Dimer unsymmetric

Electrolysis of carboxylates to afford the coupled products. Homocoupling product is obtained if two carboxylates are the same — decarboxylative dimerization unsymmetrical product will be produced if the two carboxylates are different. 

The 1-alkoxytitatranes can be synthesized by the reaction of equimolar amounts of tetraalkyl titanates and triethanolamine (105). X-ray crystallographic analysis of the soHd isolated from the reaction of one mole of triethanolamine and one mole of TYZOR TPT confirms the stmcture as a centrosymmetric dimer having a Ti isopropoxy nitrilotriethoxy ratio of 1 1 1. The titanium atoms have achieved a coordination number of six via a rather unsymmetrical titanium—oxygen bridge (106). 

Figure 11.19 Schematic diagram of the centrosymmetric dimer in [Cii2(NO ).i(py) lpy showing the two bridging nitrato groups each coordinated to the 2 Cii atoms by a single O atom the dimer also has an unsymmetrical bidentate nitrate group on each Cu.

Tetraazafulvalenes bearing two pyrazole subunits could be prepared by an original way. Tlius, treatment of benzylidene acetophenone with iso-pentylnitrite leads to an A, A -dihydroxy-bipyrazolyl-A, A -oxide, which in turn can be oxidized to TAF of type 100 (72CC961, 79JOC3211). Another type of oxidative dimerization was observed by the reaction of the electron-rich l-methyl-2,4-bis(dimethylamino)imidazole with silver salts (83TL3563). A bis-cation was isolated in 30% yield in the presence of sodium tetrafluo-roborate an unsymmetrical structure 101 was predicted from its NMR data (Scheme 40). 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

The diradical mechanism b is most prominent in the reactions involving fluorinated alkenes. These reactions are generally not stereospecificand are insensitive to solvent effects. Further evidence that a diion is not involved is that head-to-head eoupling is found when an unsymmetrical molecule is dimerized. Thus dimerization of F2C=CFC1 gives 106, not 107. If one pair of electrons moved before the other, the positive end of one molecule would be expeeted to attack the negative end of the other. 

Silyi enol ethers can be dimerized to symmetrical 1,4-diketones by treatment with Ag20 in DMSO or certain other polar aprotic solvents." The reaction has been performed with R , R = hydrogen or alkyl, though best yields are obtained when r = r = H. In certain cases, unsymmetrical 1,4-diketones have been prepared by using a mixture of two silyi enol ethers. Other reagents that have been used to achieve either symmetrical or cross-coupled products are iodosobenzene-Bp3-Et20," ceric ammonium nitrate," and lead tetraacetate." If R =0R (in which case the substrate is a ketene silyi acetal), dimerization with TiCU leads to a dialkyl succinate (34, r =0R)." ... 

Niecke et al. have prepare polyimido analogues of the metaphosphate ion, PO3, bylithiation ofthe corresponding amido compounds [16]. Thus the monomeric solvent-separated ion pair [(THF)4Li][P(NMes )3] (10) is obtained by treatment of (Mes N)2P(NHMes ) with "BuLi [16]. A monomeric contact ion pair (11) containing the unsymmetrical anion [P(N Bu)2(NMes )]" has also been reported [16]. By contrast the dilithium derivative of the trisimidometaphosphate [P(N Bu)3]" forms a dimer (12) [17], with a cubic structure reminiscent of that of (7). 

Unsymmetrical zinc phthalocyanine analogs with three 15-crown-5 ether ligands appended at the 3,4-positions were synthesized and characterized. Introduction of alkali metal ions results in cofacial dimer formation with evidence for this dimerization from NMR.841... 

Unsymmetrical dimeso derivatives dimerize while symmetrical dimeso derivatives give very low yields of dimer. 

If, on the other hand, unsymmetrically substituted carbonyl compounds such as monosubstituted benzophenones (X = OCH3, CH3, Cl), tert-butyl methyl ketone, acetophenone, acetaldehyde, or benzaldehyde are used for trapping 39a, diastere-omeric mixtures are formed in each case they could all be resolved except for the products obtained with p-methoxybenzophenone and acetophenone 33>. An X-ray structure analysis has been performed for the E-isomer 57g 36) which, in conjunction with H-NMR studies, permitted structural assignment in cases 56 and 57e, g and h35>. Additional chemical evidence for the structure of the six-membered heterocycles is provided by the thermolysis of 56 a considered in another context (see Sect. 3.1). In general the reaction 39a- 56 or 57 is accompanied by formation of phosphene dimers, presumably via [4 + 4]- and via [4 + 2]-cycloaddition 35). 

Structural similarities between the monomeric units of aplydilactone (144) and the red algal metabolites constanolactone A (145) and B (146), suggest an analogous biosynthetic origin. Dehydration of two m3 unsaturated eonstanolac-tone-type units could result in the formation of the unsymmetrical dimer (Scheme 11). Since sea hares are known to feed extensively on red algae and sequester natural products from this source, it remains possible that aplydilactone (144) derives, at least in part, from a dietary source. 

Cyclic thioethers S(CH2) , n — 3, 4, form 1 1 adducts with diphenylzinc that have similar dimeric structures as diphenylzinc,77 as shown for [Ph2ZnS(CH2)4]2 in Figure 13. The zinc-sulfur bond is quite long, 2.5025(5) A, and the bridging phenyl groups form two decidedly unsymmetrical Zn-G bonds (2.114(2) and 2.261(2) A), which are substantially longer than the bonds to the terminal phenyl groups (1.983(2) A). 

Polymerization and oligomerization reactions. l-FIalogenopropane-2-thiones give homopolycondensation,10 in different conditions l-chloropropane-2-thi-one11 forms a polymer or a cyclic trimer. a-Oxothioketones12 15 form dimers, by [4+2] unsymmetrical Diels Alder cycloaddition (Scheme 7). a,p-Unsatu-rated thioketones,16 Scheme 3 E = S, form dimers via head-to-head (R1 = Ph, R2 = Me) and head-to-tail (R1 = R2 = Ph), while selenoketones, E = Se, dimerize17 via head-to-head . 

The tin(II) derivative Sn[S2P(OPh)2]2 is a dimeric supermolecule formed through intermolecular Sn- S secondary bonds and Sn-7i-aryl interactions.57 Lead(II) dithiophosphates, Pb[S2P(OR)2]2, e.g. R = Me,58 Pr" and Cy,59 all form supramolecular chain-like arrays. The ligands are unsymmetrically bridging and form Pb- S secondary bonds. 

The mercury(II) diphenyldithiophosphinate is a cyclic dimer [Hg(S2PPh2)2]2 with both chelating and bridging ligands.133 Dimerization occurs through secondary bonds and the bridging ligands are unsymmetric. 

---