Styrenes electron-deficient

In contrast to other furoxans, the cycloreversion of 3,4-dinitrofuroxan to nitro-formonitrile oxide was observed even at room temperature. The nitrile oxide could be trapped in situ with electron-deficient nitriles (Scheme 149) (95MC231). Attempts to obtain cyclo adducts with styrene, phenylacetylene, rran.s-stilbene, and cyclohexene failed. 

Absolute rate constants for addition reactions of cyanoalkyl radicals are significantly lower than for unsubstituted alkyl radicals falling in the range 103-104 M V1.341 The relative reactivity data demonstrate that they possess some electrophilic character. The more electron-rich VAc is very much less reactive than the electron-deficient AN or MA. The relative reactivity of styrene and acrylonitrile towards cyanoisopropyl radicals would seem to show a remarkable temperature dependence that must, from the data shown (Table 3.6), be attributed to a variation in the reactivity of acrylonitrile with temperature and/or other conditions. 

The hydrothiolation of electron-deficient alkenes [X = CN, C(=0)(0Me)] and p-nitro-styrene was catalysed by the Cu complexes 98 and 99. The reactions with phenyl- or benzyl-thiol proceed with high conversions (>90%, rt, 5 mol%). 

The combined influences of polar and steric effects and of the strength of the newly formed bond93 was also recognized in the reaction of a,0-unsaturated carbonyl compounds and similar electron deficient alkenes95 with organomercurials and NaBH4. For the addition of alkyl radicals to substituted styrenes, p assumed a... 

Carbon radicals bearing a single fluorine atom are produced via denitration. They react with styrene to give the adduct in good yield (Eq. 7.86), but with electron-deficient alkenes yields are very low.133... 

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. 

In the phosphonium salt synthesis, the addition reaction of tertiary phosphines to activated alkenes has been reported (Scheme 3). PPh3 is added to electron-deficient alkenes such as enones or enals at the p-position in the presence of acids.4 The reaction of styrenes with phosphine has recently been reported by Okuma, which gave Markovnikov adducts.5 Although no catalyzed reactions of... 

Several reactions of halogen-substituted carbon-centered radicals with silanes have been studied, but limited kinetic information is available for reactions of halogen-substituted radicals with tin hydrides. A rate constant for reaction of the perfluorooctyl radical with Bu3SnH was determined by competition against addition of this radical to styrenes, reactions that were calibrated directly by LFP methods.93 At ambient temperature, the n-C8F17 radical reacts with tin hydride two orders of magnitude faster than does an alkyl radical, consistent with the electron-deficient nature of the perflu-oroalkyl radical and the electron-rich character of the tin hydride. Similar behavior was noted previously for reactions of silanes with perhaloalkyl radicals. 

Since the isopropyl styrene ether remains bound to the glass surface during catalysis, it is unavailable to serve as a stabilizing Lewis base for electron-deficient... 

Dependence of the Rate of Addition of Styrene to Acridizinium Nucleus upon Electron Deficiency at Position 6... 

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

Under optimized conditions regarding the choice of Br0nsted acid (mandelic acid 20), stoichiometry (1 1 ratio 9 and mandelic acid 20), solvent (the respective alcohol neat conditions), temperature (rt or 50°C), and catalyst loading (lmol% 9 and lmol% mandelic acid 20) electron-rich and electron-deficient styrene oxides underwent alcoholysis with simple aliphatic, stericaUy demanding as well as unsaturated and acid-labile alcohols. The completely regioselective (>99%) alcoholysis was reported to produce the corresponding P-aUcoxy alcohols 1-10 in moderate (41%) to good (89%) yields without noticeable decomposition or polymerization reactions of acid-labile substrates (Scheme 6.27). Notably, aU uncatalyzed reference experiments showed no conversion even after two weeks under otherwise identical conditions. 

A special case involves the thermal decomposition of 3,4-dinitrofuroxan (104). The cycloreversion is already observed at room temperature and the nitroformo-nitrile oxide could be trapped with electron-deficient nitriles. The cycloadditions with styrene, phenylacetylene, frani-stilbene, and cyclohexene, however, led to complex mixtures of products that could not be separated (104). In the related case of a furoxan with an a-hydrogen adjacent to the sulfonyl group, the reaction was proposed to proceed according to course (b) (Scheme 6.7). 

Hosokawa, Murahashi, and coworkers demonstrated the ability of Pd" to catalyze the oxidative conjugate addition of amide and carbamate nucleophiles to electron-deficient alkenes (Eq. 42) [177]. Approximately 10 years later, Stahl and coworkers discovered that Pd-catalyzed oxidative amination of styrene proceeds with either Markovnikov or anti-Markovnikov regioselectivity. The preferred isomer is dictated by the presence or absence of a Bronsted base (e.g., triethylamine or acetate), respectively (Scheme 12) [178,179]. Both of these reaction classes employ O2 as the stoichiometric oxidant, but optimal conditions include a copper cocatalyst. More recently, Stahl and coworkers found that the oxidative amination of unactivated alkyl olefins proceeds most effectively in the absence of a copper cocatalyst (Eq. 43) [180]. In the presence of 5mol% CUCI2, significant alkene amination is observed, but the product consists of a complicated isomeric mixture arising from migration of the double bond into thermodynamically more stable internal positions. 

