3.3- dimethyl-cyclopropene

From detailed HNMR spectroscopic analyses of 4//-azepine systems it has been concluded that 4//-azepines are energetically more favorable than their bicyclic 3-azabieyclo[4.1.0]hepLa-2,4-diene valence tautomers 85 however, the valence tautomer 10 of methyl 6-phenyl-4//-azepine-2-carboxylate (9) has been trapped as the [4 + 2] cycloadduct 11 with 3,3-dimethyl-cyclopropene.112... 

In a 250-mL, three-necked flask fitted with a mechanical stirrer, reflux condenser precooled to — 30 °C and a dropping funnel, a solution of 3,3-dimethylcyclopropene (60-400 mmol) in Et20 (20-40 mL) was added dropwise to a solution of the bis(alk-2-enyl)zinc compound 12 (30-200 mmol) at — 20°C. The mixture was warmed to 20"C and kept at this temperature for 12h. Solvent and unreacted 3,3-dimethyl-cyclopropene were removed (20 °C/1 Torr) and the residue was distilled [80 -120 °C (bath temperature)/10 " Torrj. 

When phosphane-free nickel complexes, such as bis(cycloocta-l,5-diene)nickel(0) or te-tracarbonylnickel, are employed in the codimerization reaction of acrylic esters, the codimer arising from [2-1-1] addition to the electron-deficient double bond is the main product. The exo-isomer is the only product in these cyclopropanation reactions. This is opposite to the carbene and carbenoid addition reactions to alkenes catalyzed by copper complexes (see previous section) where the thermodynamically less favored e Jo-isomers are formed. This finding indicates that the reaction proceeds via organonickel intermediates rather than carbenoids or carbenes. The introduction of alkyl substituents in the /I-position of the electron-deficient alkenes favors isomerization and/or homo-cyclodimerization of the cyclopropenes. Thus, with methyl crotonate and 3,3-diphenylcyclopropene only 16% of the corresponding ethenylcyc-lopropane was obtained. Methyl 3,3-dimethylacrylate does not react at all with 3,3-dimethyl-cyclopropene, so that the methylester of tra 5-chrysanthemic acid cannot be prepared in this way. This reactivity pattern can be rationalized in terms of a different tendency of the alkenes to coordinate to nickel(O). This tendency decreases in the order un-, mono- < di-< tri- < tet-... 

Employing bis(l,2.5,6- /4 -1,5-cyclooctadiene)nickel(0) as catalyst the reactions of 3,3-dimethyl-cyclopropene with dimethyl (E)- or (Z)-butcncdioate proceed with almost complete retention of configuration to afford the 1,2-trans- and 1,2-a. v-cyclopropane derivatives 11, respectively (for a procedure and further examples see Vol. E19b, pp 760-764)10. 

Enantiomerically pure 5-palladatricyclo[4.1.0.0 " ]heptanes have been prepared and been converted into enantio-merically pure complexes with helical chirality at palladium. The reaction of an excess of dimethyl 3,3-dimethyl-cyclopropene-l,2-dicarboxylate with [Pd2(dba)3]-CHCl3 gave diastereoselectively as side-product a new palladacycle 122 308 Purthermore, a highly diastereoselective synthesis of palladepanes 120 and 121 has been achieved. ... 

Disubstituted cyclopropenes do not react in the above sence. 1,2-dimethyl-cyclopropene polymerizes at 0 °C in the presence of [(T)3-C3H5)PdCl]2 30). 1,2-Diphenylcyclopropene cyclodimerizes nearly quantitatively in the presence of Pd(dba)2 or Pd(r)5-C5H5)(r 3-C3H5) to yield 1,2,4,5-tetraphenyl-cyclohexa-1,4-diene 72) (see p. 96). 

Disubstituted cyclopropenes do not react in the above sence. 1,2-dimethyl-cyclopropene polymerizes at 0 "C in the presence of [(T) -C3H5)PdCl]2 1,2-... 

Structurally novel cyclopropane-fused cycloalkanes can be constructed by oligomerization of cyclopropenes. Cycloaddition across the double bond reduces ring strain in cyclopropenes enormously. When catalysed by transition metals, dimethyl-cyclopropene tri- or tetramerizes stereoselectively to novel polycycloalkanes (equation... 

