E.Alexander Hill

INFRARED AND NUCLEAR MAGNETIC RESONANCE SPECTROSCOPIC STUDIES OF THE STRUCTURE AND DYNAMICS OF ALLYLIC MAGNESIUM COMPOUNDS

Abstract The infrared spectra of allyl- and methallyl-d 2 -magnesium bromides have two double bond streching bONds, corresponding to C  CH 2 and C  CD 2 groups in equilibrating allylic isomers. The methylene resonances in the 13 C NMR spectra of allylmagnesium bromide and chloride and methallylmagnesium bromide are broadened at low temperatures by an exchange process which appears to be the interconversion between the classical unsymmetrical allytic structures. Analogous changes are seen in the spectrum of 1,3-dimethylallylmagnesium chloride and in the proton NMR spectrum of allylmagnesium bromide. Rate constants and activation parameters for the exchange have been determined from the line broadenings. Unlike the Grignard reagent, the methylene resonances of diallylmagnesium in tetrahydrofuran are not significantly broadened at reduced temperature, and the deuterated reagent does not have two distinct double bond stretching bands in the IR spectrum.

The stabilities of the 2-ferrocenyl-2-propyl and ferrocenylmethyl cations

Abstract The p K R of the 2-ferrocenyl-2-propyl cation has been evaluated by a spectrophotometric indicator acidity study. The observed value, −0.01, is in substantial disagreement with a previous report of the stability of this carbonium ion. In the equilibrium between 2-ferrocenyl-2-propanol and 2-ferrocenylpropene in dilute acid, the alcohol predominates. Redetermination of the absorption spectrum of the ferrocenylmethyl cation demonstrates that the cation is completely formed in 38 to 70% sulfuric acid, in contrast with a previous p K R determination for this cation of −4.94.

Formation and rearrangement of the Grignard reagent from 2-phenylcyclobutylmethyl bromide

Abstract Reaction of cis - or trans -2-phenylcyclobutylmethyl bromide with magnesium produces the rearranged 5-phenyl-1-penten-5-ylmagnesium compound 7 in addition to the unrearranged Grignard reagent 6 . The former appears to result from very rapid rearrangement of a radical intermediate in the process of Grignard reagent formation. Rearrangement of 6 to 7 also occurs, at a rate which is accelerated by the phenyl group.

Cyclization of the Grignard reagent from 5-chloro-3-methyl-1-hexene; a reversible organomagnesium rearrangement☆

Abstract The Grignard reagent from 5-chloro-3-methyl-1-hexene cyclized reversibly to form an equilibrium mixture with two stereoisomers of the 2,4-dimethylcyclobutylmethyl-Grignard reagent. Equilibrium and rate constants for the opposing reactions have been evaluated at 80–100 and 100–120°C, respectively. Over this range, the equilibrium mixture favors the cyclic reagent by a ratio of about 3 1 . Comparison with results for related compounds indicates that methyl substitution influences the equilibrium both by destabilization of the organomagnesium function and by stabilization of the strained ring. Additional comparisons of rate and equilibrium constants are discussed.

Ring cleavage rearrangement of cyclobutylmethylboranes

Abstract Boranes derived from hydroboration of methylenecyclobutane with borane/THF, 9-borabicyclo[3.3.1]nonane, and borane-methyl sulfide rearranged on heating in situ at 100–160°C to open chain structures. Products after oxidation were the unrearranged cyclobutylmethanol, and 4-penten-1-ol, 1,4-pentanediol and 1,5-pentanediol. The unsaturated alcohol was the major product in reactions with a stoichiometric ratio of alkene to BH bonds, and the diols were formed with excess borane. With borane-methyl sulfide as hydroborating reagent, the rate of rearrangement at 100°C in triglyme was not significantly dependent upon the initial alkene/borane ratio 3 1 or 1.15 1 or the presence of excess methyl sulfide. However, an equivalent amount of pyridine prevented rearrangement. Rearrangement in THF using borane/THF also occurred at comparable rates in the presence and absence of excess borane. Little or no isomerization of the boron function into the cyclobutane ring was observed. Results are interpreted on the basis of a concerted four-center mechanism which requires a vacant boron orbital.

