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Femtosecond laser flash photolysis of 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene)tetrahydrofuran produces singlet 3-oxacyclopentylidenecarbene which reacts with acetonitrile solvent to form an ylide. This is the first direct detection of ylide formation by an alkylidenecarbene. This new type of ylide was observed to have a broad absorption band in the visible region with λmax ∼450 nm and a lifetime of ∼13.5 ps. As with other "conventional" carbenes (the divalent carbon atom is separately bound to two substituents), this ylide formation method could be also useful for detecting alkylidenecarbenes, especially those that do not absorb at wavelengths suitable for direct observation. Furthermore, the mechanisms by which 3-oxacyclopentylidenecarbene forms the ylide and the overall favorability of ylide formation, vis-a-vis ring expansion of the carbene to strained 3-oxacyclohexyne, were supported by results from density functional theory calculations.

A synthesis of the 2-azatricyclo[4.3.2.04,9]undecane ring system—a hitherto unreported bridged azatricyclic ring system—beginning from tricarbonyl(tropone)iron and allylamine was accomplished in three steps: (1) aza-Michael addition of allylamine to tricarbonyl(tropone)iron; (2) Boc-protection of the resulting secondary amine; and (3) oxidative demetallation leading to a spontaneous intramolecular Diels–Alder reaction. The effect of a variety of parameters on the intramolecular Diels–Alder reaction was investigated, including diene and dienophile substitution patterns and dienophile tether length.

A multifaceted experiment, accessible to students either in an introductory organic chemistry course or in a more advanced laboratory setting, is described for investigating the addition of dichlorocarbene to E- and Z-β-methylstyrenes. The innovative exercise, which integrates elements of synthesis, mechanisms, instrumental analysis, computational chemistry, and collaboration, provides students with an opportunity to learn important laboratory skills while highlighting key concepts covered in class. In particular, the coupling constants from 1H NMR spectroscopy are used to diagnose the stereochemistry of addition, and integral ratios are evaluated to determine percent conversion of the starting alkenes into dichlorocyclopropanes. The experimental observations are correlated with results from calculations of the singlet–triplet energy gap in the carbene.

Singlet α-methylbenzylidenecarbene has been detected and characterized for the first time by femtosecond transient absorption (fs-TA) spectroscopy. The carbene was generated by photolysis of the hydrocarbon precursor, 1-(1-phenylethylidene)-1a,9b-dihydro-1 H-cyclopropa[ l]-phenanthrene, in acetonitrile. The photolysis initially forms the singlet excited state of the precursor which then extrudes α-methylbenzylidenecarbene in 4.0 ps. The decay of α-methylbenzylidenecarbene, which was previously shown to rearrange into phenylpropyne by a 1,2-phenyl shift, occurred over 13.3 ps. Computed spectra at the TD-B3LYP/6-311+G** level of theory are consistent with the experimental observations. CASSCF(10,10)/6-311+G** calculations, using one of the carbene conformers as a model, indicated that the reference wave function is dominated by a closed-shell description.

The synthesis of stemofurans C, L and T was achieved using organomanganese arene complexes. The critical carbon-carbon bond between the C-2 position of benzofuran and the arene was established in a regioselective manner directed by the cationic manganese tricarbonyl moiety. Oxidation of the resulting dienyl complexes and cleavage of the methyl ethers gave the desired stemofuran products. The spectroscopic data for the originally proposed structure for stemofuran L did not match the synthesized material, which prompted the synthesis of an isomer that did match the spectroscopic data. This revised structure has been proposed as the correct structure for stemofuran L.

Photolysis of 1-(1-phenylethylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, in C6H6 (or C6D6), at ambient temperature, produces (α-methylbenzylidene)carbene which undergoes a facile Fritsch-Buttenberg-Wiechell (FBW)-type rearrangement to 1-phenylpropyne. The alkyne results exclusively from a 1,2-phenyl shift as evident from the use of a (13)C-labeled precursor. This experimental result is consistent with CCSD(T)/cc-pVTZ//B3LYP/6-31+G* calculations which reveal that a 1,2-phenyl shift in the singlet carbene needs to overcome a barrier of only 3.8 kcal/mol whereas the 1,2-methyl shift has to surmount a much larger barrier of 11.9 kcal/mol. The alkyne remains the predominant product when the photolysis is carried out in cyclohexene but the carbene-alkene cycloadduct could be detected, albeit in low yield, in the photolysate.

