Isocyanates alkynyl

A related cyclization of 2-(alkynyl)phenylisocyanates with terminal alkynes to oxindoles was also reported by the same group (Equation (115)).472 (E)-exo-olefinic oxoindoles are selectively obtained. It was proposed that a palladium acetylide generated by the C-H activation of terminal alkynes regioselectively inserts to the alkyne moiety and the resulting vinylpalladium intermediate adds to the C=0 part of the isocyanate to give a (Z)-oxindole. This (Z)-isomer is isomerized to the ( )-isomer under the reaction conditions through catalysis of the phosphine. 

We have reported on a tandem procedure for the synthesis of 3-allyl-N-(alkoxycarbonyl)indoles 115 via the reaction of 2-(alkynyl)phenylisocyanates 114 and allyl carbonates 5 in the presence of Pd(PPh3)4 (lmol%) and CuCl (4 mol%) bimetallic catalyst [80]. A proposed mechanism is shown in Scheme 35. Initially, the insertion of the isocyanates 114 into the complex 7, formed by the reaction of 5 with Pd(0), would form the 7r-allylpalladium intermediates 117. This intermediate, with Pd - N bonding, could be in equilibrium with the Pd - O bonded intermediates 118, which should more probably be represented as the bis-7r-allylpalladium analogue 119. Insertion of the alkyne then occurs to form the indoles 115 and the Pd(0) species is regenerated. It should be emphasized that no carboamination takes place at all in the absence of CuCl the product 116 was obtained. 

Addition of l,3-thiazolium-4-olates to (l-alkynyl)carbene tungsten complexes lb,n affords thiophene and/or pyridone complexes, 64 and 65, by extrusion of sulfur and isocyanate, respectively, from the [3+2] adduct formed initially (Scheme 21).97 [3+2] cycloaddition reactions of azomethine... 

Numerous theoretically interesting hydrocarbons, like iptycenes [137b], starphenylenes (see Scheme 41 [140]) and highly strained threefold bicycloan-nelated benzenes (see Scheme 40 [139 a, c]) were synthesized by this very versatile approach. In addition, when alkynes are brought into a cocyclization reaction with alkyl and aryl nitriles or co-alkynyl isocyanates validated routes to heterocycles become available as it was demonstrated e. g. for the total synthesis... 

Furthermore, Yamamoto and coworkers illustrated that o-(alkynyl)phenylisocya-nates 159 could also be efficiently employed in a similar Pt( 11)-catalyzed cycloisomerization reaction, serving as surrogates of the corresponding carbamate derivatives 160, to provide N-(alkoxycarbonyl)indoles 161 in moderate to excellent yields (Scheme 9.60) [219]. It is believed that a dual-role catalysis with the Pt(II) salt first triggered the initial intermolecular nucleophilic addition of alcohol to isocyanate 159, affording the key transient carbamate 160, which, upon a subsequent Pt(II)-catalyzed 5-endo-dig cyclization, generated the desired product 161. 

Alkenyl and alkynyl isocyanates have been prepared from amines and various car-bonylating agents. 3-Isocyanato-propene 416 was obtained either with diphosgene and activated charcoal by heating in ethyl acetate [264] or by using CO2 [276]. 

In this chapter, the scope, mechanism, and recent progress of zirconocene (Il)-mediated cyclization reactions are introduced. Zirconocene(II) is a very important reagent for organometallic chemistry, synthetic chemistry, and polymer chemistry. Zirconocene(II) is capable of coordinating with unsaturated compounds. Further reactions could lead to zirconocene(IV) species, zirconacycles, C-C bond formation, C-X bond formation, and synthesis of carbocycles and heterocycles. Zirconocene(lI)-mediated cyclization of bis(alkynyl)silane gives zirconacyclobu-tene-silacyclobutene complexes, which could react with alkyne, bis(alkynyl)silane, ketone, nitrile, and isocyanate and could be applied in the synthesis of various valuable products. 

An interesting palladium-catalyzed cascade reaction leading to 3,3-disubstituted oxindoles 190 starting from 2-(alkynyl)aryl isocyanates 189 with benzylic alcohols has been developed by Toyoshima et al. [76] (Scheme 6.53). This reaction integrates a cyclization step and a novel [1,3] rearrangement step. Both benzylic and allylic alcohols could be introduced efficiently to this domino process. Furthermore, the products of this reaction are an important class of heterocycles which are often found in naturally occurring and biologically active molecules. 

Heterocyclic 1,3-dipoles 32-34 also react with Fischer alkynyl carbene complexes to give pyrazolyl- and pyrrolyl-carbene complexes (Scheme 5.9) [18]. These reactions proceed along with elimination of carbon dioxide, isocyanate, and sulfur from the initially formed [3-1-2] cycloaddition products. When Fischer alkynyl carbene complexes 35 are treated with 1,3-dipole 33, pyrrolylcarbene complexes 36 are obtained. 

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