Solvent-free bond formation

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

This approach may find application in peptide bond formation that would eliminate the use of irritating and corrosive chemicals such as trifluoroacetic acid and piperidine as has been demonstrated recently for the deprotection of N-boc groups (Scheme 6.7) a solvent-free deprotection of N-tert-butoxycarbonyl group occurs upon exposure to microwave irradiation in the presence of neutral alumina doped with aluminum chloride (Scheme 6.7) [41]. 

The palladium-catalyzed coupling of aiyl and vinyl halides to organotin compounds, known as Stille coupling, is one of the most important catalytic methods of carbon-carbon bond formation. The reaction is generally conducted in polar organic solvents, such as dimethylformamide, with tertiary phosphine complexes of palladium, although phosphine-free complexes or simple Pd-salts are also frequently used as catalysts [8]. 

Despite the intense investigations, data on H H bond formation rates are rather limited since the process is usually too fast for studies by standard kinetic techniqnes. Nevertheless, some estimations are possible from low-temperatnre NMR spectra, nsually recorded in Freon solutions. These Freon solvents stop MH HX bond formation even on the H NMR time scale. For example, the ReH resonance of [Cp ReH(CO)(NO)] decoalesces in the presence of acidic alcohols at 96 K, giving two resolved lines at —7.54 and —8.87 ppm, assignable to the free and dihydrogen-bonded hydride, respectively [33]. Under these conditions, the lifetime of the MH- HX complexes, x, can be calculated via... 

Besides reactions with alkali salts, there are several groups investigating metal-ligand bond formation by mechanochemical methods. For example Orita et al. have shown that the supramolecular self-assembly of a number of complexes can take place under solvent-free conditions, leading to higher-order fabrications of two- or three-dimensional topology and even double... 

Microwave irradiation, for 15-20 min under solvent-free conditions, promoted the regiospecific three-component one-step cyclocondensation of benzoylacetonitrile, an aromatic aldehyde, and aminopyrimidinones 460 to give 6-cyano-5,8-dihydropyrido[2,3-, pyrimidin-4(3/7)-ones 461 rather than the isomers 462. The formation of 461 proceeds via a Michael-type addition of C-5 in aminopyrimidine 460 to the activated double bond of the arylidene-benzoylacetonitrile intermediate followed by cyclization with the removal of a water molecule. Compounds 461 were also prepared conventionally by refluxing the reactants in absolute ethanol for 40 8 h <2001TL5625>. 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

Type of reaction C-C bond formation Reaction condition solvent-free... 

---