Olefins substrates

In the preparation of hydroperoxides from hydrogen peroxide, dialkyl peroxides usually form as by-products from the alkylation of the hydroperoxide in the reaction mixture. The reactivity of the substrate (olefin or RX) with hydrogen peroxide is the principal restriction in the process. If elevated temperatures or strongly acidic or strongly basic conditions are required, extensive decomposition of the hydrogen peroxide and the hydroperoxide can occur. 

The product composition from these reactions is influenced by the location of the functional group in the substrate. Olefin formation is the most common side reaction and in certain cases, especially with reductions of tosyl-hydrazones (section IV-B), it may become dominant so that the reaction can be used for the preparation of mono-labeled olefins. 

The Jacobsen-Katsuki epoxidation reaction is an efficient and highly selective method for the preparation of a wide variety of structurally and electronically diverse chiral epoxides from olefins. The reaction involves the use of a catalytic amount of a chiral Mn(III)salen complex 1 (salen refers to ligands composed of the N,N -ethylenebis(salicylideneaminato) core), a stoichiometric amount of a terminal oxidant, and the substrate olefin 2 in the appropriate solvent (Scheme 1.4.1). The reaction protocol is straightforward and does not require any special handling techniques. 

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). 

Surfactant Substrate olefin (% branching) Unsulfonated organic material (wt %)b Foam half-life (min)... 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

An obvious method to investigate the formation and the nature of the catalytically active nickel species is to study the nature of products formed in the reaction of complexes such as 3 or 4 with substrate olefins. This has been investigated in some detail in the case of the catalytic dimerization of cyclooctene to 1-cyclooctylcyclooctene (17) and dicy-clooctylidene (18) [Eq. (4)] using as catalyst 7r-allylnickel acetylacetonate (11) or 7r-allylnickel bromide (1) activated by ethylaluminum sesquihalide or aluminum bromide (4). In a typical experiment, 11 in chlorobenzene was activated with excess ethylaluminum sesquichloride cyclooctene was then added at 0°C and the catalytic reaction followed by removing... 

Yet another possibility is illustrated by the propene (or ethylene) dimerization catalyzed by 7r-l,l,3,3-tetraphenylallylnickel bromide (26) activated with ethylaluminum dichloride the isolation of considerable amounts of 1,1,3,3-tetraphenylpropene (27) from the reaction mixture suggests that a hydrogen atom has been transferred from the substrate olefin to the sterically hindered 1,1,3,3-tetraphenylallyl system under formation of 3 [Eq. (7)] (81). The subsequent formation of the HNiY species from 3 can then take place by insertion of a second propene molecule and /3-hydrogen elimination, as discussed above. 

Mechanisms, exemplified by alcohol as donor (493, 496), usually invoke coordination of the substrate (olefins, saturated and unsaturated ketones, and aldehydes), then coordination of the alcohol and formation of a metal alkoxide, followed by /8-hydrogen transfer from the alkoxide and release of product via protonolysis ... 

C° (catalyst metal), Cs (substrate olefin), Cco (carbon monoxide),... 

An interesting effect of pH was found by Ogo et al. when studying the hydrogenation of olefins and carbonyl compounds with [Cp Ir(H20)3] (Cp = ri -CsMej) [89]. This complex is active only in strongly acidic solutions. From the pH-dependence ofthe HNMR spectra it was concluded that at pH 2.8 the initial mononuclear compound was reversibly converted to the known dinuclear complex [(Cp Ir)2(p-OH)3] which is inactive for hydrogenation. In the strongly acidic solutions (e.g. 1 M HCIO4) protonation of the substrate olefins and carbonyl compounds is also likely to influence the rate ofthe reactions. 

The rhodium complex [CpRh(bipy)Cl2] is reported (162) to act as one-half of a redox couple that, in concert with a manganese porphyrin system, catalyzes the epoxidation of olefins by dioxygen. In this two-phase system, the aqueous phase contains sodium formate, and the organic phase is a trichloroethane solution of [Mnm(tpp)]1+ and the rhodium complex (tpp = meso-tetraphenylporphyrin). Apparently, the rhodium complex catalyzes the reduction of [Mnin(tpp)]1+ by formate, and the manganese(II) species thus formed binds dioxygen and reacts with the substrate olefin to form the epoxide. However, the intermedi-... 

