Small-ring Compounds. XXIII. The Nature of the Intermediates in Carbonium Ion-type Interconversion Reactions of Cyclopropylcarbinyl, Cyclobutyl and Allylcarbinyl Derivatives1a

Rearrangement, Elimination, and Ring-Opening Reactions of Cyclopropyl-Substituted Nitrenium Ions: A Computational and Experimental Investigation

N-(4-Biphenylyl)-N-cyclopropyl nitrenium ion 5 and N-benzyl-N-cyclopropyl nitrenium ion (6) were generated through photolysis of their corresponding N-aminopyridinium ion photoprecursors. In the case of 5, stable products result from a combination of cyclopropyl ring expansion (N-biphenylazetium ion) and ethylene elimination (biphenylisonitrilium ion). When present in high concentrations, methanol can add to the cyclopropyl ring-forming N-3-methoxypropyl-N-biphenyl iminium ion. In contrast, the only detectable product from the N-benzyl-N-cyclopropyl nitrenium ion (6) is benzylisonitrile, resulting from the elimination of ethylene. Density functional theory (DFT) calculations predict the product distributions from the more stable biphenyl system 5 with reasonable accuracy. However, product distributions from the less stable benzyl system 6 are forecast with less accuracy.

Cyclopropylcarbinyl cation chemistry in synthetic method development and natural product synthesis: cyclopropane formation and skeletal rearrangement

In this Review, the underrecognized utilities of the cyclopropylcarbinyl cation chemistry are summarized in cyclopropane synthesis and skeletal rearrangements, and their applications in natural product total synthesis are highlighted.

Rapid Access to Arylated and Allylated Cyclopropanes via Brønsted Acid-Catalyzed Dehydrative Coupling of Cyclopropylcarbinols

A regioselective protocol for the synthesis of cyclopropyl derivatives that relies on Brookhart acid-catalyzed dehydrative coupling over substituted cyclopropylcarbinols without rearrangement is reported herein. The reactions proceed promptly at 25 °C with only 2.0 mol % catalyst loading and produce the cyclopropyl derivatives in excellent yields. This method is well tolerated with a vast range of cyclopropylcarbinols including aliphatic cyclopropylcarbinols, where no elimination product was obtained, demonstrating the protocol's utility. Further, the Hammett correlation suggested the formation of a cyclopropylcarbinyl cation followed by a coupling reaction. An extremely effective gram-scale reaction has also been demonstrated with a high turnover number.

History of the Woodward-Hoffmann Rules. The No-Mechanism Puzzle

This is Paper 2 in the 27-paper series on the history of the development of the Woodward-Hoffmann rules. Paper 2 takes the reader back to the 1950s and early 1960s, before the publication of the first Woodward-Hoffmann paper in January 1965. The scope of the pericyclic no-mechanism problem is described along with many of the key "hints" or "clues" to orbital symmetry control that were available in the literature prior to 1965. A chronology of reactions with alternating stereospecificities is provided. A second chronology of alternating theoretical hints is provided, e. g., 4n+2 versus 4n. Another chronology is provided, that of the development of frontier molecular orbital theory, with and without phases and nodes. A tabulation is provided of 36 instances in which the MOs of 1,3-butadiene were reported in the literature. A knowledge of the MOs of 1,3-butadiene, plus a knowledge of some of the alternating stereospecific reactions, could have led many chemists to the solution of the pericyclic no-mechanism problem before Woodward and Hoffmann.

Theoretical Study on the Mechanism of Spirocyclization in Spiroviolene Biosynthesis

Spiroviolene is a spirocyclic triquinane diterpene produced by Streptomyces violens. Recently, a biosynthetic pathway that includes secondary carbocation intermediates and a complicated concerted skeletal rearrangement was proposed for spiroviolene, based upon careful labeling experiments. On the basis of density functional theory (DFT) calculations, we propose a revised pathway for spiroviolene biosynthesis, involving a multistep carbocation cascade that bypasses the formation of unstable secondary carbocations by breaking the adjacent C-C bond to form a more stable tertiary carbocation (IM3) and by Wagner-Meerwein 1,2-methyl rearrangement (IM7).