The Diels–Alder reaction of γ-hydroxybutenolides with dienes gave good yields of cycloadducts under thermal and Lewis acid catalyzed conditions. The application of this methodology to a more complex system was demonstrated by the synthesis of a model system for the tricyclic himbacine core. The stereo- and regioselective Diels–Alder reaction established three of the stereogenic centers, with the fourth stereogenic center secured by diastereoselective alkylation of the cycloadduct.

Cophotolysis of noradamantyldiazirine with the phenanthride precursor of dichlorocarbene or phenylchlorodiazirine in pentane at room temperature produces noradamantylethylenes in 11% yield with slight diastereoselectivity. Cophotolysis of adamantyldiazirine with phenylchlorodiazirine in pentane at room temperature generates adamantylethylenes in 6% yield with no diastereoselectivity. (1)H NMR showed the reaction of noradamantyldiazirine + phenylchlorodiazirine to be independent of solvent, and the rate of noradamantyldiazirine consumption correlated with the rate of ethylene formation. Laser flash photolysis showed that reaction of phenylchlorocarbene + adamantene was independent of adamantene concentration. The reaction of phenylchlorocarbene + homoadamantene produces the ethylene products with k = 9.6 × 10(5) M(-1) s(-1). Calculations at the UB3LYP/6-31+G(d,p) and UM062X/6-31+G(d,p)//UB3LYP/6-31+G(d,p) levels show the formation of exocyclic ethylenes to proceed (a) on the singlet surface via stepwise addition of phenylchlorocarbene (PhCCl) to bridgehead alkenes adamantene and homoadamantene, respectively, producing an intermediate singlet diradical in each case, or (b) via addition of PhCCl to the diazo analogues of noradamantyl- and adamantyldiazirine. Preliminary direct dynamics calculations on adamantene + PhCCl show a high degree of recrossing (68%), indicative of a flat transition state surface. Overall, 9% of the total trajectories formed noradamantylethylene product, each proceeding via the computed singlet diradical.

Previous literature reports have shown that single crystals of 1,2-bis(9-anthryl)acetylene (1), in which the molecule is essentially planar, can be fabricated as organic field effect transistors. We have identified a new conformational polymorph of 1 in which the two anthracene moieties are twisted out of plane by more than 60°. Occasionally, both the planar (1α) and twisted (1β) polymorphic forms crystallize out together representing a rare example of concomitant conformational polymorphism in a pure hydrocarbon. The two polymorphic forms also show distinctly different colours and morphologies – the planar form (1α) grows as long orange-red needles whereas the twisted form (1β) appears as yellow blocks. Furthermore, a variety of electronic structure-based calculations show that the planar molecule forms the more stable polymorph (1α), despite the conformation being close to the maximum in conformational energy. This may be attributed to 1α being stabilized by greater intermolecular dispersion forces as the planar conformation allows a denser packing. The new polymorph (1β) is a long-lived metastable structure, because of the marked difference in crystal packing. Since the conformation in 1β is more stable in isolation, and probably in solution state, this discovery raises questions about the control of crystallization of anthracene-based functional materials and the assumptions about conformational polymoprhism commonly made in crystal structure prediction studies.

The hydrocarbon, 1-(propan-2-ylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, affords dimethylvinylidene upon photolysis (∼315–400 nm) at ambient temperature, and the carbene may be trapped by cyclohexene. Some of the precursor also rearranged to a non-photolabile isomer via a putative 1,5-sigmatropic shift followed by an electrocyclic ring opening.

The strained heterocyclic alkyne, 3-oxacyclohexyne, was generated photochemically for the first time using a cyclopropanated phenanthrene precursor, and trapped by cyclopentadienones as Diels–Alder adducts. The precursor initially produced the putative 3-oxacyclopentylidenecarbene that subsequently rearranged to the cycloalkyne. Computational studies indicate that the carbene favors a singlet state, and the barrier for its ring expansion by a 1,2-shift of the carbon proximal to oxygen is lower in energy than the corresponding shift of the distal carbon.

