John F. Garst

Absolute hardness as a measure of aromaticity

Abstract Absolute hardness, which is half the HOMO-LUMO gap in Hartree-Fock or Huckel theory, is shown to be an excellent measure of aromaticity, and a principle of maximum hardness is established.

Grignard reagent formation from aryl halides. There is no aryl radical intermediate along the dominant reaction channel

Abstract For Grignard reagent formation from magnesium and an aliphatic halide RX in an ether solvent, a route through R is the major pathway. Part of the evidence is that by-products of side reactions of R are formed in substantial yields. Similar reactions of phenyl and o -(3-butenyl)phenyl halides give very low (sometimes trace) yields of by-products derived from side reactions of R , despite the fact that aryl R are much more reactive than alkyl in both solvent attack and cyclization [ o -(3-butenyl)phenyl case]. Grignard reactions of aryl halides appear to proceed largely through a pathway along which R is not an intermediate. This is probably a dianion pathway, that is, one along which RX 2− is an intermediate or transition state.

Magnesium bromide in Grignard reagent formation

Abstract The progress of the reaction of magnesium with bromocyclopropane in diethyl ether at reflux varies with time in a sigmoid fashion, reflecting an initial induction period during which autocatalysis is evident. The initial addition of MgBr 2 to the medium greatly reduces or eliminates the autocatalytic induction period, suggesting that the autocatalysis in the initial absence of MgBr 2 is due to its formation. Magnesium does not react with either 1-bromo-2,2,3,3-tetramethylcyclopropane or bromopentamethylbenzene in pure diethyl ether. It reacts smoothly with either in 2.6 M magnesium bromide in diethyl ether, giving the corresponding Grignard reagents (2,2,3,3-tetramethylcyclopropylmagnesium bromide, 28%; pentamethylphenylmagnesium bromide, 49-80%). In the case of 1-bromo-2,2,3,3-tetramethylcyclopropane, by-products representing >44% solvent attack are formed. Magnesium bromide also has more subtle effects. In reactions of magnesium with bromocyclopropane in diethyl ether, the product distribution varies significantly with the initial concentration of the substrate. Initially-added MgBr 2 emulates the effect of higher initial concentrations of the substrate, suggesting that the substrate concentration effects are responses to the buildup of polar solutes (MgBr 2 , RMgBr) as the reaction proceeds. The bromocyclopropane reaction contrasts with that of 5-hexenyl bromide, for which the extent of cyclization is not very sensitive to added magnesium bromide. The early turbidity that usually forms in reactions of magnesium with organic halides in pure diethyl ether is absent for reactions in 2.6 M MgBr 2 . Turbidity not due to a precipitate (MgX 2 , RMgX, or other) may be due to a separation of dilute and concentrated liquid phases of MgBr 2 solutions. There are many parallels between Grignard reagent formation and metallic corrosion in contact with aqueous solutions, suggesting that Grignard reagent formation, like aqueous corrosion, involves local galvanic cells.

CIDNP evidence for radical intermediates in the hydroformylation and reduction of styrene by HCo(CO)4/CO

Reactions of styrene with HCo(CO)4 in CH2Cl2, CD2Cl2 and C6D6 give ethylbenzene, 2-phenylpropanal, and probably 2-phenylpropanoylcobalt tetracarbonyl with nuclear spin polarizations consistent with formation through initially singlet radical pairs [Ph⋯HCH3 •Co(CO)4]. This is the first report of CIDNP in a hydroformylation product.

REACTION OF COBALT TETRACARBONYL HYDRIDE WITH PHENYLACETYLENE

Abstract In C6H6 or hexane at room temperature, under an atmosphere of CO, excess HCo(CO)4 reacts with phenylacetylene to give ethylbenzene and 2-phenylpropanal, the same products that result from the similar reaction of styrene. This and other evidence show that styrene is an intermediate. However, when phenylacetylene is in large excess, neither ethylbenzene, 2-phenylpropanal, nor styrene is formed in more than trace quantity. Instead, a compound is formed whose spectral properties suggest that it is an alkyl- or acylcobalt carbonyl containing a 1-phenylethenyl group. This compound reacts with HCo(CO)4 to give styrene. CIDNP suggests that the reaction of phenylacetylene with HCo(CO)4 proceeds through radical pairs [1-phenylethenyl··Co(CO)4].

A simple modification of a commercial atomic force microscopy liquid cell for in situ imaging in organic, reactive, or air sensitive environments

We describe modifications to a commercial atomic force microscopy (AFM) fluid cell to enable imaging in organic and reactive environments and show initial images obtained with this cell. The fluid cell is constructed of glass and Teflon only, both of which are inert in many organic and corrosive fluids. A flange with a viton “O” ring was fitted around the base of the AFM and an airtight Plexiglas cap was placed on top to cover the AFM. The cap was purged with Ar during scanning and fluid transfer. A commercial oxygen sensor was used to measure the oxygen partial pressure. A flow system was used to introduce the solutions into the cell, minimizing atmospheric contamination. The cell was used to image a Mg surface under aqueous and tetrahydrofuran liquids for hours without leaking.

Suppressing the cyclization of (1-methyl-5-hexenyl)sodium

Abstract In reactions of 1-methyl-5-hexenyl chloride and bromide with sodium metal and sodium naphthalene in DME and THF, the cyclization of (1-methyl-5-hexenyl) sodium is suppressed by added tert -butylamine. Since the cyclization of 1-methyl-5-hexenyl radical does not appear to be affected, this demonstrates the practicality of using the 1-methyl-5-hexenyl group as a probe for radical intermediates in the presence of tert -butylamine.

On hyperfine coupling constants and molecular geometries of radical-anions of some cyclooctatetraene derivatives

Since calculations of proton hyperfine coupling constants for the radicalanions of tetraphenylene and 1,2: 5,6-dibenzocyclooctatetraene based on planar models with strong coupling between unsaturated fragments agree with experiment nearly as well as those based on non-planar models with weak coupling between unsaturated fragments, such data do not constitute evidence for the molecular geometry of the radical-anions.

The Interfacial Chemistry of Grignard Reagent Formation: Reactions of Clean Mg(0001) Surfaces

The Grignard reagent, RMgX, where R is a hydrocarbon group and X is a halogen, is one of the more important and versatile reagents for organic synthesis [1]. It is formed in a heterogeneous reaction between magnesium and an organic halide in an appropriate organic solvent [2, 3, 4, 5]