8 + 2 Cycloaddition

9 Key excerpts on "8 + 2 Cycloaddition"

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  eBook - ePub

  Reaction Mechanisms in Organic Synthesis

  • Rakesh Kumar Parashar(Author)
  • 2013(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Fig. 8.19 ).

  Figure 8.19

  8.3.2 [2+2]-Cycloaddition reactions

  Dimerization of ethene to cyclobutane, described as a [2+2]-cycloaddition reaction, does not proceed in either the forward or reverse directions under thermal conditions. But the [2+2]-cycloaddition reaction does occur under photochemical conditions. For example, ethene and maleic anhydride when exposed to UV light at −65°C gave cyclobutane diacid anhydride (8.11) in 77% yield.

  8.3.3 1,3-Dipolar additions

  [4s +2s ]-Cycloaddition reactions leading to five-membered heterocyclic adducts are classified as dipolar cycloadditions22,23 . 1,3-Dipolar cycloaddition involves two components just like the Diels-Alder reaction, one dipolar heteroatom compound analogous to diene and the other dipolarophile analogous to dienophile. The dipolar heteroatom compound (1,3-dipole) may be exemplified by ozone and diazomethane.

  8.3.4 Theoretical explanation

  Cycloaddition reactions can be explained in terms of the π-MOs of the reactants.

  Correlation diagram

  Cycloaddition reactions can be explained by using correlation diagrams. According to the orbital symmetry theory, the symmetry of the orbitals of the reactants must be conserved as they are transformed into the orbitals of the product.

  Consider a simple example of a cycloaddition reaction of two molecules of ethene to form cyclobutane. Let us classify all the MOs of reactants and the product as symmetric (S) or antisymmetric (A) with respect to symmetry planes m and C 2 . Once these symmetries are noted, correlations of reactants and product orbitals may be drawn so that orbitals of like symmetry are connected. It is assumed that ethene molecules attack each other in parallel planes (i.e. vertically). There are two symmetry planes (the mirror planes), one bisecting the π-system of the molecules (plane 1, vertical) and the other between the interacting molecules (plane 2, horizontal), as shown in Fig. 8.20

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  Microwave-Assisted Organic Synthesis

  A Green Chemical Approach

  • Suresh C. Ameta, Pinki B. Punjabi, Rakshit Ameta, Chetna Ameta, Suresh C. Ameta, Pinki B. Punjabi, Rakshit Ameta, Chetna Ameta(Authors)
  • 2014(Publication Date)
  • Apple Academic Press

    (Publisher)

  8.3 [4+2] CYCLOADDITION [4+2] Cycloadditions certainly represent one of the most important area in synthetic organic chemistry. Diels-Alder cycloaddition is an important step in a wide variety of natural products skeleton constructions. Since this is a known thermal reaction, the influence of microwave irradiation should prove to be significant. The Diels– Alder reaction of (hetero) dienes with various dienophiles generating a versatile array of useful heterocyclic systems and fused-ring systems, often plays a role in the synthesis of key intermediates of complex natural products and biologically active molecules. The [4+2] cycloaddition reactions furnishes high degrees of chemo-, re-gio- and stereoselectivities, and generate upto four stereo centers in one step. These reactions gained extreme importance for the synthesis of highly substituted and ste-reoselective polycyclic systems in a easy workup. Gomez et al. (2009) reported that the cycloaddition reactions of nitropyrroles in solvent-free conditions under microwave irradiation give 27–71% yields of the aromatic indoles followed by elimination of the nitro group and subsequent aroma-tization. Cycloaddition 147 Zhang et al. (2007) described an efficient [4+2] cycloaddition/rearrangement method that plays a crucial role in the synthesis of (-)-strychnine, a strychnos alka-loid possessing highly toxic properties including disruption of nerve-cell signaling. They applied MgI 2 -catalyzed microwave-assisted intramolecular [4+2] cycloaddi-tion-rearrangement cascade, generating the key aza-tetracycle from the furanylin-dole intermediate in a remarkable yield (95%) in 30 min. Kranjc and Kocevar (2008) have proposed an interesting microwave-assisted [4+2] cycloaddition between suitably functionalized alkynes and 2H-pyran-2-ones, followed by a microwave-assisted cyclization for the synthesis of the indole ring, where the positions on the aromatic ring can be preselected.

