Rodlike molecules and singlet energy transfer(1)(,)(2)

In continuing our investigations on rodlike molecules composed of bicyclo[2.2.2]octane units, we studied the effect of interposing a single aromatic ring in the rod. Thus, two [3]-rods were synthesized with the two outer units being bicyclooctyls, the central unit being benzenoid, and with one terminal unit bearing an alpha-naphthyl moiety and the other terminus bearing an acetyl or benzoyl group. Excitation of the alpha-naphthyl group led to fluorescence emission by both the naphthyl and the acetyl units. However, compared to the [1]- and [2]-rods previously studied, transmission of singlet excitation proved to be less efficient as determined by the fluorescence emission and also by the singlet lifetimes obtained from single photon counting measurement. Transmission to the benzoyl group proved more rapid than to the acetyl moiety. In assessing the factors controlling energy transmission, Delta-density determinations were employed to describe the distribution of electronic excitation in such systems. It was observed that despite most of the energy being located in the terminal chromophores, some is distributed in the bicyclooctyl units. The extent of this distribution provides a guide to the facility of through-bond energy transfer. Evidence is presented that energy transfer in the short rods is mainly through-bond while in the longer rods Forster through-space transfer is involved.     

Diastereoselectivity in the Epoxidation of Substituted Cyclohexenes by Dimethyldioxirane(1)(,)(2)

Three series of compounds based on the cyclohexene framework have been epoxidized by dimethyldioxirane. A pronounced dependence of epoxide diastereoselectivity on substituent has been observed. In addition there is a solvent influence on this stereoselectivity. The results have been explained by invoking steric, H-bonding, and dipole-dipole influences on the epoxide stereochemistry.     

The Intramolecular Aldol Condensation Route to Fused Bi- and Tricyclic beta-Lactams(1)(,)(2)

Staüdinger cycloaddition of activated acid chlorides to 1,3-ketoaldimines, prepared in quantitative yields from 1,3-ketoaldehydes and amino esters, gave in excellent yields cis-2-azetidinones, 6-8, having the adequate functionality to obtain fused bi- and tricyclic beta-lactams. Reaction of compounds 6 with LHMDS at low temperature gave a single diastereomer of fused bicyclic compounds with a carbapenam or carbacefam skeleton. Treatment of diastereomeric cis-2-azetidinones, 7/8, in analogous conditions resulted either in the exclusive cyclization of one of the two diastereomers to form tricyclic [4.n.m] (n = 5, 6; m = 5, 6) compounds, or in the cyclization of both diastereomers to form tricyclic [4.n.7] (n = 5, 6) 2-azetidinones. In all cases the cyclization step was totally stereoselective. Alternatively, trans-carbapenams and one example of a tricyclic system having a trans-2-azetidinone ring have been obtained by using longer reaction times and higher temperatures. Epimerization at C3 of the 2-azetidinone nucleus occurs in these reaction conditions to obtain a single diastereomer of the final products. This approach to fused policyclic 2-azetidinones is one of the scarce syntheses of this kind of compound making use of the aldol condensation.     

Solution and crystal lattice effects on the photochemistry of 6-substituted cyclohexenones(1)(,)(2)

The photochemistry of 13 4,4-diphenylcyclohexenones, substituted at carbon-6, was investigated in solution and in the crystalline state. The stereoselectivity was of particular interest. In the solution photochemistry of C-6 monosubstituted enones in benzene, there was a unique preference for migration of the cis-phenyl group with formation of bicyclo[3.1.0]hexanone photoproducts, with the original 6-substituent having an endo configuration at carbon-3 of the product. In methanol the reaction was diverted to afford 3,4-diphenylcyclohex-2-enes understood as arising from a hydrogen-bonded zwitterionic intermediate. The solid-state photochemistry was also investigated. There was a dramatic absence of the 3,4-diphenylcyclohex-2-ene products in accord with the absence of the hydrogen bonding encountered in methanol. Further, the solid-state reactivity correlated with a vector analysis using X-ray atomic coordinates. This established that the migrating phenyl group required an orientation facing the enone beta-carbon. While the interesting preference for the cis-endo migration was not intuitively predicted, ab initio computations on the alternative phenyl-bridged triplet intermediates did lead to an understanding of the selectivity.     

