Activated acetylenes

Activated Acetylenes. The unstable 1 2 adduct formed from the reaction of triphenylphosphine and dimethyl acetylenedicarboxylate at -50°C is now thought to have the cyclopropene structure (22) the 

Carbonyls. The reaction of diphenylphosphine with methyl pyruvate gave (27), isolated as the oxide, whereas reaction of tributylphosphine 

3-Oxaphospholans (29) are obtained from secondary phosphines and aldehydes or ketones. Similarly, condensation of (o-aminophenyl)-phosphine with carbonyl compounds gave substituted 1,3-benzazaphos- 

Lithium diphenylphosphide in THF reacts with epoxides stereo-specifically. Quaternization of the phosphorus of the product generally led directly to the alkene with inversion of stereochemistry relative to the starting epoxide  

In a similar rearrangement dibenzophosphorin oxides (31) have been prepared from the reaction of dibenzophosph(ni)oles with methyl pro-piolate. Some ring expansion occurs even when R = benzyl, presumably because of the difficulty of putting benzyl in an apical position in the intermediate phosphorane. 

The reaction of triphenylphosphine with an excess of dimethyl acetylene-dicarboxylate gives not only the phosph(v)ole (32) but also a cyclo-pentenylidenephosphorane (33). 

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Although the emphasis in this chapter has been on tbe synthesis and mechanism of formation of simple enamines, brief mention will be made of the addition of amines to activated acetylenes to indicate the interest and activity in this area of substituted enamines. Since such additions tend to be stereospecific, inclusion in this section seems apropos. The addition of amines to acetylenes has been much studied 130), but the assigning of the stereochemistry about the newly formed double bond could not be done unequivocally until the techniques of NMR spectroscopy were well developed. In the research efforts described below, NMR spectroscopy was used to determine isomer content and to follow the progress of some of the reactions. 

A more conventional cycloaddition occurs with activated acetylenes, however, the intermediate cyclobutene adducts undergo rearrangement to give insertion of two carbon atoms into the enamine chain (55). Thus the enamine (16) reacted with methyl propiolate to give the dienamino ester (73), presumably via the cycloaddition product (65a). 

Substituted 1-benzoxepins can be obtained by the cycloaddition of activated acetylenes to the benzofuran system. When 2-(Af-mcthylanilino)benzofurans is treated with dimethyl acetylene-dicarboxylate, substituted 1-benzoxepins 1 are obtained in reasonable yield.180,181 This reaction presumably involves a 2a,7b-dihydrocyclobuta[6]bcnzofuran as an intermediate (see Section 4.2).182... 

The 2a,7b-dihydrocyclobuta[7>]benzofuran structure, obtained by addition of activated acetylenes to benzofurans (see Houben-Weyl, Vol.E6b, p 148), isomerizes thermally to a 1-benz-oxepin182 184 185 which reacts further to 1-naphthol derivatives at higher temperature182,185 (see Section 1.2 ). Under photochemical conditions, the 1-benzoxepins undergo ring contraction to the starting material.182 184... 

The reaction takes place probably by a kind of inverse Wittig reaction , corresponding to the thermal dissociation of an oxaphosphetene resulting from a [2+2] cycloaddition between the phosphine oxide and the activated acetylenic compounds (Scheme 2) [11,12]. 

Activated Acetylenes. The reaction of triphenylphosphine with phenylacetylene has been investigated in more detail.The rearrangement has been shown to proceed via the vinylphosphonium salt (30). 

A. Preparation.—The first reverse Wittig olefin synthesis has been reported. Triphenylphosphine oxide and dicyanoacetylene at 160 °C gave the stable ylide (1 78%) the reaction was reversed at 300 °C. No comparable reaction was observed with a variety of other activated acetylenes but tri phenyl arsine oxide gave the corresponding stable arsoranes with dicyanoacetylene (— 70 °C), methyl propiolate, hexafluorobut-2-yne, dimethyl acetylene dicarboxylate, and ethyl phenylpropiolate (130 °C). 

In contrast to the reaction of an i72-CS2-rhodium complex with dimethyl acetylenedicarboxylate which gives rise to a metallocycle,186 the iron complexes 103 are converted by activated acetylenes into air-sensitive carbene complexes 104. Decomposition of the latter in air provides an unusual synthetic route to substituted tetrathiofulvene derivatives (Scheme 121).187... 

Functionalized isoxazoles were obtained in good yields from activated acetylenes and alkyl 2-nitroethanoates in the presence of triphenylphosphine <06TL1627>. The use of PPI13/DDQ offers a neutral and highly efficient method for the conversion of 2-hydroxyaryl aldoximes and ketoximes to 1,2-benzisoxazoles in excellent yields at room temperature <06TL8247>. 

Activated acetylenes are definitely more reactive towards hydroplumbation than... 

Details have appeared39 of the addition of PH-tetraoxyspirophosphoranes to aldehydes and activated ketones. These phosphoranes also add to activated acetylenes,40 e.g. diethyl acetylenedicarboxylate with (50) gives a mixture of cis- and trans-(51). Further addition of (50) to (51) at higher temperatures gives the bisphos-phorane (52). Metallation of the spirophosphorane (53) followed by reaction with... 