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] ... 

Mc2NCH2Ph reacts with PdCh to give A then A reacts with 2 -dimethyl-cyclopropene and pyridine to give a mixture of C and D. Identify A and explain what is happening. Why is it that Me2NPh does not give a product of type A, and that A does not insert ethylene. 

Cyclohexenes, 1,2-dialkyI Cyclopentenes, 1,2-dialkyl Cyclobutenes, 1,2-dimethyl Cyclopropenes, 1,2-dialkyl (Acetylene, 1,2-dimethyl)... 

The irradiation of 2,5-dimethylfuran in the presence of mercury vapor gave a complex mixture of products. Carbon monoxide and propene were removed as gaseous products. Then, cis- and ra -l,3-pentadiene, isoprene, 1,3-dimethyl-cyclopropene, 2-pentyne, 2-ethyl-5-methylfuran, hexa-3,4-dien-2-one, 1-methyl-3-acetylcyclopropene, and 4-methylcyclopent-2-enone were obtained (Scheme 8) (68JA2720 70JA1824). The most abundant product was the cyclopentenone 19,the second was the 1,3-pentadiene 12, while the third product was the cyclopropenyl derivative 18. 

A few examples of vinylogous methylene cyclopropenes are known. Thus, in an interesting reaction mode the spirohexadiene 93, prepared from dimethyl acetylene... 

As seen from Table 10, both the methyl resonance in dimethyl cyclopropenone (7.75 r) and the separation of CH2 units a and 0 to the three-ring in di-n-propyl cyclopropenone (0.85 ppm) compare well to corresponding values for the covalent cyclopropene derivatives, but differ strongly from those of the positively charged cyclopropenium species. 

Acetylene dicarboxylate and maleic anhydride failed to react with simple methylene cyclopropenes, but reacted readily with calicene derivatives, as shown by Prinz-bach293. Thus ADD combined with benzocalicene 497 to give the dimethyl tri-phenylene dicarboxylate 499, whose formation can be rationalized via (2 + 2) cycloaddition across the semicyclic double bond as well as (4 + 2) cycloaddition involving the three-membered ring (498/501). The asymmetric substitution of 499 excludes cycloaddition of ADD to the C /C2 triafulvene bond (500), which would demand a symmetrical substituent distribution in the final triphenylene derivative. 

The methyl substitution in carbenes lb-d has a pronounced influence on the yield of the bicyclic isomers 3 24,74 Thus, visible light irradiation of the 2,6-dimethylated carbene lb rapidly and with very high yield produces the cyclopropene 3b (Scheme 7).The yield is significantly higher than in the case of the parent system 3a. In contrast, methyl substitution in 3-position as in lc drastically reduces the yield of the cyclopropene 3c to approximately 10%. 

Rearrangement of the 3,5-dimethylated carbene Id would yield the destabilized cyclopropene 3d with a methyl group in the bridgehead position 1, and consequently no detectable amount of cyclopropene 3d is formed during irradiation of Id. Indeed, whereas the 3,5-dimethyl substituted carbene Id is 3.4 kcal mol-1 more stable than the 2,6-dimethyl isomer lb, the stability is reversed for the cyclopropenes, as 3d is found to be 6.5 kcal mol-1 higher in energy than 3b at the B3LYP/6-31G(d) level of theory (Table 3). 

Another method used to prepare dialkyl-substituted diazomethanes involves the photolysis of 2-alkoxy-2,5-dihydro-1,3,4-oxadiazoles (209), which can be prepared by the oxidative cyclization of A(-acetyUiydrazones. The diazoalkanes are trapped in situ by cycloaddition with dimethyl acetylenedicarboxylate (54) (Scheme 8.49). The resulting pyrazoles 210 are converted into cyclopropenes 211 by continued irradiation. 

Section I1,D,1 The kinetics have been studied for competitive formation of 3//-pyrazoles and cyclopropenes thermally from the isomeric vinyldiazo compounds 2,3-dimethyl-l-phenyl-l-diazo-2-butene and 4-methyI-3-phenyl-2-diazo-3-pentene. The higher (12 kJ/mol) ground state energy of the latter accounts almost entirely for its larger (x68) rate of cyclization to a 3H-pyrazole, relative to its isomer.172... 

---