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  Organic Reaction Mechanisms 2016

  An annual survey covering the literature dated January to December 2016

  • A. C. Knipe(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  664 Organic Reaction Mechanisms 2016 2 + 2-Cycloaddition An extensive review of 2 + 2-cycloaddition reactions of allenes, allenenes, allenamides, ketenes, alkynes, siloxy alkynes, phenylthioacetylene, and ynamides promoted by metal- based catalysts has been published. 9 A further review covers all recent aspects of the 2 + 2-photocycloaddition chemistry of synthetically relevant regio- and stereo-selective reactions for the past 20 years (1995–2015). Copper(I) and photoinduced electron trans- fer (PET) catalysis together with direct excitation or sensitization are discussed. 10 de  The iminium-ion-catalysed 2 + 2-cycloaddition of , -unsaturated aldehydes with alkenyl phenols produced highly functionalized head-to-tail coupled chiral cyclobutanes with high regiospecificity. 11 The intramolecular 2 + 2-cycloaddition of allenylic esters (10) yielded various trispirocyclic derivatives (11) containing a cyclobutane ring. The process is highly regiospecific under mild reaction conditions (Scheme 3). 12 , 13 O C Ph Ph O (10) O O O O Ph Ph Ph Ph (11) THF, 60 °C, Ar Scheme 3 A key step in the total synthesis of the sesquiterpenes Rumphellaone A and Hushinone is the gold(I)-catalysed 2 + 2-macrocycloaddition of a 1,10-enyne ((R)-6-(2-ethynyl)- benzyloxy-2-methylhept-2-ene) (12) to produce the intermediate ((2aS,5R)-2,2, 5-trimethyl-2,2a,3,4,5,7-hexahydrobenzo[c]cyclobuta[e]oxonine) (13) in a 75% average yield (Scheme 4). 14 The chiral N,N ′ -dioxide-Zn(II)(NTf 2 ) 2 -catalysed 2 + 2- de  cycloaddition of alkynones with cyclic enol silyl ethers yielded fully substituted cyclobutenes with high enantioselectivity (up to 97% ee). Both terminal and internal alkynes react in this cycloaddition. 15 ee  O (12) O (13) H [Au]-catalysed Scheme 4

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  Pericyclic Chemistry

  Orbital Mechanisms and Stereochemistry

  • Dipak Kumar Mandal(Author)
  • 2018(Publication Date)
  • Elsevier

    (Publisher)

  Chapter 5

  Cycloadditions 2: Stereochemistry of [4 + 2] and [2 + 2] Cycloadditions

  Abstract

  This chapter describes the stereochemistry of cycloaddition reactions with respect to [4 + 2] and [2 + 2] cycloadditions. The [4 + 2] cycloadditions include Diels–Alder reactions, 1,3-dipolar cycloadditions and cycloadditions with cations and anions. The stereospecificity and stereoselectivity of Diels–Alder reactions have been described wherein the product stereochemistry has been delineated using simple mnemonics. The asymmetric Diels–Alder reactions involving chiral auxiliary and chiral catalyst strategies have been presented and a mnemonic has been devised to draw product stereochemistry. The important features of intramolecular Diels–Alder reactions and examples of photochemical Diels–Alder and thermal antara/antara cycloadditions are also included. The stereochemistry of 1,3-dipolar cycloadditions are described with respect to stereospecificity, stereoselectivity, intramolecularity and asymmetric approach. The [4 + 2] cycloadditions with allyl cations, pentadienyl cations and allyl anions are presented with their stereochemistry. The descriptions of stereochemistry of [2 + 2] cycloadditions include both photochemical and thermal cycloadditions. The [2 + 2] photocycloadditions of alkenes and alpha, beta-unsaturated carbonyl compounds are described. The [2 + 2] thermal cycloadditions include mainly ketene cycloadditions with reference to regioselectivity, stereospecificity, stereoselectivity and periselectivity, and a brief description of the cycloadditions of related systems such as isocyanates, vinyl cations and allenes. The cycloaddition of singlet vinyl carbenes and cycloaddition of transition metal alkylidene in alkene metathesis are also illustrated.

