ring strain like in cyclopropyl and oxiranyl radicals, whereas the disubstituted carbon-centred species like vinyl or acyl are bent a radicals [21]. 

A variety of rhodium complexes, including [Rh(CO)2Cl]2 and [Rh(COD)Cl]2 when used in combination with a variety of bisphosphine ligands, will catalyze the ring opening of vinyl epoxides in the presence of aniline nucleophiles [19, 20]. These reactions occur under very mild and neutral conditions (at room temperature or with mild heating) and are highly regio- and stereoselective. In all cases, nucleophilic attack occurs at the allylic epoxide carbon atom and proceeds with inversion of stereochemistry (Scheme 9.11). 

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