Relative rates and approximate rate constants for inter- and intramolecular hydrogen transfer reactions of polymer-bound radicals

Measurements of relative rates and rate constants for inter- and intramolecular hydrogen transfer reactions of polymer-bound radicals are reported. The relative rate of reaction of resin-bound primary alkyl radical with tributyltin hydride is about 2 times slower than that of the benchmark reaction in solution. The data do not reveal whether this is due to a reduced rate constant or a lower concentration of tin hydride in the resin phase. Yet the difference between solid and solution reactions is small enough to be neglected, and it appears that rate constants measured in solution can be applied directly to resin-bound radicals. A resin-bound aryl radical abstracts a hydrogen atom rapidly (k = 3 x 10(6) s(-1)) from its own polymer backbone and linker, and a simplified view of the resin as a "solvent" is suggested for predicting such effects with other polymers and linkers. Rapid cyclizations of resin-bound aryl radicals will be possible, but slower cyclizations and most bimolecular reactions will be difficult due to the competing polymer/linker hydrogen transfer.     

N-alkoxy-n-acylnitrenium ions in intramolecular aromatic addition reactions

N-alkoxy-N-acylnitronium ions are generated by treatment of N-alkoxy-N-cohloroamides with silver ions in ethereal solvents. These intermediates readily cyclise onto aromatic nuclei on alkozy side-chains to give benzoxazines and benzoxazepines and on the acyl side-chains to give γ, δ and ϵ benzolactams. Spirane products are formed by ipso addition When a 4-methoxy substituent ia present on the side-chain aromatic rings. The yields and regioselectivities of these reactions have been ascribed to different transition structures for cyclisation onto the acyl and alkoxy side-chains which involve respectively an exocyolic and endocyclic N-0 π-bond. Evidence for this exeptionally high π-bond character has been obtained from MNDO calculations which predict a π-bond order of 0.9 and a rotational barrier of 29.7 kcalmol^-1     

Type-Two intramolecular diels-alder reactions of pyrazolo-o-quinodimethanes

Two isomeric series of homologous N-(acryloyloxy)alkylated pyrazolo-3-sulfolenes 10a-c and 18a-c have been efficiently synthesized from a common starting material, 4,6-dihydro-1H-thieno[3, 4-c]pyrazole (5). Thermolysis of these fused 3-sulfolenes provides the corresponding o-quinodimethanes which simultaneously undergo "type-two" intramolecular Diels-Alder reactions to form two- and three-atom-bridged tricyclic pyrazoles which are otherwise difficult to prepare. It was also demostrated that, depending on the N-substitution position of the pyrazolo-fused 3-sulfolenes, the temperature required for the thermal extrusion of SO(2) and the regioselectivity of the T2-IMDA reactions were influenced substantially.     

Activation entropy of electron transfer reactions

We report microscopic calculations of free energies and entropies for intramolecular electron transfer reactions. The calculation algorithm combines the atomistic geometry and charge distribution of a molecular solute obtained from quantum calculations with the microscopic polarization response of a polar solvent expressed in terms of its polarization structure factors. The procedure is tested on a donor–acceptor complex in which ruthenium donor and cobalt acceptor sites are linked by a four-proline polypeptide. The reorganization energies and reaction energy gaps are calculated as a function of temperature by using structure factors obtained from our analytical procedure and from computer simulations. Good agreement between two procedures and with direct computer simulations of the reorganization energy is achieved. The microscopic algorithm is compared to the dielectric continuum calculations. We found that the strong dependence of the reorganization energy on the solvent refractive index predicted by continuum models is not supported by the microscopic theory. Also, the reorganization and overall solvation entropies are substantially larger in the microscopic theory compared to continuum models.     

Protic acid-induced intramolecular reactions of 2-cyclopropylazobenzenes

Treatment of 2-cyclopropylazobenzenes with concentrated sulfuric acid affords the intramolecular N- and C-alkylation products. Treatment of azobenzenes with trifluoroacetic acid results in quantitative rearrangement to arylindazoles via the intermediate formation of acid-stable indazolium ions. It is suggested that the formation of indazolium ions in trifluoroacetic acid results from the synchronous opening of the cyclopropane ring and stabilization of the developing carbocation by an internal nucleophile (the azo-group).Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1063–1068, August, 1987.     

