Under H2 pressure, Co(II)(dmgBF2)2L2 (L = H2O, THF) generates a low concentration of an H• donor. Transfer of the H• onto an olefin gives a radical that can either (1) transfer an H• back to the metal, generating an isomerized olefin, or (2) add intramolecularly to a double bond, generating a cyclized radical. Transfer of an H• back to the metal from the cyclized radical results in a cycloisomerization. Both outcomes are favored by the low concentration of the cobalt H• donor, whereas hydrogenation and cyclohydrogenation are more likely with other catalysts (when the concentration of the H• donor is high).
CAl4
2−/− (D4h, 1A1g) is a cluster ion that has been established to be planar, aromatic, and contain a tetracoordinate planar C atom. Valence isoelectronic substitution of C with Si and Ge in this cluster leads to a radical change of structure toward distorted pentagonal species. We find that this structural change goes together with the cluster acquiring partial covalency of bonding between Si/Ge and Al4, facilitated by hybridization of the atomic orbitals (AOs). Counter intuitively, for the AAl4
2−/− (A = C, Si, Ge) clusters, hybridization in the dopant atom is strengthened from C, to Si, and to Ge, even though typically AOs are more likely to hybridize if they are closer in energy (i.e. in earlier elements in the Periodic Table). The trend is explained by the better overlap of the hybrids of the heavier dopants with the orbitals of Al4. From the thus understood trend, it is inferred that covalency in such clusters can be switched off, by varying the relative sizes of the AOs of the main element and the dopant. Using this mechanism, we then successfully killed covalency in Si, and predicted a new aromatic cluster ion containing a tetracoordinate square planar Si, SiIn4
2−/−.
Transition-metal hydrides generate α-alkoxy radicals by H• transfer to enol ethers. We have measured the rate constant for transfer from CpCr-(CO) 3 H to n-butyl vinyl ether and have examined the chemistry of radicals generated by such transfers. Radicals from appropriate substrates undergo 5-exo cyclization, with higher diastereoselectivity than the analogous allcarbon radicals. From such radicals it is straightforward to make substituted tetrahydrofurans.
As a complement to Pd(0)-catalyzed cyclizations, seven Pd(II)-catalyzed cyclization strategies are reported. α,ω-Diynes are selectively hydroborated to bis(boronate esters), which cyclize under Pd(II)-catalysis producing a diverse array of small, medium, and macrocyclic polyenes with controlled E,E, Z,Z, or E,Z stereochemistry. Various functional groups are tolerated including aryl bromides, and applications are illustrated.
Palladium(II)-catalyzed macrocyclizations of bis(vinylboronate ester) compounds are demonstrated to provide a strategically efficient approach to transannular Diels-Alder reaction substrates. In several systems reported, the macrocycle is preorganized such that cycloaddition at room temperature occurs concomitantly with cyclization. Numerous advantages over palladium(0)-catalyzed cross-coupling approaches are demonstrated.
Using
the doubly protic bis-pyrazole-pyridine ligand (N(NNH)2), we have synthesized an octahedral IrIII–H
[HIr(κ3-N(NNH)(NN–))(CO)(
t
BuPy)]+ ([1-MH]+) from an IrI starting material. This hydride
was generated by adding sufficient electron density to the metal center
such that it became the thermodynamically preferred site of protonation.
It was observed via UV–vis spectroscopy that [1-MH]+ establishes a [
t
BuPy] dependent
equilibrium with a ligand protonated square-planar IrI [Ir(N(NNH)2)(CO)]+ ([2-LH]+). This example of metal/ligand proton tautomerism is unusual in
that the position of the equilibrium can be controlled by the concentration
of exogeneous ligand (i.e.,
t
BuPy). This
equilibrium was shown to be key to the reactivity of the IrIII–H; 2 equiv of [1-MH]+ release H2, converting to the IrII dimer [[Ir(N(NN–)(NNH))(CO)(
t
BuPy)]2]2+ ([7]2+) under mild conditions
(observable at room temperature). Mechanistic evidence is presented
to support that this dinuclear reductive elimination occurs by tautomerization
of the metal hydride [1-MH]+ to a ligand protonated
species [1-LH]+, from which ligand dissociation
is facile, generating [2-LH]+. Subsequent
reaction of [2-LH]+ with [1-MH]+ allows for production of H2 and the IrII dimer [7]2+. The tautomerization
between the metal-hydride and the ligand protonated species provides
a low energy pathway for ligand dissociation, opening the needed coordination
site. The ability to control the interconversion between a metal-hydride
and a ligand-protonated congener using an exogeneous ligand introduces
a new strategy for catalyst design with proton responsive ligands.
Hydrogen
atom (H•) donors generated from H2 facilitate
the atom efficient reduction of small molecule
substrates. However, generating H• donors with X–H
bond dissociation free energies (BDFEs) below 52 kcal mol–1 is especially challenging because they thermodynamically favor the
bimolecular evolution of H2. We have recently proposed
that [CpV(CO)3H]− catalyzes the conversion
of H2 into a proton, an electron, and a hydrogen atom in
the presence of a sacrificial base. In order to understand the driving
force for H• transfer, the free energies of H+/H•/H–/e– transfer from [CpV(CO)3H]− have been
evaluated using solution phase techniques and state-of-the-art quantum
chemical calculations. Thermochemical cycles have been constructed
in order to anchor the computational values against experimental observations.
This facilitates a quantitative comparison of the thermodynamic driving
force for H+/H•/H–/e– transfer between isoelectronic anionic/neutral hydrides
of the same row (the corresponding values are already available for
CpCr(CO)3H). The overall charge greatly influences the
thermodynamics of transferring H+, H–, and e– (i.e., [CpV(CO)3H]− is a much weaker acid, a stronger hydride donor, and a stronger
reductant than CpCr(CO)3H); there is almost no change in
the thermodynamics of H• transfer (V–H BDFE
54.7 kcal mol, Cr–H BDFE 57.0 kcal mol–1).
In MeCN, the one electron oxidation of [CpV(CO)3H]− (−0.83 V vs Fc/Fc+) generates CpV(CO)3H, which spontaneously evolves H2. The resulting
CpV(CO)3 is trapped as the solvent adduct CpV(CO)3(MeCN). Because H• transfer is now coupled to metal–solvent
binding, the V–H bond is substantially weakened for CpV(CO)3H (V–H BDFE 36.1 kcal mol–1), amounting
to a strategy for obtaining very reactive H atoms from H2.
Radical cyclizations are most often achieved with BuSnH in the presence of a radical initiator, but environmental considerations demand that alternative reagents be developed-ones that can serve as a synthetic equivalent to the hydrogen atom. We have revisited [CpV(CO)H], a known replacement for BuSnH, and found that it can be used catalytically under H in the presence of a base. We have carried out tin-free catalytic radical cyclizations of alkyl iodide substrates. The reactions are atom-efficient, and the conditions are mild, with broad tolerance for functional groups. We have, for example, achieved the first 5-exo radical cyclization involving attack onto a vinyl chloride. We suggest that the radicals are generated by an initial electron transfer.
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