Olefinic systems

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

An indication of the most frequently used routes for the synthesis of common steroid olefin systems follows below. 

Fluorocylatwn of enarnines and enamides has been intensively studied by different groups [78, 79, 80 SI] The effectiveness of this particular electrophilic substitution reaction becomes obvious when the nitrogen atom of the enamine moiety is engaged in an aromatic system [82 S3] or when the olefinic system is part of an aromatic nucleus [84] (equations 37 and 38) A further extension of this reaction is demonstrated by the tnfluoracetylation of aldehyde dialkyl hydrazones [S5 86] (equation 39)... 

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

Usually the stronger acids are also the more effective co-catalysts, but exceptions to this rule are known. Trichloroacetic acid, but not the equally strong picric acid, will co-catalyze the system isobutene-titanium tetrachloride in hexane.2 8 Some Lewis acid-olefin systems will not polymerize at all in the absence of a co-catalyst, an example being isobutene with boron trifluoride.2 4 This fact, together with the markedly slower reaction usual with carefully dried materials, has nourished the current suspicion that a co-catalyst may be necessary in every Lewis acid-olefin polymerization. It is very difficult to eliminate small traces of water which could act as a co-catalyst or generate mineral acid, and it may well be that the reactions which are slower when drier would not go at all if they could be made completely dry. 

Conjugated olefinic systems have also been used in reaction with phosphorus-halogen species. For example, methylphosphonic dichloride adds regioselectively to acrylic acid in the presence of phosphorus trichloride (used as solvent), to produce the acid chloride 3-phosphonopropionyl chloride (Equation 4.35).158... 

The 13C NMR data for pentafulvene (1) and heptafulvene (2) (Table 3) and for 6,6-dimethylpentafulvene (5) and sesquifulvalene (6), afford evidence of the extent to which polar structures of the types 5a and 6a contribute to the ground state. If the chemical shifts are analyzed on the basis of electron density, these hydrocarbons are to be considered as olefinic systems with only a small contribution (10% at most) from the polar structures 5a and 6a. 

Ionic and Radical Mechanisms in Olefinic Systems, whith special Reference... 

Non-coplanarity of the aromatic (or olefinic) system and the nitro group attached to it has been thought to be a prerequisite to an efficient nitro-nitrite rearrangement si) (see ref. for a review of this well-known reaction). Exceptions to this rule have been found ... 

With Equation 3-5, the general problem can be reduced to the ozone-olefin system ... 

The Chiral lr(l)/Organic Base/Anilines/Olefins System... 

The coupling constants in unsaturated (olefinic) systems depend on the nature of the substituents attached to the C=C but for the vast majority of substituents, the ranges for -fH-c=c-H(c ) H-c=c-H(tra i) Overlap. This means that the stereochemistry... 

It has dso proved possible in the metal-olefin systems formed to isolate intermediates which have short lives in simpler systems for example, the complex PtBr4 (vp) can be seen as analogous to the unstable yellow oils sometimes reported in the oxidation of Zeise s salt to PtCU and 1,2 dichloroethane. Even weak interactions which affect the course of a reaction can be noted. Neither of the vinyl groups in PtBr2 (oS)2 is co-ordinated to the platinum atom, but bromine oxidation yields a Pt-carbon o-bond, indicating some interaction in the transition state at least. 

The same workers (46) report that HCl adds irreversibly to an Rh-olefin system when carbonylated alcoholic solutions of RhCls are refluxed with one mole of vp. The product is assigned a structure as in Fig. 35, the metal being in the - -3 oxidation state. The five-coordinate RhCl(CO)2 (vp) is held to be an intermediate in the formation of the Rh(III) system. 

Figure 7.1 Coordination of an olefin to the transition metal fragment M(PPh3)2 (M =e.g. Ni, Pd, Pt) leads to a deviation from planarity of the olefin system. The angle 9 is a measure of the deformation of the groups attached to the carbon-carbon double bonds from planarity.

Calculated transition-state geometries (Fig. 5) show a clear similarity with the alkyl -I- olefin system described in the previous section. Transition states are somewhat earlier, since these reactions are much more exothermic this can be seen, e.g., from the noticeably different C- H bond lengths of the hydrogen transfer transition state. 

Other advantages include a mechanism that allows one to rationalize and predict the stereochemical outcome for various olefin systems with a reasonable level of confidence utilising a postulated spiro transition state model. The epoxidation conditions are mild and environmentally friendly with an easy workup whereby, in some cases, the epoxide can be obtained by simple extraction of the reaction mixture with hexane, leaving the ketone catalyst in the aqueous phase. 

