4-Pyridylnitrene and 2-pyrazinylcarbene

• Curt Wentrup,
• Ales Reisinger and
• David Kvaskoff

Beilstein J. Org. Chem. 2013, 9, 754–760, doi:10.3762/bjoc.9.85

• vacuum thermolysis (FVT) conditions cyanocyclopentadiene 6 is formed (Scheme 1). Several other carbene–nitrene rearrangements have been reported [3][4][5]. In addition to the ring expansion (1–2–3), two ring opening reactions have been investigated in recent years. Type I ring opening takes place in

A computational study of base-catalyzed reactions of cyclic 1,2-diones: cyclobutane-1,2-dione

• Nargis Sultana and
• Walter M. F. Fabian

Beilstein J. Org. Chem. 2013, 9, 594–601, doi:10.3762/bjoc.9.64

• rearrangements of biacetyl and benzil [11]. Car–Parrinello molecular dynamics simulations of the hydrolysis of formamide in basic solution indicated that the traditional view of attack by hydroxide anion rather than a first-solvation-shell water molecule is more likely; however, the more powerful electrophile

Some aspects of radical chemistry in the assembly of complex molecular architectures

• Béatrice Quiclet-Sire and
• Samir Z. Zard

Beilstein J. Org. Chem. 2013, 9, 557–576, doi:10.3762/bjoc.9.61

• tendency for rearrangements and β-elimination; and a selectivity that is often complementary to that of ionic or organometallic reactions, making some protection steps superfluous. Radicals are ambiphilic species that can react with both electron-poor and electron-rich substrates, but the rates can differ

Caryolene-forming carbocation rearrangements

• Quynh Nhu N. Nguyen and
• Dean J. Tantillo

Beilstein J. Org. Chem. 2013, 9, 323–331, doi:10.3762/bjoc.9.37

• formation, applying the principles derived from previous theoretical studies on terpene-forming carbocation rearrangements [8]. In this mechanism, formation of the C1–C11 bond was expected to result in secondary carbocation B, in analogy to previously characterized pathways to sesquiterpenes containing 11

Radical zinc-atom-transfer-based carbozincation of haloalkynes with dialkylzincs

• Fabrice Chemla,
• Florian Dulong,
• Franck Ferreira and
• Alejandro Pérez-Luna

Beilstein J. Org. Chem. 2013, 9, 236–245, doi:10.3762/bjoc.9.28

• dialkylzinc reagent via the intermediate formation of a zincate [48][49][50][51] (Scheme 3, path a). The stereoselectivity of such rearrangements is often dependent on the substrate structure, so the diastereopurity of 5 is not necessarily informative about that of 6 [48][49][50][51]. An alternative

Flow photochemistry: Old light through new windows

• Jonathan P. Knowles,
• Luke D. Elliott and
• Kevin I. Booker-Milburn

Beilstein J. Org. Chem. 2012, 8, 2025–2052, doi:10.3762/bjoc.8.229

The chemistry of bisallenes

• Henning Hopf and
• Georgios Markopoulos

Beilstein J. Org. Chem. 2012, 8, 1936–1998, doi:10.3762/bjoc.8.225

• dibromocyclopropane precursors) to carbene rearrangements followed by hydrogen shifts. The above C3-dimerization pathway (Scheme 3) via organozinc compounds goes back to Vermeer et al. and appears to be not only the most general route to alkylated conjugated bisallenes [27][28][29][30][31], but also to many other

• →bismethylenecyclobutene isomerization of the dialkynylated d,l- and meso-bisallenes, 122 and 123 respectively, in the solid state (Scheme 36) [119]. These rearrangements occur in high yield and with high stereoselectivity. When 122 was heated in benzene it isomerized to 145 in quantitative yield within 2 h. A thorough

Approaches to α-amino acids via rearrangement to electron-deficient nitrogen: Beckmann and Hofmann rearrangements of appropriate carboxyl-protected substrates

• Sosale Chandrasekhar and
• V. Mohana Rao

Beilstein J. Org. Chem. 2012, 8, 1393–1399, doi:10.3762/bjoc.8.161

• ] and the Hofmann [10][11][12][13] rearrangements – are not represented in these approaches. This is perhaps surprising, but understandable when the inherent instability of the requisite substrates is considered. Thus, in the Beckmann approach, the oximation of β-keto acid derivatives would be

• rearrangements, two of the standard methods for the introduction of the amino functionality into molecules. The strategy is enabled by efficient carboxyl protection via the 2,4,10-trioxaadamantane moiety, which stabilizes the respective precursor substrates to opportunistic side reactions. Overall yields are

