Propane with electrophiles

SCHEME 11. Electrophilic character of cyclopropyl carbenoids Reaction of 2,2-diphenylcyclo-propane with -butylhthium (2 equivalents)... 

The reaction of l-metallo-l,l-bis(seleno)alkanes with electrophiles is another interesting approach to selenoacetals (Schemes 76 and 77), which inter alia allows the synthesis of l,l-bis(seleno)cyclo-propanes (Scheme 26, a, b and d Scheme 77, c) and also permits the synthesis of functionalized selenoacetals by concomitant C—C bond formation (Scheme 79). Other methods listed above are of limited use. The substitution of dihaloalkanes by selenolates is the method of choice for the synthesis of bis(seleno)methanes (Scheme 72, and can be applied to the synthesis of the relatively unhin-... 

A mixture of 1-aryl-l-chlorodiazirine and an alkene in hexane sonicated with ultrasound (Fisher Scientific Solid State Ultrasound FS-9) at 40 °C for two hours afforded 1-aryl-l-chlorocyclo-propanes with yields similar to those obtained by photolysis. Electrophilic as well as nucleophilic alkenes were used. Photolytic, thermal or ultrasonic decomposition of 3-chloro-3-phenyl-3//-diazirine in 2-vinylpyridine did not give the corresponding 1-chloro-l-phenylcyclopropane derivative (see Section 1.2.1.3.1.2.2.1). 

Irradiation of methyl-substituted phenylcyclopropanes gave rise to the formation of products 11 in which the nucleophile was attached to a carbon carrying a methyl group. Since the course of these reactions is stereospecific and the products show an inversion of configuration at the electrophilic center, an St 2-type reaction has to be assumed. In 1,1-diphenylcyclo-propanes with alkyl substituents at the other carbon atoms the nucleophilic substitution occurs at the most sterically hindered carbon. ... 

In the propane ammoxidation a lower selectivity for acrolein plus acrylonitrile is observed. The formation of partial (amm)oxidation products from propane requires more elemental steps than their formation from propene. All these intermediates can undergo a side reaction with electrophilic oxygen species yielding degradation products. 

It is interesting to note that the latter result is exceptional since l,l-bis(phenyl-seleno)cyclopropane is the only selenoacetal derived from ketones to be at least partially cleaved under these conditions and even the homologous cyclobutyl derivative is inert under these conditions. This may be due to the extra stabilization introduced by the cyclopropyl ring. The case of 2-decy 1-1,1-bis(methylseleno)cyclo-propane merits further comment. It is difficult to assess whether the cleavage of the carbon-selenium bond occurs on the methylseleno moiety cis or tram to the alkyl group, since this organometallic leads to a mixture of the two possible stereoisomers on further reaction with electrophiles (Scheme 16). 

Recent work [109, 110, 111] has shown that 1,3-dithian and 2-substituted 1,3-dithians (27) (Le., cyclic dithioacetals derived from propane-1,3-dithiol and aldehydes) can be de-protonated by strong bases and that the resulting 2-carbanion (28) can be made to react with electrophilic reagents, e.g., alkylated or acylated (Fig. 9.4). 

The addition reactions we have met so far have involved electrophilic addition across the C = C bond in alkene molecules (see page 209). Aldehydes and ketones both undergo addition reactions with hydrogen cyanide, HCN. In this case, addition of HCN takes place across the C=0 bond. However, the attack is by a nucleophile, not an electrophile. We can show this using the nucleophilic addition reaction of propanal with HCN. The HCN is generated in situ (in the reaction vessel) by the reaction of sodium cyanide, NaCN, and dilute sulfuric acid. 

The proline-catalyzed reaction has been extend to the reaction of propanal, butanal, and pentanal with a number of aromatic aldehydes and proceeds with high syn selectivity.197 The reaction can also be carried out under conditions in which the imine is formed in situ. Under these conditions, the conjugative stabilization of the aryl imines leads to the preference for the aryl imine to act as the electrophile. A good yield of the expected P-aminoalcohol was obtained with propanal serving as both the nucleophilic and the electrophilic component. The product was isolated as a 7-amino alcohol after reduction with NaBH4. 

