Reaction with cyclic-tetramer

The oxonium exchange reactions may occur with the polymer ether linkages as well as with cyclic tetramers that form, as shown above. The concentrations of the oxonium ions of the ether group on the polymer and on the cyclic tetramers, however, are very small. Polymerizations with PF5, on the other hand, or with (C2H5)30PF6 either in bulk or in methylene chloride solutions, yield no significant amounts of cyclic oligomers. ... 

Pyrroles react with the conjugate acids of aldehydes and ketones to give carbinols (e.g. 67) which cannot normally be isolated but which undergo proton-catalyzed loss of water to give reactive electrophiles (e.g. 68). Subsequent reaction may lead to polymeric products, but in the case of reaction of pyrrole and acetone a cyclic tetramer (69) is formed in high yield. 

Time dependence of the reaction products can be seen more clearly in the time-yield curves of oligomerization in methylene chloride at —40° (Fig. 4). The yield of mixture of the cyclic tetramer and hexamer (mostly the latter), passed through a maximum value of about 40% and then decreased to nearly zero after 48 hours. On the other hand, the yield of the cyclic dimer increased rather sigmoidly with reaction time. 

Thus, pyrrole and acetone react as shown above. This involves pyrrole acting as the nucleophile to attack the protonated ketone in an aldol-like reaction. This is followed by elimination of water, facilitated by the acidic conditions. This gives an intermediate alkylidene pyrrolium cation, a highly reactive electrophile that reacts with another molecule of nucleophilic pyrrole. We then have a repeat sequence of reactions, in which further acetone and pyrrole molecules are incorporated. The presence of the two methyl substituents from acetone forces the growing polymer to adopt a planar array, and this eventually leads to a cyclic tetramer, the terminal pyrrole attacking the alkylidene pyrrolium cation at the other end of the chain. 

Evaporation of the xylene filtrate and trituration of the residue with methylene chloride yields ca. 11 g of solid which consists mainly of cyclic hexamer and cyclic tetramer. The methylene chloride contains inter alia, cyclic pentamer, cyclic heptamer, and bishomo compound, and these can be obtained as pure samples in low yield by fractional crystallization procedures. The compositon of the reaction mixtures can be qualitatively established by TLC by means of the following values in 9 1 petroleum... 

Similar to other cyclic 1,3-dicarbonyl compounds, tetramic acids in solution predominantly exist in their enol form. As tram-configured enols they do not give a color reaction with FeCl3. The pKa of unsubstituted tetramic acid 2a was determined to be 6.4 [72JCS(P1)2121]. 

Aromatic aldehydes react very easily with tetramic acid under acidic conditions to give 3-benzylidene compounds (41). The yields are moderate, because often there are subsequent reactions. As a,/3-unsaturated carbonyl compounds, (41) react in a Michael addition with excess tetramic acid to form (67), but it can also react with other acyclic and cyclic 1,3-dicarbonyl compounds. In these reactions the aryl substituents may vary over a wide range. Thus, (67) and (68) can be cyclized with ammonium acetate to afford pharmacologically interesting compounds (70) and (71) (90TH1). The latter are dihydropyridines. Curiously, (69) does not cyclize under these conditions. (See Fig. 32.)... 

Unexpectedly, the cyclic tetramer ( Bu2Sn)4 (rather than Bu4Sn) is obtained as bright-yellow crystals from the reaction of Bu2SnCl2 with an excess of BuMgCl in boiling Evidence for the cyclic structure of ( Bu2Sn)4 in... 

The complex (269) is hexameric in the solid state and in solution, and the same structure presumably occurs in the complexes Ni(SR)2 obtained from reaction (197). In the cyclic hexamer the six nickel atoms form a regular hexagon with twelve symmetrically bridging SEt-groups which are situated above and below the plane of the nickel atoms.1944 A corresponding cyclic tetramer has been obtained with jV-methylpiperidine-4-thiolate anion, SCsHsNMeJ.1946 Also in the complex [Ni(SC5H9NMe)2]4 (270) the nickel atoms lie in a plane. 

In some systems, the same polymer may be produced from different monomers, e.g., plastic sulfur can be formed by polymerizing SB as well as S8 molecules. In such systems, a polymer may be stable with respect to one monomer but labile with respect to another one. For example, linear siloxane (-Me2SiO-) , which could be formed from the cyclic tetramer (-Me2SiO-)4, decomposes into the cyclic trimer (-Me2SiO-)3 and the reaction 3 (-MegSiO-)4-> 4 (-Me2SiO-)3 proceeds, therefore, through a polymer intermediate. 

Nickel (0)-allene complexes are characterized by configurational instability and a propensity to assume a high coordination number. It may not be surprising to find that the Ni(0) species is the most catalytically active of the triad. The cyclic dimers, 1,3- and 1,2-dimethylenecyclobutane, are formed only in the vapor phase reaction of allene with Ni(CO)2(Ph2PC6-H4PPh2) (143). The liquid phase reaction with Ni(0) complexes selectively produces the trimer (I), tetramer (II), andpentamer (III) (Table VIII) 123). Several intermediate Ni(0) complexes (IV-VI) were isolated. 

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