Photochemical Generation of Allenylidenes from Cyclopropanated Phenanthrenes: An Experimental and Computational Study.

To address the scarcity of generally applicable photochemical routes to allenylidenes in solution, phenanthrene-based sources have been investigated. Specifically, the syntheses of 1-vinylidene-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, 1-(2-phenylvinylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, and 1-(2-methylvinylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, photochemical precursors to propadienylidene, 3-phenylpropadienylidene, and 3-methylpropadienylidene have been carried out. Photolysis of these new precursors in olefin traps and benzene afforded the expected cyclopropane adducts of the corresponding allenylidenes. Quantum chemical calculations show that the ground state of all three carbenes is a singlet with a singlet-triplet gap of ∼29, 30, and 33 kcal/mol for propadienylidene, 3-phenylpropadienylidene, and 3-methylpropadienylidene, respectively.

Adamantylidenecarbene: Photochemical Generation, Trapping, and Theoretical Studies.

Photolysis of 1-(2-adamantylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene in benzene (or benzene-d6) at ambient temperature produces adamantylidenecarbene. The carbene undergoes dimerization to a cumulene and may also be trapped in a stereospecific fashion by cis- and trans-4-methyl-2-pentene. No products attributable to 4-homoadamantyne, resulting from ring expansion of the carbene, could be detected. Coupled cluster/density functional theory calculations place the singlet carbene ∼49 kcal/mol below the triplet and show that the former must overcome a barrier of ∼13.5 kcal/mol to rearrange into 4-homoadamantyne.

Carbenes from cyclopropanated aromatics.

Although a ripe old discipline by now, carbene chemistry continues to flourish as both theorists and experimentalists have shown sustained interest in this area of research. While there are numerous ways of generating carbenes, the thermal and/or photochemical decomposition of diazo compounds and diazirines remains, by far, the most commonly used method of producing these intermediates. There is no disputing the fact that these nitrogenous precursors have served carbene researchers well, but their use is not without problems. They are often sensitive and hazardous to handle and, sometimes, the desired nitrogenous precursor simply may not be available, e.g., for synthetic reasons, to study the particular carbene of interest. Furthermore, there is a legitimate concern that the photochemical generation of carbenes in solution from diazo compounds and diazirines may be contaminated by reactions in the excited states (RIES) of the precursors themselves. As an alternative, several laboratories, including ours, have used cyclopropanated aromatic systems to generate a wide range of carbenes. In each case, the cheleotropic extrusion of carbenes is accompanied by the formation of stable aromatic by-products such as phenanthrene, indane, naphthalene, and 1,4-dihydronaphthalene. The emergence of these "non-traditional" carbene sources, their versatility, and promise are reviewed in this work.

Detection of Ylide Formation between an Alkylidenecarbene and Acetonitrile by Femtosecond Transient Absorption Spectroscopy.

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-à-vis ring expansion of the carbene to strained 3-oxacyclohexyne, were supported by results from density functional theory calculations.

Synthetic Entry to the 2-Azatricyclo[4.3.2.04,9]undecane Ring System via Tropone.

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.

Ring Expansion of Alkylidenecarbenes Derived from Lactams, Lactones, and Thiolactones into Strained Heterocyclic Alkynes: A Theoretical Study.

Strained cycloalkynes are of considerable interest to theoreticians and experimentalists, and possess much synthetic value as well. Herein, a series of cyclic alkylidenecarbenes-formally obtained by replacing the carbonyl oxygen of four-, five-, and six-membered lactams, lactones, and thiolactones with a divalent carbon-were modeled at the CCSD(T)/cc-pVTZ//B3LYP/6-311+G** and CCSD(T)/cc-pVTZ//CCSD/6-311+G** levels of theory. The singlet carbenes were found to be more stable than the triplets. The strained heterocyclic alkynes formed by ring expansion of these singlet carbenes were also modeled. Interestingly, the C≡C bonds in the five-membered heterocycles, obtained from the rearrangement of ß-lactam- and ß-lactone-derived alkylidenecarbenes, displayed lengths intermediate between formal double and triple bonds. Furthermore, 2-(1-azacyclobutylidene)carbene was found to be nearly isoenergetic with its ring-expanded isomer, and 1-oxacyclopent-2-yne was notably higher in energy than its precursor carbene. In all other cases, the cycloalkynes were lower in energy than the corresponding carbenes. The transition states for ring-expansion were always lower for the 1,2-carbon shifts than for 1,2-nitrogen or oxygen shifts, but higher than for the 1,2-sulfur shifts. These predictions should be verifiable using carbenes bearing appropriate isotopic labels. Computed vibrational spectra for the carbenes, and their ring-expanded isomers, are presented and could be of value to matrix isolation experiments.