Valence isomerization of cyclohepta-1,3,5-triene and its heteroelement analogues

• Helen Jansen,
• J. Chris Slootweg and
• Koop Lammertsma

Beilstein J. Org. Chem. 2011, 7, 1713–1721, doi:10.3762/bjoc.7.201

• -triene [14]. Besides the 1–2 interconversion, the C7H8 system is rich in rearrangements (Scheme 3). In 1957, Woods found that bicyclo[2.2.1]hepta-2,5-diene (12) converts to cycloheptatriene (1), which was postulated to proceed via diradical 11 and norcaradiene (2) [28]. Instead, pyrolysis of 1 yielded

• –18. Benzannulated azepines 27 and 28. Reported phosphepines 29–32. Valence isomerization of cyclohepta-1,3,5-triene (1) and its heteroelement analogues. Conformational ring inversions. Rearrangements of the parent cycloheptatriene 1 and norcaradiene 2. Reactivity of oxepine (3) and benzene oxide (4

Translation of microwave methodology to continuous flow for the efficient synthesis of diaryl ethers via a base-mediated S_NAr reaction

• Charlotte Wiles and
• Paul Watts

Beilstein J. Org. Chem. 2011, 7, 1360–1371, doi:10.3762/bjoc.7.160

• ] (Figure 1). Installation of the diaryl ether can, however, be synthetically challenging, and this is illustrated by the wide number of techniques developed, which include Ullmann ether synthesis [6], Pummerer-type rearrangements [7], Buchwald–Hartwig couplings [8], phenolic additions to amines [9

Continuous flow photolysis of aryl azides: Preparation of 3H-azepinones

• Farhan R. Bou-Hamdan,
• François Lévesque,
• Alexander G. O'Brien and
• Peter H. Seeberger

Beilstein J. Org. Chem. 2011, 7, 1124–1129, doi:10.3762/bjoc.7.129

• reaction conditions using the flow reactor allowed minimization of secondary photochemical reactions. Keywords: azepinones; azides; continuous flow; nitrenes; photochemistry; Findings Although photochemical rearrangements are an important class of reactions for heterocycle synthesis [1][2], their use is

• column chromatography. Rearrangements of aryl azides bearing electron withdrawing substituents are better represented in the literature than their electron donating congeners [31], and this was reflected in our compound selection. Methyl 2-azidobenzoate (8b), the corresponding acid 8c and dimethyl 2

Triazole–Au(I) complex as chemoselective catalyst in promoting propargyl ester rearrangements

• Dawei Wang,
• Yanwei Zhang,
• Rong Cai and
• Xiaodong Shi

Beilstein J. Org. Chem. 2011, 7, 1014–1020, doi:10.3762/bjoc.7.115

Recent advances in the gold-catalyzed additions to C–C multiple bonds

• He Huang,
• Yu Zhou and
• Hong Liu

Beilstein J. Org. Chem. 2011, 7, 897–936, doi:10.3762/bjoc.7.103

• intramolecular substrate rearrangements (Scheme 18). 2.4 Propargylic alcohols and propargylic carboxylate rearrangements Pennell et al. reported Meyer–Schuster rearrangements of propargylic alcohols 102 at room temperature in toluene with 1–2 mol % PPh3AuNTf2, in the presence of 0.2 equiv of 4

• . reported a general gold-catalyzed direct oxidative homo-coupling of non-activated arenes 207 (Scheme 38). The reaction protocol tolerates a wide range of functional groups [92]. All halogens survive the reaction, which provides the potential for further reactions. 4.2 Rearrangements and ring enlargement A

• gold-catalyzed rearrangement of 6-alkynylbicyclo[3.1.0]hexen-2-enes 209 has been developed [93]. In this reaction, divergent structural rearrangements are observed in the absence/presence of nucleophiles. The process results in a novel five-to-six-membered ring expansion that involves cleavage of the

Gold-catalyzed propargylic substitutions: Scope and synthetic developments

• Olivier Debleds,
• Eric Gayon,
• Emmanuel Vrancken and
• Jean-Marc Campagne

Beilstein J. Org. Chem. 2011, 7, 866–877, doi:10.3762/bjoc.7.99

• . Direct propargylic substitutions: Scope of nucleophiles. Meyer–Schuster rearrangements. Silyl-protected propargyl alcohols in propargylic substitutions. Acetylacetone as nucleophile in direct propargylic substitution. Enantiomerically enriched propargylic alcohols. Scope of ‘activated’ alcohols in direct