Intermolecular and intramolecular photocycloaddition reactions of fluoroarenes

The meta photocycloaddition of para-fluorotoluene to cyclopentene yields 1-methyl-10-fluoro-endo- and -exo- tetracyclo[6.3.0.0^2,11.0^3,7]undec-9-ene as major photoproducts, while the intramolecular photoaddition of 5-(para-fluorophenyl)pent-1-ene gives exclusively 7-fluorotetracyclo[3.2.1.0^2,8.3^4,8]undec-6-ene. A reaction pathway via zwitterionic intermediates is proposed.     

Competitive intramolecular carbenoid reactions of pyrrole derivatives

Rhodium(II) acetate-catalyzed decomposition of 1-diazo-3-phenyl-4-(pyrrol-1-yl)-butan-2-one and its p-methoxy derivative resulted in their intramolecular cyclization to form the 6-phenyl-5,6-dihydroindolizin-7(8H)-ones and 1-(pyrrol-1-yl)methylindan-3-ones as major and minor products respectively in high yields (77–89%). The p-nitrophenyldiazo compound cyclized exclusively to the 5,6-dihydroindolizin-7(8H)-one in 76% yield.     

Asymmetric induction in intramolecular ene reactions of 1,7-dienes

Chiral, double-activated enophiles in 1,7-dienes 6a - d undergo thermal and Lewis-acid catalyzed intramolecular ene reactions yielding trans-cyclohexanes 7a-d and 8a-d. The extent of diastereoselectivity depends on the reaction temperature. The 1,7-dienes 6 are obtained via Knoevenagel condensation of citronellal 5 with acyclic 1,3-dicarbonyls and analogous compounds 4. Starting with the diene 6e, however, an intramolecular hetero-Diels-Alder reaction takes place.     

Diastereofacial selectivity in intramolecular diels-alder reactions of chiral triene-n-acyloxazolidones

Excellent diasterofacial selection has been observed in asymmetric Diels-Alder cycloadditions of E,E-triene-imides 3 and 6.     

Kinetics of crosslinking reactions. 2. Reaction rates of intermolecular and intramolecular crosslinkings by using a soluble microgel

Soluble microgels with several pendant vinyl groups were synthesised by radical copolymerization of methyl methacrylate (MMA) with p-divinyl benzene (p-DVB). The competitive reactions of intermolecular and intramolecular crosslinkings of these microgels were carried out at 40°C in the presence of 1-buten-3-ol as a degradative chain transfer agent. The rate constant of intermolecular crosslinking (k) was estimated by GPC (gel permeation chromatography) analysis on the polymer produced from intermolecular propagation between bimolecules. The k depended strongly on the internal structure of microgels. Network formation was discussd inclusive of informations for the rate constant of intramolecular crosslinking (k).     

Efficient electrocatalytic intramolecular anion radical cyclobutanation reactions

Electrochemically initiated, intramolecular anion radical cyclobutanations of bis(enones) and related substrates are presented. The formation of novel anion radical Diels-Alder adducts in minor amounts is also observed. Total yields of pericyclic products, which include both cis- and trans-cyclobutanes and a single Diels-Alder adduct, are generally high (51-88%), with electrocatalytic factors in the range of 1.5-5. Mechanistically, strong evidence for the intervention of distonic anion radical intermediates as precursors of both types of pericyclic products is presented. The scope and limitations of these reactions are rather extensively explored and defined, and in particular the tendency, in some cases, for electrogenerated base-catalyzed reactions to compete with these anion radical pericyclic reactions.     

