Hexafluoro-2-butyne, reaction with

Alkynes substituted with one or two trifluoromethyl groups are also highly reactive dienophiles [9] Indeed, hexafluoro-2-butyne is used increasingly as a definitive acetylenic dienophile in "difficult Diels-Alder reactions. It was used, for example, to prepare novel inside-outside bicycloalkanes via its reaction with cir,trnns -l,3-undecadiene [74] (equation 67) and to do a tandem Diels-Alder reaction with a l,l-bis(pyrrole)methane [75] (equation 68) Indeed, its reactions with pyrrole derivatives and furan have been used in the syntheses of 3,4-bis(tri-fluoromethyl)pyrrole [76, 77] (equation 69) and ],4-bis(trifluoromethyl)benzene-2,3-oxide [78] (equation 70), respectively. 

The complex reacts readily with ligands such as tertiary phosphines, phosphites, or arsines to give substitution of a CO ligand on the cobalt atom. With CO, however, the complex reacts to give 2-phenylindazolone and 3-phenyl-2,4 lif, 3f/)-quinazolinedione,15 whereas the reaction with CO and hexafluoro-2-butyne affords an anilinoquinoline, probably via an intermediate complex in which the alkyne and CO have inserted into the Co—C bond.16... 

At present no reactions of a A3-phosphorin are known in which the phosphorus atom is not involved. A remarkable reaction with hexafluoro-2-butyne leads to the 1 -phosphabar-relene derivatives (41) by 1,4-addition (equation 27) (68AG(E)733). Singlet oxygen seems to react in a similar way. By a totally different method, i.e. refluxing hexafluoro-2-butyne with red phosphorus, a 1,4-diphosphabarrelene derivative (42) was synthesized (61JA3432). 

Arsenin, antimonin and bismin react readily with dienophiles in the Diels-Alder reaction, and the reaction with hexafluoro-2-butyne and other acetylenic dienophiles has been used to trap these heterocycles as barrelenes (115), in particular the unstable bismin (equation 21). The reactivity increases with increasing size of the heteroatom and this may be related to decreased aromatic character of the heterobenzenes with increasing size of the heteroatom. In fact, bismin and antimonin are so reactive that at low temperatures (< -10 °C and < - 50 °C respectively) both exist as Diels-Alder dimers (116 equation 20). 

Substitution reactions at Os(CO)4( -alkene) or Os(CO)4 ()] -alkyne) take place through initial dissociation of a ligand. The complexes Os(CO)4(jj -alkyne), where the alkyne is CF3CSCCF3 or HC=CH, compounds are more reactive than Os(CO)5 in substitution and insertion reactions. The acetylene complex is 10 times more reactive than the hexafluoro-2-butyne complex. This is probably due to the ability of the alkyne to act as a four electron donor and stabilize electron-deficient intermediates. The reaction of Os(CO)4( -HC CH) with excess PMes gives a CO insertion product 0s(C0)2(PMe3)2 C(H)=C(H)-C(0) while reaction with the bulkier phosphine PBuj gives a double insertion product, 0s(C0)3(PBu ) C(0)-C(H)=C(H)-C(0) (Scheme 9). ... 

Only perfluorocyclopentene has been synthesised directly by this route it can be seen that, here, allylic rearrangements can occur to make all positions potentially vinylic and therefore reactive [64]. An analogous situation applies to hexachlorobutadiene. These reactions may also be carried out with potassium fluoride that has been exposed to Sulpholan or 18-crown-6, but then suspended in a fluorocarbon. Under these conditions, a significant proportion of hexafluoro-2-butyne is formed, presumably because the latter is extracted into the fluorocarbon, pre-empting further reaction with fluoride [65] (Figure 2.14). 

Hexafluoro-2-butyne undergoes 1,4 addition by reaction with Rh(dik)(diolefin) (dik = acac, dpm, dbm diolefin = cod, norbornadiene), with formation of a C—C bond between the alkyne and the methyne carbon atom of the /3-diketonate. Additional trimer-ization of hexafluoro-2-butyne to hexakis(trifluoromethyl)benzene and /j -arene coordination of the latter to rhodium affords 103 (equation 73). 

DMAD), methyl propiolate or l,l,l trifluoropropyne or hexafluoro-2-butyne at 145-250 C. Ene reactions with DMAD give only one product, while reactions with methyl propiolate give mixtures of re-gioisomers. Reaction of methyl propiolate with 1-heptene for 30 h at 200 C gives a 4 1 mixture of (25a) and (26a) in 30% yield. Similar reaction with isobutene gives a 94 6 mixture of (25b) and (26b) in 50% yield. Reaction of limonene with methyl propiolate for 24 h at 160 °C gives the ene adduct in 70-80% yield.2 ... 