DFT Study on the Biosynthesis of Verrucosane Diterpenoids and Mangicol Sesterterpenoids: Involvement of Secondary-Carbocation-Free Reaction Cascades

Some experimental observations indicate that a sequential formation of secondary (2°) carbocations might be involved in some biosynthetic pathways, including those of verrucosane-type diterpenoids and mangicol-type sesterterpenoids, but it remains controversial whether or not such 2° cations are viable intermediates. Here, we performed comprehensive density functional theory calculations of these biosynthetic pathways. The results do not support previously proposed pathways/mechanisms: in particular, we find that none of the putative 2° carbocation intermediates is involved in either of the biosynthetic pathways. In verrucosane biosynthesis, the proposed 2° carbocations (II and IV) in the early stage are bypassed by the formation of the adjacent 3° carbocations and by unusual skeletal rearrangement reactions, and in the later stage, the putative 2° carbocation intermediates (VI, VII, and VIII) are not present as the proposed forms but as nonclassical structures between homoallyl and cyclopropylcarbinyl cations. In the mangicol biosynthesis, one of the two proposed 2° carbocations (X) is bypassed by a C-C bond-breaking reaction to generate a 3° carbocation with a C=C bond, while the other (XI) is bypassed by a strong hyperconjugative interaction leading to a nonclassical carbocation. We propose new biosynthetic pathways/mechanisms for the verrucosane-type diterpenoids and mangicol-type sesterterpenoids. These pathways are in good agreement with the findings of previous biosynthetic studies, including isotope-labeling experiments and byproducts analysis, and moreover can account for the biosynthesis of related terpenes.

DFT Study of a Missing Piece in Brasilane-Type Structure Biosynthesis: An Unusual Skeletal Rearrangement

Brasilane-type sesquiterpenes have been known for a long time, but their biosynthetic pathways and mechanisms remain elusive. Recently, two groups independently characterized a Trichoderma terpene cyclase that produces trichobrasilenol, a brasilane-type sesquiterpene, and a plausible biosynthetic pathway was proposed based on isotopic labeling experiments. In the proposed mechanism, the characteristic brasilane-type 5/6 bicyclic skeleton is synthesized from a 5/7/3 tricyclic intermediate via a complicated concerted reaction, including six chemical events of C-C σ bond metathesis and rearrangements, ring-contraction, π bond formation, and regioselective hydroxylation. However, our density functional theory (DFT) calculations do not support this mechanism. On the basis of DFT calculations, we propose a new pathway for trichobrasilenol biosynthesis, involving a multistep carbocation cascade in which cyclopropylcarbinyl cations in equilibrium with homoallyl cations play a pivotal role. This pathway and mechanism is in good agreement with previous biosynthetic studies on brasilane-type compounds and related terpenoids, including isotope-labeling experiments and byproducts analysis.

Re-Examination of Some Carbocations. Structures, Energies, and Charge Distributions

Chemists have had a long-standing interest in reactive intermediates such as carbenes, carbon radicals, carbanions, and carbocations. Carbocations are an interesting part of this group because of their tendency to undergo rearrangement, sometimes forming bridged ions, as well as their ability in many cases of spreading out the positive charge over several atoms. We have re-examined some of these cases using high-level compound procedures, W1BD and G4, as well as by considering the charge distributions making use of the Hirshfeld method that has been shown to uniquely correlate with several types of experimental data.

Synthesis and Discovery of Estra-1,3,5(10),6,8-pentaene-2,16α-diol

A metallacycle-centered approach to the assembly of partially aromatic synthetic steroids was investigated as a means to prepare a boutique collection of unique steroidal agents. The synthesis and discovery of estra-1,3,5(10),6,8-pentaene-2,16α-diol (VII) is described, along with structure-activity relationships related to its cytotoxic properties. Overall, VII was found to have a GI50 = 0.2 μg/mL (∼800 nM) in MDA-MB-231 human breast cancer cells, be an efficacious estrogen receptor agonist with potency for ERβ > ERα (ERβ EC50 = 21 nM), possess selective affinity to the cdc-2-like kinase CLK4 (Kd = 350 nM), and be phenotypically related to paclitaxel by an unbiased panel assessment.

