Dimerization, conditions

The physical properties of polymerized fatty acids are influenced by the basestock, by the dimerization conditions and catalysis, and by the degree to which monomer, dimer, and higher oligomers are separated following the dimerization. 

The extreme stereoselectivity toward the synthesis of cis-1,4-hexadiene is attributed to the fact that only cisoid-coordinated 1,3-diene can undergo the addition reaction (65, 66). 1,3-Dienes whose cisoid conformations are stoically unfavorable do not react with ethylene under the dimerization conditions. For example, Hata (65) was able to show that, using an Fe-based catalyst system, l-tra/is-3-pentadiene (40) and 2-methyl-1 -trans-3-pentadiene (41) reacted readily with ethylene to form the expected 1 1 addition products, while l-c/s-3-pentadiene (42) and 4-methyl- 1,3-penta-diene (43) failed to interact with ethylene. The explanation is that the cisoid conformations of 40 and 41 are stoically favorable while those for 42 and 43 are not. 

Good to excellent yields of such compounds as r) -CpMo(CO)3SnMc3 are obtained by reacting (MciSnljN or (Mc3Sn)20 with metal-metal-bonded dimers. Conditions vary from 15 min at 20°C in EtjO, Co2(CO)g, yield 75% and 80%, respectively, to 15 h at 140°C in xylene [t/ -CpFe(CO)2]2, yield 50%. The reactions are as follows ... 

The test was deliberately stopped after 5500 h. Compared to the Dimersol one-phase system, the Difasol nickel consumption was ten times lower and the cocatalyst consumption twice as low. No additional fresh IL was required. This definitely demonstrated the stability of the chloroaluminate IL under dimerization conditions. 

Recently Iwabuchi et al. reported the first total synthesis of (—)-idesolide (650) (559), an efficient inhibitor of nitric oxide production induced by lipopolysaccharide in BV2 microglial cells, isolated firom the deciduous tree Idesia polycarpa (560). As idesolide may originate from dimerization of the precursor 651, two major challenges had to be addressed in this investigation. The first was installation of the required absolute configuration in 651 and the second, identifying useful dimerization conditions for this monomeric precursor. Synthesis of 651 could be achieved successfully by carrying out an oxidative kinetic resolution on the easily synthesized racemic... 

Thermal stability of thiophene 1,1-dioxides increases with more substituents on the thiophene ring. For example, dimerization of 3-chloro-4-fluorothiophene 1,1-dioxide required heating in refluxing toluene for 20 h to give aromatic product 69 in 57% yield with release of one molecule of SO2 and one molecule of HCI (Scheme 66) [88], Similarly, the dimerization conditions for 3,4-dichlorothiophene 1,1 -dioxide were also very harsh, and a mixture of products 70 and 71 was obtained. It should be noted that 71 could also be prepared from 70, though the conditions were even harsher - reflux at 200-215°C (Scheme 67) [89]. 

Figure 4.7 Selective cross coupling of polyhalogenated heterocycles under Pd(l) dimer conditions.

Recent tlieoretical [35, 36 and 37] and experimental [38] research has revealed anomalous behaviour of tire dimer anisotropy under certain excitation conditions. If tire dimer is excited by broadband light tliat covers botli excitonic transitions, or by a relatively narrow band properly positioned between tire maxima of tire excitonic transitions, tire... 

The a-methylene-/3-lactam 103 is obtained by the carbonylation of the inethyleneaziridine 102 under mild conditions[91]. The azirine 104 undergoes an interesting dimerization-carbonylation to form the fused )3-lactam 105[92]. 

Many monomeric heterocyclic anhydrobases can be isolated now using specific methods (44), but application of these methods to thiazole ring did not succeed however, appropriate conditions lead to the separation of a dimer, the structure of which has been established by its NMR Spectra and chemical reactivity (26). The most probable mechanism of its formation appears identical with the one previously described in the benzothiazolium series (24). A second molecule of quaternary salt A3... 

Rearrangement to an open chain imine (165) provides an intermediate whose acidity toward lithiomethylthiazole (162) is rather pronounced. Proton abstraction by 162 gives the dilithio intermediate (166) and regenerates 2-methylthiazole for further reaction. During the final hydrolysis, 166 affords the dimer (167) that could be isolated by molecular distillation (433). A proof in favor of this mechanism is that when a large excess of butyllithium is added to (161) at -78°C and the solution is allowed to warm to room temperature, the deuterolysis affords only dideuterated thiazole (170), with no evidence of any dimeric product. Under these conditions almost complete dianion formation results (169), and the concentration of nonmetalated thiazole is nil. (Scheme 79). This dimerization bears some similitude with the formation of 2-methylthia-zolium anhydrobase dealt with in Chapter DC. Meyers could confirm the independence of the formation of the benzyl-type (172) and the aryl-type... 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

Acryhc esters dimerize to give the 2-methylene glutaric acid esters catalyzed by tertiary organic phosphines (37) or organic phosphorous triamides, phosphonous diamides, or phosphinous amides (38). Yields of 75—80% dimer, together with 15—20% trimer, are obtained. Reaction conditions can be varied to obtain high yields of trimer, tetramer, and other polymers. 

Difluoromonosulfane and Difluorodisulfane Difluoride. Difluoromonosulfane [13814-25-0] (sulfur difluoride), SF2, and its dimer, disulfane tetrafluoride [27245-05-2] SF SF, are both extremely unstable compounds which have only a fleeting existence except under rigorously controlled laboratory conditions. These compounds may be prepared by passing SCl vapor over HgF2 at 150°C (131). Electronic and nmr examinations of SF2 have been reported (132,133). 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Some control over the spHt between methyl radical oxidation (to HCHO) and dimerization in heterogeneous oxidation can be achieved by varying conditions (116). For homogeneous oxidation, an efficiency of 70—80% to methanol has been claimed at 8—10% conversions (110). This is the high end of the reported range and is controversial. Even so, such technology appears unlikely to be competitive for regular commercial use until further advances are made (117). The critical need is to protect the products from further oxidation (118). 

Hydrocarbon resins based on CPD are used heavily in the adhesive and road marking industries derivatives of these resins are used in the production of printing inks. These resins may be produced catalyticaHy using typical carbocationic polymerization techniques, but the large majority of these resins are synthesized under thermal polymerization conditions. The rate constants for the Diels-Alder based dimerization of CPD to DCPD are weU known (49). The abiHty to polymerize without Lewis acid catalysis reduces the amount of aluminous water or other catalyst effluents/emissions that must be addressed from an environmental standpoint. Both thermal and catalyticaHy polymerized DCPD/CPD-based resins contain a high degree of unsaturation. Therefore, many of these resins are hydrogenated for certain appHcations. 

Because lactic acid has both hydroxyl and carboxyl functional groups, it undergoes iatramolecular or self-esterificatioa and forms linear polyesters, lactoyUactic acid (4) and higher poly(lactic acid)s, or the cycUc dimer 3,6-dimethyl-/)-dioxane-2,5-dione [95-96-5] (dilactide) (5). Whereas the linear polyesters, lactoyUactic acid and poly(lactic acid)s, are produced under typical condensation conditions such as by removal of water ia the preseace of acidic catalysts, the formation of dilactide with high yield and selectivity requires the use of special catalysts which are primarily weakly basic. The use of tin and ziac oxides and organostaimates and -titanates has been reported (6,21,22). 

Ethyleneknine dimer has been synthesized using catalytic amounts of an alkaU metal amide of ethyleneknine under alkaline conditions (89,90). 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

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