The hydrocarbons 1-cyclopentylidene-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene and 1-cyclobutylidene-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene undergo photolysis in solution at ambient temperature to produce cyclohexyne and cyclopentyne, respectively. These strained cycloalkynes, formed via the putative cycloalkylidenecarbenes, were intercepted as Diels–Alder adducts. Calculations at the CCSD(T)/cc-pVTZ//B3LYP/6-31+G* level of theory show that singlet cyclopentylidenecarbene has to overcome a barrier of 9.1 kcal mol−1 to rearrange into cyclohexyne (with ΔE for ring expansion=−15.1 kcal mol−1). By contrast, cyclobutylidenecarbene only needs to surmount a barrier of 1.6 kcal mol−1 to rearrange into cyclopentyne (with ΔE for ring expansion=−6.2 kcal mol−1).

Photolysis of exo-1-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-yl)cyclopropan-1-ol and exo-1-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-yl)cyclobutan-1-ol in benzene-d6 produces (1-hydroxycyclopropyl)- and (1-hydroxycyclobutyl)carbene respectively. It was observed that (1-hydroxycyclopropyl)carbene rearranges to cyclobutanone whereas (1-hydroxycyclobutyl)carbene forms cyclopentanone. Formation of both ketones is attributed to tautomerization of the corresponding enols that arise from ring expansion of the carbenes. Products assignable to intramolecular C–H insertions were not detected in the photolysates.

Several naphthalene compounds containing a methyl group in a 1,8-relationship to a metal-complexed phenyl ring bearing various substituents have been synthesized. The through-space shielding effects of the phenyl ring, as a function of substituent and complexing metal species, were monitored by observing the 1H NMR signal of the methyl group located in the shielding zone of the ring. In all cases, the methyl signal was slightly more downfield in the complexes than in the uncomplexed analogues. A comparison of available crystal structures, however, shows that the phenyl ring is slightly closer to the naphthyl methyl group in the complexes than in their metal-free counterparts. X-ray structures and DFT calculations also reveal a slight elongation in the average length of the carbon–carbon bonds of the phenyl ring upon complexation. The effect of substituents on the signal of the naphthyl methyl group is small but discernible in the uncomplexed derivatives, and consistent with our previous report. A similar ...

Cassiferaldehyde, a recently discovered naturally occurring tyrosinase inhibitor, and six of its analogs, in which the aldehyde group has been replaced by various other functionalities, have been synthesized by a short and simple sequence starting from 2,3-dihydroxybenzaldehyde. Single-crystal x-ray structures of cassiferaldehyde as well as its methyl ketone, nitrile, and carboxylic acid analogs are reported.

Photolysis of exo-2-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-yl)propan-2-ol in benzene-d(6) afforded phenanthrene and the beta-hydroxycarbene intermediate 2-hydroxy-2-methylpropylidene. The carbene showed an overwhelming preference for 1,2-methyl migration as evident from the formation of 2-butanone as the major product via the enol 2-hydroxy-2-butene. Also produced, albeit in smaller amounts, were 1-methylcyclopropanol and 2,2-dimethyloxirane from intramolecular insertion into the C-H and O-H bonds, respectively. These results stand in sharp contrast to the intramolecular reactions of simple alkylcarbenes which usually prefer insertion into C-H bonds over 1,2-alkyl migrations. Calculations at the B3LYP/6-311+G//B3LYP/6-31G level of theory give a lower activation barrier for 1,2-methyl migration leading to the eventual formation of 2-butanone than for the other two pathways. The lower activation energy for methyl migration, relative to C-H and O-H insertions, strongly supports the observed experimental product distribution of the carbene. The parent carbene exists in three distinct conformations, each with stabilizing interactions between the adjacent bonds and the empty p orbital and the filled sp(2) orbital of the carbene center. The most stable conformer is perfectly poised for a 1,2-methyl migration as the C-CH(3) group is involved in a hyperconjugative interaction with the empty p orbital and the O-H bond is simultaneously interacting with the sp(2) lone pair of the carbene.

Controlled thermal decomposition of the sodium salt of cyclopropyl methyl ketone tosylhydrazone gives 1-methylcyclobutene of excellent purity in good yield.

