Olefinic bond

In a polluted or urban atmosphere, O formation by the CH oxidation mechanism is overshadowed by the oxidation of other VOCs. Seed OH can be produced from reactions 4 and 5, but the photodisassociation of carbonyls and nitrous acid [7782-77-6] HNO2, (formed from the reaction of OH + NO and other reactions) are also important sources of OH ia polluted environments. An imperfect, but useful, measure of the rate of O formation by VOC oxidation is the rate of the initial OH-VOC reaction, shown ia Table 4 relative to the OH-CH rate for some commonly occurring VOCs. Also given are the median VOC concentrations. Shown for comparison are the relative reaction rates for two VOC species that are emitted by vegetation isoprene and a-piuene. In general, internally bonded olefins are the most reactive, followed ia decreasiag order by terminally bonded olefins, multi alkyl aromatics, monoalkyl aromatics, C and higher paraffins, C2—C paraffins, benzene, acetylene, and ethane. 

The reaction mechanisms by which the VOCs are oxidized are analogous to, but much more complex than, the CH oxidation mechanism. The fastest reacting species are the natural VOCs emitted from vegetation. However, natural VOCs also react rapidly with O, and whether they are a net source or sink is determined by the natural VOC to NO ratio and the sunlight intensity. At high VOC/NO ratios, there is insufficient NO2 formed to offset the O loss. However, when O reacts with the internally bonded olefinic compounds, carbonyls are formed and, the greater the sunshine, the better the chance the carbonyls will photolyze and produce OH which initiates the O.-forming chain reactions. 

Total synthesis of macrolide immunodepressants using 1,3-dithiane aldol couplings and a-bond olefin constructions 98ACR35. 

On one hand, /z-alkanes of the molecular range C10-C16 are important starting materials for the synthesis of anionic surfactants. It is possible to dehydrogenate these hydrocarbons to isomeric /z-olefins with internal double bonds olefins) [4], which are also important initial products for the synthesis of an-... 

No organocobalt(III) complexes have yet been reported to contain a TT-bonded olefin or acetylene, though the former probably occur as reaction intermediates (see Section VI,B,l,d). In other words, equilibria such as... 

Another, very notable, case where the two definitions are in conflict is that of het-eroannular cisoid dienes. As we have mentioned, this was just the class of molecules that stimulated the introduction of the AAR. Here, in order to have the correct results one should refer the chirality of the axial substituent to the individual double bonds (olefin-picture), as depicted in Figure 6 and in the upper parts of Figure 7(b) and (c). The case of heteroannular dienes is anyway peculiar, because in these compounds the chromophore is unusually distorted. This case is treated in the following section. 

These reactions are covered in other chapters of Volume 11 (Chapters 11.06 and 11.07). This part deals only with examples which are in connection with other sections of this chapter. Additions of metallocarbenoids to unsaturated partners have been extensively studied. Most of the initial studies have involved the transition metal-catalyzed decomposition of cr-carbonyl diazo compounds.163,164 Three main reaction modes of metallocarbenoids derived from a-carbonyl diazo precursor are (i) addition to an unsaturated C-C bond (olefin or alkyne), (ii) C-H insertion, and (iii) formation of an ylid (carbonyl or onium).1 5 These reactions have been applied to the total synthesis of natural... 

Recently considerable attention has been given to chiral 72-bonded olefin spectator ligands for the asymmetric conjugate addition.122 Hayashi and Carreira independently reported that new chiral diene ligands 83,123 84,124 and 85125 showed high enantioselectivities in Rh(i)-catalyzed conjugate addition of arylboronic acids to enones (Scheme 41). 

A final example concerns the question of 7r-allyl and related complexes with hydrogen. Again the evidence cited in the preceding section suggests that the principal reaction of jr-allyl complexes with hydrogen is to yield olefins, desorbed from the surface, although the possibility that a TT-bonded olefin is formed first is a geometrically feasible process (Fig. 25). 

