Dimers, open-chain

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... 

TABLE 8. (Partly) open-chain sulfones from sulfenes and their dimers... 

Crystalline methyl-a-cyanocinnamylideneacetate (107) forms dimer (108) upon irradiation, while the ethyl ester (109) yields an open-chain dimer (110)(114) ... 

Neglect of the formation of polymeric rings, however, is sometimes too crude an approximation. It may happen that the cyclisation constant C2 of the linear dimer is larger than the cyclisation constant Cl of the monomer. It may also happen that the concentration of the monomer is comparable to Ct but smaller than C2. When this is the case, the open chain dimer, once formed, will show a higher tendency to cyclise than to react with the monomer to give the linear trimer. Under the above conditions the stepwise polymerisation is truncated after the first step, and the system is described to a useful approximation by scheme (6). 

The structure of the compound consists of two residues, one cationic (PYS-PYSH)+, containing the S-S bond linking the two 2-mercapto-pyridine molecules one of which is protonated and Ij as a counter anion. In the crystal lattice there are four symmetry-independent cation-anion pairs. There are only a few crystal structures reported in the literature containing open chain stable cations of DS-SD dimers, such as the monocationic [(C HgN S-SNjC Hj) ) ) [15]. The two I-I bond distances of the Ij" in the four components of complex (26) are 2.887(4) and 2.944(3) A in component a, 2.874(4) and 2.957(3) A in b, 2.968(3) and 2.862(3) A in c and 2.855(4) and 2.927(3) A in d, respectively, indicating a slight asymmetry of I3 in this complex (covalent hnear asymmetric). 

In the case of thiazoline-2(3//)-thiones, the mesoionic thiazolo[2,3-h][l,3,4]thiadiazoles are obtained by two different routes (Scheme 65). On the one hand, thione 166 reacts with isothiocyanate via intermediate 167 and with a second equivalent isothiocyanate to afford the mesoionic 168 on the other hand, in the presence of isocyanate, the thione preferentially dimerizes 167 with the open-chain carbodiimide 169 to give the mesoionic 170. Addition of acid with removal of an amine group converts 170 into the symmetric heteroaromatic amine (171) (88CB1495 92T1285). The related transformation of an imidazoline into 1,3,4-thiadiazoles has also been described (90T4353). 

A plausible alternative mechanism involves as a first step the formation of a linear dimer, XB=NR—BX=NR, according to the left part of Eq. (21). This linear dimer will be more stable in entropy but less stable in energy than the corresponding cyclodimer. The second step would be addition of iminoborane to the open-chain dimer giving the borazine in either a concerted or a two-step mechanism. The intramolecular cyclization of the open-chain dimer, according to the right-hand side of Eq. (21), would compete addition of an iminoborane. We cannot definitely exclude a mechanism via open-chain dimers. 

A similar spiro-fused starting material was prepared to study the thermolysis of a 1,3-dioxane analog. As found for the dithia compound (cf. Section 8.11.6.3.1), a carbene-derived dimer was formed as the major detectible product (20%) <2002TL1927>. Other products, such as tricycle 185, have been identified in subsequent studies <2004CJC1769>. However, phenyl substitution at G-4 provided completely different thermolysis products, probably via formation of an open-chain bis-radical. Thus, 3-phenyl-7-butyrolactone and, after CO2 extrusion, phenylcyclo-propane are the major reaction products (Scheme 58) <2002CJC1187>. 

Isoprene is converted both to cis-trans-trans trimethylcyclododecatri-ene and to an open chain dimer. The methyl groups favor an internal... 

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