Alkylation with unsaturated hydrocarbons

Alkylation with Unsaturated Hydrocarbons. CH4 is thermodynamically favorable to react with lower olefins to form higher paraffins using super-acids (Sommer et al. (1982), and Olah et al. (1983)) according to the following reaction with C2H4 ... 

Organometallic chemistry (see p. 1199) is not particularly extensive even though gold alkyls were amongst the first organo-transition metal compounds to be prepared. Those of Au are the most stable in this group, while Cu and Ag (but not Au ) form complexes, of lower stability, with unsaturated hydrocarbons. 

The most common mode of reactivity of Y—H with unsaturated hydrocarbons in the absence of acidic hydrogens is 1,2-addition as shown in Scheme 5. The hydrides react with alkenes to form alkyl complexes, with 1,2-propadiene to form allyl systems, and with alkynes to form cis-alkenyl complexes (45). Crystallographic confirmation of the 1,2-addition mode of Y—H with unsaturated hydrocarbons was obtained using terf-butylnitrile as a substrate. The structure of the alkylideneamido product, [(C5Hs)2Y(/Li-N==CHCMe3)]2, was determined by X-ray crystallography. 

These metal-alkynyl complexes can be protonated to afford the free alkynes and parent cobalt hydroxo complex (comparable reactivity to their alkyl and aryl congeners), but have proven inert toward oxygenation and carbonylation. They are also thermally stable up to 100 °C. Attempts to explore the reactions of these compounds with unsaturated hydrocarbons were typically fruitless. The one exception is the reaction between 53 and its parent alkyne (HC = C02Me, Scheme 6), which under benzene reflux effects catalytic, stereospecific, linear trimerisation of the alkyne to afford ( , )-buta-l,3-dien-5-yne. The reaction was, however, slow (4.5 turnovers in 20 h) and suffered from catalytic deactivation due to hydrolysis of 53, which subsequently reacted with adventitious CO2 to irreversibly form an inert /x-carbonato complex. The catalytic cycle was concluded to involve initially a double coordination-insertion of the C = C bond of methylpropiolate into the Co-Caikyne linkage. Subsequent hydrolysis of the Co-C bond by a third equivalent of HC = CC02Me would then afford the observed product and regenerate 53. However, a definitive explanation for the stereospecificity of the process was not established. 

In this chapter the elementary reaction of CO2 with a discrete M-C bond is discussed. The coupling of CO2 with unsaturated hydrocarbons, which may also imply insertion of CO2 into an M-C bond, is discussed in Chap. 5. The insertion-deinsertion of CO2 into M-R bonds (R alkyl or aryl moiety) is relevant to carboxylation-decarboxylation processes which are very important in biological systems [38]. In this case, as seen for the insertion into M-H bonds, two mechanisms are possible, as depicted in Scheme 4.11. 

Alkyl dinitrates are important products of the reaction of NO3 radicals with unsaturated hydrocarbons, at least in laboratory experiments (Wangberg, 1993). A limited number of studies have been reported on their atmospheric degradation processes. All measurements have been conducted using the relative rate method (Zhu et al., 1991 Wangberg et al., 1996 Treves and Rudich, 2003). 

A necessary feature of the alkylation reaction section is the use of two reactors the first-stage reactor completes the major part of the alkylation reaction, and in the second-stage reactor the last traces of unsaturated hydrocarbons react, and a sizable portion of the soluble polyaromatics is removed. Modem units with lower-diene-containing feeds employ a single alkylation reactor (79). 

Removal of unsaturated hydrocarbons, of alcohols and of ethers from saturated hydrocarbons or alkyl halides by washing with cold concentrated sulfuric acid. 

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

Another approach was developed by Scott in the 1970 s (7.8) which utilises the same mechanochemistry used previously by Watson to initiate the Kharacsh-type addition of substituted alkyl mercaptans and disulphides to olefinic double bonds in unsaturated polymers. More recently, this approach was used to react a variety of additives (both antioxidants and modifiers) other than sulphur-containing compounds with saturated hydrocarbon polymers in the melt. In this method, mechanochemically formed alkyl radicals during the processing operation are utilised to produce polymer-bound functions which can either improve the additive performance and/or modify polymer properties (Al-Malaika, S., Quinn, N., and Scott, 6 Al-Malaika, S., Ibrahim, A., and Scott, 6., Aston University, Birmingham, unpublished work). This has provided a potential solution to the problem of loss of antioxidants by volatilisation or extraction since such antioxidants can only be removed by breaking chemical bonds. It can also provide substantial improvement to polymer properties, for example, in composites, under aggresive environments. 

The anhydrous salt which had been used for drying unsaturated hydrocarbons exploded on heating to 220° C for reactivation. The need to avoid contact with acidic materials is stressed. (Traces of an alkyl perchlorate may conceivably have been formed from free perchloric acid). 

The reaction described is of considerable general utility for the preparation of benzoyloxy derivatives of unsaturated hydrocarbons.2"8 Reactions of 2-butyl perbenzoate with various other classes of compounds in the presence of catalytic amounts of copper ions produce benzoyloxy derivatives. Thus this reaction can also be used to effect one-step oxidation of saturated hydrocarbons,9, 10 esters,6,11 dialkyl and aryl alkyl ethers,12 14 benzylic ethers,11,15 cyclic ethers,13,16 straight-chain and benzylic sulfides,12, 17-19 cyclic sulfides,11,19 amides,11 and certain organo-silicon compounds.20... 

The Heck reaction is a C-C coupling reaction where an unsaturated hydrocarbon or arene halide/triflate/sulfonate reacts with an alkene in presence of a base and Pd(0) catalyst so as to form a substituted alkene. Kaufmann et al. showed that the Heck reaction carried out in presence of ILs such as tetra-alkyl ammonium and phosphonium salts without the phosphine ligands, resulted in high yields of product. They attributed the activity to the stabilizing effect of ammonium and phosphonium salts on Pd(0) species. Carmichael et al. used ionic liquids containing either A,A -dialkylimidazolium and A-alkylpyridinium cations with anions such as halide, hexafluorophosphate or tetrafiuoroborate to carry out reactions of aryl halide and benzoic anhydride with ethyl and butyl acrylates in presence of Pd catalyst. An example of iodobenzene reacting with ethyl acrylate to give trans-et vy cinnamate is shown in Scheme 14. 

It has been observed [23,91,92] that when an unsaturated hydrocarbon is reacted with (a) equilibrated and (b) non-equilibrated hydrogen—deuterium mixtures, the deuteroalkane distributions are identical. Such observations indicate that the direct addition of a hydrogen molecule across the olefinic bond does not occur, and provides strong evidence for the formation of a half-hydrogenated state , that is, an adsorbed alkyl radical, first suggested by Horiuti and Polanyi [81], as a relatively stable reaction intermediate. The process of hydrogenation may thus be represented as... 

A similar mechanism has been proposed for photonitrosylations, with the difference that the reaction of the alkyl radical with NOC1 (Eq. 5) is not competitive (absence of chain reaction) and that the rate of reaction 6 cannot prevent dismutation (Eq. 4) and subsequent radical polymerization of unsaturated hydrocarbons (e.g., cyclohexene in the case of caprolactam synthesis). 

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