Dehydrodimerization reactions

The dehydrodimerization reaction involving aromatic radical-cations is fast only when electron donating substituents are present in the benzene ring. These substituents stabilise the a-intermediate. Benzene, naphthalene and anthracene radical-cations form a a-sandwich complex with the substrate but lack the ability to stabilise the a-intermediate so that radical-cation substrate reactions are not observed. The energy level diagram of Scheme 6.4 illustrates the influence of electron donating substituents in stabilising the Wheland type a-intermediate. 

It is noteworthy that acetone reacts with the organoaluminum in a manner quite different from aldehyde to form mesityloxide and acid amide. This dehydrodimerization reaction of acetone is identical with that observed between A1R3 and ketone. That a hydrogen atom bonded to a nitrogen atom of the product originates from that of the methyl group in acetone was proved by using acetone-d6 as a reactant. 

The mercury-photosensitized dehydrodimerization reaction has been known for nuuiy years, but it has only been made preparatively useful very recently. The key feature of the process is that the system is only active in the vapor phase, so that after condensation the product is protected from further conversion. This implies that the reaction can be run to essentially quantitadve conversion without a fall-off in yield. In order to run on a gram scale to tens of grams, all diat is needed is a quartz flask and a low pressure mercury lamp. Heating the substrate or substrates in the quartz flask with a small drop of mercury leads to smooth formation of the products. Aspects of the process are shown in equations (IS) to (18). 

Cirkva and Hajek have studied the photochemically or microwave-induced addition of tetrahydrofuran to perfluorohexylethene (Scheme 19.2) [105]. Whereas the thermal reaction was too slow, photochemical activation was very efficient, with no apparent thermal effects of MW radiation. Combined UV and MW radiation (Fig. 19.12) has principally been used to initiate EDL operation in the reaction mixture. Another illustration of the MW-UV-assisted reaction has been demonstrated by Niichter et al. [22] on dehydrodimerization reactions of some hydrocarbons. 

Anodic oxidation of malonate esters in alkaline solution gives the dehydrodimerization product by carbon-carbon coupling. The reaction mechanism has been... 

Electrochemical oxidation of hydrazidoyl halides (330) also affords 1,4-dihydro-1,2,4,5-tetrazines (104). A nitrilimine intermediate is not suggested for this reaction. The main process is the dehydrodimerization of the initially formed hydrazonyl radical, while a concurrent side-reaction leads to the l,4-dihydro-l,2,4,5-tetrazines (104), which are transformed into the corresponding cation radicals (336) on further oxidation (77IZV393, b-75MI22102). 

In looking at some control reactions for transition-metal catalyzed alkane photodehydrogenation, we came across mercury-photosensitized dehydrodimerization of alkanes. The very high efficiency of the procedure, when performed under reflux conditions at ambient temperature and pressure was immediately obvious. 

Dehydrodimerization. On excitation with a mercury vapor lamp, mercury is converted to an excited state, Hg, which can convert a C—H bond into a carbon radical and a hydrogen atom. This process can result in dehydrodimerization, which has been known for some time, but which has not been synthetically useful because of low yields when carried out in solution. Brown and Crabtree1 have shown that this reaction can be synthetically useful when carried out in the vapor phase, in which the reaction is much faster than in a liquid phase, and in which very high selectivities are attainable. Secondary C—H bonds are cleaved more readily than primary ones, and tertiary C—H bonds are cleaved the most readily. Isobutane is dimerized exclusively to 2,2,3,3-tetramethylbutane. This dehydrodimerization is also applicable to alcohols, ethers, and silanes. Cross-dehydrodimerization is also possible, and is a useful synthetic reaction. 

Although zwitterions are mainly considered for their novel ion conductive matrix in this chapter, they are being used as not only as solvents and catalysts for organic reactions [42] but also as organogelators [43]. Zwitterions have been screened as solvent/catalysts for several classical acid-promoted organic reactions such as the Fischer esterification, alcohol dehydrodimerization, and the pinacol/ benzopinacole rearrangement. The zwitterion containing an equimolar trifluoro-methane sulfonic acid is liquid at room temperature. Because they can work as solvent/catalysts, as shown in the reactions discussed in this chapter, zwitterionic liquids should open the door to a whole new area of applications. 

A procedure for the dehydrodimerization of volatile organic compounds on a preparative scale by Hg photosensitized reactions has been reported by Crabtree and coworkers. ... 

In this section, not only alkane isomerization and dehydrodimerization (equatirm 13) ate consictered, but also the dehydrogenation of alkanes to alkenes, as in diis case, two adjacent C—bonds ate replaced by a ir-type C— bond. Other C—C bond-forming reactions are also mentioned. 

