Vinylidene protonation

The only physical property which has been studied for substituted vinylidene sets is the nmr chemical shift of the vinylidene proton in substituted ethylenes and in tra s-l,2-disubstituted ethylenes. The first attempt at correlating chemical shift data for substituted ethylenes with the Hammett equation appears to be the work of Banwell and Sheppard (53), who reported a correlation of A2 values with the or constants, the A2 values being defined by the equation... 

Further support for the presence of these intermediates is given by the finding that H/D exchange of /raws-Pt(C=CR)2(PMe2Ph)2 (R = H or D) with MeOD proceeds only in the presence of weak acids protonation of the ethynyl group results in exchange because of the greater acidity of the vinylidene proton. 

The H NMR spectrum of PMMA prepared with AIBN at 60°C showed vinylidene proton signals at 5.4 and 6.14ppm38 (Fig. 14). No inner olefin proton signal was observed. The intensity measurement indicated that 36% of the polymer molecules contained the unsaturated terminal. The thermal stability of radically prepared PMMA was discussed in relation to the... 

The functionalities of these macromonomers could be determined by both UV-visible spectroscopic analysis ( lmax for DPE at 260 run) and by HNMR spectroscopy (vinylidene protons at 5 5.4 ppm). The functionality of the macromonomer 74 (M =3400) was estimated to be 0.6 by UV analysis and 0.83 by NMR no dimer was observed by SEC analysis [198]. The stoichiometric addition of PSLi (M =16,000) occurred quantitatively with no residual macromonomer observed by SEC [198]. 

Example 2. Vinviidene Chloride Isobutylene Copolymer. The next example is for the carbon-13 spectrum of copolymer vinylidene chloride isobutylene. Figure 5 shows the full spectrum and the peak assignment listing for the non-protonated vinylidene chloride carbon in the 84-92 ppm range. Triad assignments were made (Crowther, M. W., 1987, Syracuse University, unpublished data) using the two-dimensional COLOC (20) experiment. There are ten v-centered pentads representing different environments for the vinylidene chloride carbon. The i represents the non-protonated carbon in the isobutylene polymer unit. 

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

Wakatsuki et al. (4) proposed vinyl complex, 5, and presented DFT results supporting isomerization to a vinylidene hydride as the rate determining step. Our results indicate that the rate determining step involves H-OH bond breaking and that protonation of a bound alkyne is the rate determining step in this... 

Another approach toward C-O bond formation using alkynes that has been pursued involves the intermediacy of transition metal vinylidenes that can arise from the corresponding y2-alkyne complexes (Scheme 13). Due to the electrophilicity of the cr-carbon directly bound to the metal center, a nucleophilic addition can readily occur to form a vinyl metal species. Subsequent protonation of the resulting metal-carbon cr-bond yields the product with anti-Markovnikov selectivity and regenerates the catalyst. 

Attempts by Fish and Johnson to effect a steroid synthesis using a standard epoxide-initiated pentacyclization of a polyene afforded complex mixtures [69]. Alternatively, the allyl alcohol 326 was synthesized and treated with TFA (Scheme 19.60). Protonation affords a symmetrical tetramethylallyl cation that undergoes cyclization to give pentacycle 327 in 31% yield. Simultaneous cleavage of the isopropylidene and vinylidene groups was carried out to furnish the diketone 328 in 88% yield, which was then converted to sophoradiol (329). 

Shi, Z. Q., and Holdcroft, S. 2005. Synthesis and proton conductivity of partially sulfonated poly([vinylidene difluoride-co-hexafluoropropylene]-b-styrene) block copolymers. Macromolecules 38 4193-4201. 

Song, M. K., Kim, Y. T., Fenton, J. M., Kunz, H. R. and Rhee, H. W. 2003. Chemically modified Nafion (R)/poly(vinylidene fluoride) blend ionomers for proton exchange membrane fuel cells. Journal of Power Sources 117 14-21. 

If a given vinylidene complex is not sufficiently electrophilic, protonation at Cp can promote nucleophilic addition at C by intermediate formation of an electrophilic carbyne complex [89] (Figure 2.9, Section 2.1.8). 

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

The alkenyl(amino)aUenylidene complex 41 is also prone to undergo electrophilic additions at the Cp atom of the cumulenic chain. Thus, treatment of 41 with HBF4 OEt2 led to the spectroscopically characterized dicationic vinylidene complex 65 (Fig. 10) [52, 53]. Related Cp-protonations of complexes 35 (Fig. 6) have also been described [49]. 

A similar type of substitution, which clearly shows the electrophilic character, occurs in vinylidene carbenoids. In an early example of this reaction, Kobrich and AnsarP observed that the aUcene 70 results when the fi-configurated vinyl lithium compound 68 is treated with an excess of butyllithium and the fithioafkene 69 formed thereby is protonated (equation 41). Obviously, the nucleophilic attack of the butyl residue on the carbenoid takes place with inversion of the configuration. 

The first point of the stereochemical analysis is in the recognition of the sequence to which a given nucleus is sensitive the problem seems rather obvious for vinyl or vinylidene polymers where the sequence must extend equally from the two sides of the nucleus in question but for diene polymers or those containing heteroatoms, the problem is not so simple. In the present case, the methylene protons are sensitive to the structure of the even sequences, dyads and tetrads, whereas the methyl protons are sensitive to the odd sequences, triads, and pentads. 

While protonation affords the vinylidene expected by H migration from the original 1-alkyne, use of other electrophiles provides a convenient route to disubsti-tuted vinylidenes. The stereospecificity of this reaction with Re(C=CR)(NO)(PPh3)... 

Oxidation ]PhIO or Cu(OAc)2] of ]Fe(=C=CHMe)(dppe)Cp]" affords bis(vinyli-dene) ] Fe(dppe)Cp 2(p-C4Me2)], possibly via an intermediate radical cation ]222]. Similar oxidative coupling of cyclopropenyl Ru C=CPhCH(CN) (PPh3)2Cp affords bis(vinylidene) ] Cp(Ph3P)2Ru =C=CPhCH(CN)2 2] which, in turn, can be de-protonated to the bis(cydopropenyl) ]223]. Oxidation of ]Ru(N4Meg)(=C=CH2)] with PhNj or ]FeCp2]+ affords ] Ru(N4Meg) 2(p-C=CHCH=C)] ]51]. 

Ready addition of nucleophiles (Nu ) to metal-allenylidene complexes affords alkynyl derivatives. Subsequent protonation or alkylation, as described in Section 1.2.3 above, then gives the corresponding vinylidene complexes (Equation 1.8) ... 

In the ruthenium series, more success has been reported, particularly with RuP2Cp (Cp = Cp < [225],q -C9H7[115,226-232], Tp [130]). This can be rationalized in terms of a high contribution from the vinylidene to the stmcture of the alke-nylcarbyne formed by protonation of the allenylidene (Equation 1.9) ... 

The deprotonation of carbyne complexes is the formal reverse of the addition of a proton to the vinylidene (Equation 1.10, Table 1.4) ... 

An early approach to vinylidenes was by the formal dehydration of metal acyls, which is best achieved by treatment with an electrophile, often the proton in the form of a non- or weakly-coordinating strong acid. The reaction appears to proceed stepwise via a hydroxycarbene formed by protonation of the acyl, subsequent dehydration of which affords the vinylidene. Occasionally, mixtures of the two complexes are obtained, again suggesting the intermediacy of the carbene. 

---