Pentenes addition reactions

The regiochemistry is determined by the regiochemistry of the fluoride ion addition reaction, that is, via the most stable perfluorocarbanion intermediate Von Werner used a similar reaction to prepare silver compounds from perfluoro-2-methyl-2-butene and perfluoro 2 methyl-2-pentene [271] Silver(I) fluoride adds to bis(ttitluoromethyl)ketene in DMF without fluoride ion catalysis [270] The analogous trifluorovinylsulfurpentafluoride reacts similarly to give the isolable pentafluorosulfur derivative [272] (equation 187)... 

A simple test to distinguish between 2-pentene and cyclopentane is to add a few drops of a red Br2 solution to the unknown liquid. The reddish color will disappear if the liquid is an alkene or alkyne, e.g., 2-pentene, due to the addition of Br2 to the multiple bond. No such addition reaction occurs between Br2 and cyclopentane. 

Isolated instances of 1,4-addition reactions of other hetero-nucleophiles to 4-en-2-ynoic acids and derivatives have been reported172-174. Thus, treatment of methyl 4-methyl-4-penten-2-ynoate with phenolate provided the 3-phenoxy-substituted conjugated dienoate (equation 71)172, and the 1,4-addition of water-soluble phosphines to 4-octen-2-ynoic acid afforded dienylphosphonium salts which were transformed into the corresponding phosphine oxides (equation 72)174. 

Extreme cases were reactions of the least stabilized, most reactive carbene (Y = CF3, X = Br) with the more reactive alkene (CH3)2C=C(CH3)2, and the most stabilized, least reactive carbene (Y = CH3O, X = F) with the less reactive alkene (1-hexene). The rate constants, as measured by LFP, were 1.7 x 10 and 5.0 X lO M s, respectively, spanning an interval of 34,000. In agreement with Houk s ideas,the reactions were entropy dominated (A5 —22 to —29e.u.). The AG barriers were 5.0 kcal/mol for the faster reaction and 11 kcal/ mol for the slower reaction, mainly because of entropic contributions the AH components were only —1.6 and +2.5 kcal/mol, respectively. Despite the dominance of entropy in these reactive carbene addition reactions, a kind of de facto enthalpic control operates. The entropies of activation are all very similar, so that in any comparison of the reactivities of alkene pairs (i.e., ferei)> the rate constant ratios reflect differences in AA//t, which ultimately appear in AAG. Thus, car-benic philicity, which is the pattern created by carbenic reactivity, behaves in accord with our qualitative ideas about structure-reactivity relations, as modulated by substiment effects in both the carbene and alkene partners of the addition reactions. " Finally, volumes of activation were measured for the additions of CgHsCCl to (CH3)2C=C(CH3)2 and frani-pentene in both methylcyclohexane and acetonitrile. The measured absolute rate constants increased with increasing pressure Ayf ranged from —10 to —18 cm /mol and were independent of solvent. These results were consistent with an early, and not very polar transition state for the addition reaction. 

One of the most direct ways to produce diastereomers is by addition reactions across carbon-carbon double bonds. If the structure of the olefin substrate is such that two new chiral centers are produced by the addition of a particular reagent across the double bond, then diastereomers will result. For example, the addition of HBr to Z-3-chloro-2-phenyl-2-pentene produces 2-bromo-3-chloro-2-phenylpentane as a mixture of four diastereomers. Assuming only Markovnikov addition, the diastereomers are produced by the addition of a proton to C-3 followed by addition of bromide to the carbocation intermediate at C-2. Since the olefin precursor is planar, the proton can add from either face, and since the carbocation intermediate is also planar and freely rotating, the bromide can add to either face to give diastereomeric products. The possibilities are delineated schematically (but not mechanistically) below. 

A typical example of a stereospecific olefin addition reaction is the addition of bromine to olefins. If d.v-2-pentene is used as the substrate, only the 2R,3R and 2S,3S pair will be produced (they are enantiomers). 

