A homopropargyl radical rearrangement. Kinetics of the rearrangement of the 2,2,5,5,-tetramethyl-3-hexyn-1-yl radical

Abstract

The rate constants for rearrangement of the 2,2,5,5-tetramethyl-3-hexyn-1-yl radical to the 2,6,6-trimethyl-4-heptyn-2-yl radical have been measured from 45 to 88 °C by kinetic epr spectroscopy. The temperature dependence can be represented by[Formula: see text]where θ = 2.3RT kcal mol−1. This rearrangement, which must involve an intermediate vinyl radical of the methylenecyclopropane type, is much slower than analogous rearrangement of homoallyl radicals but comparable in rate to rearrangement of homobenzyl (neophyl) radicals.

Iron-Catalyzed Photoinduced LMCT: A 1° C–H Abstraction Enables Skeletal Rearrangements and C(sp3)–H Alkylation

“…Again, this was contrary to our expectations, since fumaronitrile is known to be a stronger electrophile than the acrylates based on previous studies of polar reactions. 60 Dimethyl fumarate (22) gives a slightly higher ir in favor of the rearrangement compared to fumaronitrile, while the cis isomer dimethyl maleate (23) heavily favors the rearrangement under both conditions. Small proportions of cyclized rearranged product 22c are observed under conditions B but not under conditions A, suggesting a possible thermally driven aldol-type reaction.…”

“…The key skeletal rearrangement step in the Dowd-Beckwith ring expansion is related to a family of radical 1,2-rearrangements wherein a primary radical β to a π-system adds to it, forming a transient cyclopropyl intermediate that then undergoes β-scission to form a more stable tertiary radical. − The rate constants of these rearrangements have been measured with various π-systems such as alkenes, alkynes, arenes, carbonyls, and nitriles, and several have been used as radical clocks. − …”

A homoallyl radical rearrangement. Kinetics of the isomerization of the 2,2-dimethyl-3-buten-1-yl radical to the 1,1-dimethyl-3-buten-1-yl radical

“…[5]). The very substantial driving force for formation of three-membered rings by free radicals when gem dimethyl groups are present ('rhorpe-lngold effect) is demonstrated once again (6).…”

“…A (6). In addition, new "radical clocks" (7) are still required as aids to the quantitative study of free radical chemistry.…”

The fragmentation and isomerization of hot (vibrationally excited) ground state unsaturated radicals

“…An example of such a mechanism is the isomerization of the cyclopropylcarbinyl radical to the allylcarbinyl radical (Scheme 2) [22][23][24][25]. This process has been observed in many different molecules and is of general occurrence [26,27] even when an acetylenic bond is the unsaturated site [28]. The same reaction is also efficient in an even longer chain.…”