Ring openings of dilithium derivatives of 2,3-diphenylcyclopropane-1-carboxylic acids

Abstract

2,3-Diphenylcyclopropane-l-carboxylic acids undergo ring opening when treated with lithium diisopropylamide (LDIA) in tetrahydrofuran. From c/j,tra«i-2,3-diphenylcyclopropane-l-carboxylic acid (la) LDIA treatment, hydrolysis, and diazomethane esterification produce methyl cis-and nww-a-benzylcinnamates (6b and 4b). From /m/«,i/'u/K-2,3-diphenylcyclopropane-l-carboxylic acid (2a) the same procedure gives methyl (£)-and (Z)-3,4-diphenyl-3-butenoates (7b and 8b). Similar treatment of 2,3:4,5-dibenzo-2,4-norcaradiene-a«n'-7-carboxylic acid (3a) produces methyl 9-phenanthrylacetate (lib) and methyl 3,4:5,6-dibenzocyclohepta-l,3,5-triene-l-carboxylate (12b). Structures of all of the ring-opened products have been verified by their spectral properties and independent syntheses. All of them can be explained by electrocyclic conversions of cyclopropyl anions to allyl anions. Conversion of 3a to 12b can proceed only by a disrotatory path, while the other ring openings should be conrotatory according to theory. Preliminary kinetic experiments suggest that the rate-limiting step for ring opening of la is loss of the second proton from the lithium carboxylate, while the rate-limiting steps for ring openings of 2a and 3a may be the ring opening itself. The relative rates of conversion of the cyclopropanecarboxylic acids to ring-opened materials are la » 2a » 3a. In contrast to the slow deprotonation of la, 2a, and 3a, methyl c/s',//-tim-2,3-diphenylcyclopropane-l-carboxylate (lb) forms an enolate with LDIA in THF readily at -78°. The lithium enolate of lb appears to undergo ring opening at -39°. The name dilithium "carboxylate enolate" is proposed for ciilithio derivatives of aliphatic carboxylic acids which are formed by loss of protons from oxygen and a carbon.

Enyne [4+4] Cycloaddition/Oxidation: Ring Contraction via Cyclopropanones and Their Anionic Ring‐Opening Reactions

“…Forr ingopening products from 14,asilicon group is the sole anionstabilizing substituent. [29,31] Fort he unusual 1,2,5-cyclooctatrienes discussed here, oxidation of the allene,e ither with dioxirane or by photocycloaddition in the presence of oxygen, results in isomerization to the corresponding cyclopropanone.W eh ave not observed oxidation of the more stable allenes,s uch as 8,o n exposure to oxygen in the absence of light. [30] Theabsence of stereochemistry at the cyclopropanone carbonyl of 14 unfortunately does not allow as tudy of ac onrotatory ring-opening pathway for this cyclopropanone.…”

“…[30] Theabsence of stereochemistry at the cyclopropanone carbonyl of 14 unfortunately does not allow as tudy of ac onrotatory ring-opening pathway for this cyclopropanone. [29,31] Fort he unusual 1,2,5-cyclooctatrienes discussed here, oxidation of the allene,e ither with dioxirane or by photocycloaddition in the presence of oxygen, results in isomerization to the corresponding cyclopropanone.W eh ave not observed oxidation of the more stable allenes,s uch as 8,o n exposure to oxygen in the absence of light. Expulsion of carbon monoxide,either photochemically or through heating, leads to formation of ac ycloheptadiene.O verall, the threestep transformation with loss of ar ing carbon represents a[4+ +4À1] combination of an enyne with a2-pyridone to yield aseven-membered ring.…”

“…The calculated dihedral angle between the CÀSi(TMS) bond and the breaking cyclopropane bond, bonds a,is728 8.The dihedral angle for bonds b,the diisopropyl Si À Cbond and its reacting cyclopropane bond, measures 998 8.B ased on this analysis we favor an electrocyclic reaction mechanism where the cyclopropane ring opens in ac onrotatory four-electron electrocyclic process.W hile not as well known as the two-electron disrotatory solvolysis of ac yclopropane substituent, [27] substantial experimental and computational analysis supports the validity of af our-electron ring-opening mechanism. [28,29] In many cases,a nion-stabilizing substituents on the termini of the allyl anion intermediate have been required. Forr ingopening products from 14,asilicon group is the sole anionstabilizing substituent.…”

Enyne [4+4] Cycloaddition/Oxidation: Ring Contraction via Cyclopropanones and Their Anionic Ring‐Opening Reactions

“…Based on this analysis we favor an electrocyclic reaction mechanism where the cyclopropane ring opens in a conrotatory four‐electron electrocyclic process. While not as well known as the two‐electron disrotatory solvolysis of a cyclopropane substituent, substantial experimental and computational analysis supports the validity of a four‐electron ring‐opening mechanism . In many cases, anion‐stabilizing substituents on the termini of the allyl anion intermediate have been required.…”

“…The ability of silicon to stabilize an α‐anion is well known, but the extent of this stabilization is limited . The absence of stereochemistry at the cyclopropanone carbonyl of 14 unfortunately does not allow a study of a conrotatory ring‐opening pathway for this cyclopropanone …”