Figure 2.3, the Michael addition reactions of P-nitrostirenes (10) with 4-pentene-l-magnesium bromide (11a) or 3-butene-1-magnesium bromide (11b) generated the nitronates (12) or (13). Satisfactory to high yields of isoxazolidine derivatives (16) and (18) were obtained when nitronates (12) or (13) were treated, in situ, with ethyl chloroformate and catalytic amount of 4-dimethylaminopyridine (DMAP). 

The elementary reactions of carbocationic polymerizations can be separated into three types. Deactivation of carbenium ions with anions and transfer to counteranion are ion-ion reactions, propagation and transfer to monomer are ion-dipole reactions, and ionization is a dipole-dipole reaction [274]. Ion-ion and dipole-dipole reactions with polar transition states experience the strongest solvent effects. Carbocationic propagation is an ion-dipole reaction in which a growing carbenium ion adds electro-philically to an alkene it should be weakly accelerated in less polar solvents because the charge is more dispersed in the transition state than in the ground state [276]. However, a model addition reaction of bis(p-methoxyphenyl)carbenium ions to 2-methyl- 1-pentene is two times faster in nitroethane (e = 28) than in methylene chloride (e = 9) at - 30° C [193]. However, this is a minor effect which corresponds to only ddG = 2 kJ morit may also be influenced by specific solvation, polarizability, etc. [276,277]. 

The direct photolysis and benzophenone-sensitized decomposition of methyl diazomalonate has been shown to produce two different spin states of biscarbo-methoxycarbene. In the direct photolysis experiments stereospecific addition to olefins and a high yield of insertion products in the presence of alkanes demonstrate the singlet character of the carbene in the sensitized decomposition the yield of insertion products decreases, and addition to cis- or ra 5 -4-methyl-2-pentene yields about 88 % trnwj-cyclopropane. In this case, as with most carboalkoxy-carbenes, the addition reaction to olefins can proceed by two paths ... 

Recently reported synthetic studies en route to the epothilones documents a series of fascinating observations by Danishefsky of a novel aldol addition reaction (Eq. (8.10)). The epothilone strategy necessitated an unusual aldol addition reaction of 38 and (5 )-2-methyl-4-pentenal. The addition reaction gave a stereochemical outcome unexpected on the basis of the accepted models for acyclic stereocontrol in carbonyl addition reactions. Thus, the addition affords adduct 39 and 40 as a 5.5 1 diastereomeric mixture with an unexpected preference for the anti-Cram adduct. By contrast, addition to (S)-phenyl acetaldehyde affords the Cram adduct as an 11 1 mixture of diastereomers. In a series of studies, Danishefsky has noted that the positioning of unsaturation in the substrate in relation to the aldehyde C=0 appears to be critical. 

Oxazaborolidenes. Corey has reported the use of a novel oxazaborolidene complex 41 prepared from borane and A-tosyl (5)-tryptophan. This complex functions in a catalytic fashion in enantioselective, Mukaiyama aldol addition reactions (Scheme 8-3) [17]. The addition of ketone-derived enol silanes 42-43 gives adducts in 56-100% yields and up to 93% ee. The use of 1-trimethylsilyloxycyclo-pentene 43 in the addition reactions to benzaldehyde affords adducts 46 as a 94 6 mixture of diastereomers favoring the syn diastereomer in 92% ee. Addition reactions with dienol silanes 44 furnishes products 47 in up to 82% ee. Corey also demonstrated the use of these adducts as important building blocks for the synthesis of corresponding dihydropyrones treatment of 47 with trifluoroacetic acid affords the cyclic product in good yields. 

Intramolecular transition metal-catalyzed hydro acylation reactions have opened up a new area of synthesizing cyclic ketones. This reaction can also be extended to intermolecular addition reactions. Miller et al. found the first example of an intermolecular hydroacylation of an aldehyde with an olefin giving ketones, when they were studying the mechanism of the rhodium-catalyzed intramolecular cyclization of 4-pentenal using ethylene-saturated chloroform as the solvent (Eqs.46,50) [112]. 