1-Methylcyclopropylcarbene, generated by photolysis of two isomeric hydrocarbon precursors, undergoes ring-expansion readily to give 1-methylcyclobutene. Experimentally, intramolecular carbon−hydrogen insertions are not observed. Trapping studies with TME demonstrates the formation of the expected cyclopropane adduct, and via a double-reciprocal analysis, the lifetime of 1-methylcyclopropylcarbene was determined to be 12 ns in 1,1,2-trichlorotrifluoroethane. Computational studies show that the barrier to ring-expansion is significantly smaller in 1-methylcyclopropylcarbene than in cyclopropylcarbene. The origin of the increased rate of ring-expansion is due to stabilization of the positive charge that occurs at the incipient tertiary carbon that is attached to the migrating carbon center.

Reactions of arc-generated carbon atoms with acyl chlorides proceed by two distinct mechanistic pathways depending on the nature of the alkyl group in the substrate. Acetyl chloride affords vinyl chloride from the putative chloromethylcarbene produced by deoxygenation, whereas pivaloyl chloride gives t-butyl chloride as the predominant product via a chain reaction from the initially generated pivaloyl radical. When the alkyl group is isopropyl, both pathways are implicated.

Benzylidenecarbene was generated from a new photochemical source, 1-benzylidene-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, in deuterated benzene at ambient temperature. The carbene undergoes a facile rearrangement to phenylacetylene and could not be trapped by olefins. Generation of the carbene bearing a (13)C label at the β-carbon produced phenylacetylene in which the label was found exclusively at the carbon adjacent to the phenyl ring. This overwhelming preference for H shift is consistent with B3LYP and CCSD(T) calculations. The label distribution observed in this work, however, contrasts previously reported high-temperature flash vacuum pyrolysis results where the interconversion of carbene and alkyne leads to the scrambling of labels over both alkynyl (sp) carbons.

A series of naphthalene derivatives, bearing a methyl group and a substituted phenyl ring in a 1,8-relationship, have been synthesized. The chemical shifts of the protons of the methyl group, which are pointed toward the shielding zone of the phenyl ring, were monitored as the phenyl substituents were varied. This work indicates that the shielding effect of the phenyl ring is not so severely altered by the substituents as to significantly influence the chemical shift of the methyl group. Nonetheless, within the small changes observed experimentally, there appears to be a tendency for electron-withdrawing X to shift the methyl signal downfield, whereas electron-donating X-groups cause a more upfield shift. Polarization and field effects are discussed as possible causes for this phenomenon. Chemical shifts computed for selected members of the series, using the recently published procedures of Rablen and Bally, are in agreement with the experimentally observed trends.

Montiporynes A and B, that were among the recently isolated diacetylenic ketones from the stony coral Montipora sp., and reported to possess in vitro cytotoxic activity against several human solid tumor cells, have been synthesized in three simple steps from readily available materials.

The rearrangement of 2-ethoxyethylidene, generated photochemically from a nonnitrogenous precursor, leads to ethyl vinyl ether. Although this product could result, in principle, from a 1,2-hydrogen shift and/or a 1,2-ethoxy shift in the carbene, a deuterium labeling study indicates an essentially exclusive preference for hydrogen migration. The experimental results are in agreement with CCSD and W1BD calculations for the simpler 2-methoxyethylidene system that show a prohibitively large barrier for the methoxy shift and a negligible barrier for the hydride shift.

Controlled thermal decomposition of the sodium salt of cyclopropyl methyl ketone tosylhydrazone gives 1-methylcyclobutene of excellent purity in good yield.

1-Methylcyclopropylcarbene, generated by photolysis of two isomeric hydrocarbon precursors, undergoes ring-expansion readily to give 1-methylcyclobutene. Experimentally, intramolecular carbon−hydrogen insertions are not observed. Trapping studies with TME demonstrates the formation of the expected cyclopropane adduct, and via a double-reciprocal analysis, the lifetime of 1-methylcyclopropylcarbene was determined to be 12 ns in 1,1,2-trichlorotrifluoroethane. Computational studies show that the barrier to ring-expansion is significantly smaller in 1-methylcyclopropylcarbene than in cyclopropylcarbene. The origin of the increased rate of ring-expansion is due to stabilization of the positive charge that occurs at the incipient tertiary carbon that is attached to the migrating carbon center.

Montiporynes A and B, that were among the recently isolated diacetylenic ketones from the stony coral Montipora sp., and reported to possess in vitro cytotoxic activity against several human solid tumor cells, have been synthesized in three simple steps from readily available materials.