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  Science of Synthesis: Metal-Catalyzed Cyclization Reactions Vol. 2

  • Shengming Ma, Shuanhu Gao(Authors)
  • 2016(Publication Date)
  • Thieme

    (Publisher)

  2.8 Metal-Catalyzed (5 + 1), (5 + 2), and (5 + 2 + 1) Cycloadditions X. Li and W. Tang General Introduction Transition-metal-catalyzed cycloaddition has emerged as a versatile strategy to access highly functionalized carbocycles and heterocycles that are present in diverse natural products and pharmaceutical reagents. [1–11] These methods are particularly attractive owing to their nature of constructing multiple carbon–carbon and carbon–heteroatom bonds in a single step with a high degree of regioselectivity and stereoselectivity. Many cycloadditions that can be catalyzed by a metal are difficult or are not feasible under ther-mal and photolytic conditions. The complexation of the metal with an alkyne, alkene, al-lene, or diene significantly modifies the reactivity of the ð -systems. A plethora of synthet-ically useful synthons have been discovered for highly selective cycloaddition reactions with the aid of transition-metal catalysts or promoters. To date, extensive efforts have been made to prepare three-to six-membered rings by (1 + 2)-, (2 + 2)-, (3 + 2)-, and (4 + 2)-cycloaddition reactions involving one-to four-carbon synthons. In contrast, five-carbon synthons have received much less attention. Significant progress has been recently made to demonstrate that (5 + n) cycloaddition is a powerful strategy for the synthesis of six-, seven-, and eight-membered carbocycles. For example, transition-metal-catalyzed vi-nylcyclopropane (5 + 2) cycloadditions have emerged as a rich and highly active area for the construction of seven-membered rings. [12–14] These cycloadditions usually involve oxi-dative cyclization to form a metallacycle; insertion of carbon monoxide, an alkyne, an al-kene, an allene, or a combination of these, into C — M bonds; and reductive elimination. The focus of this chapter is on recent advances in transition-metal-catalyzed (5 + 1)-, (5 + 2)-, and (5 + 2 + 1)-cycloaddition reactions.

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  Organic Reaction Mechanisms 2015

  An annual survey covering the literature dated January to December 2015

  • A. C. Knipe(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  10 520 Organic Reaction Mechanisms 2015 The intermolecular 2 + 2-cycloaddition reactions of coumarin-3-carboxylates (15) and acrylamides (16) produced cyclobutabenzopropancarboxylate esters (17, 18) at room temperature. The reaction is driven by FlrPic iridium complex as a photosensi- tizer and a 3 W blue LED light source (Scheme 5). 11 The novel 2 + 2-cycloaddition reactions of the in situ generated 1,1-bis(trifluoromethylsulfonyl)ethene with a variety of alkynes produced substituted cyclobutenes under mild conditions. 2-(Pyridinium-1- yl)-1,1-bis(triflyl)ethanides were selected as the precursors of the 1,1-bis(trifluoromethyl sulfonyl)ethenes. 12 The initial product of the 2 + 2-cycloaddition of digermyne, BbtGe≡GeBbt (Bbt = 2,6-[CH(SiMe 3 ) 2 ] 2 -4-[C(SiMe 3 ) 3 ]–C 6 H 2 ) with ethylene is the corresponding 1,2-digermacyclobutene, which can react with a further molecule of ethylene. 13 X R 2 O R 1 NR 4 R 5 O R 3 + (15) (16) X R 2 O X R 2 O H R 1 R 4 R 5 NOC R 3 H R 1 CONR 4 R 5 R 3 (17) (18) visible light MeCN, r.t. FlrPic (0.5 mol%) X = O, NH, NMe, NBz + Scheme 5 Aryl-substituted bis(imino)pyridine cobalt dinitrogen compounds behave as precata- lysts in the 2 + 2-cycloaddition reactions of ,-dienes to form bicyclo[3.2.0]heptanes under mild thermal conditions. 14 The 2 + 2-cycloaddition reactions of a dyad connect- ing two styrylquinoline chromophores by an o-xylene bridge (19) produced a single rctt-cyclobutane cycloadduct (20) (Scheme 6). 15 Electrophilic noble metals catalyse the cycloaddition/hydroarylation of 7-aryl-1,6-enynes (21) with e-rich arenes (24) to form 6,6-diarylbicyclo[3.2.0]heptanes (25) in good yields and under mild conditions. The mechanism involves a gold-catalysed 2 + 2-cycloaddition followed by a silver-catalysed hydroarylation of the bicyclo[3.2.0]hept-1(7)-ene intermediate (22) via the silver com- plex (23) (Scheme 7).