Cyclopropane and the cyclic compounds shown in Figure 1-12, cyclopentane and cyclohexane, are members of the alicyclic branch shown in Figure 1-4. The is the same prefix as used in the aliphatics because of the structure. Except for the cyclic formation, they are made up basically of chains of methylene groups (-CH2-). But. one difference from the aliphatic series of.organics is the ch-emical reactivity. Lower members of the alicyclic series have one chemical property similar to double-bonded olefins—they are quick to react chemically. 

Aliphatic- Straight- or branch-chain organic molecules that have saturated bonds (paraffins), double bonds (olefins), or triple bonds (acetylenes). 

There are three possible types of three-electron bonds. Oxidation of a u bond leads to a cation-radical with a, u three-electron bond. This bond contains no antibonding electrons, and the total bond strength exceeds that of a double bond by the energy of half a n bond. Olefins can acquire the 2a—In bond on one-electron oxidation, the bond constructed from the electrons 2a and In. Oxidation of organic disulfides, RSSR, to their cation-radicals (RSSR) yields species in which the unpaired electron from the oxidized sulfur interacts with the unbound p-electron pair of the second sulfur (Glass 1999). This establishes a 2n-In bond on top of the already existing o bond. The overall bond strength of this five-electron (2a—2n-In ) bond also exceeds that of the normal... 

We are currently trying to answer specifically the question of whether ir-bonded complexes do occur in certain cases where insertion reactions are observed. I think they do because I believe that the same factors which favor stabilization of this type of transition state will also tend to favor formation of 7r-bonded olefin complexes, which are only slightly removed from this. At the moment Bern Tinker is examining the insertion of olefins in mercuric complexes to see whether there is any indication of 7r-bonded intermediates. In his paper, Dr. Heck referred to some unpublished work relevant to this theme. I would certainly be interested in anything more he can tell us about that. 

The effects of hydrogen on the infrared spectra of adsorbed acetylene together with evidence from mechanistic studies of alkyne hydrogenation has led to the general conclusion that the acetylenic species active in hydrogenation is associatively bonded to the surface. However, as with monoolefins, there is still doubt as to the precise formulation of the surface—alkyne bonding. In the early work [156], it was assumed that the associatively adsorbed complex was adequately represented as a di-a-bonded olefin, which adopted a cis-configuration. 

The conversion of trans- to cis-stilbene by irradiation of either W(CO)6 or Mo(CO)6 is consistent with production of an excited metal carbonyl-stilbene complex which undergoes isomerization promptly.136, 137 The generation of the cis isomer represents movement away from the thermodynamic ratio of cis- and frans-stilbene which is 100% trans. Thus, an electronically excited state must be important in the reaction sequence. Two proposals have been made 137 (1) excitation of an M(CO) (stilbene)6 complex leads to a sigma bonded diradical intermediate as shown in reaction (49) or (2) electronic excitation may yield a perturbed IL excited state as for the styrylpyridine complexes discussed above. The sigma bonded olefin diradical... 

Quinones, Semiquinones, and Catechols. All molecules with unsaturated bonds (olefins, acetylenes, aromatics, carbonyls, quinones, etc.) have a degree of electrophilicity and electron affinity. Within a class, the extent of conjugation... 

Carbon-carbon double bonds (olefins) present significantly higher rotational barriers—typically 25-65 kcalmol 1—than single bonds, providing kinetic stability of both cis and trans isomers. This stability, together with the possibility of their interconversion by photoisomerization, have been exploited in the construction of a wide variety of rotors—and even directional molecular rotary motors—in which the rotor and base are connected via an olefin. 

A //ads, are negative, and it is convenient to define the heat of adsorption as being equal to -A//adS. Figure 3 shows initial heats for hydrogen and isobutylene adsorptions of 95 and 125 kJ/mol, respectively. In both cases, the heats of adsorption decrease with increasing coverage. The observed heat of isobutylene adsorption has been attributed to the formation of either di-er-bonded or r-bonded olefin on the tin-modified platinum surface (46). Recent microcalorimetric and infrared spectroscopic measurements characterizing adsorption of ethylene indicate that the addition of tin to... 

