Cyclopropane labeled

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. 

The species identified as XA reacts with styrene to give the expected cyclopropane. The rate constant for this reaction is ca 200 times less than the corresponding rate constant for 3BA (Table 6). Also, use of the deuterium-labeled a-methylstyrene reveals that the cyclopropanation occurs with essentially total retention of stereochemistry. Moreover, precisely the same result is obtained when this carbene is formed by triplet sensitization rather than by direct irradiation. These findings also point to a reaction originating from a singlet carbene. 

As expected, DMFL reacts with styrene to give the appropriate cyclopropane. Irradiation of the diazo-compound in benzene containing the labeled a-methylstyrene gives cyclopropane with essentially complete retention of stereochemistry (Table 6). 

Irradiation of DAAN in benzene gives 3AN. This carbene reacts with oxygen very rapidly to give an intermediate believed to be the carbonyl oxide. The triplet carbene reacts with labeled a-methylstyrene to give the cyclopropane with total loss of stereochemistry (Table 6). Direct irradiation in neat isopropyl alcohol gives the ether in low yield (relative to the yields from XA, DMFL, FL, and BFL). The other products are those expected to result from hydrogen-atom abstraction. Triplet-sensitized irradiation of DAAN in the alcohol does not give a detectable amount of the ether. 

A carbon labelling study has elucidated the rearrangement mechanism for formation of chalcone (97) which accompanies formation of (91) by the expected vicinyl elimination of trimethylsilyl and benzotriazolyl groups from 2-benzotriazolyl-2-aryl-3-ketopropylsilanes, on reaction with fluoride ion in DMF. ° Thus, it has been possible to distinguish between the two alternative mechanisms depicted in Scheme 11 (via intermediates (93) or (95), respectively, by determining the fate of the labelled quaternary carbon of substrate (89). The results are consistent with the formation of a cyclopropane intermediate (95) which subsequently ring opens, with relief of strain, to form delocalized carbanion (96), from which the chalcone (97) is obtained (labelled... 

We examine the two lowest singlet states of cyclopropane as one of the CH2 groups is pulled away from the other two. Figure 16.2 shows the basic arrangement of the molecule with the three C atoms in the x-y plane. The C atom on the right is on the j-axis, and Ri is its distance to the midpoint of the other two Cs. R2 is the distance between the two Cs that will become part of ethylene and is the angle out of planarity. We have labeled the C atoms 1, 2, and 3 to identify the three different methylenes for designating orbitals. 

Dianions of the above types may not fall into the category of homoenolate in a strictly formal sense. Nevertheless the amide dianion does show a behavior typical of the homoenolate. Thus, the reaction of the isotopically labeled stannylpropionate results in scrambling of the label probably via a cyclopropane intermediate Eq. (47) [44]. As the result of such an equilibration, isomerization of a-methyl and a-phenyl substituted propionate homoenolates may occur to give the thermodynamically more favorable isomers, respectively. 

Isotopic labelling, Eq. (49), indicated that the transmetalation occurs via direct tin-carbon bond cleavage rather than via intermediate formation of an alkoxy-cyclopropane [45],... 

Indeed, determination of the distribution of the label in the ethers shows convincingly that l-phenylhomocub-9-ylidene (6) is formed via the C —C bond migration from 9-phenylho-mocub-l(9)-ene (5).4 Nonetheless, unequivocal evidence for the presence of 1 -phenylhomocub-9-ylidene (6) was also obtained from a trapping experiment. Photolysis of cubylphenyldi-azomethane (2) in neat (Z)-but-2-ene at —78 C afforded the two cyclopropane adducts 7 and 8. As expected, a single adduct 9 was obtained when cubylphenyldiazomethane (2) was irradiated in ( >but-2-ene.3... 

Though the bulk of the products are not formed from protonated cyclopropane intermediates, there is considerable evidence that at least in 1-propyl systems, a small part of the product can in fact arise from such intermediates.23 Among this evidence is the isolation of 10 to 15% cyclopropanes (mentioned on p. 325). Additional evidence comes from propyl cations generated by diazotization of labeled amines (CH3CH2CD2, CH3CD2CH2, CH3CH2I4CH2 ), where isotopic distribution in the products indicated that a small amount (about 5%) of the product had to be formed from protonated cyclopropane intermediates, e.g.,24... 

Even more scrambling was found in trifluoroacetolysis of l-propyl-l-14C-mercuric perchlorate.25 However, protonated cyclopropane intermediates accounted for less than 1% of the products from diazotization of labeled isobutylamine26 and from formolysis of labeled 1-propyl tosylate.27... 

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