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  Organic Reaction Mechanisms 2018

  An Annual Survey Covering the Literature Dated January to December 2018

  • Mark G. Moloney(Author)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  413 11 Addition Reactions: Cycloaddition N. Dennis 3 Camphorlaurel Crt, Stretton, Queensland, 4116 Australia CHAPTER MENU [2 + 2]-Cycloaddition, 418 [2 + 3]-Cycloaddition, 420 [2 + 4]-Cycloaddition, 430 Miscellaneous Cycloadditions, 439 References, 446 The recent achievements in catalytic cycloadditions of 1, n -dipoles in the synthesis of medium-sized carbo-and heterocyclic compounds has been reviewed. 1 A review of the cycloadditions between o -carboranes and o -carborynes with unsaturated compounds, alkenes, polycyclic or heterocyclic aromatics, in the past ten years, has been pre-sented. The reaction mechanisms of o -carboryne-involved [2 + 2 + 2]-, [2 + 2 + 1]-, [2 + 2]-, [3 + 2]-, [4 + 2]-and [5 + 2]-cycloadditions are discussed. 2 The K 2 CO 3 -promoted ring-closing carbonyl-allene metathesis reaction of N -allenyl-𝛽 -enaminones produced 2,4-disubstituted pyrroles with cleavage of the C( sp )-C( sp 3 ) bond. A stepwise [2 + 2]-cycloaddition/retro [2 + 2]-reaction mechanism has been proposed. 3 The Fe(III)-catalyzed bicyclization of yne-allenones ( 1 ) with indoles ( 3 ) furnished cyclobuta [ a ]naphthalen-4-ols ( 4 ) in good to excellent yields. The reaction mechanism is thought to proceed through an initial intramolecular [2 + 2]-cycloaddition followed by a 1,6-conjugate addition. The key intermediate is a cyclobutene adduct ( 2 ) (Scheme 1). 4 The rhodium-catalyzed intramolecular cycloaddition of electron ( e )-deficient 2-phenylnaph-thalene-linked triynes ( 5 ), possessing carbonyl groups at the alkyne termini, yielded [2 + 2 + 2]-and [2 + 1 + 2 + 1]-cycloadducts ( 6 , 7 ) depending on the nature of the carbonyl groups on the alkyne termini. Thus, when the triyne possesses bulky and e -withdrawing isobutanoyl and pivaloyl groups, only [2 + 1 + 2 + 1]-cycloadducts are formed. However, when the triyne has highly coordinating dimethoxycarbamoyl groups, the [2 + 2 + 2]-cycloadducts formed exclusively (Scheme 2).

