Olefin electron-poor

Simple olefins do not usually add well to ketenes except to ketoketenes and halogenated ketenes. Mild Lewis acids as well as bases often increase the rate of the cyclo addition. The cycloaddition of ketenes to acetylenes yields cyclobutenones. The cycloaddition of ketenes to aldehydes and ketones yields oxetanones. The reaction can also be base-cataly2ed if the reactant contains electron-poor carbonyl bonds. Optically active bases lead to chiral lactones (41—43). The dimerization of the ketene itself is the main competing reaction. This process precludes the parent compound ketene from many [2 + 2] cyclo additions. Intramolecular cycloaddition reactions of ketenes are known and have been reviewed (7). 

A more efficient agent than peroxy compounds for the epoxidation of fluoro-olefins with nonfluonnated double bond is the hypofluorous acid-acetomtrile complex [22] Perfluoroalkylethenes react with this agent at room temperature within 2-3 h with moderate yields (equation 13), whereas olefins with strongly electron-deficient double bond or electron-poor, sterically hindered olefins, for example l,2-bis(perfluorobutyl)ethene and perfluoro-(l-alkylethyl)ethenes, are practically inert [22] Epoxidation of a mixture of 3 perfluoroalkyl-1-propenes at 0 C IS finished after 10 mm in 80% yield [22] The trifluorovinyl group in partially fluorinated dienes is not affected by this agent [22] (equation 13)... 

Hexafluoropropylene oxide (HFPO), which decomposes reversibly to di-fluorocarbene and trifluoroacetyl fluonde with a half-life of about 6 h at 165 °C [30], is a versatile reagent. Its pyrolysis with olefins is normally carried out at 180-2(X) °C, and yields are usually good with either electron-nch or electron-poor olefins [31, 32, 33, 34, 35, 36, 37] (Table 2). The high reaction temperatures allow the eyclopropanation of very electron poor double bonds [58] (equation 10) but can result in rearranged products [39, 40, 41] (equations 11-13)... 

Addition reactions — The fullerenes Ceo and C70 react as electron-poor olefins with fairly localized double bonds. Addition occurs preferentially at a double bond common to two annelated 6-membered rings (a 6 6 bond) and a second addition, when it occurs is generally in the opposite hemisphere. The first characteriz-able mono adduct was [C6oOs04(NC5H4Bu )2]. formed by reacting Cgo with an excess of OSO4 in 4-butylpyridine. The structure is shown in... 

One of the earliest reported thermal reactions of Fischer carbene complexes was the reaction with olefins to give cyclopropanes [127]. More recently it has been shown that photolysis accelerates inter molecular cydopropanation of electron-poor alkenes [128]. Photolysis of Group 6 imine carbenes with alkenes... 

Anionic Diels-Alder reactions have been studied less extensively with the interest having been focused mainly on the cycloaddition of enolates of a,/l-unsaturated ketones with electron-poor olefins [24] (Equations 1.8 and 1.9). These reactions are fast and stereoselective and can be regarded as a sequential double Michael condensation, but a mechanism involving a Diels-Alder cycloaddition seems to be preferred [24b,f, 25]. 

Resonance hydrids (15) and (16) would suggest that these carbenes should show nucleophilic behavior toward olefins. As predicted, only olefins with electron-poor n bonds have proven to be suitable substrates for these carbenes. 

Irradiation of mixtures of acetone and the electron-poor olefin cis- or frmy-dicyanoethylene leads to the stereospecific formation of oxetane(iaa) ... 

Diazomethane is also decomposed by N O)40 -43 and Pd(0) complexes43 . Electron-poor alkenes such as methyl acrylate are cyclopropanated efficiently with Ni(0) catalysts, whereas with Pd(0) yields were much lower (Scheme 1)43). Cyclopropanes derived from styrene, cyclohexene or 1-hexene were formed only in trace yields. In the uncatalyzed reaction between diazomethane and methyl acrylate, methyl 2-pyrazoline-3-carboxylate and methyl crotonate are formed competitively, but the yield of the latter can be largely reduced by adding an appropriate amount of catalyst. It has been verified that cyclopropane formation does not result from metal-catalyzed ring contraction of the 2-pyrazoline, Instead, a nickel(0)-carbene complex is assumed to be involved in the direct cyclopropanation of the olefin. The preference of such an intermediate for an electron-poor alkene is in agreement with the view that nickel carbenoids are nucleophilic 44). 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

Mukaiyama reaction (Lewis acid-catalyzed Michael reaction) with electron-poor olefins, ketals and acetals, and enones 32... 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

Electron-poor olefins with higher oxidation potentials may decrease the rate of electron transfer and other processes competing for deactivation of the iminium salt excited states may increase. Alternate reaction pathways involving olefin-arene 2 + 2 cycloaddition may take place in the photochemistry of 133 with electron-poor olefins (equation 62)120,121. 

