Cycloadditions with electron-rich triple bonds

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) ... 

At the present, our studies with alkynylcarbene complexes of Cr and W suggest that their cycloadditions with electron-rich olefins follow, mainly, a concerted [2+2] pathway by activation of the triple bond[6], whereas, the intramolecular Co-induced carbonylative cycloaddition of the corresponding allylamino complexes seems to be facilitated by the alternative influence of the metal on the heteroatom side. A strict control of the stereochemistry in these reactions has been observed making these complexes valuable auxiliaries in organic synthesis. Efforts to broaden the scope of application are under way. 

The cycloaddition of azides to multiple -ir-bonds is an old and widely used reaction. Organic azides are well known to behave as 1,3-dipoles in thermal cycloaddition reactions.178 The first example of this reaction was observed by Michael in 1893.179 Since then the addition of azide to carbon-carbon double and triple bonds has become the most important synthetic route to 1,2,3-triazoles, -triazolines and their derivatives.180-184 The cycloadditions of simple organic azides with electron-rich dipolarophiles are LUMO controlled.3 Since the larger terminal coefficients are on the unsubstituted nitrogen in the azide and unsubstituted terminus in the dipolarophiles, the 5-substituted A2-triazolines are favored, in agreement with experiment.185-187 Reactions with electron-deficient dipolarophiles are HOMO controlled, and... 

The Diels-Alder reaction with inverse electron demand has been one of the most intensively studied reactions of 1,2,4-triazines. In this reaction 1,2,4-triazines behave as electron-deficient dienes and react with electron-rich dienophiles to give, generally, pyridines (see Houben-Weyl, Vol. E7b, p 471 ff). [4 + 2] Cycloadditions of 1,2,4-triazines have been observed with alkenes, alkynes, strained double bonds, electron-rich double and triple bonds, but in a few cases also with electron-deficient alkynes C—N double and triple bonds can also be used as dienophiles. In addition to intermolecular Diels-Alder reactions, intramolecular [4 + 2] cycloaddition reactions of 1,2,4-triazines have also been studied and used for the synthesis of condensed heterocyclic systems. A review on the intermolecular Diels-Alder reaction was published by Boger and Weinreb 14 Sauer published a review on his studies in 1992,381 and E. C. Taylor published a summary of his own work on intramolecular Diels-Alder reactions in 1988.382... 

The Diels-Alder reaction,is a cycloaddition reaction of a conjugated diene with a double or triple bond (the dienophile) it is one of the most important reactions in organic chemistry. For instance an electron-rich diene 1 reacts with an electron-poor dienophile 2 (e.g. an alkene bearing an electron-withdrawing substituent Z) to yield the unsaturated six-membered ring product 3. An illustrative example is the reaction of butadiene 1 with maleic anhydride 4 ... 

From a formal point of view this process may be regarded as the [2-f 1] cycloaddition of a transient phosphinidene to the PC triple bond of the electron-rich phosphaalkyne. A little later a different approach to diphosphirenes was devised by Bertrand. When a toluene solution of phosphaalkene Pr NP=C(NPr2 )P(NPt2 )2 90 was treated with 2 equiv of boron trifluoride-diethylamine complex at room temperature, a clean reaction took place to afford the pale yellow crystalline diphosphirenium salt 91 (60% yield) <1991JA8160> <1994JA6146>. 

If triple bonds are implemented in cycloadditions with six-membered cychc dienes, an irreversible release of an alkene moiety claims for another type of arene synthesis. Driven by the search for novel antitumor agents, Danishefsky exploited this strategy during the synthesis of cycloproparadicicol [13]. The dimedone-derived electron-rich diene 22 cycloadds to the activated triple bond of the compound termed ynolide 23 at elevated temperatures, whereupon a concomitant extrusion of isobutene uncovers the aromatic core (Scheme 6.5). The trimethylsilyl (TMS) ether groups get cleaved during chromatography, furnishing the precursor 25 of cycloproparadicicol in 75% yield. 

The reaction of dicarbonyl(t 5-cyclopentadienyl)-carbyne complexes Cp(CO)2-n(PMe3)nMsC-R (M = Mo, W n = 0, 1 R = Me, Ph, Tol) with azides, N3R (R = CO2CH3, CH2CO2CH3), results in a [3+2]-cycloaddition to the electron rich metal carbon triple bond, leading to neutral 1-metalla-2,3,4-triazole complexes in high yields [43,44]. 

---