Dimethyl isomer, isomerization

Careful chromatographic and detailed HNMR spectroscopic analysis of the products from the thermolyses of ethyl azidoformate in o-, m- and p-xylene revealed in all cases a mixture of 1 //-azepines.80 In o-xylene, only two of the four possible isomers were separated and characterized, namely, ethyl 4,5-dimethy 1-1 //-azepine-1 -carboxylate (9 %) and ethyl 3,4-dimethyl-l H-azepine-1-carboxylate (7 %). w-Xylene yielded a 2 3 mixture of ethyl 3,5-dimethyl-l//-azepine-1-carboxylate and ethyl 2,4-dimethyl-l//-azepine-l-carboxylate. The 2,4-dimethyl isomer (20 %) can be isolated from the mixture by removal of the 3,5-dimethyl isomer as its Diels-Alder cycloadduct with ethenetetracarbonitrile. p-Xylene gave a mixture of the two possible isomeric azepines which were partially separated by column chromatography. A pure sample of ethyl 2,5-dimethyl-1//-azepine-1-carboxylate (26%) was obtained from the mixture by selective decomposition of the 3,6-dimethyl isomer with refluxing alcoholic potassium hydroxide. 

Treatment of a mixture of ethyl 2,5-dimethyl- and ethyl 3,6-dimethyl-l//-azepine-l-carboxylate in benzene solution with nitrosobenzene yields only the [6 + 2] adduct (26% mp 99-100.5 CC) of the 3,6-dimethyl isomer.80 Other isomeric dimethyl-l//-azepine-l-carboxylates, and mono-methyl-1//-azepines, fail to react. 

Figure 4. Isomerization of solvent to dimethyl isomer (-------), model predicted...

Reduction of the metal dimer [CpMo(NO)l2]2 with Na/Hg in the presence of a variety of acyclic dienes generates the (diene)MoCp(NO) complexes in moderate to low isolated yield (equation 8)12,31,89. For the majority of diene ligands, complexes 60 are formed exclusively as the s-trans isomers as evidenced by NMR spectroscopy and single-crystal X-ray diffraction analysis. In comparison, complexation of the 2,3-dimethyl-l,3-butadiene initially gives a separable mixture of the s-trans (60) and s-d.v-complex (61). The s-cis isomer isomerizes to the more thermodynamically stable s-trans isomer in solution (THF, 1/2 = 5 min C6H6, ti/2 = 24 h). 

Among the isomeric C7 alkyl cyclopentanes, the approximate relative amounts of the monoalkyl and the sum of the dialkyl isomers are ethyl isomer, 1 sum of the five dimethyl isomers, 10. 

Remarkably, CCC gave almost exclusively trans-3A-cis,cis-l,5-cyc ooctSLdicnc instead of the expected cw-3,4-dimethyl isomer if a boat-like 3,3-shift were to occur. The origin of this material is unclear. One possibility considered was formation of trans-3,4-dimQthy -cis,trans-l,5-cycloociSLdicnQ via a chair-like transition state as described in Scheme 9.77 which then underwent trans to cis isomerization (Scheme 9.79). 

Geometrical Isomerization. The l,l-Dimethyl-/i -allyl group in [(A -l,l-Me2C8H3)M(PF3)3] (M = Co or Rh) (51) isomerizes to the 1,2-dimethyl isomer on heating to 333 K. The mechanism probably involves a 1,4-hydrogen shift to give (53) with the intermediacy of (52). In the solid state [Co(diphos)2-CI]+ exists in two isomeric forms which equilibrate immediately upon separate... 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

Constitutional isomerism is not limited to alkanes—it occurs widely throughout organic chemistry. Constitutional isomers may have different carbon skeletons (as in isobutane and butane), different functional groups (as in ethanol and dimethyl ether), or different locations of a functional group along the chain (as in isopropylamine and propylamine). Regardless of the reason for the isomerism, constitutional isomers are always different compounds with different properties, but with the same formula. 

Because of their cyclic structures, cycloalkanes have two faces as viewed edge-on, a "top" face and a "bottom" face. As a result, isomerism is possible in substituted cycloalkanes. For example, there are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyl groups on the same face of the ring and one with the methyls on opposite faces (Figure 4.2). Both isomers are stable compounds, and neither can be converted into the other without breaking and reforming chemical bonds. Make molecular models to prove this to yourself. 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

The pronounced acidity of the bridgehead hydrogen atoms in 4 (R = H) facilitates the regio-selective introduction of electrophiles. Rearrangements of 4 (R = H, Me, CHO, C02Me) catalyzed by dicarbonyldichlororhodium(I) lead to 4-substituted 1-benzothiepins 5,10 except in the case of R = Me where a mixture (1 1.3) of 3- and 4-methyl-l-benzothiepin is obtained (total yield 98 %). In the case of the dimethyl-substituted derivative 8 (R1 = R2 = Me), however, the rhodium(I)-catalyzed isomerization reaction leads to the thiophene isomer. 

Cobalt, dichlorobis(AvY -dimethyl-1,2-ethanediamine)-chloride hydrate isomerization, 1, 468 Cobalt, dich orobis(l,2-ethanediamine)-base hydrolysis, 1, 304 chloride anation, 1, 469 halogen exchange, 1, 468 chloride hydrate isomerization, 1, 468 isomers, 1,191 nitrate... 

After succeeding in the asymmetric reductive acylation of ketones, we ventured to see if enol acetates can be used as acyl donors and precursors of ketones at the same time through deacylation and keto-enol tautomerization (Scheme 8). The overall reaction thus corresponds to the asymmetric reduction of enol acetate. For example, 1-phenylvinyl acetate was transformed to (f )-l-phenylethyl acetate by CALB and diruthenium complex 1 in the presence of 2,6-dimethyl-4-heptanol with 89% yield and 98% ee. Molecular hydrogen (1 atm) was almost equally effective for the transformation. A broad range of enol acetates were prepared from ketones and were successfully transformed into their corresponding (7 )-acetates under 1 atm H2 (Table 19). From unsymmetrical aliphatic ketones, enol acetates were obtained as the mixtures of regio- and geometrical isomers. Notably, however, the efficiency of the process was little affected by the isomeric composition of the enol acetates. 

On the basis of fundamental experiments (see Section IV,A,2) some indenobenzazepine alkaloids have been efficiently synthesized from the corresponding protoberberines via 8,14-cycloberbines. For example, the cycloberbine 428 derived from the protoberberine 427 was heated with methanesulfonic acid in aqueous tetrahydrofuran to afford a 2 1 mixture of cis- and trans-indenobenzazepines 429 in 92% yield (Scheme 85). The mixture was methylated with methyl iodide to give the cts-N-methyl derivative 430 and the unchanged trans secondary amine (21%), which was very difficult to methylate and which gave the /V-methyl derivative only in 6% yield even on treatment with dimethyl sulfate for 43 hr. Contrary to the ordinary cases (Section IV,A,2), the trans derivative did not isomerize to the cis isomer 430 under various acidic conditions. Debenzylation of 430 by hydrogenolysis afforded fumarofine (417), which was converted to O-methylfumarofine (316) by methylation with diazomethane (215). 

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

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