When gold can do what iodine cannot do: A critical comparison

• Sara Hummel and
• Stefan F. Kirsch

Beilstein J. Org. Chem. 2011, 7, 847–859, doi:10.3762/bjoc.7.97

• . Nevertheless, heteroatom nucleophiles having no protons attached react in gold-catalyzed carboalkoxylations [63][64][65][66] and related processes where the analogous electrophilic processes are unknown. Catalyzed propargylic ester rearrangements [67][68] also remain the realm of gold-complexes since such

Isotopic labelling studies for a gold-catalysed skeletal rearrangement of alkynyl aziridines

• Paul W. Davies,
• Nicolas Martin and
• Neil Spencer

Beilstein J. Org. Chem. 2011, 7, 839–846, doi:10.3762/bjoc.7.96

• aziridines into 2,4-disubstituted pyrroles. Two isotopomers of the expected skeletal rearrangement product were identified using 13C-labelling and led to a revised mechanism featuring two distinct skeletal rearrangements. The mechanistic proposal has been rationalised against the reaction of a range of 13C

• the most common fundamental steps in these skeletal rearrangements involves C–C bond fission through 1,2-migration. This step is triggered by the generation of carbocationic character, which is generally stabilised in some form, either by an adjacent gold atom or through extended delocalisation. While

• , and hence the isotopomeric reaction selectivity, is controlled by the relative ability of the substituents to stabilise the respective cations in favour of a particular pathway. The observed skeletal rearrangements are consistent with either a 1,2-aryl shift or a 1,2-(metal-stabilised)-vinyl shift in

Highly efficient gold(I)-catalyzed Overman rearrangement in water

• Dong Xing and
• Dan Yang

Beilstein J. Org. Chem. 2011, 7, 781–785, doi:10.3762/bjoc.7.88

• have been used for different types of [3,3]-sigmatropic rearrangements [14][15], only Pd(II) and Hg(II) salts have found wide application in Overman rearrangements. In recent years, gold catalysts have been successfully applied to a series of [3,3]-sigmatropic rearrangements, such as the rearrangement

Synthetic applications of gold-catalyzed ring expansions

• David Garayalde and
• Cristina Nevado

Beilstein J. Org. Chem. 2011, 7, 767–780, doi:10.3762/bjoc.7.87

• on transition metal promoted rearrangements of bicyclo[1.1.0]butanes [11]. Thus Ru–carbonyl complexes promote the rearrangement of 1,2,2-trimethylbicyclo[1.1.0]butane (1) to yield diene 2 and the cyclopropyl derivative 3 (Scheme 1, reaction 1: a,b for 2 and 3), whilst pentafluorophenylcopper tetramer

• into the angular triquinane ventricosene in six steps (Scheme 23). 6 Ring expansions involving propargyl acyloxy rearrangements Propargyl carboxylates 80 can be π-activated by gold towards 1,2-acyloxy migration and/or [3,3]-sigmatropic rearrangement. Two different, but mechanistically related

• organic chemist. Transition metal promoted rearrangements of bicyclo[1.1.0]butanes. Gold-catalyzed rearrangements of strained rings. Gold-catalyzed ring expansions of cyclopropanols and cyclobutanols. Mechanism of the cycloisomerization of alkynyl cyclopropanols and cyclobutanols. Proposed mechanism for

When cyclopropenes meet gold catalysts

• Frédéric Miege,
• Christophe Meyer and
• Janine Cossy

Beilstein J. Org. Chem. 2011, 7, 717–734, doi:10.3762/bjoc.7.82

• secondary benzylic cyclopropyl cation). The gold carbene 31 was captured by the aromatic group at C3 via an intramolecular Friedel–Crafts reaction. Subsequent elimination of AcOH from compound 33 then delivered methylene indene 30 (Scheme 14) [21]. Other gold-catalyzed rearrangements of cyclopropenes that

The arene–alkene photocycloaddition

• Ursula Streit and
• Christian G. Bochet

Beilstein J. Org. Chem. 2011, 7, 525–542, doi:10.3762/bjoc.7.61

• photocycloaddition followed by rearrangements. Stable [2 + 2] photocycloadducts. Ortho photocycloadditions with alkynes. Intramolecular ortho photocycloaddition and rearrangement thereof. Intramolecular ortho photocycloaddition to access propellanes. Para photocycloaddition with allene. Photocycloadditions of

