Simple Alkynes

The dienoplules for reaction with butadiene can be alkenes, allenes, and alkynes. Simple alkenes like ethylene are poor dienoplules resulting in sluggish reactions. Substituted olefins, X—C=C—X, are more reactive when X and/or X are C=C, Ar, COOR, COOH, COH, COR, COCl, CN,... 

Radical Addition of Perhaloalkanes to Various Alkynes Simple Alkynes... 

Linear Addition Reactions toCsC Bonds 2.1.1.5.1. Radical Addition of Perhaloalkanes to Various Alkynes Simple Alkynes... 

Iron complexes favor the codimerization of BD with alkynes in a 1 1 ratio to (substituted) cyclohexadienes [39]. Two BD molecules and one alkyne give cyclodecatrienes with zerovalent nickel catalysts and good electron-donating ligands such as Ph3P [7, 40]. Ten-membered rings are in fact the principal products of such a reaction the variety of dienes seems to be limited to BD, iso-prene and 1,3-pentadiene, whereas numerous alkynes - simple alkyl-substituted alkynes, alkynes with aprotic functional groups, dialkynes, and cyclic alkynes... 

Out first example is 2-hydroxy-2-methyl-3-octanone. 3-Octanone can be purchased, but it would be difficult to differentiate the two activated methylene groups in alkylation and oxidation reactions. Usual syntheses of acyloins are based upon addition of terminal alkynes to ketones (disconnection 1 see p. 52). For syntheses of unsymmetrical 1,2-difunctional compounds it is often advisable to look also for reactive starting materials, which do already contain the right substitution pattern. In the present case it turns out that 3-hydroxy-3-methyl-2-butanone is an inexpensive commercial product. This molecule dictates disconnection 3. Another practical synthesis starts with acetone cyanohydrin and pentylmagnesium bromide (disconnection 2). Many 1,2-difunctional compounds are accessible via oxidation of C—C multiple bonds. In this case the target molecule may be obtained by simple permanganate oxidation of 2-methyl-2-octene, which may be synthesized by Wittig reaction (disconnection 1). 

No intennolecular reaction of malonate or /3-keto esters with halides has been reported, but the intramolecular reaction of /3-diketones such as 790 and malonates proceeds smoothly[652,653]. Even the simple ketone 791 can be arylated or alkenylated intramolecularly. In this reaction, slow addition of a base is important to prevent alkyne formation from the vinyl iodide by elim-ination[654]. 

Propargylic (or 2-alkynyl) compounds are derivatives of alkynes. However, Pd-catalyzed reactions of propargylic derivatives, particularly esters and halides, are very different mechanistically from those of simple alkynes, except in a few cases. Therefore, the reactions of propargylic esters and halides are treated in this section separately from those of other alkynes. However, some reactions of propargylic alcohols, which behave similarly to simple alkynes, are treated in Section 6. 

Evidence from a variety of sources however indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations and their involve ment m these additions has been questioned Eor example although electrophilic addi tion of hydrogen halides to alkynes occurs more slowly than the corresponding additions... 

Its reactions with olefins, governed by steric rather than electronic factors, are very sluggish. Even simple 1-alkenes require 8 h at 25°C for complete reaction. In contrast, alkynes are hydroborated with great ease to alkenylboranes, high steric requirements of the reagent preventing dihydroboration (117). 

The addition proceeds in three discrete steps and the intermediates can be isolated. Simple alkenes are less reactive than alkynes and do not undergo the addition to aHylic boranes, but electron-rich alkyl vinyl ethers react at moderate temperatures to give 1,4-dienes or dienyl alcohols (440). 

Few isothiazoles undergo simple cycloaddition reactions. 4-Nitroisothiazoles add to alkynes (see Section 4.17.7.4). With 5-thiones (84) and dimethyl acetylenedicarboxylate, addition to both sulfur atoms leads to 1,3-dithioles (85) (77SST(4)339, 80H(14)785, 81H(16)156, 81H(16)595). Isothiazol-3-one 1-oxide and the corresponding 1,1-dioxide give normal adducts with cyclopentadiene and anthracene (80MI41700), and saccharin forms simple 1 1 or 1 2 adducts with dimethyl acetylenedicarboxylate (72IJC(B)881). 

