Methylcyclopropane intermediate

C-labeled t-butyl carbocation (CH3)3 C, obtained at 195 K in FS03H/SbF5/S02FCl from labeled t-BuCl, exhibits complete scrambling of the label after 20 h at 343 K. No deprotonation occurs, and the scrambling is thought to involve a delocalized protonated methylcyclopropane intermediate or transition state. "... 

The photolysis of a-diazosulfones dissolved in alkenes provides sulfonyl-substituted cyclopropanes in high yields. This is exemplified by the preparation of l-(p-methoxyphenylsulfonyl)-2,2,3,3-tetra-methylcyclopropane in 75% yield from -methoxybenzenesulfonyl-diazomethane and 2,3-dimethyl-2-butene. A similar addition to [Pg.101]

These results are inconsistent with a radical rebound mechanism because this mechanism is a two-step process that requires the involvement of intermediates. Instead the results suggest that the hydroxylation is a concerted process, much like a singlet carbene reaction, which does not involve intermediates. However, this conclusion is in conflict with the properties of singlet carbene reactions discussed above. Subsequent studies on a number of substituted methylcyclopropanes and other stained hydrocarbon systems established that these findings were not anomalous. 

If the intermediate were corner-protonated, however, and undergoing exchange with the acid, it would be most reasonable to expect all the protons at the comer to exchange with the acid before ring cleavage occurred. Thus, corner protonated methylcyclopropane would be expected to exchange three protons with the acid if it exchanged any. 

Finally, the formation of chlorocyclobutenes 25 and 27 by treatment of trimethylcyclopropene 24 with dichlorocarbene,2n and by reaction of l,l-dibromo-2,2-dichloro-3-chloromethyl-3-methylcyclopropane (26) with methyllithium 212 can be explained by disrotatory outward ring opening of an intermediate dichlorobicyclobutane with concomitant migration of an endo-ch a-rine and methylation, respectively. 

The reaction of either cis- or trans-1 with potassium /-butoxide in tetrahydrofuran at 25 °C leads to a /-butyl ether (2), apparently arising by attack of /-butoxide ion on an intermediate l,4-di-/-butylmethylenecyclopropene. If the reaction is carried out at low temperature and the volatile materials are distilled directly into a cold trap, the cyclopropene can be trapped, albeit in low yield (10 %), by added cyclopentadiene or detected directly by low-temperature NMR23. In a related example, a l,l-dihalo-2-bromo-3-methylcyclopropane (2a) leads to products which are also apparently derived through an intermediate 1 -chloro-3-methylenecyclopropene which undergoes nucleophilic addition (See Ref. 80). 

The fact that the decomposition of ionized methylpropene (106) at longer lifetimes is preceded not only by hydrogen randomization but also by participation of the C(2) carbon atom (indicated with an asterisk in Scheme 14) in the expulsion of ethylene has been explained by invoking the intermediate existence of methylcyclopropane (107) and 2-butene (108) (Scheme 14). This is in line with the finding that direct ionization of neutral methylcyclopropane gives 107 which is known to undergo ethylene loss. The isomerization sequence described in Scheme 14 can be viewed as a further example for a mass spectrometric dyotropic rearrangement ". ... 

The formation of methylenecyclopropene was surmised from experiments where 1,2-dihalo-1-methylcyclopropanes are treated with base . Thus the reaction of 99 with potassium r-butoxide yielded r-butoxymethylenecyclopropane 100. When meth-anethiol is added to the reaction mixture, 101 is isolated (equation 80). These results can be rationalized in terms of methylenecyclopropene as a reactive intermediate. 

Many cyclopropyl chlorides and bromides have been converted to alkoxycyclopropanes by treatment with a strong base, in most cases potassium rerf-butoxide, in an appropriate organic solvent (Table 13). Under such conditions, hydrogen halide elimination takes place, yielding strained cyclopropene intermediates, which are trapped by nucleophilic attack of the alkoxide. Overall, a simple substitution occurs when a bond is formed between the alkoxide group and the carbon atom to which the halide was attached. This is the case when l-chloro-5-methyl-exo-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (1) was reacted with potassium /ert-butoxide l-/er/-butoxy-5-methyl-ent/o-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (2) was isolated in 94% yield.If a C-O bond is established at the other olefinic carbon atom, a C H bond is concomitantly formed at the carbon atom, to which the halide was attached. The result is a double substitution which is discussed elsewhere (see Section 5.2.1.3 ). When the substrate contains more than one halogen atom, several elimination reactions usually take place. Thus, treatment of 1 -bromo-2-chloro-2-methylcyclopropane (3) with an excess of potassium /er/-butoxide gave l-ter/-butoxy-2-methylenecyclopropane (4) in 30% yield. 

When a side chain larger than methyl is not present, diene products are not formed. Thus, for example, 1,1 -dichloro-2,3-dimethylcyclopropane does not give bis(methylene)cyclopropane but rather products derived from double dehydrochlorination followed by addition of base (see Section 5.2.2.1.3.). l,l-Dichloro-2-methylcyclopropane gives only polymeric material, presumably due to the fact that the cyclopropene intermediate has no avenue of escape for the double bond from the three-membered ring. ... 

As a representative example, treatment of bicyclo[1.1.0]butane (1) with Zeise s dimer in acetone at 23°C for 20 minutes gives butadiene in quantitative yield. At — 45°C (48 hours) a 1 1 complex is formed in 97% yield, which upon further treatment with pyridine gives 2-[bis(pyridine)dichloroplatina]bicyclo[l.l.l]pentane (2) in 94% yield. Thermal decomposition of this complex 2 unselectively leads to a mixture of the starting hydrocarbon 1, cyclobutene (3), butadiene (4), 2-methylcyclopropene (5) and methylene cyclopropane (6). Reduction of the metallacyclic intermediate 2 with lithium aluminum deuteride leads to a bisdeuterated methylcyclopropane 7 accompanied by a bisdeuterated cyclobutane 8 and the starting material 1. ... 

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