An overview of the key routes to the best selling 5-membered ring heterocyclic pharmaceuticals

• Marcus Baumann,
• Ian R. Baxendale,
• Steven V. Ley and
• Nikzad Nikbin

Beilstein J. Org. Chem. 2011, 7, 442–495, doi:10.3762/bjoc.7.57

Molecular rearrangements of superelectrophiles

• Douglas A. Klumpp

Beilstein J. Org. Chem. 2011, 7, 346–363, doi:10.3762/bjoc.7.45

• reactive intermediates may also undergo a wide variety of rearrangement-type reactions. Superelectrophilic rearrangements are often driven by charge–charge repulsive effects, as these densely charged ions react so as to maximize the distances between charge centers. These rearrangements involve reaction

• steps similar to monocationic rearrangements, such as alkyl group shifts, Wagner–Meerwein shifts, hydride shifts, ring opening reactions, and other skeletal rearrangements. This review will describe these types of superelectrophilic reactions. Keywords: dication; rearrangement; superacid

• superelectrophiles are often densely charged species, they are also known for their tendencies to undergo rearrangement and charge migration reactions. These types of conversions will be examined in this review article, including ring opening reactions, carbon–carbon bond shifts, skeletal rearrangements, and charge

Rh-Catalyzed rearrangement of vinylcyclopropane to 1,3-diene units attached to N-heterocycles

• Franca M. Cordero,
• Carolina Vurchio,
• Stefano Cicchi,
• Armin de Meijere and
• Alberto Brandi

Beilstein J. Org. Chem. 2011, 7, 298–303, doi:10.3762/bjoc.7.39

• -annelated heterocyclic compounds [35], we started to investigate some metal-catalyzed rearrangements. The first choice was the readily available so-called Wilkinson catalyst Rh(PPh3)3Cl, because of its documented efficiency in catalyzing the rearrangement [26] and of the possibility to extend its use to

• other interesting transformations, such as the [5 + 2] cycloadditions of vinylcyclopropanes to alkynes developed by Wender and co-workers [36][37]. It is known, that rhodium-catalyzed rearrangements of unactivated VCPs, without any functional substituent, usually afford dienes. In order to evaluate the

• temperatures. This explains the low isolated yields in their syntheses. Analogously to other Rh(I)-catalyzed VCP rearrangements [46][47], the mechanism of the rearrangement likely involves insertion of the Rh(I) species into the cyclopropane ring of the VCP system, with or without incorporation of the double

Effects of anion complexation on the photoreactivity of bisureido- and bisthioureido-substituted dibenzobarrelene derivatives

• Heiko Ihmels and
• Jia Luo

Beilstein J. Org. Chem. 2011, 7, 278–289, doi:10.3762/bjoc.7.37

• unit. Stereoselective DPM rearrangements of dibenzobarrelene derivatives have been reported in special media, such as chiral mesoporous silica [31] or ionic-liquids [32]; however, most examples for stereoselective DPM rearrangements of dibenzobarrelene derivatives have been observed in the solid-state

• asymmetric photoreactions have been conducted with remarkable enantioselectivity in homogeneous solution, whereas reports of asymmetric di-π-methane rearrangements in solutions are relatively rare. Chiral auxiliaries attached as ester or amide functionalities at the vinylic positions of dibenzobarrelene

• induce only low enantioselectivities in the DPM rearrangement in solution [37]; and the ionic auxiliary strategy, which generates impressive enantioselectivity in the solid-state, fails to induce any stereoselectivity in DPM rearrangements in solution. Considering these observations, it remains a

Synthesis of cationic dibenzosemibullvalene-based phase-transfer catalysts by di-π-methane rearrangements of pyrrolinium-annelated dibenzobarrelene derivatives

• Heiko Ihmels and
• Jia Luo

Beilstein J. Org. Chem. 2011, 7, 119–126, doi:10.3762/bjoc.7.17

• these cases, the dibenzosemibullvalene structure was indicated by the characteristic 1H NMR spectroscopic shifts of the 4b-H and 8b-H protons. It has been demonstrated with several examples that solid-state photoreactions are an excellent tool to induce highly selective di-π-methane rearrangements of

• dibenzobarrelene derivatives 2a–g. Di-π-methane rearrangements of dibenzobarrelene derivatives 2a–f (counter ions omitted for clarity). Di-π-methane rearrangement of dibenzobarrelene derivative 2g. Synthesis and solid-state photoreactivity of the sulfonate salt 2b-4. Phase-transfer catalyzed alkylation reactions