Dimethyl derivative dimers

The bulk polymerization of 4-methyl-1,3-dioxepane at 0 °C yields 60% polymer. Further substitution decreases polymerizability as with 1,3-dioxolanes. 2,4-Dimethyl-1,3-dioxepane gives oligomers in limited yield, the 2,2-dimethyl-derivative dimerizes and the 4,4-dimethyl derivative does not polymerize at all53). 

On the other hand y-pyrones or 1,3-diketones could be obtained from the reactions of ketone derived enamines with diketene 423-426). The addition of dimethyl ketene dimer to aldehyde or ketone derived enamines produced cyclohexanediones 425,426). 

Pyridine, and its monomethyl and 3,5-dimethyl derivatives " combine exothermically with dimethyl acetylenedicarboxylate in ether yielding some ether soluble materials, including trimethyl pyrrocoline-1,2,3-tricarboxylate (Section III,F,3) and deep red ether-insoluble gums. A number of crystalline compounds have been isolated from these gums by fractional crystallizations and will now be considered in detail. In the case of pyridine, Diels et al. ° isolated a red labile 1 2 molar adduct, which they formulated as (75), which isomerized rapidly on standing to a yellow stable adduct (76). These formulations are no longer accepted. Diels and Alder also suggested that the acetylenic ester first dimerized to the diradical (74) which then combined with the pyridine. 

In contrast, the 6,8-dichloro 7 and 6,8-dimethyl derivatives, in which [6 + 4] dimerization would locate a substituent at the bridgehead position, form the symmetrical [6+6] adducts, e.g. 8.154... 

Griffin, G. W., A. F. Vellturo, and K. Furukawa The chemistry of photodimers of maleic and fumaric acid derivatives. I. Dimethyl fumarate dimer. J. Amer. chem. Soc. 83, 2725 (1961). 

The 2,6-dimethyl derivatives 353a gives the tricyclic ketone 354 resulting from an intramolecular electrocyclic reaction. However, the parent thieno-tropone (51) and its 2- (353b) or 7-methyl derivatives give [4 + 2] dimers (e.g., 355) of the head-to-head type. These dimers are formed by reaction of the 5,6-ene bond of one molecule with the 5,7-diene system of another. The dimerization seems to be the normal photochemical reaction of this group of thienotropones and to be suppressed by a 6-methyl substituent (e.g., in 353a). 

Successive reduction of l,3-dimethyl-4,5,6,7-tetrahydrobenzo[c]-thiophen-4-one (Section III,C) with sodium borohydride and dehydration of the resulting alcohol with polyphosphoric acid gives the unstable 1,3-dimethyl derivative [(18) R = H] in excellent yield,35 and successive treatment of the same ketone with methylmagnesium iodide and acid gives the 1,3,7-trimethyl derivative [(18) R=Me] (Table IV) and a second compound which is reported to be a dimer. 36 When l,3-dimethyl-4,5,6,7-tetrahydrobenzo[c]thiophen-4-one is treated with a mixture of phosphorus oxychloride and dimethyl-formamide (Vilsmeier-Haack formylation), it gives compound 19, which yields 20 on being treated successively with thioglycolic acid... 

On the other hand, electrolysis of the dimethyl derivative in the presence of (CH, )4NBr as electrolyte, led to the appearance of two products, one of which was the photochemically dissociable dimer, the other the dihydroderivative of 1,3-di-methylpyrimidone-2 3). 

When introducing a phenyl substituent into the a-position of the vinylsilane, as in 17, R = Ph, the reaction with lithium metal affords the usual product of Schlenk-dimerization 37, which yields a mixture of d, /, and meso dimethyl derivatives 38 (92%). Even when heating a-trimethylsilylstyrene in either hexane or toluene no reaction to 18, R = Ph, can be enforced. 

For the dimerization of 2-methyl-, 2,2-dimethyl- and 2,2,3,3-tetramethylmethylenecyclo-propane the bis(cycloocta-l,5-diene)nickel(0) catalyst was modified with dialkyl fumarate, maleic anhydride, or various trialkylphosphanes. For all of the substituted methylenecy-clopropanes but the tetramethyl derivative, dimers of the spiro[2.4]heptane and dispiro[2.1.2.11-octane type were obtained in good yields. 

Me3SiNMe2 yields the N-dimethyl derivative 456. Spectroscopic identification or attempted isolation of the P radical of452 by reaction of452 with sodium in octane failed, but the dimer phosphine 457 was isolated. 458 is obtained in quantitative yield by reaction of 452 with Fe2(CO)9. 

Iso- and pseudoindenes have been prepared. The former was generated and found to dimerize. The latter, as its 2,2-dimethyl derivative, was prepared and found to give the isoindene with an activation energy of 18.9 kcal/mol (Scheme 10.5). ... 

The experimental evidence is suggestive, but inconclusive. Although cyclopropene is a highly strained molecule, with an estimated strain energy of 26 kcal/mol,[28] it does not dimerize spontaneously. The an e-isomer of TCH, the 6z5- em-dimethyl derivative of which has been formed by catalytic dimerization of em-dimethyl cyclopropene,[29] does not revert thermally to two monomer molecules. Instead it isomerizes to vibrationally excited cyclohexadiene [30] ... 

Reaction of compound 56 with Li20 yields the anhydro dimer 57, which is the N,N-dimethyl derivative of anhydro dimer 53, and this provides another route to the anhydro dimers. In analogy with the reaction to form the 1-azonia-2-boratanaphthalene 56, l-oxonia-2-boratanaphthalenes 58a—fare formed by the reaction of phenylethyne with arylchloro(phenyloxy)boranes (Scheme 23). 

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