Alkenes Table

Thus with dihalocarbenes we have the interesting case of a species that resem bles both a carbanion (unshared pair of electrons on carbon) and a carbocation (empty p orbital) Which structural feature controls its reactivity s Does its empty p orbital cause It to react as an electrophile s Does its unshared pair make it nucleophilic s By compar mg the rate of reaction of CBi2 toward a series of alkenes with that of typical electrophiles toward the same alkenes (Table 14 4) we see that the reactivity of CBi2... 

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

The notion that car bocation formation is rate-determining follows from our previous experience and by obser-ving how the reaction rate is affected by the structure of the alkene. Table 6.2 gives some data showing that alkenes that yield relatively stable carbocations react faster than those that yield less stable carbocations. Protonation of ethylene, the least reactive alkene in the table, yields a primary carbocation protonation of 2-methylpropene, the most reactive in the table, yields a tertiary car bocation. As we have seen on other occasions, the more stable the car bocation, the faster is its rate of formation. 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

Various metal complexes catalyze the addition of catecholborane and pinacolbo-rane to aliphatic terminal alkenes (Table 1-2). Neither the borane reagents nor the catalysts alter the high terminal selectivity, but a titanium catalyst does (entry 3). Although Cp2TiMe2 [30] exhibits high terminal selectivity for vinylarenes, aliphatic alkenes afford appreciable amounts of internal products, whereas an analogous Cp 2Sm(THF) [31] allows selective addition of catecholborane to the terminal car-... 

Table 4.3 provides some data on the regioselectivity of addition of diborane and several of its derivatives to representative alkenes. Table 4.3 includes data for some mono- and dialkylboranes that show even higher regioselectivity than diborane itself. These derivatives are widely used in synthesis and are frequently referred to by the shortened names shown with the structures. 

As is true for most reagents, there is a preference for approach of the borane from the less hindered face of the alkene. Because diborane itself is a relatively small molecule, the stereoselectivity is not high for unhindered alkenes. Table 4.4 gives some data comparing the direction of approach for three cyclic alkenes. The products in all cases result from syn addition, but the mixtures result from both the low regioselectivity and from addition to both faces of the double bond. Even 7,7-dimethylnorbornene shows only modest preference for endo addition with diborane. The selectivity is enhanced with the bulkier reagent 9-BBN. 

COD = cyclooctadiene) not only for the ketone substrate, but also for terminal and cyclic unactivated alkenes (Table 1). 

BCP is unreactive towards electron-rich olefins, but reacts smoothly with a series of electron-deficient alkenes (Table 43) [13b, 143]. 

Two alkenylidene derivatives 149 and 563 have been reacted with several electron-deficient alkenes (Table 45) [36, 149]. 

The method of catalyst immobilisation appeared to affect its performance in catalysis. Catalyst obtained by method II showed a low selectivity in the hydroformylation of 1-octene (l b aldehyde ratio was even lower than 2) at a very high rate and high yields of isomerised alkenes (Table 3.2, entry 2), whereas procedure IV resulted in a catalyst that was highly selective for the linear aldehyde (with a l b ratio of 37) (entry 5). In accordance with examples from literature it is likely that procedure II gave rise to the ionic bonding of ligand-free rhodium cations on the slightly acidic silica surface [29],... 

Addition of diphenyl disulfide (PhS)2 to terminal alkynes is catalyzed by palladium complexes to give l,2-bis(phe-nylthio)alkenes (Table 3)168-172 The reaction is stereoselective, affording the (Z)-adducts as the major isomer. A rhodium(i) catalyst system works well for less reactive aliphatic disulfides.173 Bis(triisopropylsilyl) disulfide adds to alkynes to give (Z)-l,2-bis(silylsulfanyl)alkenes, which allows further transformations of the silyl group to occur with various electrophiles.174,175 Diphenyl diselenide also undergoes the 1,2-addition to terminal alkynes in the presence of palladium catalysts.176... 

While transition metal-catalyzed hydroboration is a well-established reaction, the same cannot be said for the transition metal-catalyzed hydroalumination. The synthetic utility of this reaction is only just beginning to emerge. Lautens has led the way in the use of hydroaluminations as the key step in the total synthesis of complex natural products. The synthesis of the anti-depressant sertraline130 involved the formation of the tetrahydronaphthalene core, and this is best achieved using the nickel-catalyzed hydroalumination of oxabicyclic alkenes (Table 16). 

