Cycloaddition reactions triple-bond systems

CN triple-bond systems, cycloaddition reactions with, 24, 25... 

The [3+2] cycloaddition of azides to double and triple bond systems has found considerable interest over the last couple of years. The reaction can either be performed under thermal conditions or by copper(i) catalysis <2001AG(E)2004, 2002AG(E)2596>. In an attempt to broaden the chemistry of such cycloaddition processes, Sharpless et al. reported the generation of tetrazole derivatives 61 by an intramolecular process (Scheme 12). In... 

It seems that the homo-Diels-Alder reactions proceed markedly more rapidly than the Diels-Alder reactions since the primary [4-1-2] adducts carmot be detected spectroscopically. Furthermore, it is noteworthy that all 1 -alkynes react regiospecifically in the initial [4-1-2]-cycloaddition process. This addition proceeds in such a way that, independent of the polarity situation of the acetylene, the carbon atom bearing the substituent is positioned in the immediate vicinity of the phosphorus atom of the phosphirane increment. This suggests that steric factors are responsible for the direction of the addition. This is further supported by the observation that 11a does not participate in cycloaddition reactions with any disubstituted triple bond system, except for cyclooctyne, which possesses the necessary reactivity for a cycloaddition on account for the cisoid-dis-... 

The authors have also elaborated a microwave-enhanced one-pot procedure [90] for the Huisgen 1,3-dipolar cycloaddition reaction. In a typical procedure, a pyrazinone with a triple bond connected to the core via C - O linkage, was reacted with a suitable benzylic bromide and NaNs in presence of the Cu(I) catalyst in a t Bu0H/H20 system under microwave irradiation (Scheme 26). The cycloaddition was found to proceed cleanly and with full regioselectivity. As the azide is generated in situ, this procedure avoids the isolation and purification of hazardous azides, which is especially important when handling the ahphatic ones, which are known to be toxic and explosive in nature. 

The various transitions of triafulvenes to pentafulvenes achieved by addition of electron-rich double bonds is complemented by the reaction of triafulvenes with ynamines and yndiamines299, which gives rise to 3-amino fulvenes 539. This penta-fulvene type deserves some interest for its merocyanine-like inverse polarization of the fulvene system and its formation is reasonably rationalized by (2 + 2) cycloaddition of the electron-rich triple bond to the triafulvene C /C2 bond (probably via the dipolar intermediate 538) ... 

Thiazyl halide monomers undergo a variety of reactions that can be classified under the general headings (a) reactions involving the 7i-system of the N = S triple bond, (b) nucleophilic substitution, (c) halide abstraction, and (d) halide addition. The cycloaddition of NSF with hexafluoro-1,3-butadiene provides an example of a type (a) reaction. 

Most metal mediated [2 + 2 + 2] cycloadditions involve two triple bonds which coordinate to a metal center to form a reactive metallocyclopentadiene species (vide infra). The corresponding reactions involving at least two double bonds and an intermediate met-allocyclopentane species are almost completely limited to norbornadiene systems. These reactions can be considered as homo Diels-Alder reactions. 

Cycloaddition reactions represent a very versatile route to alicyclic compounds. The most important for six-membered rings is the Diels-Alder reaction, and its great utility lies in the fact that it is both regioselective and stereospecific. The reaction involves compounds containing a double or triple bond, usually activated by conjugation with additional multiply-bonded systems (carbonyl, cyano,... 

Of the systems showing 1,3-dipolar reactivity, the nitrile oxide is possibly the longest known and is one of the most reactive.81,82 Such reactivity shows itself in the great variety of both acyclic- (via 1,3-addition) and heterocyclic structures (via 1,3-cycloaddition) that can be synthesized from them.59 In the reactions of nitrile oxides with terminal alkynes, both modes of reaction occur, leading to alkynyloximes (25) and isoxazoles (26). The former readily cyclize to the latter but have been detected or isolated in some cases.83,84 In some substituted alkynes 1,3-addition of a substituent may compete with cycloaddition of the triple bond. For example, the reaction between benzonitrile oxide and 3-aminopropyne gives a mixture of the 1,3-adduct (28) (ca. 40%), the 1,3-cycloadduct (29) (ca. 5%), and the product of both modes of reaction (30) (ca. 15%).85 Competition can also occur... 

Dipolar cycloadditions 6.12 + 6.13 —> 6.14, however, are a large group of [4 + 2] cycloadditions isoelectronic with the allyl anion+ alkene reaction. There is much evidence that these reactions are usually concerted cycloadditions. They have a conjugated system of three p orbitals with four electrons in the conjugated system, but the three atoms, X, Y, and Z in the dipole 6.12 and the two atoms A and B in the dipolarophile 6.13, are not restricted to carbon atoms. The range of possible structures is large, with X, Y, Z, A and B able to be almost any combination of C, N, O and S, and with a double 6.12 or, in those combinations that can support it, a triple bond 6.15 between two of them. 

All the reactions described so far have mobilised six electrons, but other numbers are possible, notably a few [8 + 2] and [6 + 4] cycloadditions involving 10 electrons in the cyclic transition structure. A conjugated system of eight electrons would normally have the two ends of the conjugated system far apart, but there are a few molecules in which the two ends are held close enough to participate in cycloadditions to a double or triple bond. Thus, the tetraene 6.17 reacts with dimethyl azodicarboxylate 6.18 to give the [8 + 2] adduct 6.19, and tropone 6.20 adds as a 6-electron component to the 4-electron component cyclopentadiene to give the adduct 6.21. 

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