Olefins electron-donor substitution

The coupling of bromo- or iodobenzene to styrene yields regioselectively a mixture of E- and Z-stilbenes 12 and 13. An electron-withdrawing substituent at the olefinic double bond often improves the regioselectivity, while an electron-donor-substituted alkene gives rise to the formation of regioisomers. 

Thiocoumarin (127) underwent [2 + 2]-photocycloaddition reactions in better yields than 126. In contrast to coumarin, cis- and trans-fused products are being found, however, for example, in the reaction with 2,3-dimethyl-2-butene, possibly because the thiopyran ring is more flexible than the pyran ring due to the longer C—S bonds. HTproduct is favored with electron donor-substituted olefins [121]. Electron acceptor substitution in 3-position, as in 3-cyano-l-thiocoumarin (128), leads to an improved performance in [2 + 2]-photocycloaddition reactions [122]. 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

Photoaddhion of electron donor olefins such as vinyl ethers and stilbene to variously methyl and halogeno-substituted 1,4-benzoquinones resulted in the formation of dihydrobenzofurans via a dienone-phenol rearrangement of the primary product spirooxetanes <96H(43)619>. High-temperature water seems to be an alternative to use of acid catalysts or organic solvents by the cyclization of allyl phenyl ethers to dihydrobenzofurans <96JOC7355>. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

Ruthenium catalysts, coordinated with an N-heterocyclic carbene allowed for the ROMP of low-strain cyclopentene and substituted cyclopentenes (10,23). Suitable ruthenium and osmium carbene compounds may be synthesized using diazo compounds, by neutral electron donor ligand exchange, by cross metathesis, using acetylene, cumulated olefins, and in an one-pot method using diazo compounds and neutral electron donors (24). The route via diazo compounds is shown in Figure 1.7. 

Carbon-13 shifts of enamines [342] follow the behavior described for other donor substituted alkenes (Sections 4.4.3 and 4.6.2). Electron release by the dialkylamino group has two consequences The inductive electron withdrawal at the a alkene carbon is reduced (Za 10-15 ppm) compared with the a increments of aliphatic amines (Table 4.43). Further, electron density at the fi olefinic carbon increases, as indicated by considerable shieldings in pyrrolidino- and morpholinoalkenes. 

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

A different approach must be used for the photochemical hydrophosphination of electron-poor olefins, and this involves a PET reaction. Silyl phosphites (e.g., 30) were used as electron donors, whereas conjugated ketones have the double role of electron acceptors and absorbing species. Thus, the irradiation of a mixture containing 2-cydohexenone and 30 generated an ion pair. The phosphoniumyl radical cations decomposed to give trimethylsilyl cations (which in turn were trapped by the enone radical anion) and phosphonyl radicals. A radical-radical combination afforded the 4-phosphonylated ketones in yields ranging from 78% to 92% (Scheme 3.20) [49]. This reaction was exploited for the preparation of substituted phosphonates, which serve as key intermediates in the synthesis of a class of biologically active compounds. 

A simple example of photopolymerization based on PET is realized, if the monomers polymerized are the electron donor (D) and electron acceptor (A) themselves [12]. Therefore, the electron densities of both compounds must be very different. With vinyl monomers, combinations of donor and acceptor substituted olefins are especially suitable systems. 

The presence of donors which can occupy the alkene coordination site slows down the reaction. A kinetic analysis of the epoxidation of a number of alkenes has shown that the coordinating power of the olefin, and its tendency to undergo the intramolecular attack, increase with substitution of the alkene with electron donor groups provided they are not too bulky ° ... 

This similarity between cyclopropane and olefin was first illustrated by the displacement of ethylene from [(C2H4)PtCl2]2 with various substituted cyclopropanes (equation 37) . The reaction was found to depend markedly on the electron-donor capacity of the cyclopropane ring . ... 