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  Organic Reaction Mechanisms 2019

  • Mark G. Moloney(Author)
  • 2023(Publication Date)
  • Wiley

    (Publisher)

  473 11 Addition Reactions: Cycloaddition N. Dennis 3 Camphorlaurel Crt, Stretton, Queensland, 4116 Australia CHAPTER MENU 2 + 2-Cycloaddition, 475 2 + 3-Cycloaddition, 477 2 + 4-Cycloaddition, 491 Miscellaneous Cycloadditions, 502 References, 513 An extensive review detailing the formal cycloadditions of allenes and allenamides, catalysed by gold(I) and palladium(II) catalysts, has been presented. Formal [4 + 3]-, [4 + 2]-, [2 + 2]-, [2 + 3]-, [3 + 2]-, [2 + 1]-, [5 + 2]-, and [2 + 2 + 2]-annulations are discussed. 1 A detailed review of transition metal vinylidene- and allenylidene-mediated catalysis in organic synthesis includ- ing cyclopropanation and cycloaddition has been presented. 2 A review of the 6-azaelectrocyclization of 1-, 2-, and 3-azatrienes and 1-azadienes in the total synthesis of natural products, bioactive heterocycles, and complex molecules is presented in detail. 3 The 2 + 1-cycloaddition of oxa(aza)bicyclic alkenes/norbornene (1) in the presence of sodium azide (NaN 3 ) and arylsulfonyl chlorides afforded aziridine products (2, 3) in good yields (up to 82%). However, oxa(aza)bicyclic alkenes (1) with NaN 3 and chloroalkanes produced exo-1,2,3-triazolines (4) via a 3 + 2-cycloaddition reaction in excellent yields (up to 95%) (Scheme 1). 4 The platinum-catalysed 3 + 2- and 2 + 2-cycloadditions of allenes with alkene derivatives follow a stepwise process. Unsubstituted allenes and alkenes favour 3 + 2-cycloaddition, while substituted allenes and unsubstituted alkenes favour 2 + 2-cycloaddition. The selectivity between 3 + 2- and 2 + 2-cycloadditions is controlled by the substituent on the allene moiety. 5 Tetrafluorothiophene-S,S-dioxide reacts with terminal alkynes to yield 2 + 2-cycloadducts and 4 + 2-Diels–Alder cycloadducts. The 2 + 2-cycloadducts are formed in a stepwise reac- tion via a singlet diradical intermediate. The [4 + 2]-adducts are produced in an asynchronous orbital symmetry allowed Diels–Alder reaction.

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  Lecture Notes On Fullerene Chemistry: A Handbook For Chemists

  A Handbook for Chemists

  • Roger Taylor(Author)
  • 1999(Publication Date)
  • ICP

    (Publisher)

  9 Cycloadditions Of all fullerene reactions, cycloadditions have received by far the most study. This popularity stems from the ability to control the reaction so that only one addend becomes attached to the cage, making analysis of the produces relatively easy. The object of many of these syntheses is to produce intermediates for further reaction, though some of the functional groups do not undergo their normal reactions either readily or at all, when attached to fullerenes. This is due to both the strong electron withdrawal by the cage, and steric constraints. Six types of reaction are known: [1 + 2], [2 + 2], [3 + 2], [4 + 2], [6 + 2] and [8 + 2] cycloadditions. The [4 + 2] group (Diels-Alder reactions) have been the subject of most attention. The number of compounds that have been made by cycloaddition is already too vast to be described in detail here, consequently the reactions shown are selected either to be representative of general features, or have some aspect of special interest. There have been recent reviews on some of these reactions. 1-3 9.1 [1 + 2] Cycloadditions: Reactions That Produce Methano- and Homofullerenes and Their Heteroanalogues These additions involve either carbon, oxygen, nitrogen and silicon, numerous derivatives based upon these additions being now known. Addition of carbon has been the most studied to date, but addition of nitrogen (which leads to azafullerenes described in Chap. 13) and of oxygen (which can occur spontaneously) are rapidly gaining in importance. Two possible products are obtainable, arising from insertion into a 6,5 a-bond giving 9.1, or addition to a 137 9 Cycloadditions 9.1 [1 + 2] Cycloadditions: Reactions That Produce Methano- and Homofullerenes and Their Heteroanalogues 138 Lecture Notes on Fullerene Chemistry: A Handbook for Chemists 6,6 n-bond giving 9.2, the reason for these preferences having been given in Sec.

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