The addition of phthalimidylnitrene (374) to simple alkynes affords 1-azirines in yields of 1-15% (Scheme 10). In this reaction, which is of no real preparative value, the symmetrical 2-azirines (375) were suggested as the most plausible intermediates and unequivocal proof of the existence of such species was demonstrated from a series of 1,2,3-triazole pyrolysis reactions <71CC1518). Extrusion of nitrogen from the regioisomeric 4,5-disubstituted 1,2,3-triazoles (376) during flash vacuum pyrolysis furnished identical product mixtures which included both regioisomeric 1-azirines (377). 

Further details must be added for a complete description, but these outlines encompass most reactions of alkynes with simple electrophiles. 

Various carbene-transfer reactions can be used with both electron-rich and electron-poor alkynes to make fluorinated cyclopropenes [9. 13, 79, 80, 81, 82] (Table 4). Haloacetylenes are too thermally unstable for most cycloaddition conditions, and simple fluorinated cyclopropenes are made by other methods [32, 45, 83, 84] (equations 30-32). 

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]... 

Alkynes are hydrocar bons that contain a carbon-car bon triple bond. Simple alkynes having no other functional groups or rings have the general formula C H2 -2. Acetylene is the simplest alkyne. 

Cycloadditioiis in which 1,2-dithietes acted formally as dienes are among the most typieal reaetions of 1,2-dithietes. Tire dithiete 144 is highly reaetive and eapable of reaetions even with simple alkenes and alkynes (60JA1515 61JA3434,61JA3438).Tlius, 144 reaeted with aeetylene to form 191 and 192 with the initial formation of 193, and with tetramethylethylene to give 194. Other [4 + 2] eyeloadditions of 144 involved those with ethylene, cyelohexene, fra s-stilbene, ethyl vinyl ether, butyl vinyl sulfide, 3-hexyne, and DMAD. 

From reaction of simple alkynes and alkenes four regioisomeric products 7a-d may be formed ... 

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. 

Alkynes. These contain a single triple bond and have the general formula C H, with n > 2., - Ikynes are also referred to as acetylinic compounds. The simple alkynes are alternatively named as derivatives of acetylene, e.g.,... 

Aliphatic (Section 3.2) A nonaromalic hydrocarbon such as a simple alkane, alkene, or alkyne. 

The total synthesis of ( )-estrone [( )-1 ] by Vollhardt et al. is a novel extension of transition metal mediated alkyne cyclotrimeriza-tion technology. This remarkable total synthesis is achieved in only five steps from 2-methylcyclopentenone (19) in an overall yield of 22%. The most striking maneuver in this synthesis is, of course, the construction of tetracycle 13 from the comparatively simple diyne 16 by combining cobalt-mediated and ort/io-quinodimethane cycloaddition reactions. This achievement bodes well for future applications of this chemistry to the total synthesis of other natural products. 

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

The simple cyclopropylmethoxycarbenechromium complex 142 reacts with alkynes to afford cyclopentenones 143 and 144 via the cyclopentadiene intermediate 145, which is hydrogenated with the aid of the chromium(O) residue and water (Scheme 31) [100-103]. Formation of 145 can be regarded as... 

Aryl- and alkenylcarbene complexes are known to react with alkynes through a [3C+2S+1C0] cycloaddition reaction to produce benzannulated compounds. This reaction, known as the Dotz reaction , is widely reviewed in Chap. Chromium-Templated Benzannulation Reactions , p. 123 of this book. However, simple alkyl-substituted carbene complexes react with excess of an alkyne (or with diynes) to produce a different benzannulated product which incorporates in its structure two molecules of the alkyne, a carbon monoxide ligand and the carbene carbon [128]. As referred to before, this [2S+2SH-1C+1C0] cycloaddition reaction can be carried out with diyne derivatives, showing these reactions give better yields than the corresponding intermolecular version (Scheme 80). 

The tin hydrides find important applications as reducing agents. Many of their reactions (particularly the reduction of alkyl halides and the hydrostannation of simple alkenes and alkynes) arc known to proceed through RaSn- intermediates, and this aspect of their chemistry is referred to in Section II,G. 

The simple hexaalkylditins, RsSnSnRs, do not disproportionate on heating, but, in oxolane (tetrahydrofuran) or acetonitrile in the presence of a base such as a Grignard reagent, or in the more strongly basic solvent hexamethylphosphoric triamide (HMPT), disproportionation readily occurs at room temperature, and, in HMPT, addition occurs to such alkynes as phenylacetylene and diphenylbutadiyne. The disproportionation is considered to proceed by nucleophilic attack upon tin (259, 260), e.g.,... 

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