Butadiyne stability

Butadiyne, H-CSC-C C-H, as a polymerizable monomer, has received very little attention from polymer chemists although its discovery dates back to Bayer in 1885. This structurally simple, highly reactive bifunctional molecule would be expected to have been a monomer of considerable interest in the field of polymer chemistry. Possibly, limited butadiyne stability may account for the small amount of polymerization research. The The compound is a liquified gas at room temperature (BP = 10 C), discolors slowly in sealed vessels at 20 C and may explode if heated. Storage and instability problems may be circumvented. Prevention of explosion may e accomplished by addition of an inert diluent such as butane. The monomer may also be stored in t e form of a labile complex with N-methyl-pyrrolidone. Its thermal condensation or polymerization was briefly recorded as an observation by Bayer and described in a little more detail by Miiller in 1925. Prevention of this thermal polymerization has been the subject of several patents with methylene blue, pyridine and vinylpyridine claimed as inhibitors. 

In contrast, the trimer 89 with ethyne and butadiyne links stabilizes the thermodynamically disfavored endo transition state, and the endo adduct 86 is rapidly and almost exclusively formed. 

In some derivarization reactions with mono-metallaied bmadiyne, considerable amounts of disubstimted diacetylene are farmed. Their presence can hamper the purification of the desired mono-substitution products, particularly when the boiling point is high and the thermal stability limited. The formation of disubstitution products can be effectively suppressed, however, by using a large excess of butadiyne. The preparation of butadiynvl tributyltin is an illustrative example. 

The radical anion of /3-trimethylsilylstyrene also undergoes dimerization but coupling takes place at the carbons a to silicon 33). The kinetics of the alkyne dimerization, followed by ESR, showed the reaction to be second order in radical anion 43). With Li+, Na+, K+, or Rb+ as the counterions, the rate increases in the order Si < C < Ge 45). Consistent with the increased stability of the trimethylsilyl-substituted radical anion, the radical anion of 1,4-bis(trimethylsilyl)butadiyne, produced by reduction with Li, Na, K, Rb, or Cs in THF is stable at room temperature even on exposure to air, whereas the carbon analog, 1,4-di-r-butyl-1,3-butadiyne radical anion, dimerizes by second-order kinetics at -40° (42). The enhanced stability of the trimethylsilylalkynyl radical anions has been attributed to p-drr interactions (42). 

Many recent investigations in this field of chemistry have been directed toward the synthesis and study of diyne titanium complexes. The TiCp2 precursor compound Cp2Ti(Me3SiC2SiMe3) reacts with 1,4-disubstituted 1,3-butadiynes to give five-membered titanacyclocumulenes, the structures and stability of which depend strongly on the diyne substituents. Mononuclear or binuclear homobimetallic derivatives can be formed. For the mononuclear complexes, an equilibrium between the cyclocumulene and an alkyne structure is possible (Scheme 561). Binuclear complexes may exhibit different structural types (Scheme 562). 

Schroder and co-workers have successfully isolated a unique poly-catenated undulating molecular ladder that forms interwoven two-dimensional sheets (252). The reaction of [Cu(MeCN)4]PF6 with 1,4-bis(4-pyridyl)butadiyne (L93) in MeCN-CH2Cl2 yielded the complex [Cu2(MeCN)2(L93)3](PF6)2. The compound exists as a network of molecular ladders in which the two independent Cu1 centers are each coordinated in a tetrahedral geometry to three L93 groups and one MeCN molecule. The lattices are polycatenated to give a remarkable two-dimensional layer structure (Fig. 77). The sheets of interwoven molecules are separated by PF3 counteranions and solvent molecules. The structure is further stabilized by the tt—tt interactions between adjacent, symmetry-related ladders. 

The proper interpretation of the above results clearly underscores the importance of neutral hyperconjugation for the stability of unsaturated compounds. Jarowski et al. resolved the seeming paradox by pointing out that the reference compounds for 1,3-butadiyne and 1,3-butadiene are stabilized significantly by C-H/jt neutral hyperconjugation, which is absent in 1,3-butadiyne and 1,3-butadiene (Fignre 6.129). 

Figure 6.129 Stereoelectron ic origins of "disappearing" conjugative stabilization in butadiynes.

For some time, there has been interest in the controlled release properties of stabilized liposomes.(i-3) A number of methods for stabilizing liposomes have been developed. Liposomes have been prepared from lipids which contain polymerizable groups such as methacrylate, butadiyne, or diacetylene and subsequently polymerized.(4-6) Another strategy is to n e liposomes from lipids with amino add headgroups and then potymerize the liposome Ity potycondensatioiL(7) Alternatively, stabilized liposomes have been prepared from prepolymerized amphiphiles.(8) Recently, the design and preparation of... 

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