Reported herein is an unprecedented reactivity of propargyl alcohols as “Three-Carbon Synthons” in a Rh(III)-catalyzed C-H functionalization of acetanilides, leading to the synthesis of core structures of isocryptolepine, gamma-carbolines, dihydrochromeno[2,3-b]indoles, and diindolylmethanes derivatives. The transformation involves a Rh(III)-catalyzed C-H functionalization and heteroannulation to yield indoles followed by a cascade cyclization with both external and internal nucleophiles to afford diverse products. The role of the hydroxy group, the key function of the silver additive, the origin of the unique reverse regioselectivity and the rate-determining step, are rationalized in conformity with the combination of experimental, noncovalent interaction analysis and DFT studies. This protocol is endowed with several salient features, including one-pot multistep cascade approach, exclusive regioselectivity, high bond-forming efficiency, and synthesis of a variety of molecular frameworks.
A combined synthetic-theoretical study has been undertaken to determine the factors that influence transannulation in azaphosphatranes. The commonly used proazaphosphatrane P(i-BuNCH2CH2)3N and several of its oxidized congeners are used as model systems. The haloazaphosphatranes of P(i-BuNCH2CH2)3N were synthesized, including a rare fluoroazaphopshatrane, and used as references for computational investigations. Comparisons of the experimental and theoretical observations highlight the flexibility observed in transannulated atranes and the potential for multiple local energetic minima depending on the identity of the equatorial substituents for a given azaphosphatrane. Theoretical calculations also identify the role of the ethylene linker in azaphosphatrane bonding, the influence of transannulation on P–electrophile interactions, and the contribution of electrostatic interactions to transannulation.
Monovalent boron, free borylene species of the form B-R are notoriously unstable. Consequently, there are substantial gaps in the literature concerning the potential utility of those species in organic and inorganic synthesis either as ligands or as critical intermediates in reactions. We show that the relative stability of borylene complexes varies widely, depending on the electron donating ability of the R group. We find that borylenes can form, in the gas phase, weak sigma hole type interactions to saturated carbon centers and stronger dative bonds to tetravalent silicon and germanium. An insertion reaction of the form FHM + BR → FHMBHR competes against dative bonding, however, and the reaction is barrierless in several cases when M = Si and in a few cases when M = Ge. For M = C, the barriers are high enough to stabilize monovalent boron complexes. In each case, the barrier heights to M-H bond activation and BR insertion are very sensitive to the nucleophilicity of BR. We confirm, at the MP2(full) and CCSD(T) levels, a substantial preference in borylenes for the singlet over the triplet state. An account is provided at the B3LYP-D3 and MP2(full) levels for the facile insertion reaction on the singlet surface when M = Si and for the stability of FHM·BR type complexes and the higher barriers to insertion when M = C and Ge.
The fundamental challenge of C−F bond formation by reductive elimination has been met by compounds of select transition metals and fewer main group elements. The work detailed herein expands the list of main group elements known to be capable of reductively eliminating a C−F bond to include tellurium. Surprising and novel modes of both sp2 and sp3 C−F bond formation were observed alongside formation of TeIV cations during two separate attempts to synthesize/characterize fluorinated organotellurium(VI) cations in superacidic media (SbF5/SO2ClF). Following detailed low‐temperature NMR experiments, the mechanisms of the two unique reductive elimination reactions were probed and investigated using density functional theory (DFT) calculations. Ultimately, we found that an “indirect” reductive elimination pathway is likely operative whereby Sb plays a key role in fluoride abstraction and C−F bond formation, as opposed to unimolecular reductive elimination from a discrete TeVI cation.
The bonding preferences in the mixed dihalides (MXY) of groups 2 and 12 metals, including the extent of any anomalous bending, are assessed and established. The deviation from linearity in group 2 metal binary dihalides is well-known, runs contrary to simple bonding models, and is believed to be decisive for structural preferences in the extended solids. Yet the bonding in the ternary, MXY, molecules has not been investigated systematically until now. The structure and bonding in these ternary systems (and, for completeness, the binary cases as well) are determined herein at high levels of theory. A softness criterion formulated by Szentpály and Schwerdtfeger, and tested initially on binary dihalides with predictions for mixed systems, is confirmed to apply broadly for binary and ternary species of the group 2 and 12 metals. For each M, a function of the form E(Θ) = Ae–kΘ is shown to predict the barriers to linearization for all of the bent molecules. The extended solids of some of the ternary dihalides are of interest for their optical properties. The bonding in the molecular (MXY) units may offer we think some new perspectives from which to rationalize the bonding preferences in those crystal structures.
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