Discrete Existence of Singlet Nitrenium Ions Revisited: Computational Studies of Non-Aryl Nitrenium Ions and Their Rearrangements

Nitrenium ion species are examined using computational methods (DFT, MP2, coupled-cluster, and a composite method, CBS-APNO) with a particular emphasis on nonaromatic species (i.e., those lacking an aromatic or heteroaromatic ring in direct conjugation with the formal nitrenium ion center.) The substitution of the N+ center with alkyl, alkoxy, vinyl, acyl, and sulfonyl, among others, was evaluated. For these species, three properties are considered. (1) The stability of the nitrenium ions to unimolecular isomerizations such as 1,2 alkyl or H shifts; to the extent that the singlet states could be characterized as discrete minima on the potential energy surface (PES), (2) the effect of the substituents on singlet-triplet energy splitting as well as (3) the relative stabilities of the nitrenium ions as defined by N-hydration enthalpies (RR'N+ + H2O → RR'NOH2 +). Nearly all simple alkyl and di-alkyl nitrenium ion singlet states are predicted to rearrange without detectable barriers, largely through 1,2 H or alkyl shifts. Methyl and N,N-dimethylnitrenium ion singlet states could be characterized as formal minima on the PES. However, these species show small or insignificant barriers to isomerization. Disubstituted nitrenium ions that include an alkyl group and a conjugating substituent such as alkoxyl, vinyl, or phenyl show meaningful barriers to isomerization and are thus predicted to possess nontrivial lifetimes in solution. Alkyl groups substantially stabilize the singlet state relative to the situation in the parent nitrenium ion NH2 + to the point where the two states are nearly degenerate. Other groups that interact with the nitrenium ion center decrease the ΔE st in the order formoyl < vinyl < phenyl < alkoxy ∼ sulfonyl < cyclopropyl ∼ cyclobutyl. The latter two substituents interact strongly with the (singlet) nitrenium ion center through the formation of a nonclassical bonding reminiscent of the bisected cyclopropylcarbinyl ion case for carbocations. When singlet-state stability is evaluated in the context of N-hydration enthalpies, it is found that the ordering is acyl < vinyl < alkoxyl < phenyl < cyclopropyl and cyclobutyl.

Intramolecular cyclopropylmethylation via non-classical carbocations

Cyclopropyl-cyclopropyl rearrangement can be achieved selectively by intramolecular trapping of cyclopropylmethyl carbenium ions with an internal nucleophile. This can be exploited as a useful method for the introduction of a cyclopropyl group into complex molecules using readily accessible disubstituted cyclopropane intermediates.

Nitrenium ion analogues of nonclassical carbocations: cyclopropylnitrenium, allylnitrenium, and azetidenium ions and mechanisms for their interconversion

Cyclopropylnitrenium 3S, allylnitreium 6S, and azetidenium (i.e., the nitrogen analogue of cyclobutylcarbenium) ions were examined using density functional theory and a complete basis set method. Similarly to the carbon analogues, the singlet states of these species have several local minima with nonclassical bonding. Structures characterized include 3S, an N analogue of the bisected cyclopropylcarbinyl cation, 11S, an N analogue of the bicyclobutonium ion, and 6S, an unsymmetric 2-azidinylcarbinyl cation.

The energetic viability of an unexpected skeletal rearrangement in cyclooctatin biosynthesis

Results of density functional theory calculations on possible mechanisms for formation of the diterpenoid cyclooctatin are described.

Solid solvents: activation parameters for the rearrangement of cyclopropylcarbinyl bromide on mordenite zeolite

Rearrangement of cyclopropylcarbinyl bromide over proton and ammonium-exchanged mordenite (H-MOR and NH₄-MOR) was studied at different temperatures.

Ab initio/GIAO-CCSD(T) study of structures, energies, and 13C NMR chemical shifts of C4H7(+) and C5H9(+) ions: relative stability and dynamic aspects of the cyclopropylcarbinyl vs bicyclobutonium ions

The structures and energies of the carbocations C 4H 7 (+) and C 5H 9 (+) were calculated using the ab initio method. The (13)C NMR chemical shifts of the carbocations were calculated using the GIAO-CCSD(T) method. The pisigma-delocalized bisected cyclopropylcarbinyl cation, 1 and nonclassical bicyclobutonium ion, 2 were found to be the minima for C 4H 7 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level the structure 2 is 0.4 kcal/mol more stable than the structure 1. The (13)C NMR chemical shifts of 1 and 2 were calculated by the GIAO-CCSD(T) method. Based on relative energies and (13)C NMR chemical shift calculations, an equilibrium involving the 1 and 2 in superacid solutions is most likely responsible for the experimentally observed (13)C NMR chemical shifts, with the latter as the predominant equilibrating species. The alpha-methylcyclopropylcarbinyl cation, 4, and nonclassical bicyclobutonium ion, 5, were found to be the minima for C 5H 9 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level ion 5 is 5.9 kcal/mol more stable than the structure 4. The calculated (13)C NMR chemical shifts of 5 agree rather well with the experimental values of C 5H 9 (+).

