Linear additions to acetylenic

The stereochemistry of the reactions of vinyl cations with nucleophiles is predictably different depending on their geometry, (a) High stereospecificity is expected from bridged ions (trans addition to acetylenes and retention of configuration in substitution reactions of vinyl derivatives) and is experimentally observed in the case of thiirenium ions, (b) From free linear cations with two j3 substituents of equal size, complete racemization is expected and is fully verified in the substitution products from l,2-dianisyl-2-phenylvinylbromide (section II,C,2). 

Calculations on electrophilic additions to acetylene were reported for a number of hydrocarbon cations, including CH" [129], C3H3" [130, 131], and phenylvinylium ion [132]. It was predicted that the cyclopropenylium cation, the most stable form of C3H, forms an ion-molecule complex with acetylene but does not undergo further addition [131]. The linear propargyl cation, however, reacts with acetylene without an apparent barrier, to form many different C5H5 isomers [130,131]. Wang et al. computed the AMI potential energy surfaces and carried out RRKM and microcanonical variational transition-state analysis for the rate of reaction of phenylvinylium ion with acetylene [132]. 

Areas of acetylenic chemistry reviewed recently include the base-catalysed isomerization of acetylenes, nucleophilic additions to acetylenes, additions to activated triple bonds, synthetic and naturally occurring acetylene compounds as drugs, allenic and acetylenic carotenoids, linear polymers from acetylenes, carbonylation of mono-olefinic and monoacetylenic hydrocarbons, and the combustion and oxidation of acetylene. Several books have also appeared. ... 

Fahey, R. C., The Stereochemistry of Electrophilic Additions to Olefins and Acetylenes, 3, 237. Farina, M., The Stereochemistry of Linear Macromolecules, 17, 1. 

Following his self-consistent field MO calculations on acetylene, Burnelle (1964) examined the role of excited states and molecular vibrations in determining 88 of additions. He found that, when a proton is brought close to acetylene, the energy of the trans-hent structure falls below that of the linear form. For similar addition to ethylene, Bumelle (1965) found that the first stable intermediate derived from the 90° twisted form of ethylene. Since such a geometry could only lead to SS = 0—there is no preferred orientation for attack— this particular model was less successful for ethylene than for acetylene. 

The C-H and C-C cr-bonds are all trigonal sp hybrids, with 120° bond angles. The two unhybridized p-orbitals form a 7r-bond, which gives the molecule its rigid planar structure. The two carbon atoms are connected by a double bond, consisting of one o and one tt. The third canonical form of 5/ -hybridization occurs in C-C triple bonds, for example, acetylene (ethyne). Here, two of the p AO s on each carbon remain unhybridized and can form two n -bonds, in addition to two (r-bonds. Acetylene H-C=C-H is a linear molecule, as shown below, since the p-hybrids are oriented 180° apart. 

The synthesis of highly strained linear and angular dicyclopropabenzenes has been attempted without success, since the precursors required for the cleavage could not be synthesized. Thus, iV77-l,6 7,12-diinethano[14]annulene 3 underwent addition to acetylenedicarbonitrile in the wrong sense and did not lead to the required norcaradiene. Cycloaddition of dimethyl acetylenedicarboxlate to cyclopropa-1,6-methano[10]annulene would provide a precursor 4 for the linear dicyclopropabenzene however, the desired cycloadduct was not obtained. In this context, cyclopropa-1,6-methano[10]annulene gave no cycloadduct with 4-phenyl-4//-1,2,4-triazol-3,5-dione. On the other hand, the addition product 5 of 1.6-dimethylenecyclohepta-triene with 1 -bromo-2-chlorocyclopropene reacted with the acetylene derivative, but the twofold dehydrochlorination did not lead to the required norcaradiene. ... 

There have been extensive experimental and theoretical studies devoted to the structural and bonding characterization of weakly bound van der Waals complexes of acetylene. Structures of these complexes can often be determinated experimentally by means of Fourier transform microwave and infrared spectroscopic techniques. On the theoretical side, advanced treatments are required to understand the complex nature of the weak bonding in terms of the relative contributions of polarization and dispersion interactions, interactions of multiple moments, and electrostatic interactions involved in these completes. To determine the interaction energy in a weak complex, it is necessary to use large basis sets with the inclusion of electron correlation interactions. Theoretical calculations have been reported for van der Waals complexes of acetylene with COj [160], CO [161, 162], AICI3 [163], NH3 [164], He [165], Ar [166], H2O [167], HCN [168], HF [169-172], HCl [173, 174], and acetylene itself in the forms of non-covalent dimer [175-180], trimer [175,181], tetramer [175, 182, 183], and pentamer [175]. These calculations are very useful for the determination of multiple isomeric forms of the complex. For example, calculations at the MP2/6-31G level along with IR spectra indicate that the HCN-acetylene complex exists in a linear form in addition to the T-shaped structure observed previously by microwave studies (see Fig. 1-5) [168]. 

In addition, the linear approach was represented by sequential dithiane coupling [21] of the epoxides for the necessary fragments to either side of the ketone function at C21. Another approach uses microbial reduction (baker s yeast) to set the stereocenter at C25 before elaboration of that fragment into a methyl acetylenic ketone [89]. This acetylenic ketone was condensed with the aldehyde partner representing Cl5-19 to give the aldol adduct which was cyclized in acid to afford a precursor similar to those obtained from acetylide addition to lactone B 3. Yet another linear assembly pathway involves the alkylation of the portion containing C23-27 of 22,23-dihydroavermectin B to the dianion of 2,4-pentanedione followed by another condensation to 3-be-nzyloxypropanal [108]. Subsequent acidic cyclization and standard chemistry provided the thermodynamic spiroketal. 

Hartmann " has observed that the ratio of cyclobutene di-cyclopropyl adducts (e.g., 46 47) obtained upon irradiation of MA and acetylene or methylacetylene showed a linear relationship with inverse of MA concentration (Fig. 6-4). The passage of the plot through the origin can be reconciled only if the addition of triplet MA to acetylene occurs and is /mn5-stereospecific. 

Utilizing the methodology of selective thiol radical mono-addition to phenyl-acetylene derivatives, Voit and coworkers [236] recently synthesized a series of high refractive index hb polymers (refractive index of 1.68-1.75 with low optical dispersions of 0.004) by using different dithiol and trialkyne monomers. The hb structures produced materials with better performance in terms of light reflection and chromatic dispersion compared with linear analogs that were also synthesized for comparison. 

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