The magnitude of the LCAO MO coefficients of the interacting orbitals permits a prediction of the expected oxetane regiochemistry. Both in the HOMO of a donor-substituted olefin and in the LUMO of an acceptor-substituted olefin, the coefficient of the unsubstituted carbon atom is the larger one in absolute value. Therefore, electron-poor olefins regioselectively afford the oxetane with the substituted carbon next to the oxygen. (Cf. Barl-trop and Carless, 1972.) In contrast, an electron-rich olefin predominantly yields the oxetane with the unsubstituted carbon next to the oxygen. (Cf. Scheme 22.)... 

Ethylene, with x = 4.4 eV and 77 = 6.2 eV, is a typical organic molecule in being intermediate in EN so that reaction with both electrophiles and nucleophiles is possible. For substituted olefins, x ranges from 3.0 eV for (CH3)2 = C(CH3)2 to 7.3 eV for (NC)2C — C(CN)2. More EN olefins react best with electron donors, or nucleophiles, as we have already seen in Table 3.2. The least EN olefins react best with reagents like Br2 and H3O. In the case of Br2, with x = 6.6 eV, there is good agreement between the rate constants and AjV calculated from Equation (3.1), as shown in Table 3.8. ... 

With cyclohexyl radicals the opposite behavior is seen. Relative rate constants arc reduced and the preference for tail addition is reinforced. For olefins substituted with electron-donor substituents, nucleophilic radicals give the greatest tail v.v head specificity. The converse generally also applies. 

Thus different mechanisms for the oxidation of substituted aromatics can be proposed, depending on the oxidant resulting in the formation of species B or C. Species C is well known to oxidize electron donor molecules such as olefines ... 

In the presence of methanol as solvent and 1,4-dicyanobenzene as acceptor, photoinduced electron transfer from 1,4-bis(methylene)cyclohexane gives 4-(methoxymethyl)-1 -methylenecyclohexane and 4-(4-cyanophenyl)-4-(methoxy-methyl)-l-methylenecyclohexane which arise by nucleophilic attack of the solvent on the radical cations, followed either by reduction and protonation, or by combination with the radical anion of the electron acceptor.These observations are in accordance with the proposed mechanism of the nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction. The same group has also investigated the use of cyanide ion as nucleophile and report that irradiation of a mixture of 1,4-dicyanobenzene in the presence of biphenyl as donor, KCN, and 18-crown-6 gives a mixture of (79) and (80). These workers have also extended the scope of NOCAS to fluoride ion. In particular, use of 2,3-dimethylbut-2-ene and 2-methylbut-2-ene gives 4-cyanophenyl substituted... 

Phosphinocarbene or 2 -phosphaacetylene 4, which is in resonance with an ylide form and with a form containing phosphoms carbon triple bond, is a distillable red oil. Electronic and more importantly steric effects make these two compounds so stable. Carbene 4 adds to various electron-deficient olefins such as styrene and substituted styrenes. Bertrand et al. have made excellent use of the push-pull motif to produce the isolable carbenes 5 and 6, which are stable at low temperature in solutions of electron-donor solvents (THF (tetrahydrofuran), diethyl ether, toluene) but dimerizes in pentane solution. Some persistent carbenes are used as ancillary ligands in organometallic chemistry and in catalysis, for example, the ruthenium-based Grubbs catalyst and palladium-based catalysts for cross-coupling reactions. 

Direct application of Ru3(CO)i2 in photochemical synthesis has been described in detail [120]. Thermal reactions of this cluster in presence of two-electron donors L affords [Ru3(CO)9L3]. The discovery in 1974 that irradiation of the cluster under those conditions produces mononuclear products instead of the substituted clusters initiated a wealth of research in Ru-clusters as precursors in photochemical synthesis [121]. Much research has been devoted to the preparation of mononuclear f/ -olefin complexes, as well as alkyne complexes. For example, [Ru(CO)3(PPh3)2] has been reported as an active catalyst for olefin polymerisation, and as such, many investigations have dealt with the reactivity of this compound. Other directions of research include formation of metallacycles, generation of new cluster species, and mixed transition metal/non-metal clusters. 

In the compounds involved in reaction scheme in Fig. 2.67, the olefine is a two electron donor in the first substitution product, three electron donor in the second, and four electron donor in the isomers a and b. 

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