Olefins substituted

The problem of the synthesis of highly substituted olefins from ketones according to this principle was solved by D.H.R. Barton. The ketones are first connected to azines by hydrazine and secondly treated with hydrogen sulfide to yield 1,3,4-thiadiazolidines. In this heterocycle the substituents of the prospective olefin are too far from each other to produce problems. Mild oxidation of the hydrazine nitrogens produces d -l,3,4-thiadiazolines. The decisive step of carbon-carbon bond formation is achieved in a thermal reaction a nitrogen molecule is cleaved off and the biradical formed recombines immediately since its two reactive centers are hold together by the sulfur atom. The thiirane (episulfide) can be finally desulfurized by phosphines or phosphites, and the desired olefin is formed. With very large substituents the 1,3,4-thiadiazolidines do not form with hydrazine. In such cases, however, direct thiadiazoline formation from thiones and diazo compounds is often possible, or a thermal reaction between alkylideneazinophosphoranes and thiones may be successful (D.H.R. Barton, 1972, 1974, 1975). 

The dipolar ion can react in several ways according to the solvent and the stmcture of the olefin. In inert solvents, if the carbonyl compound is highly reactive (eg, an aldehyde), the dipolar ion can be added to the carbonyl fragment to give the normal ozonide or 1,2,4-trioxolane (7) for example, 1,1-and 1,2-dialkylethylenes react in this manner. Tri- or tetraalkyl-substituted olefins produce a smaH, if any, yield of an ozonide when the ozonolysis is... 

Ritter Reaction (Method 4). A small but important class of amines are manufactured by the Ritter reaction. These are the amines in which the nitrogen atom is adjacent to a tertiary alkyl group. In the Ritter reaction a substituted olefin such as isobutylene reacts with hydrogen cyanide under acidic conditions (12). The resulting formamide is then hydroly2ed to the parent primary amine. Typically sulfuric acid is used in this transformation of an olefin to an amine. Stoichiometric quantities of sulfate salts are produced along with the desired amine. 

The dienoplules for reaction with butadiene can be alkenes, allenes, and alkynes. Simple alkenes like ethylene are poor dienoplules resulting in sluggish reactions. Substituted olefins, X—C=C—X, are more reactive when X and/or X are C=C, Ar, COOR, COOH, COH, COR, COCl, CN,... 

The olefins that undergo metathesis include most simple and substituted olefins cycHc olefins give linear high molecular-weight polymers. The mechanism of the reaction is beheved to involve formation of carbene complexes that react via cycHc intermediates, ie, metaHacycles. Industrial olefin metathesis processes are carried out with soHd catalysts (30). 

Olefins by elimination from quaternary ammonium salts (less substituted olefin preferred). 

A variety of synthetic polymers, including polycarbonate resins, substituted olefins, and polyelectrolyte complexes, are employed as ultrafiltration membranes. Many of these membranes can be handled dry, have superior organic solvent resistance, and are less sensitive to temperature and pH than cellulose acetate, which is widely used in RO systems. 

A considerable number of examples will be found in the text in which halo-genated and other substituted olefins are produced. Their modes of fonnation do not usually differ in principle from the corresponding unsubstituted case. However, some special methods have been used, for example, the direct preparation of halo olefins from ketones with phosphorous hahdes, or via hydrazones. 

Halogen-substituted olefins may be obtained from hydrazones by the action ofN-halocompounds. 327, /. 328.372 (Chloro compounds of this type may also be made by the direct action of phosphorous pentachloride on the ketone, e.g. ref. 329, 330.)... 

Perfluoroolefins isomerize photochemically to yield less substituted olefins [272] Photolysis of polyfluorotriarylmtrones leads to polyfluorotriaryloxaziridines [173] (equation 43)... 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

This model prediets that tri-substituted and tetra-substituted olefins would also be poor substrates. Thus it was not until 1994 that a study in the epoxidation of higher substituted olefins appeared. Indeed Jaeobsen revealed that tri-substituted olefins, and even tetra-substituted olefins ean be excellent substratesA new model was put forth that encompasses a skewed side-on approach of tri-substituted olefins to the Mn-oxo eomplex. The observation that certain tetrasubstituted olefins undergo epoxidation with good enantioseleetivity suggests that further studies are needed in order to fully understand the transition state geometry of the catalyst and substrate. 

