Internal vinylidene

Unimolecular /3-H transfer to the metal in propylene polymerization is key to understanding growing-chain-end isomerization237 and formation of internal vinylidenes.217... 

Scheme 13 Formation of internal vinylidene unsaturations. A via bimolecular /3-H transfer B via unimolecular /3-H transfer.

Figure 12 shows the most common terminal unsaturations of iPP, generated by chain-release reactions after primary insertion. The internal vinylidene, likely produced by allylic activation, is also shown.217,246... 

Etherification using a metal vinylidene has also been combined with G-G bond formation through the reaction of an alkynyl tungsten complex with benzaldehyde (Scheme 14). The addition of an internal alcohol to the incipient /3,/Udialkylvinylidene that is generated leads to dehydration and the formation of a Fischer-type alkylidene complex. Further reactions of this carbene with a range of nucleophiles have provided access to various furan derivatives.374,375... 

This reaction is quite different from the other P-H addition reactions in that it involves external nucleophilic attack of HPPh2 on the vinylidene ligand as shown in Scheme 13. The ZIE ratio depends on the structures of the substrate and the catalyst. Ru-Cp" (Cp =77 -CsMes) species selectively forms the Z isomer while Ru-Cp (Cp r -CsHs) favors the E isomer. Since the key intermediate is the vinylidene species that has an electrophilic carbon, the reaction is applicable to other alkynes that are vinylidene precursors. Thus, phenylacetylene also reacts similarly to give Ph2PCH=CHPh ZIE=93I7), while internal alkynes are totally unreactive. 

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

H-shift is invoked for the formation of 6 and regeneration of catalyst 8. The proposed mechanism is unusual insofar as the tt-bonds of electroneutral alkynes and arenes seldom participate in Diels-Alder reactions. The intermediacy of metal vinylidenes is supported by the failure of internal alkynes to dimerize under the reported conditions. More importantly, mechanistic restrictions imposed by the porphyrin ligand set severely restrict conceivable alternative mechanisms. 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

On the basis of DFT calculations, a catalytic cycle involving a copper vinylidene intermediate has been proposed (Scheme 9.22) [44]. The reaction is initiated by copper acetylide (138) formation. Sharpless and coworkers next invoke an unusual [3 + 3]-cycloaddition that would be forbidden by orbital symmetry, were it not stepwise. Coordination of an azide to complex 138 generates a zwitterionic complex (139). Internal nucleophilic attack of the acetylide moiety of 139 on the electrophilic... 

Finally, it can be noted that some cross-dimerization of terminal alkynes with internal alkynes, where ruthenium vinylidene intermediates are postulated, have also been reported [74, 75]. 

The American Conference of Governmental Industrial Hygienists (ACGIH) (1997) has recommended 20 mg/m as the threshold limit value for occupational exposures to vinylidene chloride in workplace air. Similar values have been used as standards or guidelines in many countries (International Labour Office, 1991). 

The World Health Organization has established an international drinking-water guideline for vinylidene chloride of 30 gg/L (WHO, 1993). 

WHO (1990) Vinylidene Chloride (Environmental Health Criteria 100), Geneva, International Programme on Chemical Safety... 

At a desired conversion, the reaction is quenched, usually with an aqueous base solution, such as, for example, NH4OH, before a significant isomerization into internal double bonds can take place. The molecular weights of high vinylidene PIB are comparatively low, i.e., in the range of 1 k Dalton (14). 

This is further indicated in the reactions of 3-butyn-l-ol with [Fe( /2-CH2=CMe2)(CO)2( -C5H5)]+, which afford a mixture of dihydrofuran complex (64) and the oxacyclopentylidene complex (65) (84). The formation of these two derivatives involves a common tp-alkyne intermediate, which either forms 64 directly by internal nucleophilic attack of the oxygen on the complexed C=C triple bond, or rearranges to the vinylidene. This forms 65 by a similar attack of the hydroxy group on the a-carbon, followed... 

The reaction with 4-pentyn-l-ol gave only [Fe t/2-CH2=C(CH2)30) (CO)2(t/-C5H5)]+, and 3-hexyn-l-ol afforded (64, R = Et) (84) no evidence for the participation of the vinylidene tautomers was found. With ruthenium (45) and platinum (47) complexes, on the other hand, rearrangement to the vinylidene is faster than internal attack on the >/2-alkyne, and only the cyclic carbene complex is formed. 

Internal plasticizers are synthesized by copolymerization of suitable monomers. Polymeric non-extractable plasticizers, mostly copolymers having substantially lower glass transition temperatures due to the presence of plasticizing ( soft ) segments such as poly(ethylene-co-vinyl acetate) with approximately 45 % vinylacetate content, ethylene-vinyl acetate-carbon monooxide terpolymer, or chlorinated PE, are available for rather special applications in medicinal articles (Meier, 1990). In this case, the performance of the internally plasticized polymers is the principal advantage. However, copolymerization may account for worse mechanical properties. A combination with external plasticizers may provide an optimal balance of properties. For example, food contact products made from poly(vinylidene chloride) should have at most a citrate or sebacate ester based plasticizers content of 5 % and at most 10 % polymeric plasticizers. 

In the presence of a base (Et3N) the ruthenium vinylidene complex RuCl(Cp )(PPh3)(=C=CHPh) promotes the selective cross-coupling of a bulky terminal alkyne with internal alkynes at room temperature to yield functionalized enynes (Eq. 10) [77]. 

Addition of internal alkynes to (t)5-C5H5)(PR3)2RuCI does not lead to the formation of the corresponding disubstituted vinylidene (68). The failure of this reaction could reflect the relative difficulty of a 1,2-alkyl shift for internal alkynes as compared to the 1,2-proton shift for the successful rearrangement of terminal alkynes (Scheme 9). Alternatively, if the deprotonation-reprotonation route is important in the rearrangement of terminal alkynes (vide supra), then clearly internal alkynes would not undergo a similar isomerization. 

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