Dehydrodimerization

R. DiCosimo, J.D. Burrington, and R.K. Grasselli, Oxidative dehydrodimerization of propylene over a Bi203-La203 oxide ion-conductive catalyst, J. Catal. 102, 234-239 (1986). 

Similar mercury-photosensitized alcohol dehydrodimerizations leading to glycols have been documented (Equation (17)).26... 

Scheme 6.3. Dehydrodimerization reactioti for anisole. (a) Rate-determining step at iow concentrations of anisole, (b) Rate-determining at high concentrations of anisole.

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. 

Anodic oxidation of 1,2,3-trimethoxybenzene in acetone containing dilute sulphuric acid gives 2,6-dimethoxybenzoquinone but in contrast 1,2,4-trimethoxy-benzene affords the dehydrodimer 11 in good yield [82]. Dehydrodimerization becomes an important process in the oxidation of methoxybenzenes in dichlo-... 

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

Anodic oxidation of the alkali metal salts of a-oximinoesters results in dehydrodimerization [32],... 

The classical route for producing 2,2 -dipyridyl consists in the dehydrodimerization of pyridine on Raney-Ni using a process developed by the Imperial Chemical Industries [63AHC(2)179 68CI(L)49, 80MI4]. 2,2 -Bipyridyl reacts with ethylene bromide to give l,l -ethylene-2,2 -bipyridylium bromide (diquat). The production of one ton of the diquat (which is widely used as a herbicide) requires 1.2 tons of pyridine [Eq.(17)]. 

The so-called toluene-to-styrene process [Eq. (12.33)], first reported in a patent by Monsanto,148 applies bismuth or lead oxides in dehydrodimerization of toluene to stilbene, which, in turn, is cleaved with ethylene to styrene 149... 

Decarboxylation of the adduct gave the corresponding 2-methyl derivative, the structure of which was proved by an independent synthesis.127 2,3-Dimethylquinoxaline on treatment with phenyllithium and Cu2Cl2 undergoes dehydrodimerization to yield compound 115. This has been further converted into the pentacyclic compound (116).128... 

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. 

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). 

Several fluorinated diols 6 have been synthesized from highly fluorinated alkanols 5 via mercury-photosensitized dehydrodimerization in the vapor phase163 in a special reactor.164 In almost all cases, the corresponding dl- and meso-diols are obtained in a 50 50 ratio.163... 

Anodic dehydrodimerization and additive dimerization of electron-rich olefins. 

Hydrodimerization of olefinsIn addition to dehydrodimerization of alkanes 15. 198), hydrodimerization of alkenes can be effected by mercury-photosensitiza- jon, and has the advantage that it is applicable to a wide range of unsaturated wbstrates alcohols and derivatives, ketones, and others. Since the hydrogen adds to ae alkene to give the most stable intermediate (tert > sec > primary), this dimeriza-son can be regioselective. The last example shows that cross-dimerization is possible In this case the hydrodimer of both components is also formed, but in lower ld. 

Scheme 1. Dehydrodimerization of electron-rich aryls (general pictogram).

Scheme 3. Dehydrodimerization by phenyliodine(l 11) bis(trifluoroacetate) (a) PIFA, BF3-OEt2, CH2CI2,-40°C, 1.5 h (b) PIFA, BF3-OEt2, CH2CI2, 25°C, 30min.

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. 

Tl(III) trifluoroacetate, sometimes with the requirement that boron trifluoride etherate should be present, causes fast dehydrodimerization of many aromatic compounds to give biaryls and/or diphenylmethanes in competition with thallation (McKillop et al., 1980). This system thus seems to be a typical exponent of (96). Table 16, entry no. 16, presents a crude estimate of the possibility of non-bonded electron transfer between Tl(III) trifluoroacetate and naphthalene, showing that it indeed appears to be feasible. Note however that both observed and estimated rate constants are based upon rather uncertain... 

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. 

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). 

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