The overall performance of each of the six levels used to estimate the barriers for radical additions to substituted alkenes (Tables 6.15 - 6.20) is summarized in Table 6.21. The CBS-RAD procedure gives the best overall performance (MAD of 3.2 kJ/mol) and generally underestimates... 

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). 

Ligand 19 was found to be highly selective for the hydrogenation of 1-aryl, I -alkyl alkenes, and ligand 18b was found to be selective for 1-aryl, I -hindered aryl alkenes (Table 7). 

Binaphthol- and biphenyl-derived ketones (9 and 10) were reported by Song and coworkers in 1997 to epoxidize unfunctionalized alkenes in up to 59% ee (Fig. 3, Table 1, entries 9, 10) [37, 38]. Ketones 9 and 10 were intended to have a rigid conformation and a stereogenic center close to the reacting carbonyl group. The reactivity of ketones 9 and 10 is lower than that of 8, presumably due to the weaker electron-withdrawing ability of the ether compared to the ester. In the same year, Adam and coworkers reported ketones 11 and 12 to be epoxidation catalysts for several trans- and trisubstituted alkenes (Table 1, entries 11,12). Up to 81% ee was obtained for phenylstilbene oxide (Table 1, entry 25) [39]. 

RuCl(TAZO)(p-cymene) (TAZO =2-methyl-5-oxo-7-phenyl-3-thioxo-3,4,5, 6-tetrahydro-2//-l,2,4-triazepine TAZS =2-methyl-7-phenyl-3, 5-dithioxo-3,4, 5, 6-tetrahydro-2H-l, 2, 4-triaze-pine). These were made from [RuCljCp-cymene)] and (TAZO). As RuCl(TAZO)(p-cymene)/Oj/MejCHCHO/CH3Clj they epoxidised natural terpenic alkenes (Table 3.1) [959]. 

Epoxy alcohols were oxidised by TPAP/NMO/PMS/CH Cl without affecting the epoxy group Fig. 2.2 also illustrates the tolerance of the system to alkenes (Tables 2.1 and 2.2) [16]. 

The structure of the dipolarophile is also a very important component. The most widely utilized dipolarophiles are monosubstituted alkenes bearing an electron-withdrawing group. A survey of dipolarophiles shows good compatibility with activated monosubstituted alkenes (Table 2.47) (224). Less activated alkenes such... 

As for other organics in the atmosphere, the OH radical is a major oxidant for alkenes. Table 6.8 gives the rate constants for some OH-alkene reactions as well as their temperature dependence in Arrhenius form. Several points are noteworthy (1) the reactions are very fast, approaching 10-l() cm3 molecule-1 s-1 for the larger alkenes (2) the rate constants have a pressure dependence (3) the apparent Arrhenius activation energies are negative. ... 

The / -methylene of an enone is typically 10-15 ppm downfield from the corresponding alkene (6). This general observation is attributed to a reduction of electron density at the / -methylene carbon due to conjugation with the carbonyl. In the case of 2,4,4-trimethylpentene-3-one, the / -methylene carbon of the enone resonates only 3.5 ppm downfield from the / -methylene carbon of the analogous alkene. Table I contains the carbon-13 chemical shift data for a series of enones and their corresponding alkenes that were measured under the same conditions. These C-13 NMR data indicate that 2,4,4-trimethylpentene-... 

A rhodium catalyst derived from the 6-DPPon ligand 1 displayed behavior typical of a bidentate ligand upon hydroformylation of terminal alkenes [9]. Thus, excellent regioselectivity in favor ofthe linear aldehyde isomer was noted for hydroformylation of a range of functionalized terminal alkenes (Table 2.1). Among them even those... 

Furthermore, electrophilic anti addition of /V-(phenylselanyl)phthalimide in the presence of triethylamine tris(hydrogen fluoride) has been investigated using alkenes (Table 14)216 and alkynes.217... 

The reactivity of polyfluorinated cycloalkenes towards vanadium(V) fluoride is similar to that of internal alkenes (Table 7). Perfluorocyclobutene (4) reacts with vanadium(V) fluoride at 20-22 C however, at 50-60 C the fluorination proceeds more efficiently. 

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