At vinyl carbon

All the mechanisms so far discussed take place at a saturated carbon atom. Nucleophilic substitution is also important at trigonal carbons, especially when the carbon is double bonded to an oxygen, a sulfur, or a nitrogen. Nucleophilic substitution at vinylic carbons is considered in the next section at aromatic carbons in Chapter 13. 

A number of reports on the investigation of the stereochemistry of solvolytic displacement at vinyl carbon have appeared. Rappoport and Apeloig (173) have studied the solvolytic behavior, under a variety of conditions, of cis and trans 1,2-dianisyl-2-phenylvinyl halides, 178. 

Figure 6.23 Direct replacement of fluoride by phosphorus at vinylic carbon.

Figure 6.24 Replacement of chloride by phosphrus at vinylic carbon in an orga-nomanganese complex.

Figure 6.25 Oxygen-to-carbon rearrangement at vinylic carbon.

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

Nucleophilic substitutions at vinylic carbon atoms usually proceed with retention of con-flguration. See, for example G. Modena, Ace. Chem. Res. 4, 73 (1971). Rationales have been proposed by W. D. Stohrer, Tetrahedron Lett., 207 (1975) S. I. Miller, Tetrahedron 33, 1211 (1977)... 

Also, reactions at vinylic carbons proceed with retention,101 indicating that the intermediate 14 has sp2 hybridization and not the sp hybridization that would be expected in the analogous carbocation. A cyclopropyl anion can also hold its configuration.102... 

Aluminum chloride, 15 Sodium amide, 278 at vinylic carbons... 

Scheme 9.14 Stereoconvergence in nucleophilic substitutions at vinylic carbon.

Alkenylations of heteroatom nucleophiles with alkenyl(aryl)iodonium salts occur by a variety of mechanisms, including SN1, SN2, alkylidenecarbene, and addition-elimination pathways [ 126,127]. Reactions that occur with retention of configuration at vinylic carbon are sometimes attributed to a ligand-coupling... 

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. 

Modena and co-workers examined the relevance of the symmetry of the lowest unoccupied molecular orbital (LUMO) of the electrophile in substitutions at vinyl carbon of thiirene <1995JA2297>. The computational levels included 3-21G //3-21G, 6-31G //3-21G, and 6-311G //3-21G. In attack at the vinyl carbon of a thiirenium ion, for example, the thiirenium ion is the electrophile and the attacking nucleophile is a neutral species with a lone pair, or an anion. It was found that in cases where the first vacant a- and n-levels differ in energy by more than 0.01 hartree, there is a good correspondence between the symmetry of the lowest unoccupied orbital and the stereochemical... 

Heating crotonylperoxide in presence of [D9]-2-methyl-2-nitrosopropane 39 a two-fold spin trap process must be assumed. The generated 1-propenyl radical 38 adds to nitroso compound 39 forming vinylaminyloxide 40. The latter one, reactive at vinyl carbon (see below, p. 91), is trapped by excess 39 giving aminyloxide 4160. ... 

The final chapter of this section is by Rappoport and is concerned with nucleophilic reactions at vinylic carbon. Two reaction types are considered, those of neutral vinyl derivatives and those of vinyl cations. Correlation of rates for these reactions with both Ritchie and Swain-Scott equations was attempted without success. Rappoport concludes that these reactions are subject to a complex blend of polar, steric, and symbiotic effects and that a quantitative nucleophilicity scale toward vinylic carbon cannot be constructed . This conclusion is reminiscent of the earlier observation of Pearson (see the introduction to the section on the Brpnsted equation) and the later observation of Ritchie (Chapter 11) regarding the difficulty of correlating nucleophilic reactivity with a single equation. Rappoport finds another familiar situation when he explores the relationship between reactivity and selectivity for the vinyl substrates sometimes the RSP is obeyed and sometimes it is not. 

The main evidence for this cyclic mechanism over an alternative stepwise mechanism lies in the failure of Maruyama et al. to observe any partial reactions catalysed by PEPC. The mechanism is also consistent with the stereochemistry of carboxylation at vinyl carbon determined by Rose et al. (1969). O Leary et a/. (1981) have studied the kinetic (carbon) isotope effect associated with PEPC-catalysed carboxylation of PEP, employing isotopi-cally equilibrated HC03 /C02. Comparison of these KIEs with those for standard C—C bond forming and breaking processes led to the proposal of a stepwise mechanism, involving the intermediacy of carboxyphosphate (Scheme 42b). 

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