Direct Observation of an Alkylidenecarbene by Ultrafast Transient Absorption Spectroscopy.

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.

Photochemical generation and trapping of 3-oxacyclohexyne.

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.

Photochemical Generation of Strained Cycloalkynes from Methylenecyclopropanes.

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 ).

An Experimental and Computational Investigation of (α-Methylbenzylidene)carbene.

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.

Through-Space Shielding Effects of Metal-Complexed Phenyl Rings.

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 trend is absent in the corresponding metal complexes, as exemplified by the chromium series, and the effect of the metal appears to be more dominant than that of the substituents. These observations were found to be in line with NMR shift calculations.

Experimental and computational mechanistic investigation of chlorocarbene additions to bridgehead carbene-anti-Bredt systems: noradamantylcarbene-adamantene and adamantylcarbene-homoadamantene.

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.

Formation and Rearrangement of a Congested Spiropentane from the Trapping of Dibenzonorcarynyliden(e/oid) by Phencyclone.

The low-temperature treatment of 1,1-dibromo-1a,9b-cyclopropa[l]phenanthrene with butyllithium appeared to produce dibenzonorcarynyliden(e/oid) which could be intercepted with phencyclone to produce a hindered spiropentane. The spiropentane readily rearranges, thermally and photochemically, into a triphenylene phenol derivative. The spiropentane and its rearrangement product were characterized by X-ray crystallography.

Formation of 3-Oxa- and 3-Thiacyclohexyne from Ring Expansion of Heterocyclic Alkylidene Carbenes: A Mechanistic Study.

The rearrangement pathways of two alkylidene carbenes appended to an oxa or thiacyclopentane into the corresponding heterocyclohexynes were elucidated using 13C-labeling experiments. Both carbenes exhibited a preference for migration of the allylic carbon bound to the heteroatom. Anomeric interactions involving a heteroatom lone pair and antibonding orbital of the migrating bond and inductive destabilization of the minor migratory pathway are discussed as plausible reasons for the observed trends.

The benzylidenecarbene-phenylacetylene rearrangement: an experimental and computational study.

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 ß-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.

Influence of substituents on the through-space shielding of aromatic rings.

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.

Experimental and theoretical study of the 2-alkoxyethylidene rearrangement.

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.

Crystal structure of 4,5-di-bromo-phenanthrene.

The synthesis and crystal structure of the title compound, C14H8Br2, is described. The mol-ecule is positioned on a twofold rotation axis and the asymmetric unit consists of half a mol-ecule with the other half being generated by symmetry. The presence of two large bromine atoms in the bay region significantly distorts the mol-ecule from planarity and the mean planes of the two terminal rings of the phenanthrene system are twisted away from each other by 28.51 (14)°. The torsion angle between the two C-Br bonds is 74.70 (14)° and the distance between the two Br atoms is 3.2777 (13) Å. The mol-ecules pack in layers in the crystal, with the centroids of the central rings of the phenanthrene units in adjacent layers separated by a distance of 4.0287 (10) Å. These centroids are shifted by 2.266 (6) Šrelative to each other, indicating slippage in the stacking arrangement. Furthermore, the distance between the centroids of the terminal and central rings of the phenanthrene units in adjacent layers is slightly shorter at 3.7533 (19) Å. While all of the mol-ecules within each layer are oriented in the same direction, those in adjacent layers are oriented in the opposite direction, leading to anti-parallel stacks.

Crystal structure of a cyclotetramer from a strained cyclic allene.

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.

Rearrangement pathways of 2-hydroxy-2-methylpropylidene: an experimental and computational study.

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.