Cyclizations chloride dimer

Since the introduction of the titanocene chloride dimer 67a to radical chemistry, much attention has been paid to render these reactions catalytic. This field was reviewed especially thoroughly for epoxides as substrates [123, 124, 142-145] so only catalyzed reactions using non-epoxide precursors and a few very recent examples of titanium-catalyzed epoxide-based cyclization reactions, which illustrate the principle, will be discussed here. A very useful feature of these reactions is that their rate constants were determined very recently [146], The reductive catalytic radical generation using 67a is not limited to epoxides. Oxetanes can also act as suitable precursors as demonstrated by pinacol couplings and reductive dimerizations [147]. Moreover, 5 mol% of 67a can serve as a catalyst for the 1,4-reduction of a, p-un saturated carbonyl compounds to ketones using zinc in the presence of triethylamine hydrochloride to regenerate the catalyst [148]. 

A similar pre-orientation involving unsaturated carbon chains was operative on generating twelve-membered enediyne 23 and arenediyne lactams 24 [7]. The seco methylesters 21 and 22 were cleaved with LiOH, the corresponding carboxylic acids underwent cyclizations after activation with 2-fluoro-pyridinium tosylate 25 [8]. Dimerization products were found as by-products (<10%). It should be pointed out, that the lactamization succeeded in a single step in about 75% yield by treating the seco-methylesters 21 and 22 with Me3Al in refluxing methylene chloride. Obviously, the latter route was more convenient (Scheme 5). 

Yields of the coupling products with alkynyl bromides are relatively low as compared with those obtained with allyl chloride or benzyl chloride. One reason for this is the instability of the product. The dienediynes 51 are slowly cyclized under the influence of light to give pentacyclic dimers 52 (Eq. 2.36), one of which has been characterized by X-ray analysis. 

Azetidones (p-lactams) are generally obtained in high yield from (3-halopropion-amides (Table 5.18) and the low yield from the reaction of N-phenyl (3-chloropropi-onamide can be reconciled with the isolation of A-phenyl acrylamide in 58% yield [34]. The unwanted elimination reaction can be obviated by conducting the cyclization in a soliddiquid system under high dilution [35, 36]. Azetidones are also formed by a predominant intramolecular cyclization of intermolecular dimerization to yield piperazine-2,5-diones, or intramolecular alkylation to yield aziridones. Aone-pot formation of azetidones in 45-58% yield from the amine and P-bromocarboxylic acid chloride has also been reported [38]. 

The complex, XLYI, may add to another molecule of isobutylene to yield a higher polymer complex or eliminate aluminum chloride to yield the dimer in the latter case intramolecular migration (a 1,5-shift 1) of hydrogen must be postulated in order to form an olefin. On the other hand, cyclization may readily occur (particularly after a 1,2-shift of a proton from a methyl group) with the resultant formation of a naphthene. 

Monomer structure can affect the competition between cyclization and linear polymerization. For example, phthalic acid (ortho isomer) is more prone to cyclization than terephthalic acid (para isomer) at the very-low-molecular-weight end, for example, the dimer stage. The ortho structure makes more likely the conformations that are more favorable for cyclization. Stiff linear chains such as those formed in the reaction between an aromatic diamine and aromatic diacid chloride are much less prone to cyclization than the flexible chains formed from the corresponding aliphatic monomers. 

The cyclopropanation of alkenes, alkynes, and aromatic compounds by carbenoids generated in the metal-catalyzed decomposition of diazo ketones has found widespread use as a method for carbon-carbon bond construction for many years, and intramolecular applications of these reactions have provided a useful cyclization strategy. Historically, copper metal, cuprous chloride, cupric sulfate, and other copper salts were used most commonly as catalysts for such reactions however, the superior catalytic activity of rhodium(ll) acetate dimer has recently become well-established.3 This commercially available rhodium salt exhibits high catalytic activity for the decomposition of diazo ketones even at very low catalyst substrate ratios (< 1%) and is less capricious than the old copper catalysts. We recommend the use of rhodium(ll) acetate dimer in preference to copper catalysts in all diazo ketone decomposition reactions. The present synthesis describes a typical cyclization procedure. 

As discussed, condensation reactions form the basis of the synthesis of the cyclic trimer, (NPC12)3. The reaction between phosphorus pentachloride and ammonia or ammonium chloride proceeds in a stepwise fashion, as shown in reaction sequence (18), by elimination of hydrogen chloride first to form a monomer (3.40), then a linear dimer (3.41), trimer (3.42), tetramer, and so on. Cyclization could occur to give cyclic chlorophosphazenes at any stage beyond the dimer, but in practice is less likely as the chains grow beyond a certain length. Several authors have extended this process to produce relatively low molecular weight poly(dichlorophosphazene).36-39... 

Cyclization involving the 5-amino group is also possible. An intermediate (56) for this reaction has been prepared from dimeric malononitrile and benzohydroximoyl chloride the intermediate (56) is cyclized under alkaline conditions (80CB1195). 

Symmetrical 3,5-dialkyl-l,2,4-trithiolanes (178) can be synthesized in reasonable yield by chlorination of dialkyl disulfides (175) to a-chiloroalkyl sulfenyl chlorides (176), which are then reacted with potassium iodide to give di-a-chloroalkyl disulfides (177). Subsequent cyclization with sodium sulfide gave (178) (72T3489). When (176) was treated with one molar equivalent of sodium sulfide, the reductive dimerization and cyclization was effected in one step (78HCA1404). Treatment of perfluoropropene with sodium hydrogen sulfide in THF resulted in the formation of 3,5-bis(2,2,2-trifluoroethyl)-l,2,4-trithiolane (179) (72IZV2517). 

By comparing the results for chlorinated polypropylene with those for polypropylene, it can be concluded that the two materials undergo very different pyrolytic reactions. Typical for polypropylene is the formation of fragments of the polymeric backbone with formation of monomer, dimer, etc., or with cleavage of the backbone in random places and formation of compounds with 3n, 3n-1, and 3n+1 carbon atoms (see Section 6.1). Pyrolysis of the chlorinated compound leads to a significant amount of HCI and also char. Very few chlorinated compounds are identified in the pyrolysate, since the elimination of HCI leaves very few chlorine atoms bound to carbons. Some aromatic hydrocarbons are formed by a mechanism similar to that of poly(vinyl chloride) pyrolysis. The elimination of HCI leads to the formation of double bonds, and the breaking of the carbon backbone leads to cyclization and formation of aromatic compounds. The reactions involved in this process are shown below for the case of formation of 1,3-dimethylbenzene ... 

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