The dioxygen complex 6 shows almost no electrophilicity as it does not react with alkenes like styrene, cyclohexene or o. jS-unsaturated alkenes such as 2-cyclohexene-l-one or 1,4-naphthochinones. Only with electron-deficient alkenes like TCNE or with benzoyl chloride reactivity was observed. 

When DISN reacts with electron-rich styrenes such as p-methoxystyrene, good yields of reduced pyrazines 117, often accompanied by their oxidized forms, are obtained. However, reaction with electron-deficient styrenes like p-fluorostyrene give the 2-amino-3-(2-arylaziridin-l-yl)maleonitriles (118) (72JA3242 84JOC813). 

This has led to the discovery of electron-deficient styrenes, such as those shown in Figure 2. As detailed later, these ligands have been shown to be effective for the Ni-catalyzed alkylation of organozincs with primary alkyl iodides and bromides91. More recently, Fu88k has reported that a catalyst consisting of Ni(COD)2 and s-Bu-Pybox (3) is satisfactory even for the reaction of primary alkylzincs with secondary alkyl bromides... 

Following a report in 1992 on the reaction of /3-hydrogen-containing alkyl iodides with alkyl-, aryl- and alkenyl-9-BBN derivatives in the presence of Pd(PPh3)4 and K3PO4208, Knochel and coworkers made an interesting and potentially important observation on the effect of proximal -bonds shown in Scheme 2790. This may have paved the way to their subsequent discovery of the Ni-catalyzed reaction of alkylzincs with primary alkyl iodides in the presence of an electron-deficient styrene or acetophenone90,91 (Scheme 74)... 

SCHEME 74. Ni-catalyzed reaction of organozincs with primary alkyl iodides promoted by electron-deficient styrenes or acetophenone... 

With monosubstituted electron-deficient ethylenes, compound 123 gives a mixture of the exo and endo adducts (e.g., 124 and 125 R = CO2Me, CN, COMe). With styrenes the endo isomer (126) predominates. These adducts (126) are transformed into isomers 127 by trifluoromethanesulfonic acid. Ethyl vinyl ether gives exclusively the endo adduct (124 R = OEt). Similar... 

Cycloadditions of betaines are not restricted to electron-deficient alkenes. Pyridinium-3-olates also react with conjugated olefins (e.g., styrenes) and with electron-rich olefins (e.g., ethyl vinyl ether). In the latter case, the betaine LUMO/alkene HOMO interaction becomes dominant and reaction is only observed with pyridinium-3-olates having a low-energy LUMO... 

Many carbenes, like carbocations, rearrange to more stable structures by the migration of a neighboring group to the electron-deficient carbon. Thus phenylmethylcarbene rearranges to ethenylbenzene (styrene) ... 

The eclipsed tetra-BINAP porphyrin 171 was conveniently synthesized by condensation of the meso-tetrakis(2,6-dihydroxyphenyl)porphyrin 173 with the (S)-BINAP derivative 174 in the presence of K2C03. After removal of the staggered isomer iron was inserted by addition of Fe(CO)5/I2, and the resulting Fe(III)-complex 171 was used as a catalyst (0.2%) to epoxidize a series of six styrene derivatives in the presence of an excess of PhIO. In every case the corresponding (R)-epoxides were preferentially formed in yields up to 72%. The best ee-values were obtained for the electron deficient substrates 2-nitrostyrene (80% ee) and pentafluorostyrene (74% ee), [114],... 

The Meerwein arylation is at least formally related to the atom transfer method because a net introduction of an aromatic ring and a chlorine across a double bond is accomplished (Scheme 62). Facile elimination of HC1 provides an efficient route to the kinds of substituted styrenes that are frequently prepared by Heck arylations. Standard protocol calls for the generation of an arene diazonium chloride in situ, followed by addition of an alkene (often electron deficient because aryl radicals are nucleophilic) and a catalytic quantity of copper(II) chloride. It is usually suggested that the copper salt operates in a catalytic redox cycle, reducing the diazonium salt to the aryl radical as Cu1 and trapping the adduct radical as Cu11. 

Hydrazones have also been used as azomethine imine precursors to achieve cycloadditions.157 Proto-nated hydrazones act under suitable conditions as quasi-azomethine imines in polar [3+ + 2] cycloadditions. Thus, r.cetaldehyde phenylhydrazone (201) was found to react with styrene in the presence of sulfuric acid in a regiospecific manner to give pyrazolidine (203 Scheme 47) as a diastereomeric mixture.157 The most commonly used azomethine imine has a phenyl group attached to one end of the dipole and hence has a raised HOMO relative to the unsubstituted system. Because the coefficients at the terminal atoms of the dipole are smaller in the LUMO than they are in the HOMO, the phenyl group does not lower the energy of the LUMO as much as it raises the energy of the HOMO. With electron-deficient di-polarophiles like methyl acrylate, the reaction is dipole HOMO-controlled, and mixtures can be expected. In fact, a 1 1 mixture of regioisomers was obtained in the reaction of (201) with acrylonitrile (equation 9).157... 

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