Rooney (6) favored a mechanism involving interconversion of intermediates as ligands of the same metal atom. Moreover, he suggested that ir-bonded olefin further interconverts on certain metal atoms, especially on Pd surfaces, with ir-allyl complexes and that the latter process may occur with trans elimination and addition of hydrogen atoms. This was suggested to explain initial exchange on both faces of a ring as in cyclopentane. The idea fitted well with the known propensity of Pd to form ir-allyl complexes and evidence was obtained that such... 

The only way of using exchange with deuterium to solve Problem B is to study compounds that have eclipsed vicinal pairs of hydrogens but with only the remotest chance of forming rr-bonded olefinic complexes. Caged compounds are necessary and a very suitable choice is the heptacyclotetradecane shown in Fig. 9... 

In Chapter 6, elimination reactions were presented. In the context of elimination reactions, the formation of double bonds was noted regardless of the elimination mechanism discussed. Continuing from the concept of using elimination reactions to form sites of unsaturation, one may reason that addition reactions can be used to remove sites of unsaturation. Thus, elaborating upon addition reactions, this chapter provides an introduction to relevant mechanisms applied to both carbon-carbon double bonds (olefins) and carbon-oxygen double bonds (carbonyls). 

M—has also been reported for olefins and acetylenes ir-bonded to rhodium and to platinum (6, 21, 46, 87). In the case of rhodium, iy(i°3Rh—is between 10 and 16 Hz for a 7r-bonded olefin (see Table XXVII), while for the cr-bonded carbon in [(C5H5)Rh(ff-C3Hs)-(w-CsHb)], 7( ° Rh—is 26 Hz. It was suggested the bonding of the olefin results from a 60% contribution from a dsp -vnet X orbital and sp -carbon orbital 21). For the olefins and acetylenes w-bonded to platinum 7( Pt—is between 18 and 195 Hz (see Table XXIX) compared to the range of 360 to 1000 Hz reported for carbon cr-bonded to platinum. It was found that 7( Pt— C) is less for a 7r-bonded acetylene than for a rr-bonded ethylene. This was considered as evidence for the Chatt-Dewar-Duncanson molecular orbital model 39, 63) of TT-bonding (XIV), rather than the formally equivalent valence-bond treatment, (XV) and (XVI) 46). However, no allowance appears to have been made for the effect on the hybridization at the carbon of the pseudo-... 

P. A. Bartlett, Olefin cyclization processes that form carbon-carbon bonds Olefin cyclization processes that form carbon-heteroatom bonds, Asymmetric Synthesis, (J. D. Morrison, Ed.), Academic Press, New York, 1984, Vol. 3, Part B, Chap. 5 and 6, pp. 341-454. 

Thermal decomposition of methyl xanthates is similar to the pyrolysis of acetates for the formation of the double bond. Olefins are obtained from primary, secondary, and tertiary alcohols without extensive isomerization or structural rearrangement. The other products of the pyrolysis of the methyl xanthates are methyl mercaptan and carbon oxy-sulfide. The xanthates prepared from primary alcohols are more difficult to decompose than those prepared from secondary and tertiary alcohols. Over-all yields of 22-51% have been obtained for a number of tertiary alkyl derivatives of ethylene. Originally the xanthates were made by successive treatment of the alcohol with sodium or potassium, carbon disulfide, and methyl iodide. In a modification of this procedure sodium... 

The mechanisms of all metal-catalyzed hydrosilations are thought to be very similar. The pathway probably involves an adduct composed of the silane, the alkene, and the metal. Transfer of the silicon to the carbon is believed to occur after the 7r-bonded olefin rearranges to a a complex. Whereas the mechanism displayed in the following scheme involves olefin insertion into Pt-H, equally possible is insertion into Pt-Si followed by reductive elimination of the alkyl silane. 

For the alkylation and alkenylation of C-H bonds, olefins and acetylenes are used as reactants. This type of coupling protocol is not applicable to arylation. Recently, a nitrogen-directed arylation of aromatic C-H bonds, leading to biaryl compounds has been developed. In 2001, Oi demonstrated that ruthenium(II)-phosphine can be used as a catalyst in the regioselective arylation of 2-arylpyridines using aryl halides (Eq. 9.34) [4 ]. The predominant ortho selectivity indicates that the coordination of... 

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