In preparation for the International Geophysical Year (IGY), the British Antarctic Survey (BAS) of the Royal Society set up a research station near Halley Bay (named in honor of noted astronomer Edmond Halley). During the IGY, the Halley Bay station played a critical role in discovering the polar vortex above the Antarctic and, nearly 30 years later, provided important evidence for massive loss of stratospheric ozone within the vortex that could be attributed to the release of anthropogenic chlorofluorocarbons (CFCs) into the atmosphere. These findings provided the experimental confirmation for the theoretical predictions made in the mid-1970s by F. Sherwood Rowland and his colleague, Mario Molina. Discovery of this “ozone hole” in 1985, made possible by the Dobson spectrometers set up during the IGY, led to the swift enactment of the important intergovernmental treaty brokered by the United Nations, known as the Montreal Protocol in 1987. The Montreal Protocol initially called for a 50 percent reduction in CFC production by the end of the twentieth century, but subsequent meetings set up an accelerated schedule for major industrialized nations to phase out these products completely by 1996.

The low-temperature treatment of 1,1-dibromo-1a,9b-cyclopropa[l]phenanthrene (1) with butyllithium and copper(II) chloride in THF affords a dibenzoannellated 1,2,4,6-cycloheptatetraene which undergoes a rare cyclotetramerization. The crystal structure of this "formal" 2 + 2 + 2 + 2 cyclotetramer (2) reveals a central eight-membered ring folded in a zigzag fashion with hydrogen atoms and exocyclic double bonds occupying axial positions. B3LYP/6-31+G** calculations indicate that the strained cyclic allene is significantly distorted and could be formed by ring expansion of a putative cyclopropylidene intermediate.

An improved procedure for preparing the title compound, resulting in substantially higher yields under simpler conditions, is described.

The diepoxide mycotoxin (2R, 3R, 8R, 9R)-4,6-decadiyne-2,3:8,9-diepoxy-1,10-diol (repandiol) was both isolated from the mushroom Hydnum repandum and synthesized de novo. Repandiol was found to form interstrand cross-links within a restriction fragment of DNA, linking deoxyguanosines on opposite strands primarily within the 5′-GNC and 5′-GNNC sequences preferred by diepoxyoctane. However, repandiol was a significantly less efficient cross-linker than either of the diepoxyalkanes (diepoxyoctane and diepoxybutane) to which it was compared.

An improved procedure for preparing the title compound, resulting in substantially higher yields under simpler conditions, is described.

The absolute rate constant of reaction of dichlorocarbene with pyridine and with tetramethylethylene was determined in Freon-113 (CF 2 ClCFCl 2 ), dioxane, and tetrahydrofuran. No evidence of specific solvation was obtained.

The action of excess DDQ on 1,4,5,8-tetrahydro-4a,8a-methanonaphthalenes bearing a methyl group and one other substituent (cyclopropyl, methyl, phenyl) at the 9 position resulted in a peculiar oxidative rearrangement to 1-vinylnaphthalene derivatives. Two closely related mechanisms, both of which involve a 1,5-sigmatropic shift of the one-carbon bridge (Berson-Willcott rearrangement), are considered for this novel process.

The synthesis of ethyl trans-cinnamate, by the reaction of benzaldehyde with the stabilized phosphorus ylid (carbethoxymethylene)triphenylphosphorane, serves as a useful experiment to illustrate the Wittig reaction in the introductory organic chemistry laboratory. The reaction is highly stereoselective and affords the product in excellent yield and purity. The stereochemistry of the product is established by examining the coupling constants displayed by the olefinic protons in the 1H NMR spectrum. The experiment highlights several important aspects of organic chemistry such as synthesis, stereochemistry, instrumental analysis, and molecular modeling.

The diastereoselective synthesis of ethyl (E)-3-methyl-3-phenylglycidate, a strawberry flavoring agent, is carried out by epoxidizing ethyl trans-β-methylcinnamate with m-chloroperbenzoic acid. This epoxidation is appropriate for the introductory organic laboratory and augments the small number of such experiments currently available for undergraduate education. In the course of performing this exercise, students are exposed to many important facets of organic chemistry such as synthesis, reaction mechanism, stereochemistry, chromatography, quantitative analysis, spectroscopy, and computational chemistry. The 1H NMR spectrum of this compound is especially interesting and presents instructive examples of diastereotopic protons and shielding effects of the aromatic ring current.