D. A. Evans. B. Breit Cyclopropyl-carbinyl Carbocations Chem 206 Carbocation Stabilization via Cyclopropylgroups Solvolysis rates represent the extend of that cyclopropyl orbital overlap contributing to the stabilization of the carbenium ion which is involved as a See Lecture 5. slide 5-05 for discussion of walsh orbitals Krel=1 A rotational barrier of about 13.7 kcal/mol is observed in -Me nmr in super acids OTS 8(CH3 )=2.6 and 3. 2 ppm OTS following example X-ray Structures support this orientation 1222A 103 1517A 1302A 478A1474A Bridgehead Carbocations 1464A OTs 1541A Tso 1409A 1444A Bridgehead carbocations are highly disfavored due to a strain increase in 1534A achieving planarity. Systems with the greatest strain increase upon passing from ground state to transition state react slow Carbocation Stabilization- Aromaticity Some Huckel aromatic cations(4n+2)T-electrons are isolable as salts with non-nucleophilic anions R R.F.Chds,JACS1986,108,1692 in SbF5-S02CIFH Me Me C R O R X Me Me Me OTs Me Me OTs OTs TsO X OTs OTs TsO Cl Cl TsO X H H D. A. Evans, B. Breit Cyclopropyl-carbinyl Carbocations Chem 206 Carbocation Stabilization via Cyclopropylgroups A rotational barrier of about 13.7 kcal/mol is observed in following example: NMR in super acids d(CH3) = 2.6 and 3.2 ppm R. F. Childs, JACS 1986, 108, 1692 1.464 Å 1.409 Å 1.534 Å 1.541 Å 1.444 Å 24 ° 1.302 Å 1.222 Å 1.474 Å 1.517 Å 1.478 Å X-ray Structures support this orientation Carbocation Stabilization - Aromaticity Some Hückel aromatic cations (4n+2) p-electrons are isolable as salts with non-nucleophilic anions. Solvolysis rates represent the extend of that cyclopropyl orbital overlap contributing to the stabiliziation of the carbenium ion which is involved as a reactive intermediate: krel = 1 krel = 1 krel = 106 krel = 10-3 krel = 1 krel = 108 + = BF4, SbCl6 ++ ++ generated in SbF5-SO3 in SbF5-SO2ClF stable, isolable salts Bridgehead Carbocations 1 10-7 10-13 104 Bridgehead carbocations are highly disfavored due to a strain increase in achieving planarity. Systems with the greatest strain increase upon passing from ground state to transition state react slowest. why so reactive? See Lecture 5, slide 5-05 for discussion of Walsh orbitals