Heteroaromatic carbenes (review)

We correlate information on the chemistry of heteroaromatic carbenes. We consider methods for generation of electrophilic and nucleophilic heteroaromatic carbenes, their reaction pathways, and their role in organic synthesis.Translated from Khimiya Geterosiklicheskikh Soedinenii, No. 9, pp. 1155–1170, September, 1992.     

The mechanism of H2 activation by (amino)carbenes

We have computationally investigated the mechanism of the H(2) activation reaction by (amino)carbenes compounds. Describing the electronic activity taking place during the reaction through the Reaction Electronic Flux, it has been possible to elucidate the mechanism of the hydrogen activation process and assign the energetic cost associated to every chemical event that drives the process along the reaction coordinate; this is crucial information to rationalize the reported experimental results. It has been observed that the substituent effect may induce early charge-transfer phenomena that increases the energy barrier and lowers the exothermicity of the reaction. Reversibility of the process is discussed in light of specific interactions defining the components of the reverse activation energy.     

Armed/Disarmed Effects and Adamantyl Expansion of Some Caged Tricyclic Acetals en Route to Tetrodotoxin(1)(,)(2)

Transformation of the previously prepared tricyclic ketone 4 into an advanced intermediate, 2a, of the Kishi-Goto synthesis of tetrodotoxin requires, among other things, cleavage of the internal acetal. In our attempts to carry this out, we were confronted by two major obstacles, one resulting from armed/disarmed effects encountered during acid-catalyzed acetolyses. Thus ester protecting groups proximal to the acetal moiety inhibited cleavage, e.g., 4 --> 8a and 7 --> 8b. Although the corresponding ether analogs 9a and 9b did undergo acetolysis, the products obtained, 10a and 10b, respectively, revealed the second obstacle, namely the proclivity of the caged systems to undergo adamantyl expansion. The latter result was found to depend upon the presence of properly positioned nucleophilic substituents. Thus 11b underwent adamantyl expansion to 12b but its C7 epimer 15 experienced facile cleavage to bicyclic product 16. As an alternative to solvolysis for cleavage of the internal acetal, reductive elimination was examined. For example, compound 28a, obtained from 4 by standard procedures, reacted with zinc to give, after protection and saponification, gamma,delta unsaturated carboxylic acid 29a, which underwent smooth iodolactonization. Replacing the iodide of this product with an hydroxyl (32a --> 32b) by a free radical process has succeeded albeit in disappointing yield. Nevertheless the resulting hydroxy lactone is a promising synthon of the advanced Kishi-Goto intermediate.     

Persistent (amino)(silyl)carbenes

(Amino)(silyl)carbenes, prepared by substitution reactions at a carbene center, can survive several days at 0 degrees C. These species are not push-pull carbenes as their phosphino analogues and therefore are excellent ligands for transition-metal centers.     

Reactions of Quino‐Ketenes with C‐Nucleophiles: Syntheses of (2‐Hydroxyphenyl)methanols and 2‐Hydroxyphenyl Ketones

In this study, we have developed a new, efficient, and one‐step protocol for the synthesis of derivatives of 2,4‐dihydroxyphenyl ketones and/or 2,4‐dihydroxyphenyl/naphthalenyl methanols starting from benzodioxinones/naphthodioxinones and organolithium compounds. Reactions have been performed under thermal conditions without catalyst. This work is the first example of the preparation of these products starting from 2‐hydroxy acids.     

Iridium(III)-induced isomerization of 2-substituted pyridines to N-heterocyclic carbenes

Steric strains (F-strain or front-strain) in the acid-base interaction between the Lewis acid [TpMe2Ir(C6H5)2] and 2-substituted pyridines Lewis bases, are responsible for the stabilization of the C-adducts (2) of the respective pyridine carbene tautomers over the expected N-adducts.     

