Preparation of Helicenes through Olefin Metathesis

Self Cite

173

The asymmetric synthesis of [7]helicene was accomplished in good ee (80%) by kinetic resolution by means of asymmetric olefin metathesis. Three key factors contributed to the success of the kinetic resolution: the use of new Ru-based olefin metathesis catalysts bearing C1-symmetric N-heterocyclic carbene ligands, simple olefins as additives to control the nature of the propagating alkylidene and hexafluorobenzene as a solvent.

Section: Introductionmentioning

confidence: 99%

Enantioselective Synthesis of [7]Helicene: Dramatic Effects of Olefin Additives and Aromatic Solvents in Asymmetric Olefin Metathesis

Grandbois

Collins

2008

Self Cite

173

“…The situation changed dramatically during the last decade when fascinating optical [1] and electronic [2] properties as well as attractive applications in bioorganic chemistry [3] and asymmetric synthesis [4] emerged for these helical-shaped molecules. In parallel, new synthetic methods that circumvented the problems often related with the classical photocyclization of stilbenes, [5] including Diels-Alder reactions, [6] cyclotrimerization of alkynes, [7] carbenoid couplings, [8] radical cyclizations, [9] Pd-mediated methodologies, [10] and olefin metathesis [11] were developed. Recently, bridged triarylamine heterohelicenes of the type 1-3 have been prepared [12, 13] and studied [13, 14] in the belief that the introduction of molecular helicity, sterically driven by the increasing overlap of the terminal a and b aryl rings, influences [2b] the well known photochemical and physical properties of triarylamines.…”

mentioning

confidence: 99%

Efficient Thia‐Bridged Triarylamine Heterohelicenes: Synthesis, Resolution, and Absolute Configuration Determination

Lamanna

Faggi

Gasparrini

et al. 2008

Dedicated to the memory of Professor Giuseppe (Pino) CapozziFor many years helicenes were regarded as an academic curiosity and a nice example of chirality without stereogenic centers. The situation changed dramatically during the last decade when fascinating optical [1] and electronic [2] properties as well as attractive applications in bioorganic chemistry [3] and asymmetric synthesis [4] emerged for these helical-shaped molecules. In parallel, new synthetic methods that circumvented the problems often related with the classical photocyclization of stilbenes, [5] including Diels-Alder reactions, [6] cyclotrimerization of alkynes, [7] carbenoid couplings, [8] radical cyclizations, [9] Pd-mediated methodologies, [10] and olefin metathesis [11] were developed. Recently, bridged triarylamine heterohelicenes of the type 1-3 have been prepared [12, 13] and studied [13, 14] in the belief that the introduction of molecular helicity, sterically driven by the increasing overlap of the terminal a and b aryl rings, influences [2b] the well known photochemical and physical properties of triarylamines. [15] As a continuation of our efforts related to sulfur heterocycles chemistry, [16] we focused on compound 3, which was prepared by building the triarylamine skeleton by employing a Buchwald-Hartwig type cross-coupling of an open iodo-aniline in the final step of a multistep reaction sequence. [13] We envisaged that derivatives like 3 could be accessible by a cascade of regioselective electrophilic aromatic sulfur insertions, [17] by applying the chemistry of the phthalimidesulfenyl chloride (4) (PhtNSCl, Pht = phthaloyl) to properly substituted triarylamines.The reaction of tris(4-methylphenyl)-(5 a) and tris(4-methoxyphenyl)amine (5 b) with one equivalent of 4 occurred smoothly at room temperature to give mono-sulfenylation ortho to the amine nitrogen atom. The bis-sulfenylation can however be achieved under more forcing reaction conditions (Scheme 1), which allowed the isolation of derivatives 6 a and 6 b in 83 % yield. In line with our previous results, [18] the introduction of a N-thiophthalimide group strongly deactivates the aromatic system, preventing further substitutions. Moreover, using triarylamines as substrates, protonation at the amine nitrogen, due to the HCl formed during sulfenylation, represents a supplementary obstacle to the polysubstitution. Indeed, we were unable to carry out an exhaustive sulfenylation of all three aromatic rings of 5 a or 5 b.The electrophilic character of sulfenamide sulfur in N-thiophthalimides can be increased by using Lewis acids.[17] Satisfyingly, reacting derivatives 6 a and 6 b with BF 3 ·Et 2 O or AlCl 3 triggers an intramolecular attack of the aromatic ring on the adjacent sulfur atom, leading to the formation of hetero[4]helicenes 7 a and 7 b in 85 and 87 % yield, respectively [a] Dr.

“…[3,4] Therefore, the fact that much attention has been recently paid to the synthesis of aromatic compounds using RCM of acyclic precursors is to be expected. [5][6][7] In the last few years, we have devoted much of our effort [8] to this field and reported that phenols can be obtained by RCM/tautomerization of 1,4,7-trien-3-ones 1 via cyclohexa-2,5-dienones [8a,c] [Eq.…”

Section: Introductionmentioning

confidence: 99%

Synthesis of Substituted Phenols by Using the Ring‐Closing Metathesis/Isoaromatization Approach

Yoshida

Narui

Imamoto

2008

Ring-closing olefin metathesis (RCM) of 4-methylene-1,7-octadien-3-ones 2, followed by isomerization of the carbon--carbon double bond of 6-methylene-2-cyclohexenones 3 from exo to endo, produced various phenols 4. As an application of the method, the RCM/Mizoroki-Heck reaction of 2 was proven to be also effective for the synthesis of phenols having an additional substituent at ortho-benzylic position.