In 1909, Patemo and Chieffi noted that mixtures of tri- or tetra-substituted olefins and aldehydes formed trimethylene oxides when exposed to sunlight. Biichi later repeated Patemo s experiments by irradiating 2-methyI-2-butene in the presence of benzaldehyde, butyraldehyde, or aeetophenone and rigorously purifying and identifying the resulting products. The reaction thus bears the name of its two primary pioneers and has come to represent any photo-catalyzed [2 + 2] electrocyclization of a carbonyl and an alkene. 

The Diels-Alder reaction of a diene with a substituted olefinic dienophile, e.g. 2, 4, 8, or 12, can go through two geometrically different transition states. With a diene that bears a substituent as a stereochemical marker (any substituent other than hydrogen deuterium will suffice ) at C-1 (e.g. 11a) or substituents at C-1 and C-4 (e.g. 5, 6, 7), the two different transition states lead to diastereomeric products, which differ in the relative configuration at the stereogenic centers connected by the newly formed cr-bonds. The respective transition state as well as the resulting product is termed with the prefix endo or exo. For example, when cyclopentadiene 5 is treated with acrylic acid 15, the cw fo-product 16 and the exo-product 17 can be formed. Formation of the cw fo-product 16 is kinetically favored by secondary orbital interactions (endo rule or Alder rule) Under kinetically controlled conditions it is the major product, and the thermodynamically more stable cxo-product 17 is formed in minor amounts only. 

Branching occurs especially when free radical initiators are used due to chain transfer reactions (see following section, Free Radical Polymerizations ). For a substituted olefin (such as vinyl chloride), the addition primarily produces the most stable intermediate (I). Intermediate (II) does not form to any appreciable extent ... 

Table 5.3 Deprotonation with s-BuLi and treatment with orga-noaluminiums or organozincs to give a range of triphenylsilyl-substituted olefins.

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

With few exceptions, radicals are observed to add preferentially to the less highly substituted end of unsymmetrically substituted olefins (i.e. give predominantly tail addition - Scheme l.l). 

TABLE 1. Sulfones from sulfinic acids RS02H and acceptor-substituted olefins, acetylenes or quinones ... 

Imide-terminated telechelics are also synthesized by metathesis depolymerization, and it is found that phthalimide-substituted olefins allow for productive depolymerization when only one methylene spacer separates the nitrogen atom and the olefin (Fig. 8.21). This combination of steric hindrance around the nitrogen lone pair and decreased electron donation from resonance prevents the negative neighboring group effect. However, secondary acyclic amines are unable to produce telechelics through metathesis depolymerization because of unfavorable catalyst-amine interactions. 

In similarly substituted olefins Kf is strongly influenced by steric effects, as shown by the comparison of tetraisobutylethylene with adamantylideneadamantane and (i,/-D3-trishomocubylidene-D3-trishomocubane. In particular, the comparison between cyclohexene and the two tetrasubstituted cage olefins indicates that Kf increases at least by a factor of 103 on passing from a 1,2 disubstituted to a tetrasubstituted olefin. This dependence is likely to be similar in other solvents, because solvent effects on Kf are modest. 

The geometrical structure of the perepoxide quasi-intermediate was suggested to play critical roles in determining diverse selectivities of the reactions of O with substituted olefins [92]. ... 

Other substituted olefins such as acrylonitrile, fumaronitrile, crotono-nitrile, cinnamonitrile, and diethylfumarate also formed adducts with Co (DMG)2 complexes containing py, H2O, or PBuj and, in one case, with [Co (DMG-BF2)2py]. Second-order rate constants were reported for the formation of several Tr-olefin-Co(I) complexes from organocobalt(III) complexes containing, for example, NCCH2CH2- with DMG, DPG, DMG-BF, py, H2O, and PBuj. 

Constrained geometry chromium alkyls catalyzed the polymerization of ethylene however, the reaction was relatively slow, and elevated pressures (PC2H4 = 500 psi) were required to generate significant amounts of polymer. Not surprisingly then, no homopolymoization or copolymerization of a-olefins was observed. Instead, catalytic isomerization and dimerization of the alkyl-substituted olefins was found. 

The first cross metathesis to form a tetra-substituted olefin was achieved recently [146]. Howell and co-workers used lactams as substrates for CM with mono- and di-substituted olefins. The authors suggest that the limitations of the method are primarily due to steric reasons. Varying the electron density of the lactam showed no great influence on the reactivity while steric influences like a-branched allylic crosspartners or a methyl-group in the C4-position of the lactam both led to no reaction (Scheme 3.13). 

Scheme 3.13 CM to form tetra-substituted olefins (Boc = tert-butyloxycarbonyl)...

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