邻羟基苯基(乙炔基)次膦酸酯的合成方法

邻羟基(乙炔基)次膦酸酯是一个多官能团的有用中间体,本文提出了此化合物的有效而切实可行的合成方法     

Substituted 2-norbornyl carbenes

A series of endo-6-substituted bicyclo[2.2.1]hept-2-yl carbenes have been generated by pyrolysis of the sodium salt of the tosylhydrazone precursors. The endo-6-trimethylsilyl and endo-6-thiomethoxy substituted carbenes give 1,3-migration of these substituents to the carbene center. However groups such as methyl, CH₂SiMe₃, phenyl, and methoxy are ineffective 1,3-migrating groups. The facile 1,3-migration of trimethylsilyl and thiomethoxy has been rationalized in terms of stabilized transition states where the migrating group interacts effectively with the carbene vacant orbital. In the cases of endo-6-thiomethoxy and methoxy derivatives, the carbene also inserts effectively into the methyl group of the substituent. Heteroatom stabilization of the transition state for C-H insertion facilitates these processes. Certain exo-6-substituted bicyclo[2.2.1]hept-2-yl carbenes have also been generated. While the exo-6-thiomethoxy and carbomethoxy systems give exclusive 1,3-hydrogen migration, the exo-6-sulfonyl derivative gives, in addition to 1,3-hydrogen migration, an alkene product derived from 1,2-hydrogen migration. The lowered propensity for 1,3-hydrogen migration is attributed to the strong electron-withdrawing sulfonyl group, which decreases the hydridic 1,3-interaction with the carbene vacant orbital.     

Reaction of heteroaromatic compounds with carbenes (review)

The literature data on the reactions of heteroaromatic compounds of various classes with halo-, alkyl-, alkoxy-, and ketocarbenes are examined. The mechanisms of these reactions and the effect of the methods used to generate the carbenes and the nature of the heterocycle on the direction of the reaction and the structures and yields of the final products are discussed. An estimate of the possibilities of the preparative utilization of the indicated reactions is given.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 723–743, June, 1982.     

Rhenium(V) oxo complexes with N-heterocyclic carbenes

Air-stable rhenium(V) oxo complexes are formed when [ReOCl(3)(PPh(3))(2)] is treated with N-heterocyclic carbenes of the 1,3-dialkyl-4,5-dimethylimidazol-2-ylidene type, L(R) (R = Me, Et, i-Pr). Complexes of the compositions [ReO(2)(L(R))(4)](+), [ReOCl(L(R))(4)](2+), or [ReO(OMe)(L(R))(4)](2+) can be isolated depending on the alkyl substituents at the nitrogen atoms of the ligands and the reaction conditions applied. Despite the steric overcrowding of the equatorial coordination spheres of the metal atoms by each of the four carbene ligands, stable complexes with six-coordinate rhenium atoms are obtained. Steric demands of the alkyl groups allow control of the stability of the mono-oxo intermediates. Air-stable cationic complexes of the compositions [ReOCl(L(Me))(4)](2+), [ReOCl(L(Et))(4)](2+), and [ReO(OMe)(L(Me))(4)](2+) have been isolated, whereas reactions of [ReOCl(3)(PPh(3))(2)] or other rhenium(V) precursors with the more bulky 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (L(i)(-)(Pr)) directly yield the dioxo complex [ReO(2)(L(i)(-)(Pr))(4)](+). X-ray structures of [ReO(2)(L(i)(-)(Pr))(4)][ReO(4)], [ReO(2)(L(i)(-)(Pr))(4)][PF(6)], [ReO(2)(L(Me))(4)][ReO(4)](0.45)[PF(6)](0.55), [ReO(MeOH)(L(Me))(4)][PF(6)](2), and [ReOCl(L(Et))(4)][PF(6)](2) show that the equatorial coordination spheres of the rhenium atoms are essentially planar irrespective of the steric demands of the individual carbene ligands.     

Stable non-N-heterocyclic carbenes (non-NHC): recent progress

This account summarizes the results that have been obtained by our group since 2000 in the area of stable singlet acyclic carbenes. It includes aryl- and alkyl-(phosphino)carbenes, aryl(amino)carbenes, and (amino)(phosphino)carbenes. Our most recent achievements, the transformation of stable carbenes into other stable carbenes are also discussed, along with preliminary results concerning the ligand properties of these species.