Acetylenes across triple bonds

Unlike ethynylation, in which acetylene adds across a carbonyl group and the triple bond is retained, in vinylation a labile hydrogen compound adds to acetylene, forming a double bond. 

Vinylation. Acetylene adds weak acids across the triple bond to give a wide variety of vinyl derivatives. Alcohols or phenols give vinyl ethers and carboxyHc acids yield vinyl esters (see Vinyl polymers). 

However, although acetylene is an excellent feedstock because two molecules can be added across the triple bond, it is intrinsically hazardous because acetylene can spontaneously decompose... 

B-Bromo and B-iodo-9-borabicyclo[3.3.1]nonane add similarly in a cis fashion to terminal triple bonds 471 They do not react, however, with alkenes and internal acetylenic bonds. In contrast to the results mentioned above, phenyl-substituted chloroboranes (PhBCl2, Ph2BCl) do not participate in haloboration. Instead, the C—B bond adds across the multiple bond to form phenylalkyl-(alkenyl) boranes.466,468... 

Starch can be vinylated with acetylene in the presence of potassium hydroxide in an aqueous tetrahydrofuran medium.1 1 The mechanism possibly involves the addition of the potassio derivative of starch across the carbon-carbon triple bond of acetylene, with subsequent hydrolysis of the organometallic intermediate to give the vinyl ether. Such a mechanism has been postulated for the formation of vinyl ethers from monohydric alcohols and acetylene, in the presence of an alkali metal base as catalyst.1 2 The vinylation of amylose is very similar to the vinylation of amylopectin, except for the relative ratio of mono- to di-substitution. With amylopectin, the proportion of disubstitution is greater. In both starches, the hydroxyl group on C-2 is slightly more reactive than the hydroxyl group on C-6 there is little substitution at the hydroxyl group on C-3. 

A number of small unsaturated molecules or functional groups have been found to add across the M-M triple bond in Cp2Mo2(C0) to give adducts Cp2Mo2(C0) (un), where un = acetylenes ( [), allenes (42), cyanamides (43) and thioketones (44). In all of these adducts, the unsaturated fragments act as 4-electron donors to the dimetal center, thus reducing the M-M bond order from three to one. 

The fruitfulness of the idea of a stepwise addition with an independent variation of the addends was brilliantly illustrated by Normant s studies, which resulted in the elaboration of a general method of alkene synthesis based on the reaction of alkyne carbometallation. Basically this reaction represents a case of the well-known nucleophilic addition to a carbon-carbon triple bond. In the Normant reaction, however, the initial addition of a nucleophile (an organome-tallic reagent) across the triple bond results in the formation of a stabilized carbanion-like intermediate equivalent to a vinyl carbanion. This intermediate can similarly be further reacted with an external electrophile. Most typically, copper-modified Mg or Li reagents, which are unable to react with acidic acetylenic hydrogens, are used in this sequence. 

As a result of this process, the sequential addition of a carbon nucleophile and a carbon electrophile across a triple bond is achieved. For example, addition of cuprate 101 (Scheme 2.32) across the triple bond of acetylene produces the vinylcuprate intermediate 102. Quenching of the latter with electrophile 103 gave acetoxydiene 104, the active constituent of the pheromone Cossus cossus. The sequence exemplified in Scheme 2.32 enables independent variations in the structure of all participants involved, namely the alkyne, the organome-tallic nucleophile, and the electrophile. Therefore this approach can serve as a unified protocol for the one-pot assemblage of various alkenes from simple precursors. 

Reaction with acetylenes (1, 1021-1022). Gensler et at. as well as several other groups report that they were unable to. repeat the synthesis of sterculic acid by addition of methylene across the triple bond of stearolic acid. Gensler has effected the synthesis in about 30% overall yield by the following six-step route ... 

Novel photochemical (and thermal) reactions of macrocyclic oxa-sila-acetylenic ring systems (expected to show unusual optical properties because of electronic effects arising from orbital overlap of the acetylenic n system with the silicon a bonds and the oxygen lone-pair electrons) were described. While thermolysis in the presence of a transition metal carbonyl compound gave cyclization to both benzenoid and fulvene species, photolysis in the presence of the transition metal carbonyl compound (which catalyzes 1,2-silyl shifts across a carbon-carbon triple bond) gave fulvene and vinylidene products, the latter being readily photolyzed to the fulvene 159 (equation 101). 

