Alkynes, strained

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

Best efficiencies under the standard quasi-catalytic PK conditions are gained with electron-poor alkynes, strained olefins (e. g., norbomene), and open-chain olefins with low substitution (e. g., ethylene). Ethylene has the advantage that it can be applied in large excess by pressurizing the reaction system. Equations (13) and (14) are standard examples. 

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... 

Photolysis of 1-tosylamino-1,2,3-triazole anions leads to loss of two molecules of nitrogen and the generation of an alkyne. Strained cycloaUcynes, for example 22, have been generated in this way and trapped as Diels-Alder type adducts. ... 

The novel intramolecular reaction of the alkenyl bromide with the terminal alkyne in 328, followed by intramolecular Diels-Alder reaction, afforded the highly strained dynemicin A structure 329 in one stepf237]. 

In addition to alcohols, some other nucleophiles such as amines and carbon nucleophiles can be used to trap the acylpalladium intermediates. The o-viny-lidene-/j-lactam 30 is prepared by the carbonylation of the 4-benzylamino-2-alkynyl methyl carbonate derivative 29[16]. The reaction proceeds using TMPP, a cyclic phosphite, as a ligand. When the amino group is protected as the p-toluenesulfonamide, the reaction proceeds in the presence of potassium carbonate, and the f>-alkynyl-/J-lactam 31 is obtained by the isomerization of the allenyl (vinylidene) group to the less strained alkyne. 

The skeletal rearrangement of various strained cyclic compounds is carried out with a catalytic amount of soluble complexes of PdCl2. Namely, the rearrangements of bulvalene (67) to bicyclo[4.2.2]deca-2,4,7,9-tetraene (68)[54], cubane (69) to cuneane (70)[55], hexamethyl Dewar benzene (71) to hexa-methylbenzene (72)[56], and 3-oxaquadricyclanes[57] and quadricyclane (73) to norbornadiene[58-60] take place mostly at room temperature. Reaction of iodocubane (74) with a terminal alkyne catalyzed by Pd(0) and CuBr unexpectedly affords an alkynylcyclooctatetraene 75, without giving the desired cubylalkyne 76. Probably the rearrangement is a Pd-catalyzed reaction[61]. 

Hydroxy-THISs react with electron-deficient alkynes to give nonisol-able adducts that extrude carbonyl sulfide, affording pyrroles (23). Compound 16 (X = 0) seems particularly reactive (Scheme 16) (25). The cycloaddition to benzyne yields isoindoles in low- yield. Further cyclo-addition between isoindole and benzyne leads to an iminoanthracene as the main product (Scheme 17). The cycloadducts derived from electron-deficient alkenes are stable (23, 25) unless highly strained. Thus the two adducts, 18a (R = H, R = COOMe) and 18b (R = COOMe, R = H), formed from 7, both extrude furan and COS under the reaction conditions producing the pyrroles (19. R = H or COOMe) (Scheme 18). Similarly, the cycloadduct formed between 16 (X = 0) and dimethylfumarate... 

AUylic organoboranes react via cyclic transition states not only with aldehydes and ketones, but also with alkynes, aHenes, and electron-rich or strained alkenes. Bicyclic stmctures, which can be further transformed into boraadamantanes, are obtained from triaHyl- or tricrotylborane and alkynes (323,438,439). 

The behavior of strained,/Zuorimiret/ methylenecyelopropanes depends upon the position and level of fluorination [34], l-(Difluoromethylene)cyclopropane is much like tetrafluoroethylene in its preference for [2+2] cycloaddition (equation 37), but Its 2,2-difluoro isomer favors [4+2] cycloadditions (equation 38). Perfluoromethylenecyclopropane is an exceptionally reactive dienophile but does not undergo [2+2] cycloadditions, possibly because of stenc reasons [34, 45] Cycloadditions involving most possible combinations of simple fluoroalkenes and alkenes or alkynes have been tried [85], but kinetic activation enthalpies (A/f j for only the dimerizations of tetrafluoroethylene (22 6-23 5 kcal/mol), chlorotri-fluoroethylene (23 6 kcal/mol), and perfluoropropene (31.6 kcal/mol) and the cycloaddition between chlorotnfluoroethylene and perfluoropropene (25.5 kcal/mol) have been determined accurately [97, 98] Some cycloadditions involving more functionalized alkenes are listed in Table 5 [99. 100, 101, 102, 103]... 

The Pauson-Khand reaction was originally developed using strained cyclic alkenes, and gives good yields with such substrates. Alkenes with sterically demanding substituents and acyclic as well as unstrained cyclic alkenes often are less suitable substrates. An exception to this is ethylene, which reacts well. Acetylene as well as simple terminal alkynes and aryl acetylenes can be used as triple-bond component. 

Treatment of 10-bromodibenz[fe,/]oxepin with potassium rm-butoxide at 100 C in dioxane results in elimination of hydrogen bromide to give 10,11-didehydrodibenz[6,/ oxepin. This highly strained alkyne intermediate can be trapped with dienophiles, for example, with 2,5-dimethylfuran to give 4.160... 

Strained Cyclic Alkynes, and Strained Cyclic Cumulenes. 

An X-ray structure analysis of 74 (R=C4Hg) revealed that the unsaturated portion of the molecule was planar, with the angles between adjacent acetylenic bonds deviating by 13 -15° from 180°, the value for a strain-free molecule. Since the connection of the alkyne moieties to the aromatic rings was only shifted slightly (2-3°), distortion of the acetylene linkages appears as the major source of instability in these macrocycles. 

Strained cyclic alkynes are the most powerful olefinic 7r-donors for gold(i) centers. This has been demonstrated for the cycloheptynes shown in Scheme 80. The Au-C distances are in the range from 2.050 to 2.100 A.318 There is also evidence for the coordination of AuCl to Me3Si-C=C-SiMe3, but no details are available.319... 

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