Cleavage of cyclopropanes by diborane

Mild Ring‐Opening 1,3‐Hydroborations of Non‐Activated Cyclopropanes

“…It proceeds via a four‐membered concerted transition state: the hydrogen and the boron atoms added on the same face of the double bond (Figure b) . However, such a pathway for the hydroboration of cyclopropanes is very sluggish . High reaction temperatures and poor functional group compatibility make this transformation less practical.…”

Mild Ring‐Opening 1,3‐Hydroborations of Non‐Activated Cyclopropanes

“…typically occurs in an anti-Markovnikov manner. [14] High reaction temperatures and poor functional group compatibility make this transformation less practical. [13] However, such ap athway for the hydroboration of cyclopropanes is very sluggish.…”

“…[13] However, such ap athway for the hydroboration of cyclopropanes is very sluggish. [14] High reaction temperatures and poor functional group compatibility make this transformation less practical. Because boron halides were known to be applied in electrophilic borylation reactions with arenes,h eteroarenes, [15] alkynes [16] and alkenes, [17] we envisioned that the alkene-like p-donating properties of nonactivated cyclopropanes could facilitate similar electrophilic ring-opening and borylation.…”

Stereochemistry and reactivity in cyclopropane ring-cleavage by electrophiles

“…Even if we allow free rotation of C1, the pentavalent mercurated carbon atom, the stereochemical results do not change. Such an intermediate(64) would place positive charge on C2 and C3 and could account for the large ~ values observed. We therefore favor a cornermercurated intermediate with the rate-determining step for the overall reaction leading to this intermediate.…”

“…It is hard to see why these two fundamentally different processes should have nearly the same activation energy. Ring-Opening from a corner-mercurated intermediate (64) inv-inv ret-inv C3. Attack from the rear at C2 (path A) gives 65 in which both electrophile and nucleophile have reacted with inversion.…”