These observations support the view that the protonation occurs by initial oxidative addition at the metal atom and subsequent transfer of hydrogen to the coordinated alkyne which is argued on the basis of theory as well as experiment . In fact, the reaction of CH3COOH with (Ph3P)2Pt(DMA) (DMA = dimethylacetylene dicarboxylate) in the presence of PPh3 proceeds by addition of triphenylphosphonium acetate across the acetylenic triple bond ... 

Although benzeneselenol adds across the triple bond of phenylacetylene in the presence of molecular oxygen to give the corresponding vinyl selenide (equation 15), inactivated acetylenes proceed very slowly under similar conditions". However, the addition of PhSeH to inactivated acetylenes has been achieved in the presence of a catalytic amount of diselenide upon irradiation (equation 16)". Evidence that these reactions proceed via a radical mechanism is also given. Tris(phenylseleno)borane and tris(methylseleno)borane react also with terminal acetylenes to give (Z)-vinyl selenides". ... 

Addition of thiols in basic solution to the C=C—C=0 bond system in a,/3 imsaturated ketones, such as 4-benqrlidene-l-buQrl-pyrrolidine-2,3-dione, has been observed, forming with benzenethiol in piperidine, l-butyl-3-hydroxy-4(a-phenylthiobenzyl)-3-pyrrolin-2-one . Addition of thiols primarily to the C=C bond in C=C—C=0 systems in quinones and lactones has been observed . The reactions were studied in neutral or alkaline solution and probably involve attack by the thiolate anion. In compounds containing both carbonyl or carboxyl groups and acetylenic triple bonds, addition occurs primarily across the acetylene bond. Cyclization of the initial product so formed is also observed . ... 

It is a short step from acetylene to ethylene, but instead of constructing a correlation diagram for addition of H2 across the acetylenic triple bond, let us consider the dimerization of two coplanar methylene molecules. For consistency with the axis convention of Figs. 4.1-4.4, the methylene molecules on the the left side of Fig. 4.7 are placed in in the zx plane and allowed to approach each other along their common C2 axis (x), leaving the Py orbitals free for tt bonding. The symmetry of the field exerted on the electrons by the nuclear frame is D2/1 no hypothetical external field need be postulated. 

In spite of the fact that compounds having Si—bonds (silylboranes) were already prepared in the early 1960s,somewhat surprisingly, transition-metal-catalyzed reaction of the silylboration had not been reported until 1996. Suginome, Nakamura, and Ito have found that silaboration of acetylenes proceeds smoothly in the presence of a catalyst Pd(OAc)2/t-octyl NC. Under reflux in toluene, addition of the Si—bonds across the carbon-carbon triple bond of 1-octyne took place in the presence of the catalyst prepared from 2 mol % of Pd(OAc)2 and 30 mol % of 1,1,3,3-tetramethylbutyl isocyanide (Scheme 21). 

Thus, as shown in the first and second examples in Table 6.8, the addition of ethanethiol (thioethane, ethyl mercaptan, CH3CH2SH) as well as ethanol (CH3CH2OH) across the triple bond of ethyne (acetylene, HOCH) is catalyzed by the corresponding metal salts and requires high temperature (at which high pressures will develop). These reactions are not generally observed in the presence of acid. 

In contrast to the difficulty in producing cyclobutadiene by the simple [2 + 2] cycloaddition of alkynes, the addition of 1,3-dipolar species across the carbon-carbon triple bond of alkynes is quite facile. Thus, in addition to the fourth example of Table 6.9 showing that alkyl azides can add across the triple bond, reactions such as that of ethyne (acetylene, H-C=C-H) with isocyanic acid (fulminic acid. 

The hydride complex (CXLVIII) can be used for monoalkylation of acetylenic derivatives as well. The reaction course consists of cis addition of the rhodium hydride across the carbon-carbon triple bond, oxidative addition of alkyl iodide, and reductive elimination of the organic moiety (Schwartz et al 1972). 

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