Ring cleavage rearrangement of methyl-subsituted cyclopropylmethylboranes

Abstract 2,2,3-Trimethyl- and 2,30dimethylmethylenecyclopropane were prepared and hydroborated using various borane reagents. Changes in the borane solutions as a result of heating were studied by NMR and by oxidation to alcoholds. Ring-cleavage rearrangement reactions were observed, analogous to rearrangements previously found for cyclopropylmethyl Grignard reagents and for unsubstituted cyclopropylmethylboranes Methyl substitution slows the rearrangement involving cleavage of the bond to the substituted ring carbon, and has an especially large effect when cis to the metallomethyl group. In the 2,2,3-trimethylcyclopropylmethyl system, the ring-cleavage rearrangement does not go to completion, but instead approaches an equilibrium in which a substantial concentration of the trans isomer remains. Rearrangement in the 2,3-dimethylcyclopropylmethyl system occurs with retention of configuration at the ring carbon to which the boron migrates. The results of the present work suggest a mechanistic reinterpretation of the carboboration of bicyclobutane.

Rearrangement of the Grignard reagent from 1-methylcyclobutylmethyl bromide: origins of the methyl substituent effect

Abstract 1-Methyl substitution leads to a small decrease (ca. 0.45) in the rate of the ring cleavage rearrangement of cyclobutylmethylmagnesium bromide. Comparison with literature data for 3- and 5-membered rings indicates that the methyl substituent shifts the equilibrium in the direction of ring cleavage, mostly by stabilization of the double bond of the open isomer, and possibly partly by destabilization of the organometallic function of the cyclic isomer. The reaction rate is decreased in both directions, probably because of increased transition state steric repulsions.

Ethanolysis and base promoted elimination reaction of bridgehead α-amino sulfides : 3-thioethoxy-4-azahomoadamantanes

Abstract Solvolysis of N-methyl-3-thioethoxy-4-azahomoadamantane ( 4c ) in ethanolic sodium ethoxide followed first order kinetics. Reaction presumably proceeds via an S N 1 mechanism with resonance stabilization of the bridgehead carbonium ion by the electrons on adjacent nitrogen. Addition of ethoxide to the bridgehead carbonium ion yields N-methyl-3-ethoxy-4-azahomoadamantane ( 4c ). Solvolysis of N-protio compound ( 4b ) under similar conditions was much more rapid, and followed first order kinetics for both substrate and base. The mechanism apparently involves an elimination-addition pathway in which ethoxide promoted elimination of ethyl mercaptan is followed by net addition of ethanol across the bridgehead imine to form ether 4d .

A polar substituent effect on the ring-cleavage rearrangement of 1-arylcyclobutylmethyl Grignard reagents

The kinetics of the ring-cleavage rearrangements of 1-phenylcyclobutylmethylmagnesium chloride and its p-methyl and p-chloro analogs have been determined. The first-order rate constants are correlated by the Hammett equation, with ϱ = −0.47. The results are consistent with a concerted mechanism with a cyclic transition state having significant polar character, although polar effects on the stabilities of reactant and product may also contribute. The phenyl group itself slows the reaction by a factor of 0.031, which is interpreted principally in terms of steric destabilization of the transition state.

The effect of solvent polarity on the rate of rearrangement of 9-(trans-2,3-dimethylcyclopropylmethyl)-9-borabicyclo[3.3.1]nonane

The rate of the ring-opening rearrangement of the borane derived from hydroboration of trans-2,3-dimethylmethylenecyclopropane with 9-BBN was measured in hexane, tetrahydrofuran, and methylene chloride. The modest increase in rate which was observed with increasing polarity is interpreted to imply a transition state only slightly more polar than the reactant.