Learning from Molecules in Distress

Hoffmann, Roald; Hopf, Henning

doi:10.1002/anie.200705775

Cited by 90 publications

(69 citation statements)

References 54 publications

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“…1 Such targets, often displaying high degrees of symmetry, push the limits of organic synthesis despite their apparently simple structures. 2 In the transformation of complex intermediates into the striking Platonic hydrocarbons cubane 3 and dodecahedrane 4 (Figure 1), new boundaries were established for the assembly of molecules with atomic-level precision. Like many classical natural products targeted throughout the mid-to-late 1900s, 5−7 these geometric beauties highlighted that our imagination is truly the only limit to what is available via organic synthesis.…”

Section: ■ Introductionmentioning

confidence: 99%

“…26 As an initial foray into [n]CPP chemistry, Parekh and Guha first reported the synthesis of macrocyclic p,p′-diphenylene disulfide (1) in 1934. 12 Unfortunately, heating with copper led only to partially desulfurized compounds rather than the strained target molecule [2]cycloparaphenylene (Figure 2a, 2). Setting their sights on less-strained macrocycles, Vogtle and coworkers proposed several more targets nearly 60 years later.…”

Section: ■ Introductionmentioning

confidence: 99%

“…(a) Parekh and Guha's attempts toward[2]cycloparaphenylene(2). (b) Attempted pyrolysis of macrocyclic aryl sulfide 3 to obtain[6]cycloparaphenylene.…”

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confidence: 99%

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Syntheses of the Smallest Carbon Nanohoops and the Emergence of Unique Physical Phenomena

2015

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The design and construction of non-natural products have fascinated and perplexed organic chemists for years. Their assembly, akin to what has been accomplished for the total synthesis of natural products, has stretched the limits of what can be prepared in the laboratory. Unlike many natural products, however, carbon-rich structures often lack heteroatoms, further complicating their construction. Consider some of the classical molecules in this genre: cubane and dodecahedrane. While highly symmetric, their assembly is far from trivial. These fascinating hydrocarbon targets have fueled the development of carbon-carbon bond-forming reactions, as new methods are needed to access these types of compounds. Among these carbon-rich structures, polycyclic aromatics such as helicenes, fullerenes, and some fullerenes share common ground due to the distortion of one or more aromatic rings out of planarity. Recently added to this group are the [n]cycloparaphenylenes ([n]CPPs), "carbon nanohoops". Here, a linear string of benzene rings connected at the para positions is wrapped back upon itself to form a cyclic structure. Clearly a simple linear p-oligophenylene cannot be cyclized in this manner without extremely harsh reaction conditions. In order to access these structures using solution-phase organic chemistry, clever synthetic strategies that can compensate for this severe distortion are required. Although cycloparaphenylenes can be considered the smallest possible fragment of an armchair carbon nanotube (CNT), they were envisioned as synthetic targets long before CNTs were discovered in 1991. CPP synthesis was first attempted in 1934, almost 70 years before Iijima's first report on CNTs. The long-forgotten targets reemerged in 1993 with a report from Vögtle, though he ultimately was unsuccessful in achieving their synthesis. More than a decade later, in 2008, CPPs succumbed to total synthesis by Jasti and Bertozzi, allowing access to three different-sized carbon nanohoops in milligram quantities. Since then, the Jasti group has embraced the smallest CPPs as inspiring synthetic targets, challenging us to develop new methodology to construct increasingly strained macrocycles. Having recently synthesized [5]-, [6]- and [7]CPP, the three smallest nanohoops synthesized to date, we have been able to realize a variety of new physical phenomena unique to these structures. Perhaps most significantly, unlike linear p-phenylenes and inorganic quantum dots, the HOMO-LUMO gaps of the CPPs narrow with decreasing CPP size. The smallest CPPs discussed in this Account illustrate this feature exceptionally well, as their HOMO-LUMO gaps become narrower than those of even the longest p-polyphenylenes. The smaller CPPs are fascinating from a structural standpoint as well because of the high amount of distortion in each benzene ring. From the synthesis of [7]CPP (84 kcal/mol of strain energy) to that of [5]CPP (119 kcal/mol of strain energy), our laboratory has been able to test the boundaries of synthetic and physical organic chemistry. In ...

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Section: ■ Introductionmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

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Syntheses of the Smallest Carbon Nanohoops and the Emergence of Unique Physical Phenomena

2015

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“…In contrast, other strained molecules 88,89 including sterically-hindered amides, 90–94 anomeric amides 95–97 and anti-Bredt olefins, 98–105 have been frequently reviewed in recent years.…”

Section: Introductionmentioning

confidence: 99%

Chemistry of Bridged Lactams and Related Heterocycles

2013

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CONTENTS 1. Introduction 5702 2. General Properties of Bridged Lactams 5703 2.1. Distortion Parameters of Bridged Lactams 5703 2.2. Bond Lengths of Bridged Lactams 5703 2.3. Spectroscopic Properties of Bridged Lactams 5703 2.4. Analogy of Bridged Lactams to Bridgehead Olefins 5704 2.5. Chemical and Biological Significance of Distorted Amides 5704 3. Synthesis of Historically Important Bridged Lactams 5704 3.1. Quinuclidone Derivatives 5704 3.2. Adamantanone Derivatives 5707 4. Synthesis of Bridged Lactams with the N−(CO) Bond on a Two-Carbon or Larger Bridge, ([m. (≥2).n] Type) 5708 4.1. Condensation Reactions Forming the N− C(O) Bond 5708 4.2. Heck Reactions 5711 4.3. Diels−Alder Reactions 5712 4.4. Carbene Insertion Reactions 5713 4.5. Reactions via Radical Intermediates 5714 4.6. Miscellaneous Examples 5714 5. Synthesis of Bridged Lactams with the N−(CO) Bond on a One-Carbon Bridge, ([m.1.n] Type) 5715 5.1. Carbene Insertion Reactions 5715 5.2. Schmidt Reactions 5716 5.3.

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“…1 In particular, the geometries, stabilities, and reactivities of bridgehead alkenes have been a source of intrigue since Bredt proposed his rule almost a century ago. 2−4 Increasing numbers of natural products containing bridgehead alkenes have been isolated and characterized, and their total syntheses achieved.…”

Section: ■ Introductionmentioning

confidence: 99%

Experimental and Computational Mechanistic Investigation of Chlorocarbene Additions to Bridgehead Carbene–Anti-Bredt Systems: Noradamantylcarbene–Adamantene and Adamantylcarbene–Homoadamantene

et al. 2015

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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.

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Learning from Molecules in Distress

Cited by 90 publications

References 54 publications

Syntheses of the Smallest Carbon Nanohoops and the Emergence of Unique Physical Phenomena

Syntheses of the Smallest Carbon Nanohoops and the Emergence of Unique Physical Phenomena

Chemistry of Bridged Lactams and Related Heterocycles

Experimental and Computational Mechanistic Investigation of Chlorocarbene Additions to Bridgehead Carbene–Anti-Bredt Systems: Noradamantylcarbene–Adamantene and Adamantylcarbene–Homoadamantene

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