With non-activated acetylenes (R- -R (a)) the best resnlts are provided by irradiation with visible light (400 nm) at 40-70°C, giving exclnsively the E isomers. On irradiation in the near-nltraviolet (300 nm), lower yields are obtained, probably becanse of the reverse reaction to the starting materials. 

The addition to activated acetylenes (R- -R (b)) proceeds even npon irradiation at 300 nm. In this case, the intermediate vinyl radical is stabilized by the carbonyl, vinyl or aryl gronps, and the reverse reaction to the starting materials is snppressed. 

A final example Illustrates the versatility that transition metal sulfur systems may provide. An activated acetylene has been shown(68 ) to Insert Into a metal-sulfur bond In Mo202S2(S2)2 forming a vinyl disulfide chelating ligand. 

The reactions of polysulfido complexes with activated acetylenes have been studied in some detail (4, 5). 1,4- (MeCp2Ti)2(S2)2 reacts with dimethylacetylene dicarboxylate (DMAD) to yield MeCp2TiS2C2R2... 

Reduction of a, -acetylenic ketones with chiral borane NB-Enanthrane prepared by addition of 9-borabicyclo[3.3.1]nonane to the benzyl ether of nopol yielded optically active acetylenic alcohols in 74-84% yields and 91-96% enantiomeric excess [770]. Another way to optically active acetylenic alcohols is reduction with a reagent prepared from lithium aluminum hydride and (2S, 3R)-( -I- )-4-dimethylamino-3-methy 1-1,2-dipheny 1-2-butanol. At —78° mainly R alcohols were obtained in 62-99% yield and 34-90% enantiomeric excesses [893]. 

Whereas, cii-divinyl epoxides are reactive and well known to undergo thermal Cope rearrangement, the trans isomers are significantly more stable. White and co-worker (24) showed that thermolysis of divinyl epoxides such as 88 could generate the corresponding carbonyl ylide and that it could be intercepted by the addition of an activated acetylene to give the corresponding dihydrofuran 89, albeit in modest yield. 

Decomposition of diazoketone 110 with rhodium acetate produced the highly electrophilic rhodium stabilized metallocarbenoid that suffers attack by the Lewis basic oxygen of the pendant ketone, producing cyclic carbonyl ylide 111. This ylide was trapped by the addition of an activated acetylene such as DMAD to furnish... 

The enhancement of thermal stability as measured by these isothermal aging experiments by the addition of bisbenzocyclobutenes to dienophilic thermosetting monomers has also been observed with bis activated acetylenes and dicyanates. The isothermal weight loss results obtained on a one to one mole... 

Reaction of triazolopyrimidinium ylides (256) with active acetylenes gave the 1 2 adducts 260. The formation of 260 may occur in two ways the shortest pathway consists of the double 1,3-dipolar cycloaddition of the diylide 256A with two molecules of the acetylene at two different sites to form the tetracyclic adduct 257, followed by ring opening under basic conditions to give 260. The second pathway consists of cycloaddition between the ylide carbanion of 256 and the bridged carbon C-4 to form the 1 1 adduct 258, which isomerizes to the more stable compound 259, which may be formed directly by the cycloaddition at the ylide carbanion and C-2. The second cycloaddition afforded the 1 2 adduct 257 [87JCS(P1)2531] (Scheme 49). 

Although there is no direct evidence for the postulated intermediate 3 in this example, there is evidence for the existence of analogous ylids in the aziridine series (see Section II, B), and in some reactions of pyridine-type heterocycles with DMAD intermediates corresponding to 3 have been trapped. The formation of intermediates similar to 3 is the commonest first stage in reactions of nitrogen-containing heterocycles with activated acetylenes. 

For many years15 activated acetylenes have been known to undergo Diels-Alder reactions, which take place8,9 by concerted processes. The stereochemistry of the pyrazoline 9, obtained as sole product from the cw-aziridine 7 by heating in the presence of DMAD, can only be accounted for if opening of the ring to the ylid 8 is followed by a concerted, or extremely rapid, combination with the acetylene.16... 

Pyridazine betaines (189) do not appear to react with activated acetylenes.327 Pyridazine ylids, prepared in situ from salts such as 190,... 

Acetylene Polymers Homopolymers of optically active acetylenes, including (/ )-153 synthesized by [RhCl(norbomadiene)]2 catalyst, show intense CD bands in the UV-visible region, probably based on a predominant helical sense of the main chain [204]. Excess single-handed helicity of the main chain can be induced for polymers of achiral acetylenes (154 and 155) by adding chiral molecules. The chiral induction is based on acid-base interaction or complex formation between the polymer and the additives [205-2081. 

The thiirane donates sulfur to the thioketone, thus furnishing olefin and thiobenzophenone 5-sulfide (122) which can be intercepted in 1,3-dipolar cycloadditions to activated acetylenes or thiones455. 

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