Chain reactions can be divided roughly into two types polymerization and nonpolymerization. In polymerizations (Scheme 6), an initiating radical (R ) adds to a substrate olefin (ordinarily termed the monomer), to yield a new radical, which adds to another olefin, and so forth. The kinetic chain, that is, the sequence of events begun by a given R- radical from the initiator, corresponds in this scheme to the actual growth of the polymer molecule, and terminates simultaneously with the growth of the molecular chain as two radicals combine or disproportionate. 

Many other variations of the basic structure 10 have been explored, including an-hydro sugars and carbocyclic analogs, the latter derived from quinic acid 13 [23-26]. In summary, the preparation of these materials (e.g. 14-16) requires more synthetic effort than the fructose-derived ketone 10. Occasionally, e.g. when using 14, catalyst loadings can be reduced to 5% relative to the substrate olefin, and epoxide yields and selectivity remain comparable with those obtained by use of the fructose-derived ketone 10. Alternative ex-chiral pool ketone catalysts were reported by Adam et al. The ketones 17 and 18 are derived from D-mannitol and tartaric acid, respectively [27]. Enantiomeric excesses up to 81% were achieved in the epox-idation of l,2-(E)-disubstituted and trisubstituted olefins. 

The same group reported the striking observation that oxygen transfer from ox-one to substrate olefins can also be catalyzed by secondary amines alone [49]. Pyrrolidines proved particularly efficient in this process, which was originally believed to involve the amine radical cation. Subsequent work [50, 51] identified the proto-nated amine as the active species and assigned a dual role to it. It is most probable that the ammonium cation acts both as a phase-transfer catalyst and forms a com-... 

Under these optimized conditions, di-tert-butylsilylene could be transferred to a range of acyclic and cyclic olefins (Schemes 7.9 and 7.10).11,74 The method was not sensitive to the steric nature of the R substituent nearly quantitative silylene transfer to olefins bearing ra-butyl, isopropyl, or tert-butyl groups was observed. Vinylsilanes were also tolerated as substrates. Olefins containing silyl ether, benzyl ether, and pivolate substituents were all effective traps of di-tm-butylsilylene. 

Method C H-abstraction by DTBP substrate/olefin/DTBP = 20/2/1 12 h, 130 °C. Method D DTBP, benzene, 140 °C olefin/DTBP = 2/1. 

The cycloaddition of enones to olefins is a reaction of considerable synthetic interest 14°). Oxetane formation and cyclobutane formation are sometimes competitive 141>, but the latter reaction is the more common. The photodimerization of enones 142> is a special case of such cycloaddition. It has been shown that triplets are involved in these cycloadditions, since intersystem crossing quantum yields are unity 143> and cycloaddition is totally quenchable by triplet quenchers. Careful kinetic analysis indicates an intermediate which can partially revert to ground state reactants, since quantum yields are lower than unity even when extrapolated to infinite substrate olefin concentration. That a diradical is... 

Figure 8 Microenvironmental polarity control upon enantiodifferentiating polar photi addition of alcohol (ROH) to aromatic olefin (D) sensitized by naphthalenedicarboxylai with saccharide auxiliaries (A ) the local polarity is enhanced around the saccharic moieties, facilitating electron transfer from exited sensitizer (A ) to substrate olefin (I to produce a radical cation (D +). The radical cation produced cannot escape from tl high polarity region around the saccharide to the low-polarity bulk solution and is accor< ingly attacked by ROH in the chiral environment of saccharide to produce the adduct i high ee.

The Murai reaction (Scheme 4), the replacement of an ortho-CH on an aromatic ketone by an alkyl group derived from a substrate olefin, is catalyzed by a variety of Ru complexes. This C bond formation occurs via chelate directed C-H bond activation (cyclometalation) in the first step, followed by alkene insertion into RuH and reductive elimination of the alkylated ketone. In a recent example of the use of a related cyclometalation in complex organic synthesis, Samos reports catalytic arylation (Suzuki reaction) and alkenylation (Heck reaction) of alkyl segments of a synthetic intermediate mediated by Pd(II). 

Photo-oxidation of olefins in pyridine solution in the presence of FeCIs has been found to take one of three courses, depending on the substitution pattern of the substrate olefin. These routes lead to formation of a-chloroketones, gem-dichloroketones, and a,-dichloroketones, and have all been illustrated by their use in actual syntheses. a-Chloroketones have also been produced by irradiation of pyridine solutions of vinylsUanes or vinyl sulphides under similar conditions. DCA-sensitized photo-oxidation of l,2-diphenyl-3,3,4,4-tetramethylcyclobutene in MeCN leads to the corresponding ozonide in a process which occurs by initial formation of an oxirane. Jasmine lactone (12 ... 

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