Abstract:Vinyl and fofmyl groups undergo rearrangements of the type X = CHCMe2-CHI + CMe2-CH2-CH=X (X = CHI or 0) with comparable rates (106-107 I/mol. s, 20°C). This result is in line wi.th high-level ab initio calculations performed on the basic systems, i. e. X = CH -CHI -CH2, which give barriers of activation of 14.9 (CH? = CH -CH2 -cH2) and 19.2 kcal/mol (0 = CH -CH2 -CH2). respectively. The calculations suggest that the rearrangements proceed via cyclopropane-like intermediates.Experimentelle und theoretische Unt… Show more
“…Interestingly, the cyclopropyloxyl radical 46 was found to be in a very shallow potential well and, at some levels of theory, not even a minimum (Figure ). The conclusion, which is in accord with the above study of Chatgilialoglu and co-workers, is that the ring expansion process does not necessarily proceed via a discrete cyclopropyloxyl radical intermediate and that there is a low-energy direct path . It is also readily seen from Figure that the unambiguous generation of any fused cyclopropyloxyl radical akin to 46 will result in preferential cleavage of the endocyclic bond for kinetic, as well as thermodynamic, reasons.…”
Section: Cyclization Reactions Of Acyl Radicalssupporting
“…Interestingly, the cyclopropyloxyl radical 46 was found to be in a very shallow potential well and, at some levels of theory, not even a minimum (Figure ). The conclusion, which is in accord with the above study of Chatgilialoglu and co-workers, is that the ring expansion process does not necessarily proceed via a discrete cyclopropyloxyl radical intermediate and that there is a low-energy direct path . It is also readily seen from Figure that the unambiguous generation of any fused cyclopropyloxyl radical akin to 46 will result in preferential cleavage of the endocyclic bond for kinetic, as well as thermodynamic, reasons.…”
Section: Cyclization Reactions Of Acyl Radicalssupporting
“…Comparison rate constants for related shifts are provided in Figure . This comparison is done at 25 °C, and to estimate the rate constant for the acylgermane at this temperature, we simply divided the 80 °C rate constant by 10.…”
An in depth study of the capability of acylgermanes to function as
acceptors in radical cyclizations is
reported. Radicals add to acylgermanes, and rapid fragmentation of
the resulting α-germylalkoxy radicals provides
ketones and germyl radicals. The germyl radicals in turn propagate
the chain by addition or abstraction, so the
reaction occurs by a unimolecular chain transfer (UMCT) process.
In contrast, acylsilanes also function as radical
acceptors, but they do not participate in UMCT processes because a
“radical-Brook” rearrangement intervenes.
Cyclizations in 5-exo and 6-exo modes show
good to excellent scope, and rate constants for cyclization can
be
varied over 2 orders of magnitude by changing the germanium
substituents. Acyltriarylgermanes are among the
best radical acceptors yet identified, and this quality makes them
superior reagent equivalents of the carbonyl radical
acceptor synthon. Parent cyclizations in the 4-exo and
7-exo modes fail. Attempted 3-exo
cyclization results in a
1,2-acyl shift, which can be conducted alone or in tandem with a
subsequent cyclization to the rearranged acylgermane.
The results provide a foundation for future synthetic applications
of radical cyclizations to acylgermanes.
“…The radical clock 2 could also be used to study autoxidation reactions where alkoxy radicals are intermediates . The 2,2-dimethylcyclopropoxy radical has been proposed as an intermediate in the mercuric acetate oxidation of 1-trimethylsiloxy-2,2-dimethylcyclopropane 2 …”
Detailed molecular orbital calculations were directed to the cyclopropylcarbinyl radical (1), the cyclopropoxy radical (2), and the cyclopropylaminium radical cation (3) as well as their ring-opened products. Since a considerable amount of data are published about cyclopropylcarbinyl radicals, calculations were made for this species and related ring-opened products as a reference for 2 and 3 and their reactions. Radicals 1-3 have practical utility as "radical clocks" that can be used to time other radical reactions. Radical 3 is of further interest in photoelectron-transfer processes where the back-electron-transfer process may be suppressed by rapid ring opening. Calculations have been carried out at the UHF/6-31G*, MP4//MP2/6-31G*, DFT B3LYP/6-31G*, and CCSD(T)/cc-pVTZ//QCISD/cc-pVDZ levels. Energies are corrected to 298 K, and the barriers between species are reported in terms of Arrhenius E(a) and log A values along with differences in enthalpies, free energies, and entropies. The CCSD(T)-calculated energy barrier for ring opening of 1 is E(a) = 9.70, DeltaG* = 8.49 kcal/mol, which compares favorably to the previously calculated value of E(a) = 9.53 kcal/mol by the G2 method, but is higher than an experimental value of 7.05 kcal/mol. Our CCSD(T)-calculated E(a) value is also higher by 1.8 kcal/mol than a previously reported CBS-RAD//B3LYP/6-31G* calculation. The cyclopropoxy radical has a very small barrier to ring opening (CCSD(T), E(a) = 0.64 kcal/mol) and should be a very sensitive time clock. Of the three series studied, the cyclopropylaminium radical cation is most complex. In agreement with experimental data, bisected cyclopropylaminium radical cation is not found, but instead a ring-opened species is found. A perpendicular cyclopropylaminium radical cation (4) was found as a transition-state structure. Rotation of the 2p orbital in 4 to the bisected array results in ring opening. The minimum onset energy of photoionization of cyclopropylamine was calculated to be 201.5 kcal/mol (CCSD(T)) compared to experimental values of between about 201 and 204 kcal/mol. Calculations were made on the closely related cyclopropylcarbinyl and bicyclobutonium cations. Stabilization of the bisected cyclopropylcarbinyl conformer relative to the perpendicular species is much greater for the cations (29.1 kcal/ mol, QCISD) compared to the radicals (3.10 kcal/mol, QCISD). A search was made for analogues to the bicyclobutonium cation in the radical series 1 and 2 and the radical cation series 3. No comparable species were found. A rationale was made for some conflicting calculations involving the cyclopropylcarbinyl and bicyclobutonium cations. The order of stability of the cyclopropyl-X radicals was calculated to be X = CH2 >> X = O > X = NH2+, where the latter species has no barrier for ring opening. The relative rate of ring opening for cyclopropyl-X radicals X = CH2 to X = O was calculated to be 3.1 x 10(6) s(-1) at 298 K (QCISD).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
