Vinyl carbons, retention

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... 

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. 

The reaction of Pd complexes (such as Pd(PhsP)4) with organic halides and related compounds has been used to prepare a number of stable Pd alkyl and vinyl compounds. This reaction with alkyl halides has the characteristics of an Sn2 reaction. Primary halides react faster than secondary halides. Also, when a chiral halide is used, such as (X)-(-F)-benzyl-o -D chloride, the benzyl palladium product is formed with inversion of configuration at the benzylic carbon (equation 8). With vinylic halides, retention of configuration at the double bond is observed... 

NMR spectroscopy. In the olefinic region, only a single peak at 5.70 ppm (1 H) was observed. For comparison, the vinyl -CH of ( )-3-trimethylsilyl-3-octene is found at 5.60 ppm (9). Two nonequivalent vinyl carbons, as well as two distinct allylic carbons, were observed in the NMR spectrum. No peaks could be assigned to either 1,2 or 3,4 structures in either the or NMR spectra. Further, only a single silicon resonance was seen in the Si NMR spectrum. Finally, protodesilation of the polymer by treatment with HI yielded (Z)-l,4-poly(butadiene) (III), whose structure was confirmed by IR spectroscopy (8). Protodesilation of vinyl silanes occurs stereospecif-ically with retention of configuration (10, 11). The data are consistent with -11 (equation 1). 

This method has also been quite useful for the synthesis of triorganoarsenic compounds. Arsenide anions, which can be prepared by several methods (vide infra), react with alkyl and aryl halides. Several examples are shown in equations 18 , 19 , 20 °, 21 , 22 23 , 24 and 25 . In the reaction with alkyl halides inversion of configuration at the sp carbon atom takes place and in the reaction of vinyl halides retention of configuration has been observed (equations 18 and 21). Photostimulated reaction with aryl halides has been reported to proceed via the Sjj l mechanism thus, the ketone group is compatible with the reaction conditions as shown in equation 23 . 

A lithium atom may also be replaced by copper to make a cuprate 161. As in the previous example, using pentynyl copper ensures that no vinyl group is wasted. Such cuprates are superior to simple copper derivatives in conjugate addition to give 162 with enones. All these reactions happen with retention at the vinylic carbon. 

Towards electrophiles, the reactivity of vinylsilanes is similar to that of the corresponding alkene. However, incorporating a silicon substituent at the vinylic carbon of a ir-nucleophile markedly affects the cyclization outcome. Specifically, iminium ion-vinylsilane cyclizations occur cleanly to substitute, preferentially with retention of double-bond configuration, the iminium ion carbon for the silyl substituent. Both endocyclic and exocyclic modes of intramolecular electrophilic substitution have been demonstrated (Scheme 35). 

First, the process may not involve a nucleophilic attack on the vinylic carbon. This fact is well recognized in vinylic substitution (2, 3), and a typical example is the substitution of ( )- and (Z)-(3-halovinyl sulfones (3 and 4 X = Cl or Br) by PhS- and MeO- in MeOH (4-6). Both reactions of both substrates are of a second order and give retention of configuration, and the element effects kBJka are 2.3 (E) and 2.2 (Z) with PhS- and 0.84 (E) with MeO-, values that are consistent with rate-determining nucleophilic attack on the vinylic carbon (2). However, for the Z isomer, kBJka with MeO- is 185, and because a-hydrogen exchange is rapid under the substitution conditions, the reaction of the Z-bromide probably proceeds via elimina-... 

Vinyl Retention Carbon Retention Carbon Retention Carbon... 

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. ... 

Vinylic halides can be coupled to give 1,3-butadienes by treatment with activated copper powder in a reaction analogous to the Ullmann reaction (13-14). " This reaction is stereospecific, with retention of configuration at both carbons. 

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 retention. 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 8 . 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) . The... 

Vinyl halides (example 17, Table VII) were first observed by Kroper to form acrylic esters by reaction with carbon monoxide under pressure and tetracarbonylnickel in methanol at 100°C. These reactions were later shown to occur under much milder conditions. Highly stereospecific reactions were observed c/s-vinyl halides gave cis-carbonylation products and trans-vinyl halides trans-carbonylation products (example 18, Table VII). Retention of configuration of alkyl substrates in carbonylation seems to be a general feature in carbon monoxide chemistry (193a). 

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