Cyclopropylmethyl/Cyclobutyl Rearrangements on Surfaces: Evidence for Transient Cation Formation near O-Covered Mo(110)

Rearrangement reactions of C4-alkoxides on O-covered Mo(110) have been studied using temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy. Cyclobutoxide on Mo(110), prepared from the corresponding alcohol or bromide, is described for the first time in detail. Several reaction mechanisms are considered for the ring-opening rearrangement of cyclopropylmethoxide during high-temperature annealing. In light of compelling new data, previous results are reinterpreted to support the formation of transient cations near O-covered Mo(110). For the first time, we present strong evidence for clean, heterolytic bond cleavage reactions over a metal surface. Our revised reactivity model is based on spectroscopic and reactivity data that show the rearrangement of cyclopropylmethyl groups to cyclobutyl groups and vice versa. Selectively deuterated 1,1-D2-cyclopropylmethanol was studied as a test of mechanism and as a probe for the lifetime of reactive intermediates. Isotopic scrambling observed for this substrate is consistent with the formation of a relatively long-lived carbocation during rearrangement. The intermediacy of transient cations is further invoked to explain the rearrangements that are now recognized to occur as alkyl bromides are transformed into alkoxides on Mo(110)-(1 x 6)-O. The observed ring expansion/contraction reactions are characteristic of a cationic process; carbon-centered radicals are not known to rearrange in this manner. However, in none of the cases discussed could contributions from radical pathways be completely ruled out. Our results are compared to analogous reactions in the vapor and solution phases. General trends governing rearrangement mechanisms on Mo(110) are presented with respect to metal-surface coverage, heteroatom incorporation, and temperature. Trends are discussed in the context of heterogeneous hydrocarbon oxidation.

Electrophilic Intermediates and Their Reactions in Superacids

Acid-catalyzed reactions have played a major role in hydrocarbon chemistry involving electron-deficient intermediates such as carbocations, carbodications, onium ions, etc. The pioneering discovery of the use of superacids by George A. Olah, in the early 1960s, to characterize such intermediates under so-called long-lived stable ion conditions led to the understanding of their structures and reactivity patterns much more clearly. Continuing studies in this area in the past 30 years have resulted in a paradigm shift in comprehending the stability and reactivity of electrophilic intermediates in superacid media.

An Expeditious Enantioselective Synthesis of Antimycin A3b

A straightforward enantioselective route to (+)-antimycin A3b is presented, which used a TiCl4-mediated asymmetric aldolization to construct C-7/C-8 and BnOH/DMAP to remove the chiral auxiliary with concurrent protection of the carboxylic group, respectively. Closing the dilactone ring was achieved in 62% yield (previously 0.8%, 13.4%, or 20%) in the presence of the C-8 ester functionality. The overall yield (34.5%) was significantly higher than that (0.019-3.6%) of the earlier routes.

Physical organic chemistry in Canada 1900–2005

Some of the history of the development of physical organic chemistry in Canada from the 1920s is presented, including many of the individuals involved, and their major areas of interest.Key words: history of physical organic chemistry in Canada, organic reaction mechanisms, free radicals, kinetics.

Rearrangements Concerted with Fragmentation of Cyclopropylmethoxychlorocarbene and Cyclobutoxychlorocarbene in Hydrocarbon Solvents and Ar Matrices

Fragmentations of cyclopropylmethoxychlorocarbene (6) and cyclobutoxychlorocarbene (10) lead to rearrangments that afford mixtures of cyclopropylmethyl chloride (7), cyclobutyl chloride (8), and 3-butenyl chloride (9). Isotopic substitution studies show that these rearrangments are accompanied by partial exchange of the methylene groups within 6 and 10. Surprisingly, these processes that are typical of carbocations persist in hydrocarbon solvents such as pentane and cyclohexane-d(12). Quantum chemical calculations reveal that the cis-conformers of the incipient oxychlorocarbenes C(4)H(7)OC(..)Cl decay to C(4)H(7)Cl + CO via transient hydrogen bonded C(4)H(7)(delta)(+)...Cl(delta-) complexes which possess significant ion pair character, even in the gas phase or in nonpolar solvents. In contrast to benzyloxychlorocarbene, no free radicals are formed upon generation or photolysis of 6 or 10 in Ar matrixes, although acid chlorides (the recombination products of these radical pairs) are observed. The IR spectra obtained in these experiments show the presence of several conformers of the two C(4)H(7)OC(..)Cl.