Dynamic salt effect on intramolecular charge-transfer reactions

The dynamic salt effect in charge-transfer reactions is investigated theoretically in this paper. Free-energy surfaces are derived based on a nonequilibrium free-energy functional. Reaction coordinates are clearly defined. The solution of the reaction-diffusion equation leads to a rate constant depending on the time correlation function of the reaction coordinates. The time correlation function of the ion-atmosphere coordinate is derived from the solution of the Debye-Falkenhagen equation. It is shown that the dynamic salt effect plays an important role in controlling the rate of charge-transfer reactions in the narrow-window limit but is balanced by the energetics and the dynamics of the polar-solvent coordinate. The simplest version of the theory is compared with an experiment, and the agreement is fairly good. The theory can also be extended to charge-transfer in the class of electrolytes that has come to be called "ionic fluids."     

beta-Acyloxysulfonyl tethers for intramolecular Diels-Alder cycloaddition reactions

[reaction: see text]. beta-Hydroxy sulfone-based tethers were employed for the first time to achieve thermally mediated intramolecular Diels-Alder cycloaddition. The reactions proceeded with complete regioselectivity and high (10/1) to complete endo/exo-selectivity and resulted in the preferential formation of one of the two possible endo-cycloadducts. The yields and stereoselectivities were proportional to the bulk of the R(1) substituent on the beta-acyloxysulfonyl tether.     

Divergent intramolecular reactions between phosphines and alkynes

A divergent intramolecular reaction of phosphine tethered alkyne in protic solvent was developed.This provided a novel and simple access to a large variety of(Z)-alkenylphosphine oxides and phospholane oxides.Our preliminary studies suggested that these divergent reactions are closely related to the reaction condition and molecular structure.A possible mechanism of C-P bond cleavage of a pentacoordinated hydroxyphosphorane intermediate was proposed.     

Remote control of diastereoselectivity in intramolecular reactions of chiral allylsilanes

During investigations of cyclization reactions between chiral allylsilanes and N-acyliminium ions, it was discovered that a suitably positioned benzyloxy group on the allylsilane component caused a reversal in the diastereoselectivity of these reactions relative to that normally observed with alkyl-substituted allylsilanes. This effect was subsequently observed in two other reaction types. Investigations into this effect led to the proposal of product formation through thermodynamic control facilitated by neighboring group interactions with a transient cationic species. This hypothesis was experimentally supported by the isolation of an intermediate in the proposed mechanistic pathway.     

Asymmetric intramolecular Diels-Alder reactions of N-acyl-camphor-sultam trienes.

Treatment of triene-imides 4 with EtAlCl₂ at ?20° gave crystalline cycloadducts 5 which furnished enantiomerically pure bicyclic alcohols 9 with regeneration of the chiral auxiliary.     

Synthesis and intramolecular reactions of trans-cyclohexyl-1,2-bisacrylate

Photocycloaddition of dimethyl cyclobut-1-ene-1,2-dicarboxylate (1) with cyclohexene (7) afforded two photoadducts 8 and 9 in 44% and 28% yields, respectively. Spontaneous thermal isomerization of 8 gave (4Z,10Z)-dimethyl cyclodeca-4,10-diene-1,4-dicarboxylate (10), which subsequently isomerized to produce trans-1,2-cyclohexanebis-alpha-acrylic acid dimethyl ester 11. Hydride reduction of the bisacrylate 11 gave the trans-octahydro-1H-inden-2-ols 12a and 15 via a novel, stereoselective, intramolecular reaction. Reaction of the bisacrylate 11 with methyllithium afforded the bis-tertiary alcohol 16. In contrast, lithium dimethylcuprate reacted with the bisacrylate 11 to give the trans-hexahydro-1H-inden-2-one 17 in high yield via a novel, stereoselective, intramolecular reaction.     

Siloxacyclopentenes as dienophile-linked directing groups in intramolecular Diels-Alder reactions

The synthesis and intramolecular Diels-Alder reactions of trienes 5 and 6 with a siloxacyclopentene-constrained dienophile are described. These reactions provide the primary cycloadducts 7 and 8 with high diastereoselectivity. These cycloadducts possess trans-relationships between the ring fusion proton and the adjacent C(1) alkoxy group and can be further elaborated to alcohols 9 and 11 (via protiodesilylation) or to 10 and 12 (via Fleming-Tamao oxidation) depending on the substitutent R.     

Synthesis of spirocyclopentene-indolines by intramolecular alkylidine insertion reactions

Intramolecular 1,5 CH insertion of unstabilized carbenes into the tertiary carbon of a substituted indoline generates spiro[cyclopent[2]ene-1,2′-indolines] in good yields and excellent enantioselectivities.