As the photocatalytic carbon-carbon bond is formed, hydrogen evolves when the photocatalytic activation is done on colloidal ZnS [149, 150]. This dehydrodimerization also takes place with saturated ethers, with reactivity related to C H bond strength. Thus, 2,5-dihydrofuran (an allylic ether) is more easily activated than the isomeric 2,3-dihydrofuran (a vinyl ether). With the former substrate, all three dia-stereomeric coupling products are observed. Water is required for the reaction, and the primary photochemical product is thought to be a surface-bound hydroxyl radical. 

One-electron oxidation of aromatic compounds (ArH) leads primarily to corresponding radical cation which exist either in monomeric (ArH +) or dimeric form [(ArH)2 ] the latter usually formulated as r-dimer [70]. However, radical cations are reactive species and can undergo further reaction yielding more persistent radical cations e.g. oxidation of rert-butylbenzene or of toluene or o-xylene yielded radical cation of 4,4 -di-rerf-butyl biphenyl, 4,4 -bitoluene or 3,3, 4,4 -tetramethyl biphenyls, products of further a-coupling, proton loss and further one-electron oxidation [71]. This is a well-known pathway of biaryl dehydrodimerization, explored in anodic and metal-ion oxidation of ArH [72, 73]. Other compounds with high reactivity in (T-coupling are alkoxy and amino substituted ArH [73]. Thus a risk with characterization of radical cations is that hardy survivors and not primary radical... 

All these photocorrosion processes are, of course, undesirable and it is obvious that their relative importance depends strongly on the presence of surface states which may facilitate recombination or redox reactions with adsorbed substrates. It is well known from ESR [69, 70, 94] and emission spectra [94] that most of these metal sulfide powders contain surface states. They are introduced during preparation of the powder as a result of lattice defects [72, 96], trapped holes [94], surface impurities [97] and metallization [38], and during the actual catalytic reaction as a consequence of irradiation and substrate adsorption. The stabilizing effect of plati-nization is exemplified by Figure 6 for the ZnS-catalyzed reduction of water in the presence of sodium formate [98]. Note that platinum does not accelerate the reaction but doubles the time of constant catalytic activity from 1 to 2 days. Similarly, the apparent product quantum yield of the 2,5-DHF dehydrodimerization is not increased but slightly decreases when platinizes ZnS is the photocatalyst [97]. 

Some of the most successful applications of LSV and CV are concerned with the study of the kinetics and mechanisms of the reactions of electrode generated intermediates and a large share of the electrochemical literature deals with this aspect of voltammetry [8,9,13-38,72]. The majority of electrochemical reactions include radical ions as the primary intermediates, and the reaction schemes describing the conversion of a substrate A to products are typically composed of one or two one-electron transfers and one or two chemical steps. The examples include cathodic hydrogenations, (-l-2e , +2H ") (see Chapter 6) and hydrodimerizations (-l-e , -t-H ) (see Chapter 21), and anodic additions (—2e , - -2Nu ) (see Chapter 24), dehydrodimerizations (—e , —H ) (see Chapter 22), and substitutions (—2e , +Nu , —H ) (see Chapter 24), where Nu is nucleophile)... 

Mercury photosensitized ( Pi-excited state) dehydrodimerization of hydrocarbons [103] has been developed into a useful organic synthetic method by using a simple reflux apparatus in which the radical reaction products are protected from further transformation simply by condensation (vapor-pressure selectivity) [104]. The selectivity of C-H cleavage increases from primary to tertiary carbons (350 1) and the method permits the formation of highly substituted C-C bonds (eq. (13)). One limitation for product formation is the appearance of four sets of obligatory 1,3-syn methyl-methyl steric repulsions (e. g., 2,3,4,4,5,5,6,7-octa-methyloctane). 

The reaction may proceed as homo- or cross-dehydrodimerization [105] and takes place with a wide range of substituted substrates such as higher alcohols, ethers, silanes, and partially fluorinated alcohols and ethers, but also with ketones, carboxylic acids, esters, amides, and amines [106]. Besides the formation of 1,2-diols from saturated alcohols, unsaturated substrates are also dimerized under hydrogen to form l,n-diols other than the 1,2-isomers [107]. The regio-selectivity of the diols is controlled by the formation of the most stable radical, which then dimerizes. 

A typical example for the oxidative dehydrodimerization of alkenes by heterogeneous catalysis is the conversion of isobutene to 2,5-dimethyl-1,5-hexadiene (DMH) catalyzed by metal oxides. The overall reaction scheme is ... 

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