We
introduce a previously unexplored parameter—halenium
affinity (HalA)– as a quantitative descriptor
of the bond strengths of various functional groups to halenium ions.
The HalA scale ranks potential halenium ion acceptors
based on their ability to stabilize a “free halenium ion”.
Alkenes in particular but other Lewis bases as well, such as amines,
amides, carbonyls, and ether oxygen atoms, etc., have been classified
on the HalA scale. This indirect approach enables
a rapid and straightforward prediction of chemoselectivity for systems
involved in halofunctionalization reactions that have multiple nucleophilic
sites. The influences of subtle electronic and steric variations,
as well as the less predictable anchimeric and stereoelectronic effects,
are intrinsically accounted for by HalA computations,
providing quantitative assessments beyond simple “chemical
intuition”. This combined theoretical–experimental approach
offers an expeditious means of predicting and identifying unprecedented
reactions.
A remarkable solvent-controlled enantiodivergence is seen in the hydroquinidine 1,4-phthalazinediyl diether ((DHQD)2PHAL)-catalyzed chlorocyclization of unsaturated carbamates. Eyring plot analyses of this previously unreported reaction are used to probe and compare the R- and S-selective pathways. In the CHCl3/hexanes solvent system, the pro-R process shows a surprising increase in selectivity with increasing temperature. These studies point to a strongly solvent-dependent entropy-enthalpy balance between the pro-R and pro-S pathways.
For four related 1,1-disubstituted olefins, (DHQD)2PHAL-catalyzed asymmetric chlorocyclization delivers Cl+ uniformly to one π face, but cyclizes with strong but differing net syn vs. anti addition.
The outcome of a tandem aza-Payne/hydroamination reaction is modified via the use of a latent nucleophile. The latter initially serves as an electrophile to intercept the aziridine alkoxide and afterward turns into a nucleophile thereby performing the aziridine ring opening, out competing the intramolecular aza-Payne pathway. Subsequent hydroamination in the same pot provides N-Ts enamide carbonates, which can be easily converted into biologically significant 3,4-dihydroxylactams.
The carboxylation of aziridine derivatives bearing a silylated propargylic alkoxy group proceeds in the presence of Bu4NF and NaHCO3 to afford the target fused enamine carbonates in good yields.
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