Vinylhalides

CM products from vinylhalides are highly desirable especially because of the possible use in metal catalysed coupling reactions. Johnson and co-workers, performed detailed studies of the possible deactivation pathways [161]. The Fischer-carbene complexes of the vinyl halides have an increased stabihty compared to their alkylidene counterparts and the Fischer carbenes may be deactivated either by migration of the phosphine or by elimination of HX leading to a carbide. 

Despite those challenges, both Johnson [161] and Grela [162] performed several cross metathesis reactions with vinylhalides using phosphine free catalysts. Turnover numbers (TON) above 20 were very few, while in many cases the TON stayed below ten. The diastereoselectivity of CMs with vinylhalides is shghtly in favour of the Z product which is similar to their acrolein-counterparts. 

The Heck reaction, i.e. the palladium(O) catalyzed vinylation of aryl- or vinylhalides (or the corresponding triflates), belongs undoubtedly to the most important metal-catalyzed C-C coupling reactions [3, 4]. Accordingly, it enjoys increasing application as a key reaction in total synthesis [5]. 

Many different routes are available for the synthesis of vinylboranes and several of them are shown in Scheme 7. Hydroboration and diboration reactions of alkynes and borylated alkynes provide access to the frill series of mono-, di-, tri-, and tetraborylated olefins. 1,2-Diborylated olefins (33) are obtained via diboration of alkynes and 1,1-diborylated olefins (34) are accessible through hydroboration of borylalkynes. An alternative route to 1,1-disubstituted products involves the diboration of carbenoids formed in situ from vinylhalides and butyl hthium. In certain cases, metal-catalyzed dehydrogenative borylation of olefins may be used. Borylalkynes serve as precursors to triborylated (35) and tetraborylated (36) olefins. Thus, the sparingly soluble tetraborylethylene derivative (36) forms in good yield through platinum-catalyzed diboration of diborylacetylene in toluene at 40 °C if the base-free catalyst [Pt(cod)2] is used. If the reaction, however, is performed at higher temperature, ftnther diboration of (36) leads directly to the hexaborylated ethane (23) shown above. Intramolecular B-O interactions were postulated for (36) based on HF-SCF calculations. ... 

In further work it turned out that the method for preparation of Z-halides has a general value whereas preparation of -isomers suffered from some limitations. Some examples of synthesis of Z-vinylhalides are shown in Table 30. 

Scheme 7.11. Palladium catalyzed coupling of vinylhalides 47-49 with styrene under normal and high pressure conditions.

THE KINETICS AND MECHANISMS OF THE DEHYDROHALOGENATION OF THE POLY(VINYLHALIDES), PVC, PVDC, AND PVF... 

The dehydrohalogenation of poly(vinylhalides) is important because of its presumed relationship to thermal stability. Most reports agree that PVC and PVDC dehydro-chlorinate by a Zipper Mechanism but kinetic studies have not followed the implications of that mechanism. The rate of PVC dehydrochlorination has been reported to increase, decrease, and remain constant with time and the catalytic effect of hydrogen chloride has not always been observed. PVDC has been reported to follow first order kinetics, but the acceleratory phase of the dehydrochlorination was not adequately accounted for. 

The reactivities of organotin hydrides [24] and chromium (II) complex [25] toward alkyl halides are also in the order of tertiary > secondary > primary alkylhalides. However, this trend is much stronger with the ate complex of 9-BBN than that with these reagents. Consequently, the high selectivity, gentleness, and convenience exhibited by the reagent has the practical synthetic application (Eq. 25.20) Benzylic halides are also reduced easily (entries 11-13, Table 25.10), whereas aryl and vinylhalides are inert (entries 19,20). Benzylic geminal dihalides are reduced stepwise (entries 16, 17). The reaction of 1-phenylallyl-chloride [26] which also contains 33% of cinnamyl chloride with an equiv of ate complex of 9-BBN affords mixture of allylbenzene (36%) and 3-metliylstyrene (66%). 

Bromination of 7-trimethylsilylbicyclo[2,2,l]hept-5-ene-2-carboxylate and its epoxide occurs with the silyl group encouraging rearrangement to the nor-bomene. Alumina acts as a dehalosilylating agent for the stereoselective synthesis of vinylhalides from cis- and /ra/i5-vinylsilanes [equation (21)3. ... 

Shibata et al. demonstrated an intermolecular three-component coupling of aryl or vinylhalides, diarylacetylenes, andmonosubstitutedalkenes, leading to the corresponding 1,3-butadiene or 1,3,5-hexatriene derivatives in the presence of palladium species [40] (Scheme 6.23). 

Trofimov, B.A. and A.I. Mikhaleva. 1980. Reaction of ketoximes with vinylhalides as a novel route to pyrroles and N-vinylpyrroles. 2h Org Khim 16 (3) 672. 

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