Surprisingly few examples of the use of allenes as the ene components in ene reactions have been reported. In a series of papers, Taylor and coworkers reported that alkylallenes undergo thermal ene reactions with DMAD, hexafluoro-2-butyne and 3,3,3-trifluoropropyne to give cross-conjugated trienes. Similar reactions occur between allenes and hexafluoroacetone and electron-deficient azo compounds. " ... 

When bis(trifluoromethyl)tetramethylbicyclo[2.2.2]octatriene, ob-tained from the reaction of durene with hexafluoro-2-butyne, interacts with Co(CO)2(C5H5), the only product obtained is that of structure (168) (343). 

The reaction with triphenylphosphine oxide cannot be extended to other negatively substituted acetylenes. However, the reaction with triphenylarsine oxide proceeds much more readily, and this reagent reacts with methyl propiolate, dimethyl acetylene-dicarboxylate, ethyl phenylpropiolate, and hexafluoro-2-butyne to give products (2) in good yield (55-90%). 

The cationic complex (VII, X = Cl) forms a symmetrical ( Pnmr i.r.) complex with diphenylacetylene, which is very labile. The complexes [Rh2Cl2(/Lt-CO)(/Lc-acet)(dppm)2], where acet is hexafluoro-2-butyne or acetylenedicarboxylic acid dimethyl ester, are also symmetrical, and the acet ligand is coplanar with the rhodium atoms. The C C bond length is increased to 1.32 A, and so the complex is truly described as a cis-dimetallated alkene. Comparison with the reactions with CO suggests that the activation of an alkyne (or alkene) to hydrogenation may occur when the alkyne is initially coordinated to the catalyst at a terminal position, e.g., (XII), with subsequent oxidative addition and hydrogen transfer steps. A complex of the type (XII) may instead be only a minor but active component of a solution of which the alkyne-bridged complex is the major but less active component. ... 

However, the thermolysis of diacylfuroxans (429) yielded two types of nitrile Af-oxides. An uncrowded diacylfuroxan such as (429a) rearranged to the a- acyloximino nitrile A-oxide (430) the diacylfuroxan with bulky substituents such as in (429b) gave rise to the half molecule acyl nitrile Af-oxide (431). Both types of nitrile Af-oxides (431) and (430) have been trapped with DMAD and hexafluoro-2-butyne to give isoxazoles in good yield. These reactions are shown in Scheme 97. 

Other examples of addition reactions that form carbon-nitrogen bonds are the reactions of hexafluoro-2-butyne with various heterocydes [77], aromatic... 

Thermal insertion reactions of dihydndobis(T) - cyclopentadienyl)molybdenum with tnfluoromethylacetylene give exclusively syn addition At —40 °C, hexafluoro-2-butyne inserts to the Mo-H bond to give anti addition [40] equation 30)... 

In addition to complex-formation, the interaction of transition-metal atoms with organic substrates at low temperatures can result in rearrangement of the organic moiety without complexation. Two such reactions have already been briefly mentioned, namely, the polymerization of hexafluoro-2-butyne by Ge and Sn atoms (72) and the polymerization of styrene by Cr atoms (i 1). In this section we shall briefly summarize some of these transition-metal-atom-promoted, organic rearrangements. 

A more complex reaction is involved in the cooligomerization of acetylenes and tert-butyl isocyanide using nickel acetate as the catalyst (Scheme 20)43 the nature of intermediate complexes leading to the formation of 2-cyano-5-terf-butylaminopyrroles has not been established. Cocyclization of tert-butyl isocyanide with coordinated hexafluoro-2-butyne gives rise to coordinated cyclopentadienone anils for molybdenum systems,44 hence the nature of acetylene substitutents and of the organometallic catalyst play crucial roles in these processes. The pyrrole products from the former reaction can be decomposed by sulfuric acid and the overall sequence provides a simple synthesis of 5-amino-2-cyanopyrroles (Scheme 20). 

The reaction of tetrafluoroallene (61) with hexafluoro-2-butyne (5 equiv.) under autogenous pressure gave a mixture containing the desired [2 + 2]-cycloadduct 66 and the allene dimer 67 [64]. 

In the reversible Wittig reaction, triphenylarsine oxide reacted with electron-deficient acetylene derivatives to form stable ylides. Thus triphenylarsine oxide reacted readily with methyl propiolate, ethyl phenylpropiolate, dimethyl acetylenedicarboxylate, and hexafluoro-2-butyne as well as dicyanoacetylene to give arsonium ylides (12). The reaction temperatures required ranged from -70°C in the case of dicyanoacetylene to 130°C in the case of ethyl phenylpropiolate (15). 

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