Olefins electron-acceptor substitution

The formation of trans-products is observed to a lesser extent in the reaction of 3-alkoxycarbonyl-substituted cyclohexenones, in the reaction with electron-deficient alkenes and in the reaction with olefinic reaction partners, such as alkynes and allenes, in which the four-membered ring is highly strained (Scheme 6.11). The ester 26 reacted with cyclopentene upon irradiation in toluene to only two diastereomeric products 27 [36]. The exo-product 27a (cis-anti-cis) prevailed over the endo-product 27b (cis-syn-cis) the formation of trans-products was not observed. The well-known [2 + 2]-photocycloaddition of cyclohexenone (24) to acrylonitrile was recently reinvestigated in connection with a comprehensive study [37]. The product distribution, with the two major products 28a and 28b being isolated in 90% purity, nicely illustrates the preferential formation of HH (head-to-head) cyclobutanes with electron-acceptor substituted olefins. The low simple diastereoselectivity can be interpreted by the fact that the cyano group is relatively small and does not exhibit a significant preference for being positioned in an exo-fashion. 

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]. 

Electron acceptor-substituted benzenes such as benzonitrile and a,a,a-trifluorotoluene generally give ort/io-cycloadducts with olefins [18]. Some exceptions are also found. For example, photo addition of l,3-dioxol-2-one to benzonitrile gives only mera-adduct of emio-configuration having electron acceptor substituent at C-2 and C-4 carbon atoms. Possibly the electron acceptor ability of benzonitrile is higher at ortho- and para-positions [19]. The following examples are illustrative ... 

Isonitrile complexes, having a similar electronic structure to carbonyl complexes, can also react with nucleophiles. Amino-substituted carbene complexes can be prepared in this way (Figure 2.6) [109-112]. Complexes of acceptor-substituted isonitriles can undergo 1,3-dipolar cycloaddition reactions with aldehydes, electron-poor olefins [113], isocyanates [114,115], carbon disulfide [115], etc., to yield heterocycloalkylidene complexes (Figure 2.6). 

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

The order of reactivity of these three catalysts towards alkenes (but also towards oxygen) is 1 > 3 > 2. As illustrated by the examples in Table 3.18, these catalysts tolerate a broad spectrum of functional groups. Highly substituted and donor- or acceptor-substituted olefins can also be suitable substrates for RCM. It is indeed surprising that acceptor-substituted alkenes can be metathesized. As discussed in Section 3.2.2.3 such electron-poor alkenes can also be cyclopropanated by nucleophilic carbene complexes [34,678] or even quench metathesis reactions [34]. This seems, however, not to be true for catalysts 1 or 2. 

A wide range of olefins can be cyclopropanated with acceptor-substituted carbene complexes. These include acyclic or cyclic alkenes, styrenes [1015], 1,3-dienes [1002], vinyl iodides [1347,1348], arenes [1349], fullerenes [1350], heteroare-nes, enol ethers or esters [1351-1354], ketene acetals, and A-alkoxycarbonyl-[1355,1356] or A-silyl enamines [1357], Electron-rich alkenes are usually cyclopropanated faster than electron-poor alkenes [626,1015],... 

Intramolecular coordination is apparently responsible for most examples of regioselective Wacker oxidations of internal olefins, but electronic effects are also operating [28], specifically in acceptor-substituted olefins. Steric effects are currently not well explored [8], Recent theoretical studies on the mechanism of the Wacker and related reactions are available elsewhere [29, 30],... 

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. 

The cyanoaromatic-sensitized electron-transfer photooxygenation (Foote-type) is a useful preparative reaction with a very limited class of unsaturated olefins, namely those substituted by aromatics and, at the same time, totally inert towards singlet oxygen. On the other hand, in the previous sections, it has been many times underlined that electron-transfer reactions may compete with singlet oxygen formation and, above all, that the reactions of 02 may be the only observable outcome in the presence of singlet-oxygen acceptors. 

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 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.)... 

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... 

The photo-NOCAS reaction is restricted to mononuclear, electron-deficient aromatics as the electron acceptors. Using polynuclear systems such as 1,4-dicyanonaphthalene, for example, leads to the ole-fin-nucleophile adduct radical adding, rather than substituting, at the ipso-position of the arene (Scheme 18). This is most Hkely due to the reduced rearomatization energy gained in binuclear, as opposed to mononuclear, systems. The photo-NOCAS reaction is Hmited to aliphatic olefins as electron... 

Reactions of highly electron-rich organometalate salts (organocuprates, orga-noborates, Grignard reagents, etc.) and metal hydrides (trialkyltin hydride, triethylsilane, borohydrides, etc.) with cyano-substituted olefins, enones, ketones, carbocations, pyridinium cations, etc. are conventionally formulated as nucleophilic addition reactions. We illustrate the utility of donor/acceptor association and electron-transfer below. 

The quantitative treatment of the electron-transfer paradigm in Scheme l by FERET (equation (104)) is restricted to the comparative study of a series of structurally related donors (or acceptors). Under these conditions, the reactivity differences due to electronic properties inherent to the donor (or acceptor) are the dominant factors in the charge-transfer assessment, and any differences due to steric effects are considered minor. Such a situation is sufficient to demonstrate the viability of the electron-transfer paradigm to a specific type of donor acceptor behavior (e.g. aromatic substitution, olefin addition, etc.). However, a more general consideration requires that any steric effect be directly addressed. 

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). 

Addition of dimethylsulfonium methylide (122) to various Michael acceptors (121), followed by alkylation, has been reported to produce functionalized 1-substituted alkenes (124), arising via the unprecedented elimination (123), rather than the usual cyclopropanation products. In silyl substituted substrates, where a facile Peterson-type olefination is possible from the adduct, elimination took place instead. Aryl-substituted Michael acceptors (121 R1 = Ar) underwent a similar olefination to give 1-substituted styrene derivatives with moderate yields along with a side product, which arose by nucleophilic demethylation from the adduct of dimethylsulfonium methylide and arylidene malonates. Hammett studies revealed that selectivity for olefination versus demethylation increases as the aryl substituent becomes more electron deficient.164... 

In analogy to the cycloaddition of electron-rich alkenes with electron-deficient olefins , cycloadditions between donor-acceptor pairs of cyclopropanes and unsaturated compounds have been obtained. They include for instance the cycloaddition of substituted cyclopropanes with ethylene-tetracarbonitrile (TCNE), (equation 23 ). [10]Paracyclophadienes have been obtained from the reaction of tetrasubstituted 1,2-dicyclopropylethylenes with TCNQ. ... 

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