Olefin structures

Some years ago, we tackled (ref. 7) the particular question of bromine bridging, related mainly to stereochemistry, postulating that bromonium ions and bromo-carbocations are formed in separate pathways as shown in Scheme 3. The relative rates of reaction by these pathways depend on the olefin structure. As demonstrated later... 

Wavelengths of about X -- 200 nm exdte olefinic structures in particular, causing n n transitions. 

Wavelengths of about A. = 200 nm excite olefinic structures in particular, causing jt 71 transitions. 

The difference in activity of the E and Z olefins against HRV-14 was explained by examining the relatively low energy virus-bound conformations. The result of an overlay of WIN-54954 (based on x-ray crystallography data), minimize E- and Z-olefinic structures and the butyne analogue, suggested that the E isomer showed a reasonable fit while the Z isomer did not. Furthermore, when the Z isomer was inserted into the HRV-14 pocket, unfavorable interactions occurred. 

For cyclopropanations with ethyl diazoacetate, a rather weak influence of the olefin structure has been noted 59 60, (Table 7). The preference for the sterically less crowded cyclopropane is more marked for 1,2-disubstituted than for 1,1-disubstituted olefins. The influence of steric factors becomes obvious from the fact that the ratio Z-36/E-36, obtained upon cyclopropanation of silyl enol ethers 35, parallels Knorr s 90> empirical substituent parameter A.d of the group R 60). These ZjE ratios, however, do not represent the thermodynamic equilibrium of both diastereomers. 

Branching in the olefinic structure at one or both of the double-bonded... 

Luef, W., Strained Olefins Structure and Reactivity ofNonplanar Carbon-Carbon Double Bonds, 20, 231. 

Computation allows one to circumvent nature s reluctance to offer the dihydride to direct detection. The first papers using molecular mechanics to study asymmetric hydrogenation appeared in the late 80 s [53-55], However, molecular mechanics is not the ideal technique for any reaction that involves bond-breaking or bond-forming, such as all catalytic reactions, and only a limited amount of reliable information was obtained from these early studies. An MP2/QC/5IXT) study of (PH3)2Rh(olefin) structures was published in... 

The gas-phase reactivity of various terpenes has been measured. Stephens and Scott were the first to include two terpenes (pinene and a-phel-landrene) with their study of the relative reactivity of various hydrocar ns. Both monoterpenes showed the high reactivity predicted by their olefinic structure. Conversion of nitric oxide to nitrogen dioxide in e presence of isoprene is at a rate intermediate between those for ethylene and trans-2-butene, and Japar et al, reported rate constants for the a-pinene and terpinolene-ozone reactions. Grimsrud et a/. measured the rate con-... 

Because of the multiple conjugated olefinic structure in the molecule, pure crystalline carotenoids are very sensitive to light and air and must be stored in sealed containers under vacuum or inert gas to prevent degradation. Thus, commercial utilization as food colorings was initially limited however, stable forms were developed and marketed as emulsions, oil solutions and suspensions, and spray-dried forms. 

Alkene (olefinic) structures introduce several new modes of vibration into a hydrocarbon molecule a C=C stretching vibration, C—H stretching vibrations in which the carbon atom is present in the alkene linkage, and in-plane and out-of-plane bending of the alkene C—H bond. The spectrum of Figure 3.10 is that of a typical terminal alkene. 

T he epoxidation of olefins using organic hydroperoxides has been studied in detail in this laboratory for a number of years. This general reaction has also recently been reported by other workers (6,7). We now report on the effects of five reaction variables and propose a mechanism for this reaction. The variables are catalyst, solvent, temperature, olefin structure, and hydroperoxide structure. Besides these variables, the effect of oxygen and carbon monoxide, the stereochemistry, and the kinetics were studied. This work allows us to postulate a possible mechanism for the reaction. 

Effect of Olefin Structure. The reaction rate of the epoxidation depends on olefin structure. In general, the more alkyl substituents bonded to the carbon atoms of the double bond, the faster the reaction rate. This was shown by a reaction of 2-methyl-2-pentene, cyclohexene, and 2-octene with cumene hydroperoxide under the same conditions (Table V). The yield of epoxide was quantitative. The results indicate that 2-methyl-2-pentene reacts faster than cyclohexene and 2-octene. 

Cundall has done extensive work on benzene231,237 and acetone243 sensitized isomerizations of the 2-butenes, and in every case reported a photostationary or radiostationary trans/cis ratio of 1.27-1.37. Sato, however, has measured a value of unity for the benzene photosensitized isomerization.510 With higher homologs, from 2-pentene to 2-octene, benzene-sensitized isomerizations yield trans/cis ratios of 1.0,238 while acetone-sensitized isomerization of the 2-pentenes in solution yields a ratio of 1.65.244 At present no explanation is possible for the differences between 2-butene and 2-pentene. Until much more information is gathered relating to rates of triplet energy transfer as functions of olefin structure, sensitizer, and medium, the natural decay ratios of each olefin s common triplet cannot be deduced from photostationary trans/cis ratios. 

Some Effects of the Olefinic Structure on the Orientation of the Hydroformylation Reaction... 

With Rh, the phenomena are strikingly different since the aromatic ring does not influence the selectivity which is always very high, no matter what olefinic structure (except for 1,1-diphenylpropene). Isomer selectivity depends on several factors, mainly the structure and the stereochemistry of the alkenylbenzene. With conjugated alkenylbenzenes, addition of the CHO group occurs preferentially on the a carbon when the a and / carbons are monosubstituted. When the ft carbon is disubsti-tuted, because of steric requirements, hydroformylation is less selective and occurs on both the a and y carbon ... 

Vicinal fluoroalkyl ethers are also obtained from olefins and methyl hypo-fluorite at low temperatures [168] and from the unstable reagents generated from additions of xenon difluoride to methanol [169] or other alcohols [170] Alcohols tend to give complex product mixtures depending on the olefin structure, and best results are often achieved when reactions are run in the presence of boron tri-fluonde Additions of xenon difluoride to trifluoromcthanesulfomc fluorosulfonic, or nitric acids give fluoroxenonium reagents that are stable to about -10 °C and... 

Farmer, E.H. and Sutton, D.A. 1946. Peroxidation in relation to olefinic structure. Trans. Faraday Soc. 42 228-232. 

By trapping PX at liquid nitrogen temperature and transferring it to THF at —80° C, the nmr spectmm could be observed (9). It consists of two sharp peaks of equal area at chemical shifts of 5.10 and 6.49 ppm downfield from tetramethylsilane (TMS). The fact that any sharp peaks are observed at all attests to the absence of any significant concentration of unpaired electron spins, such as those that would be contributed by the biradical (11). Furthermore, the chemical shift of the ring protons, 6.49 ppm, is well upheld from the typical aromatic range and more characteristic of an olefinic proton. Thus the olefin structure (1) for PX is also supported by nmr. 

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