Alkene adduct

Using the very bulky rhodium porphyrins Rh(TTEPP)- and Rh(TTiPP)- (which contain triethylphenyl and triisopropylphenyl groups), neither of which can dimerize. direct evidence for an alkene adduct and its subsequent dimerization to the four-carbon bridged product has been obtained. Reaction of Rh(TTEPP)- with ethene... 

When the diphosphine is chiral, binding of a prochiral alkene creates diastereomeric catalyst-alkene adducts. (Diastereomers result because binding of a prochiral alkene to a metal center generates a stereogenic center at the site of unsaturation.) Through a powerful combination of3lP and l3C NMR methods, Brown and Chaloner first demonstrated the presence of two diastereomeric catalyst-enamide adducts with bidentate coordination of the substrate to the metal (Figure 1) [19]. 

Radical addition of dibromodifluoromethane to alkenes followed by sodium borohydride reduction is a convenient two-step method for the introduction of the difluoromethyl group.5 Either one or both carbon-bromine bonds in the intermediate dibromides may be reduced, depending on the reaction conditions. In the case of acyclic dibromodifluoromethane-alkene adducts, the reduction occurs regioselectively to yield the relatively inaccessible bromodifluoromethyl-substituted alkanes. The latter are potential building blocks for other fluorinated compounds. For example, they may be dehydrohalogenated to 1,1-difluoroalkenes an example of this methodology is illustrated in this synthesis of (3,3-difluoroallyl)trimethylsilane. 

The two co-ordination sites, however, are equivalent due to the C2 symmetry of the diphosphine complex. This reduces the number of diastereomers from four to two. In Figure 4.5 the two enantiomeric intermediate alkene adducts have been drawn. Note that the faces are indicated by si, si and re, re because the configurations of two carbon atoms have to be assigned. After hydrogenation, only one chiral centre has formed at the a-carbon atom, because the (3-carbon atom now carries two hydrogen atoms and thus does not form a chiral centre. 

Kinetics. Detailed studies by Halpem [7] and Brown [8] have revealed that the most stable intermediate of the two alkene adducts is (Figure 4.9) not the one that leads to the major observed enantiomeric product. 

One might ask the question why a reaction involving such a small dihydrogen molecule can lead to such enormous differences in rate for the diastereomeric alkene adducts present (major and minor). Tentative answers were developed by Brown, Burk and Landis [9], Their studies included the use of iridium instead of rhodium since the iridium hydride intermediates can be studied spectroscopically. Consider the oxidative addition in Figure 4.10. 

For entries 3-5 the increase in molecular weight observed can be assigned to the increase in the rate of insertion and the rate of termination remains practically the same. An increase of the rate of polymerisation with the steric bulk of the ligand is usually ascribed to the destabilisation of the alkene adduct while the energy of the transition state remains the same. As a chain transfer reaction presumably P-hydride elimination takes place or traces of water might be chain transfer agents. Chain transfer does occur, because a Schulz-Flory molecular weight distribution is found (PDI 2, see Table 12.2). Shorter chains are obtained with a polar ortho substituent (OMe, entry 2) and in methanol as the solvent, albeit that most palladium is inactive in the latter case. 

This mechanism has been confirmed by the mass spec-trometric observation of the OH-alkene adducts themselves (e.g., Morris et al., 1971 Hoyermann and Sievert, 1983). Only at low pressures for the smaller alkenes is decomposition back to reactants significant. 

However, this does illustrate the importance of understanding the fundamental mechanisms in order to extrapolate to atmospheric conditions reliably. A number of experimental techniques used for studying gas-phase kinetics and mechanisms require low pressures and, under these conditions, decomposition of the OH-alkene adduct can predominate. As long as the fundamental mechanisms are understood and the kinetics determined as a function of pressure, extrapolation to atmospheric conditions is possible. Clearly, confirmation using studies at atmospheric pressure is also important. 

The lifetime of the excited N03-alkene adducts is sufficiently long that rotation about the C-C bond leads to the same yields of trans- and cw-epoxides regardless of the configuration of the reactant alkene for example, the reactions of both cis- and trans-2-butene give about 80% of the trans form of the product epoxide and 20% of the cis form (Benter et al., 1994). 

Hydroxy ketone HKET NO.-alkene adducts reacting to form OLNN (OLN) ... 

Organic nitrate ONIT N02-alkene adducts reacing via OLND... 

Azole approach. 5-JT[l,3,4]Thiadiazolo[3,2-a]pyridin-5-ones (723) can be prepared by 1,3-dipolar cycloaddition reactions between electron-deficient alkenic or alkynic dipolarophiles and the thiocarbonyl ylide dipole present in anhydro-5-hydroxy-2-methyl-6-phenylthiazolo[2,3-6][l,3,4]thiadiazolium hydroxide (720). Sulfur is extruded from the original acetylene adduct (722) whereas H2S is eliminated from the alkene adduct (721) to form the same product (723) (79JOC3808). 

Addition of alkynic 1,3-dipolarophiles to the thiocarbonyl ylide systems (48) gives the adducts (49) which readily lose sulfur giving the benzoheterocycles (50) and this is a useful synthetic route to these systems. The heterocycles (50) have also been made by elimination of hydrogen sulfide or methylamine from analogous alkenic adducts. 