D. Addition of Thiolacetic Acid to Electron-Poor Olefins.107... 

Japanese workers (50,51) were the first to observe optical activity in the addition of thiols to electron-poor olefins (eq. [9]) The e.e. was not determined, but these observations led us to attempt using a cinchona alkaloid as the catalyst in the addition of thiophenol to cyclohexenone. The reaction lends itself admirably to a scope, limitations, and mechanism study, and the results have been published in detail (19). An important mechanistic difference between the addition of the dodecanethiol to isopropenyl methyl ketone and the addition of thiophenol to a cyclohexenone (eq. [1]) lies in the sequence of chirality-producing steps. In the former case, chirality is produced when the proton adds to the a-caibon atom of the ketone—after thiol addition has taken place. In the latter... 

Although thiolacetic acid additions are free-radical reactions (60), it was found recently that the addition to electron-poor olefins can be base catalyzed (61) (eqs. [14], [15]). Thus the (S)-(-) adduct is obtained with an e.e. of 54% when cyclohexenone is treated with thiolacetic acid in benzene in the presence of catalytic amounts of cinchonine. The reaction appears to be quite general, although very high e.e. s (>80%) have not yet been achieved. 

Examination of the reactivity of acyclic (diene)Fe(CO)3 complexes indicates that this nucleophilic addition is reversible. The reaction of (C4H6)Fe(CO)3 with strong carbon nucleophiles, followed by protonation, gives olefinic products 195 and 196 (Scheme 49)187. The ratio of 195 and 196 depends upon the reaction temperature and time. Thus, for short reaction time and low temperature (0.5 h, —78 °C) the product from attack at C2 (i.e. 195) predominates while at higher temperature and longer reaction time (2 h, 0 °C) the product from attack at Cl (i.e. 196) predominates. This selectivity is rationalized by kinetically controlled attack at the more electron-poor carbon (C2) at low temperature. Nucleophilic attack is reversible and, under conditions where an equilibrium is established, the thermodynamically more stable (allyl)Fe(CO)3" is favored. The regioselectivity for nucleophilic attack on substituted (diene)Fe(CO)3 complexes has been reported187. The... 

Not much is known about the reactivity of the phosphinocarbene 2i. Problems arise, at least in part, from the high 1,3-dipolar reactivity of the diazo precursor li, which hides any carbene reactivity. Indeed, although li is stable in a toluene solution at 60°C for hours, the addition of an electron-poor olefin, such as a perfluoroalkyl-monosubstituted alkene, induces the exclusive formation of the thermodynamically more stable anti-isomer of the cyclopropane 14 (see Section V,B,3,a).36 This clearly demonstrates that the cyclopropanation reaction does not involve the carbene 2i, but that an initial [2 + 3]-cycloaddition occurs leading to the pyrazoline 13, which subsequently undergoes a classical N2 elimination.37... 

The epoxidation of alkenes is one of the most impoi4ant oxidation methods. Electrochemical epoxidation of electron-poor olefins such as enoates (154 155) and enones has been accomplished by using silver(III)oxo bis(2,2 -bipyridine) and similar complexes (Scheme 61) [241], )-Dimethyl glutaconate is electrolyzed in an MeCN-LiCl04/Ag0Ac)(bpy)-(Pt) system to give the trans-epoxide in 90% yield. 

If the olefin was electron-poor, then the reactions could be channeled to products of type 24 even without the methyl group on the internal carbon of the olefin (Scheme 25) [41]. 

The reactions reviewed here can be divided into two classes those involving electron-rich alkenes or alkynes and which are of the radical type, and those involving electron-poor olefins (Michael acceptors) and which are of the radical or carbanion type. 

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). 

In addition to reactions characteristic of carbonyl compounds, Fischer-type carbene complexes undergo a series of transformations which are unique to this class of compounds. These include olefin metathesis [206,265-267] (for the use as metathesis catalysts, see Section 3.2.5.3), alkyne insertion, benzannulation and other types of cyclization reaction. Generally, in most of these reactions electron-rich substrates (e.g. ynamines, enol ethers) react more readily than electron-poor compounds. Because many preparations with this type of complex take place under mild conditions, Fischer-type carbene complexes are being increasingly used for the synthesis [268-272] and modification [103,140,148,273] of sensitive natural products. 

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],... 

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