Nonstereospecificity

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

Mixtures of 1 -phenyl-3,3,3-tnfluoropropene which are 97-98% E and 2-3% Z react quickly with elemental chlonne without irradiation or heating The reaction IS nonstereospecific and produces mixtures of diastereomers m ratios between 1 1 5 and 1 3 i [9] (equation 3)... 

As a result the polymer chain growth seems to stop due to the impossibility of the monomer coordination on the titanium ion. The propagation centers are nonuniform and the adsorption of the aluminumorganic compound takes place mostly at sterically more accessible nonstereospecific active centers. 

In investigating the mechanism of addition to a double bond, perhaps the most useful type of information is the stereochemistry of the reaction. The two carbons of the double bond and the four atoms immediately attached to them are all in a plane (p. 8) there are thus three possibilities. Both Y and W may enter from the same side of the plane, in which case the addition is stereospecific and syn they may enter from opposite sides for stereospecific anti addition or the reaction may be nonstereospecific. In order to determine which of these possibilities is occurring in a given reaction, the following type of experiment is often done YW is added to the cis and trans isomers of an alkene of the form ABC=CBA. We may use the cis alkene as an example. If the addition is syn, the product will be the erythro dl pair, because each carbon has a 50% chance of being attacked by Y ... 

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. ... 

If the mechanism for nucleophilic addition is the simple carbanion mechanism outlined on page 975, the addition should be nonstereospecific, though it might well be stereoselective (see p. 166 for the distinction). For example, the E) and (Z) forms of an alkene ABC=CDE would give 6 and 7 ... 

The diion mechanism c has been reported for at least some of the reae-tions in categories 3 and as well as some ketene dimerizations. For example, the rate of the reaction between l,2-bis(trifluoromethyl)-l,2-dicyanoe-thene and ethyl vinyl ether was strongly influenced by changes in solvent polarity.Some of these reactions are nonstereospecific, but others are stereo-specific. As previously indicated, it is likely that in the latter cases the di-ionic... 

The lUPAC designation is Dn + De (or Dn+Dh). This mechanism normally operates without an added base. Just as the E2 mechanism is analogous to and competes with the Sn2, so is the El mechanism related to the SnE In fact, the first step of the El is exactly the same as that of the SnI mechanism. The second step differs in that the solvent pulls a proton from the P carbon of the carbocation rather than attacking it at the positively charged carbon, as in the SnI process. In a pure El reaction (i.e., without ion pairs), the product should be completely nonstereospecific, since the carbocation is free to adopt its most stable conformation before giving up the proton. 

Lansbury, RT. Mancuso, N.R. Tetrahedron Lett, 1965, 2445 have shown that some Beckmann rearrangements are authentically nonstereospecific. 

The complete product stereospecificity (presumably retention ) observed in this case is in strong contrast to the complete nonstereospecificity observed in... 

Evidence for the intermediate formation of a free radical comes from labeling experiments, which show that both reduction and isomerization, which sometimes proceeds concurrently, are nonstereospecific 94,10). 

The stereochemistry of these reactions depends on the lifetime of the dipolar intermediate, which, in turn, is influenced by the polarity of the solvent. In the reactions of enol ethers with tetracyanoethylene, the stereochemistry of the enol ether is retained in nonpolar solvents. In polar solvents, cycloaddition is nonstereospecific, as a result of a longer lifetime for the zwitterionic intermediate.177... 

Arguably the most challenging aspect for the preparation of 1 was construction of the unsymmetrically substituted sec-sec chiral bis(trifluoromethyl)benzylic ether functionality with careful control of the relative and absolute stereochemistry [21], The original chemistry route to ether intermediate 18 involved an unselective etherification of chiral alcohol 10 with racemic imidate 17 and separation of a nearly 1 1 mixture of diastereomers, as shown in Scheme 7.3. Carbon-oxygen single bond forming reactions leading directly to chiral acyclic sec-sec ethers are particularly rare since known reactions are typically nonstereospecific. While notable exceptions have surfaced [22], each method provides ethers with particular substitution patterns which are not broadly applicable. 

Reactions of aromatic ketones with electron-rich olefins are in general nonstereospecific, as evidenced by the same product distribution being obtained from reaction with both cis- and Oww-2-butene<92> ... 

The yields of cyclopropanes in this case are low in relation to the amount of acetophenone formed. However, similar cyclopropane product ratios are obtained when photolysis is carried out in the presence of Michler s ketone as sensitizer. Thus the carbene intermediate produced in the direct irradiation is thought to be a triplet, as suggested by the nonstereospecificity of its addition. Whether this intermediate arose from singlet diazoacetophenone (via singlet decomposition and intersystem crossing of the singlet carbene) or by decomposition of the triplet molecule was not determined. 

The stereospecificity of addition suggests a singlet(10) carbene although the ground state of cyclopentadienylidene is known to be a triplet. Attempts to produce a triplet species, which would be expected to react nonstereospecific-ally, in a 4-methyl-cw-2-pentene matrix at 77°K or by dilution of mixtures of the azo compound and olefin with hexafluorobenzene or octafluorocyclo-butane (inert diluents) were unsuccessful. It was concluded that the singlet carbene produced upon photolysis reacts more rapidly with the olefinic... 

A singlet carbene was proposed to account for this stereoselectivity. Attempts to produce triplet carbene by collisional deactivation with octafluorocyclo-butane were unsuccessful and stereospecific addition to olefin still occurred. However, nonstereospecific addition to olefins and larger amounts of olefinic (insertion) products result from irradiation of the phenyldiazomethane in a frozen m-butene matrix at — 196°C ... 

Treatment of the photolysate with methanol gave two diastereomeric methoxysilanes in about 3 1 proportions, consistent with methanol addition being a nonstereospecific process. During extended photolysis (8 hours) to convert all of 25 to 26 it was observed that the concentration of 26 decreased, and a set of signals characteristic of a different silene grew in (no change of relative proportions occurred in the dark). The chemical... 

The epoxidation of electon-defident olefins using a nucleophilic oxidant such as an alkyl hydroperoxide is generally nonstereospecific epoxidation of both cis- and /nmv- ,/3-unsatii rated ketones gives the trans-epoxide preferentially. However, the epoxidation of cis-ofi-unsaturated ketones catalyzed by Yb-(40) gives civ-epoxides preferentially, with high enantioselectivity, because the oxidation occurs in the coordination sphere of the ytterbium ion (Scheme 26).132... 

Flowever, Mansuy el al. reported that treatment of olefins with [A-(p-toluenesulfonyl)imino]-phenyliodinane, PhI=NTs, in the presence of a Mn or Fe porphyrin, provided the corresponding aziridine or allylic sulfonamide, depending on the substrate and the catalyst used (Scheme 33).145-147 The reaction has been considered to proceed through a metal-A-toluenesulfonylnitrenoid species. Similarly to epoxidation via the oxo-Mn species, this aziridination is nonstereospecific, and the participation of a radical intermediate has been suggested.146... 

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