In the first step the prochiral alkene entity coordinates to the cationic chiral rhodium centre with either one of the two enantiotopic faces (due to the asymmetry in the diphosphine ligand) which leads to the formation of two possible structures. Only one diastereoisomer of the intermediate alkene adduct is shown on Fig. 6.22 the second diastereoisomer can be easily imagined by taking the... 

The intermediate alkene adduct observed in the NMR spectra is not the one leading to the major catalytic pathway, see also Fig. 6.25. 

Hydrogen Halides. Hydrogen halides add to double bonds to form alkyl halides in high yield [71]. This spontaneous addition is usually second order in acid, suggesting that either dimers of the acid are involved or halide anions react with the acid/alkene adduct by an Ade3 mechanism. Depending on the acid and nucleophile involved, either syn or anti addition is possible [Eq. (26)]. 

Nitrosyl halides add to alkenes references are scattered through the litnnture back to 1875 (ref. 194 and references cited therein). The adducts vary enormously in their stability, but when their structures allow they, like nonhalogenated nitroso compounds, isomerize to oximes or dimnize. The orientation of the reaction is consistent with an electrophilic medumism, in which the reagent is polarized as NO Hat. Bicyclic substrates and reaction media of low polarity favor syn addition, suggesting a four-center transition state (Scheme 81). Aziridine synthesis via NOCl/alkene adducts is discussed in Section 3.5.2.1. 

Benzenesulfenyl chloride alkene adducts may be transformed to many useful molecules. Intermediates, such as (3), can be treated with base to pi uce vinyl or allyl sulfi s (equation 2). Alternatively, the adducts can be oxidized and treated with base to yield vinyl sulfones in high overall yield (equation 3). The thiirane intermediates or adducts, i.e. (3), may be alkylated with alkyl-titanium and -aluminum reagents which replace the chloride substituent with retention of configuration. 

Benzeneselenenyl chloride adds to alkynes to produce mixtures of trans-alkene adducts. For example, the addition of benzeneselenenyl chloride to the alkyne (18) produces the alkene (19), which can be transformed to yield the unusual diene (20 equation 16). Alkynic alcohols give anti-Miukovnikov addition products under kinetic control. The reaction is thought to proceed dirough the selenirenium ion (21 equation 17). Selenium electrophiles add to a, -alkynic caibonyl moieties to produce cis adducts in good yield (equation 18). ... 

The organometallic chemistry of aluminum is dominated by the chemistry of aluminum(lll), but lower oxidation state compounds are now accessible. The first examples of this class of compounds are carbonyl complexes such as Al(CO), A1(C0)2, and Al3(CO), which were generated upon exposure of aluminum atoms to CO in matrix-isolation experiments near 20 K. The number, relative intensities, and frequency of the carbonyl stretches in the IR spectra, along with isotopic labeling and EPR studies were used to verify these compositions. These complexes exhibit vco values of 1868, 1985 and 1904, and 1715 cm , respectively, indicative of Al- CO 7t backbonding. The carbonyl species are unstable at higher temperatures and no stable carbonyl complex of aluminum, in any oxidation state, has been reported. The monomeric aluminum-alkene adducts A1( -C2H4) and k rf-CeHe) were similarly identified in inert matrices at low temperature. No room-stable alkene complexes of aluminum have been reported. 

The stereoselective synthesis of unsaturated oxetanes has recently been achieved by Feigenbaum and coworkers.Previous studies have indicated that photochemical cis-trans isomerization of enals is rapid and results in the formation of equivalent amounts of stereoisomeric alkene adducts. " For example, irradiation of rran.r-crotonaldehyde and 2,6-dimethylfuran produced a 1 1 mixture of alkenic isomers (174) and (175) in 64% yield. Irradiation of 4-trimethylsilylbutyn-2-one and furan provided with S 1 stereoselectivity the bicyclic oxetane (176) in which the methyl group occupies the exo position, presumably because of the small steric requirement of the triple bond. Desilyation of the protected al-kyne produced an alkynic oxetane which was hydrogenated under Lindlar conditions to bicyclic vinyl-oxetane (177) attempts to use the unprotected butyn-2-one gave low isolated yields of oxetane because of extensive polymerization. The stereochemical outcome thus broadens previous alkynyloxetane syn-theses and makes possible the preparation of new oxetane structures that may be synthetically useful. 

Several strategies have recently been developed for the direct observation of non-chelated cationic zirconocenium-alkene adducts. The first is the use of a non-inserting alkoxide ligand which lowers the metal Lewis acidity. 

The development of living ring-opening metathesis polymerization (ROMP cf. Section 6.10) catalysts was greatly influenced by the initial use of hydrated late transition metal salts. In the aqueous polymerization of 7-oxanorbornene derivatives, RuC13 and [Ru(H20)6]2+ were found to produce polymer with the latter having a smaller induction period [68], A key intermediate and product in the reaction is the alkene adduct, [Ru(H20)5(alkene)]2+ with the use of RuC13 and... 

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