Treatment of 1-pyridinium sulphonate with sodium or potassium hydroxide generates sodium or potassium salts of 5-hydroxy-2,4-pentadienal (glutaconaldehyde), which are starting materials for a variety of transformations (equation 178)171b 301. For example, the reaction of the potassium salt with a carbon electrophile has been used for the preparation of a dienol aldehyde (equation 179)mb which was an intermediate in the total synthesis of a mutagen, (S)-3-(dodeca-l,3,5,7,9-pentaenyloxy)propane-l,2-diol. 

Experimental results [1361] and theoretical treatment [28] indicate that the cyclo-propanation of alkenes by electrophilic carbene complexes is a concerted process. Z-Olefins normally lead to the formation of the corresponding c7. -cyclopropanes, and -olefins yield fran -cyclopropanes. The relative configuration of the carbene-bound substituent and the substituents of the alkene in the final cyclopropane seems to be mainly determined by the steric bulk of these groups. In cyclopropanations of terminal alkenes with ethyl diazoacetate low diastereoselectivities are often observed [1024,1351]. These can be improved by increasing the steric demand of the substituents at the carbene or at the alkene [1033,1362]. High diastereoselectivities can, e.g., often be achieved with terf-butyl, neopentyl or 2,6-di(rerr-butyl)phenyl diazoacetate [1362] as carbene complex precursors (Figure 4.19). 

Alkane metathesis was first reported in 1997 [84]. Acyclic alkanes, with the exception of methane, in contact with a silica supported tantalum hydride ](=SiO)2TaH] were transformed into their lower and higher homologues (for instance, ethane was transformed into methane and propane). Later, the reverse reaction was also reported [85]. Taking into accountthe high electrophilic character ofa tantalum(III) species, two mechanistic hypotheses were then envisaged (i) successive oxidative addition/reductive elimination steps and (ii) o-bond metathesis. Further work has shown that aLkyhdene hydrides are critical intermediates, and that carbon-carbon... 

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

The enantiomerically pure a-stannyl ether, (R)-l-benzyloxymethoxy-l-tributylstannyl-propane, can be obtained by resolution of the precursor compound. The tin - lithium exchange reaction, as well as the electrophilic substitution, occurred with retention of configuration to give (I )-2-(benzyloxymethoxy) butane only28. A later study examined more examples and also confirmed this result27. 

It is possible to obtain anti-Markovnikov products when HBr is added to alkenes in the presence of free radical initiators, e.g. hydrogen peroxide (HOOH) or alkyl peroxide (ROOR). The free radical initiators change the mechanism of addition from an electrophilic addition to a free radical addition. This change of mechanism gives rise to the anh-Markovnikov regiochemistry. For example, 2-methyl propene reacts with HBr in the presence of peroxide (ROOR) to form 1-bromo-2-methyl propane, which is an anh-Markovnikov product. Radical additions do not proceed with HCl or HI. 

Among isolable metal homoenolates only zinc homoenolates cyclize to cyclo-propanes under suitable conditions. Whereas acylation of zinc alkyls makes a straightforward ketone synthesis [32], that of a zinc homoenolate is more complex. Treatment of a purified zinc homoenolate in CDC13 with acid chloride at room temperature gives O-acylation product, instead of the expected 4-keto ester, as the single product (Eq. (22) [33]). The reaction probably proceeds by initial electrophilic attack of acyl cation on the carbonyl oxygen. A C-acylation leading to a 4-keto ester can, however, be accomplished in a polar solvent Eq. (44)-... 

Perfluoro-tert-butyl hypofluorite [perfluoro(2-fluoroxy-2-methylpropane), 14], readily prepared by the low-temperature fluorination of potassium or sodium perfluoro-/cr/-butoxide,812 reacts with hexafluoropropenc at low temperature to give 15 with more than 95 % regiosclcctivity, which may be explained by electrophilic attack and the formation of a fluorocarbeniuni ion, whose intervention is suggested in order to explain the production of perfluoro(l-/m-butoxy-propane) (15).12... 

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