Electrophiles acetylenes

The A -unsubstituted complex 16 reacts with various electrophilic acetylenes to yield adducts 25 as mixtures of Z- and -stereoisomers the adducts afford the corresponding TV-substituted 1,2-diazepines 26 on decomplexation.88... 

The nitrogen atoms in ADC compounds are highly electrophilic. Nucleophilic attack on nitrogen is easy, and as with electrophilic acetylenes, such as dimethyl acetylenedicarboxylate, it seems likely that some cycloaddition reactions of ADC compounds with unsymmetrical substrates proceed via a stepwise mechanism. PTAD is a powerful electrophile, although TCNE is more reactive, and chlorosulfonyl isocyanate is more reactive still.58... 

Electron-rich heterocyclic systems such as indolizines react readily with DEAZD (and PTAD) to give substitution products (Eq. 16).141 None of the formal [8 + 2] cycloaddition products (e.g., 89) are observed. This is in direct contrast to the reaction of indolizines with electrophilic acetylenes which gives high yields of cycloaddition products, presumably via a stepwise mechanism, in the presence of palladium on charcoal.142 This example of... 

Addition of nitroalkanes 402 (R = Me, Bu, Ph etc.) to methyl acrylate without a solvent in the presence of Amberlyst-21 gives good yields of the esters 403436. An analogous reaction with electrophilic acetylenes, e.g. dimethyl acetylenedicarboxylate, in the presence of potassium fluoride and tetrabutylammonium chloride yields adducts 404 as mixtures of geometrical isomers437. 

Pinho e Melo et al. (89) employed an intramolecular miinchnone cycloaddition to constmct several l/7-pyrrolo[l,2-c]thiazole derivatives from N-acylthiazolidines and acetic anhydride. Martinelli and co-workers (90,91) employed an intramolecular miinchnone cycloaddition to craft a series of 4-keto, 5,6,7-tetrahydroindoles (168-171) in two steps. The requisite acetylenic precursors were prepared from glutaric anhydride (or 3-methylglutaric anhydride). The overall sequence is illustrated for the synthesis of 168. An electrophilic acetylenic unit appears to be necessary for successful intramolecular 1,3-dipolar cycloaddition. 

Stang PJ (1995) Alkynyliodonium Salts Electrophilic Acetylene Equivalents. In Stang PJ, Diederich F (eds) Modern Acetylene Chemistry. VCH, Weinheim, p 67, chap 3... 

Different modes of reaction are known for the reaction of 1,1-enediamines with electrophilic acetylenes. Simple 1,1-enediamines undergo cycloaddition reaction which will be discussed in Section V.A119, whereas conjugated 1,1-enediamines act as nucleophiles towards activated acetylenes. 

Cycloaddition of electrophilic acetylenes to dienamines is referred to in Section VII.B (Scheme 16) and VII.C (Scheme 30). 

Metallo 2-vinylcyclopropanes were introduced independently as a tool for the efficient preparation of 1,2-divinylcyclopropane skeletons in 1976 by the three groups mentioned above ". Either 2-vinylcyclopropyl lithium addition to 3-alkoxyenones " followed by acid treatment will lead to the desired rearrangement precursors (equation 166), or the corresponding cyclopropyl cuprates will add to j5-iodoenones and electrophilic acetylenes " forming directly divinylcyclopropanes (equation 167). 

Triphenylarsine oxide reacts with a number of electrophilic acetylenes having electron-withdrawing substituents in what are, in effect, reverse-Wittig reactions, thereby providing stable arsonium ylides (equation 31) Reaction is presumably initiated by Michael-type reaction of the oxide with the alkyne, as exemplified in equation 32. As would be expected from such a mechanism, use of an unsymmetric alkyne, as in the foregoing example, results in virtually regiospecific attack by the oxide to give the product shown. 

The pentasulfido complex reacts with tertiary phosphines to give dimers [Ti(MeCp)2]2Sj, (x = 4 and 6). The pentasulfido complex also gives heterocycles of the type Ti(MeCp)2S4CR2 when treated with ammonium sulfide in the presence of certain alkylating agents. In fact, the Me2CS4 chelate is a minor side product in the synthesis of Ti(MeCp)2Sj. The compound Ti(MeCp)2S5 reacts with electrophilic acetylenes to give dithiolenes... 

The original meaning of the term has been extended to the reversal of any commonly accepted reactivity pattern. For example, reaction of R-C=CX (X = halide) as a synthon for "R-C=C+ (i.e., electrophilic acetylene) is an umpolung of the normal, more common acetylide, R-C=(T (i.e., nucleophilic) reactivity. 

Furthermore, TTF (394)-(397) are accessible by the reaction of carbon disulfide with strained (398) or (399) (Scheme 74) <74JA945,89LA435) or of electrophilic acetylene derivatives (400) or (401) (Scheme 75) in the presence of an acid <70JA1412,73JA4379) or under high pressure <81JOC446>, thus... 

With two heteroatoms in the ring, it is again useful to look for a recognisable fragment containing both. Uracil (47), one of the bases in the nucleic adds, can be disconnected to urea and a suitable electrophilic fragment. Michael addition to an electrophilic acetylene is a suitable reaction. 

The great majority of o-acetylide transition metal complexes are prepared by interaction of a metal halide with acetylide, RC C", or the formal oxidative addition of terminal alkynes or alkynyl stannanes to the metal center. As amply demonstrated in the previous section, alkynyliodonium salts may serve as electrophilic acetylene equivalents. In other words, transition metal complexes may act as nucleophiles in reactions with alkynyliodonium species. Indeed, the reaction [81] of the square planar Vaska s complex, 106, and its Rh analog, 107, with a variety of alkynyliodonium triflates in toluene results in 89-96% isolated yield of the hexa-coordinate o-acetylide complexes, 108 and 109 [Eq. (58)]. Reaction is essentially instantaneous and occurs with retention of stereochemistry around the metal center. 

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