Ring-Opening Reactions of MCPs with Sulfonamides Promoted by Metal Triflate Lewis Acids

The reaction of aromatic MCPs with sulfonamides catalyzed by Lewis acids affords the corresponding ring-opened homoallylic sulfonamides in good yields and the reaction of aliphatic MCPs with sulfonamides gives the corresponding pyrrolidine derivatives under the same conditions. Through deuterium-labeling experiments, we found that the reaction process is involved with the rearrangement of a cyclopropylcarbinyl cation and a nonclassic carbonium ion.

Quantum-mechanical calculations of the stabilities of fluxional isomers of C4H(7)(+) in solution

Although numerous quantum calculations have been made over the years of the stabilities of the fluxional isomers of C(4)H(7)(+), none have been reported for other than the gas phase (which is unrealistic for these ionic species) that exhibit exceptional fluxional properties in solution. To be sure, quantum-mechanical calculations for solutions are subject to substantial uncertainties, but nonetheless it is important to see whether the trends seen for the gas-phase C(4)H(7)(+) species are also found in calculations for polar solutions. Of the C(4)H(7)(+) species, commonly designated bisected-cyclopropylcarbinyl 1, unsym-bicyclobutonium 2, sym-bicyclobutonium 3, allylcarbinyl 4, and pyramidal structure 6, the most advanced gas-phase calculations available thus far suggest that the order of stability is 1 > or = 2 > or = 3 4 6 with barriers of only approximately 1 kcalmol for interconversions among 1, 2, and 3. We report here that, when account is taken of solvation, 2 turns out to be slightly more stable than 1 or 3 in polar solvents. The pattern of the overall results is unexpected, in that despite substantial differences in structures and charge distributions between the primary players in the C(4)H(7)(+) equilibria and the large differences in solvation energies calculated for the solvents considered, the differential solvent effects from species to species are rather small.

Domino reactions of amidines with methyl 2-chloro-2-cyclopropylideneacetate as an efficient access to cyclobutene-annelated pyrimidinones

[reaction: see text] An efficient one-step synthesis of 2,4-diazabicyclo[4.2.0]octa-1(6),2-dien-5-ones 3 from methyl 2-chloro-2-cyclopropylideneacetate (1) and amidines 2a-c as well as N,N-dimethylguanidine (2d) is described. Similar to the benzocyclobutenes, the cyclobutene-annelated pyrimidones 3 undergo thermal ring opening and the resulting o-quinodimethane analogues readily cycloadd dienophiles to yield tetrahydroquinazolone derivatives.

About the stereoelectronics of the intramolecular addition of allylsilanes to aldehydes

(Z)-omega-Trimethylsilyl-(omega-2)-alken-1-ols are readily accessible by consecutive superbase metalation and silylation of (omega-1)-alken-1-ols. These versatile intermediates may be oxidized to give the corresponding (Z)-omega-trimethylsilyl-(omega-2)-alkenals which, in the presence of trifluoroacetic acid, can be converted into 2-vinylcycloalkanols such as 2-vinylcyclohexanol (2), isopulegol (4), and bis(2-vinylcyclobutyl) ether (8). The stereochemical outcome of these cyclization reactions suggests the interference of a novel electrodynamic effect.

Correlation of the rates of solvolysis of cyclopropylcarbinyl and cyclobutyl bromides using the extended Grunwald-Winstein equation

The reactions of cyclopropylcarbinyl bromide (1) and cyclobutyl bromide (2) in hydroxylic solvents proceed with both solvolysis and rearrangement. Depending on the solvent, the reactions of 1 are 10-120 times faster than those of 2, and both are faster than the previously studied allylcarbinyl bromide (3). Specific rates are reported for the reactions of 2 proceeding to solvolysis products and 3. Reactions of 1 proceed to solvolysis products and both 2 and 3; since 2 slowly undergoes further solvolysis, specific rates are obtained by a modified Guggenheim treatment. The two sets of specific rates are analyzed using the extended Grunwald-Winstein equation to give sensitivities toward changes in solvent nucleophilicity of 0.42 for 1 and 0.53 for 2 and corresponding sensitivities toward changes in solvent ionizing power of 0.75 and 0.94. A mechanism is proposed involving a rate-determining ionization with an appreciable nucleophilic solvation of the incipient carbocation.