When the catalyst was used for simple olefin systems, it was not as active as with the amino acid precursors. Table III shows the relative rates for a variety of substrates, special care being taken in each case to purge oxygen. The slow rate of a-phenylacrylic acid was unexpected, but, it may be the result of a stable olefin-rhodium complex similar to the one Wilkinson (15) experienced with ethylene. Such a contention is consistent with the increased speed of hydrogenation with increased pressure. 

Whereas FMO theory correctly predicts the regioselectivity for cycloadditions in simple alkyl-substituted olefinic systems,51,58 extension of similar calculations for cycloadducts (7a,b-lla,b)120 predicts the formation of regioisomer a, although, except in the case of 7 and 8, the b isomer is the predominant one. The differences between prediction and experiment in stereoselectivity have been attributed primarily to double bond rehybridization arising from double bond distortion in bridgehead olefins,142 which also explains their enhanced reactivity.96,120 Also double-bond deformation that will alter the normal mixing of alkyl substituent orbitals with localized rc-bond orbitals may explain the unexpected formation of 8b.120 Attempts to explain the formation of the b isomers, based on a two-step diradical mechanism, also have failed.120... 

New chiral centers are produced by addition reactions to other trigonal centers as well. Hydrogenation of 3-methyl-3-hexene gives 3-methylhexane. Clearly the addition of hydrogen to one face of the planar olefinic system gives one enantiomer and addition to the opposite face gives the opposite enantiomer. Likewise reaction of styrene with chlorine or bromine (X2) or potassium permanganate produces products with a new chiral center. Formation of the two possible enantiomers results from addition to either face of the olefin. 

This radical could then eliminate an H atom to form a C=S bond, also analogous to olefinic systems [71, 72], to form the species observed at m/e = 80,... 

Many unsaturated compounds found in nature contain one or more acetylenic bonds, and these are predominantly produced by further desaturation of olefinic systems in fatty acid-derived molecules. They are surprisingly widespread in nature, and are found in many organisms, but are especially common in plants of the Compositae/Asteraceae, the Umbelliferae/Apiaceae, and fungi of the group... 

The ortho cycloaddition is thermally forbidden in a suprafacial-suprafacial manner and the photochemical reaction is forbidden with S benzene and ground-state alkene. On the basis of these considerations, it could be understood that the ortho addition had only been observed with systems where the alkene is the lowest excited singlet species (as with maleimides [37,74,75] or where either the alkene or the arene has marked acceptor properties (the only examples known at that time were benzene-acrylonitrile [127] and benzonitrile + a mono-olefin [1,73], Benzene-acrylonitrile and benzonitrile-olefin systems do not display charge-transfer absorption, but charge transfer could well follow excitation. Bryce-Smith further stated that irradiation of benzene in the presence of simple mono-olefins normally provides B2u (Si) benzene as the lowest excited singlet species, which leads to meta rather than ortho addition, but the latter process might, in principle, be able to occur under conditions where a Biu (S2) state of benzene is populated. 

The process of obtaining alkyl sulphonate in an autoclave with a mixer has been studied. Basic reagents were water solution of sodium hydrosulfite 36-38% and industrial olefin fractions at 240-320 °C. NaN03 and oxygen from air were used as initiators of the reaction of free radicals. System factors are x, reaction time, h x2 temperature of reaction, °C x3 mole ratio of sodium hydrosulfite and olefin x3 mole ratio of NaN03 and olefin x5 volume ratio of N-propanol and olefin. System response is a yield of alkyl sulfonate as a per cent of theoretical yield. FRFE design... 

An interesting alternative shows that the conversion of 3 into the secondary adducts 7 can be effected by treatment with additional equivalents of the cuprates In such a conversion both alkyl groups of J are exploited in functionalization of the olefinic system. The nucleophilic properties of the new complexes appear to be remarkable. Although 7 are reactive towards water and alkyl halides giving and 5 respectively, no reactions under standard conditions were observed with aldehydes and ketones. 

Fang N., Teal P. E. A., Doolittle R. E. and Tumlinson J. H. (1995a) Biosynthesis of conjugated olefinic systems in the sex pheromone gland of female tobacco homworm moths, Manduca sexta (L.). Insect Biochem. Mol. Biol. 25, 39 18. 

A 71-allylic together with an olefin system are considered to be involved in bonding to the iron tricarbonyl residue of bicyclo[3.2.1]octadienyliron tricarbonyl cation (VIII) (196). This is apparent from the X-ray structural determination, although a delocalized form cannot be discounted (IX). 

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