Bicyclic pentenes are used as trapping agents in addition reactions of 3-butynyl radicals81. Addition occurs exclusively trans to the annulated cyclopentyl ring, while the regioselectivity is low when polar substituents are not present. Under thermal conditions, the reaction finishes after iodine atom abstraction to yield the overall trans-addition product. With photochemical initiation and in the presence of hexabutylditin, further cyclization and iodine atom abstraction occurs to form exocyclic vinyl iodides. 

Recombination of the trapped radicals at the melting point necessarily produces dimer molecules by addition (Reaction 8) or pentene molecules by disproportionation (Reaction 9). 

Irradiation of 26 in cyclohexane solution containing either cis- or trans-4-methyl-2-pentene forms aziridines with complete (>98%) retention of olefin stereochemistry.Stereospecific aziridine formation is usually considered as evidence of a concerted, singlet nitrene addition reaction. 

Cyclizations of 5-alkyl-substituted 4-penten-l-oxyl radicals are faster and frequently more selective than those of terminal unsubstituted derivatives [48, 53]. This finding is in accord with the electrophilic nature of alkoxyl radicals in addition reaction to C-C double bonds. 5-Alkyl- or 5-phenyl-substituted 4-pentenoxyl radicals, such as intermediates 25 or 26, were generated from a number of different sources. For example, alkyl nitrites [39], A-alkoxypyridine-2(l//)-thiones [46], and A-alkoxy-(/ -chlorophenyl)thiazole-2(3/f)-thiones [54] in photochemically induced reactions, and A-alkoxyphthalimides [55] or type IV radical precursors [53] in thermally initiated reactions have been applied for this purpose (Scheme 6). 

In discussing processes in olefins, it is convenient to divide the reactions into two classes simple particle transfer and carbon-addition reactions. The relative importance of these two types of reaction is also dependent on the structure of the reactants. The presence of the isobutene type structure (CH2=C(CH3)CH2—) in either the reactant ion or neutral molecule favours the simple particle transfer reaction, mainly because of the large cross-section for the formation of parent-plus-one ions. The 2-butene neutral molecule is more like isobutene than 1-butene with regard to the relative importance of simple particle transfer and carbon addition reactions [284]. However, the magnitude of the cross-section for simple particle transfer reactions in 2-butene (2-P + 2-M reaction) is much closer to that in 1-butene (1-P + 1-M reaction) than to that in isobutene (iso-P + iso-M reaction) [284, 299]. The same is true for pentene isomers i.e. the cross-sections for simple particle transfer reactions in 1-pentene and 2-pentenes are almost the same and are much lower than that in 2-methyl-l-butene. The simple particle transfer cross-sections for other branched pentenes, i.e. 2-methyl-2-butene and 3-methyl-l-butene, are even smaller than those for 1- and 2-pentenes, while the proportion of transfer reactions is higher than the corresponding proportions for 1- and 2-pentenes. The proportion is, of course, highest for 2-methyl-l-butene which has the isobutene type structure. 

With pentene isomers, the carbon addition products are mainly Cg, C7 and Cg ions [132, 290]. However, Cg ions are absent in branched pentenes except for 2-methyl-l-butene where Cg ions constitute more than 10% of the total product ions at 1.7 eV ion exit energy [132]. No Cg ions are observed in any of the isomers [132, 290]. The photoionization study by Koyano et al. [132] reports an observation of a very small concentration of dimeric ion(s) (Cj 0 ion(s)) with all of the pentene isomers. With hexenes, the main carbon addition products are C7, Cg and C9 ions [290], the relative importance of these ions being dependent on the structure. The relative probabilities of these carbon addition reactions... 

Conceivably, in an olefin addition reaction, if the more substituted end of the ethylenic linkage is sterically shielded such that the approach of the nucleophile or the radical is essentially blocked, the product cannot be expected to be the result of a simple addition reaction, but would always be complicated by the intervention of other pathways, such as elimination/rearran ment. To illustrate this point, the reaction of chlorine-free hypochlorous acid with u/wym-dineopentylethylene (110) and 2,4,4-trimethyl-l-pentene (112) may be cited (110) gives a complex reaction product in which (111) predominates (48%) and no oxygen-containing material was detected in the total product, while (112) which is less hindered furnishes 34% of the normal product (113) and 46% of elimination product (114) (65). 

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