Addition reactions, cyclic alkynes

Complex 3c, a catalytic precursor for addition reactions to alkynes (65), reacts at room temperature with a variety of terminal alkynes in alcohols to produce stable alkoxyl alkyl carbene ruthenium(II) derivatives 109 in good yields (Scheme 7). Reaction of 3c (L = PMe3), with trimethylsilyacetylene in methanol gives the carbene ruthenium complex 110, by protonolysis of the C—Si bond, whereas with 4-hydroxy-l-butyne in methanol the cyclic carbene complex 111 is obtained (65,66). 

Heteronuclear addition reactions to alkynes usually produce the corresponding trans-alkene derivatives. However, in the case of strained cyclic alkynes, these would be even more heavily strained than the starting alkyne. Thus, in most reactions, cis-products are observed. The initial addition of the electrophile may, however, still proceed in an anh -manner, as was shown by Krebs et al. [1 b] for the reaction of a cycloheptyne derivative with trichloromethylsulfenyl chloride (Scheme 8-21). This reaction produced the first seven-membered rm s-cycloalkene derivative to be isolable at room temperature. 

What intermediate could we use to replace the highly unstable vinyl cations in addition reactions involving alkynes Perhaps cyclic intermediates are involved. Moreover, alkynes are known to form complexes wth HX acids. A cyclic protonium ion or complex could accommodate the complicated kinetics, which show that more than one molecule of halide is involved, and also account for the generally observed predominance of trans addition (Fig. 10.64). We will write cyclic ions in the answers to problems, but you should know that the intermediacy of vinyl cations in these reactions is not a fully resolved issue. There is still lots to do in organic chemistry. 

NbBrs, and NbCls-Pl Sn evidently proceeds via cyclotrimerization of diynes, which most probably involve cyclic carbometallation, details are not very clear.246 2463 Related reactions of Ta and Mo complexes were also investigated in this study. Formation of tantallacyclopropenes by complexation of alkynes with Ta complexes has also been reported247 (Scheme 51). In addition to the Ta-catalyzed polymerization of diynes mentioned above, Ta-catalyzed or -promoted cyclotrimerization reactions of alkynes to produce benzene derivatives, a Ta-promoted ethylene... 

Furthermore, the successful [3+2+1] cycloaddition of alkynes bearing a cyclopropane ring and a carbene complex unit has been reported. These benzannulations result in the formation of bimetallic naphthohydroquinone chromium tricarbonyl complexes [48]. Additionally, (non-strained) cyclic alkynes are potent reaction partners in the cycloaddition of chromium carbene complexes [49]. 

In the photoaddition of a saturated hydrocarbon to ethyl propiolate (equation 19) it is likely that the excited state of the acetylenic ester initiates reaction by abstracting a hydrogen atom from the hydrocarbon. The addition of cyclic ethers to an alkyne seems similar (equation 20), although a ketone sensitizer is required for addition of tetrahydropyran or dioxan . When reactions of this type involve a conjugated acetylenic ester, the first-formed a,p-unsaturated ester can normally undergo further photochemical reaction to produce the p,y isomer (see equations 19 and 20). 

Similar reactions have been carried out on acetylene.In an interesting variation, thiocarbonates add to aUcynes in the presence of a palladium catalyst to give a p-phenylthio a,p-unsaturated ester.Aldehydes add to alkynes in the presence of a rhodium catalyst to give conjugated ketones.In a cyclic version of the addition of aldehydes, 4-pentenal was converted to cyclopentanone with a rhodium-complex catalyst. An intramolecular acyl addition to an alkyne was reported using silyl ketones, acetic aid and a rhodium catalyst. In the presence of a palladium catalyst, a tosylamide group added to an alkene unit to generate A-tosylpyrrolidine derivatives. 

Nickel tetracarbonyl undergoes a rapid oxidative addition of the Si-Si bond of 1, highly strained fluorinated disilane, at room temperature to give ffve-membered cyclic bis(organosilyl)nickel(II) complex 2, which then reacts with terf-buty-lacetylene to give six-membered disilacyclohexadiene derivatives 3 as a mixture of the regioisomers (Eq. 1) [10]. A similar bis-silylation reaction of alkynes with bis(organosilyl)nickel(II) complex has been reported in the reaction of bis(trichlorosilyl)(bipy)nickel(II) (bipy 2,2 -bipyridyl), which is prepared by dialkyl(bipy)nickel(II) with trichlorosilane [11]. 

Addition reactions of the Si-Si bonds across carbon-carbon triple bonds have been most extensively studied since the 1970s by means of palladium catalysts. In the early reports, palladium complexes bearing tertiary phosphine ligands, mostly PPh3, were exclusively employed as effective catalysts, enabling the alkyne bis-silylation with activated disilanes, i.e., disilanes with electronegative elements on the silicon atoms such as hydro [36], fluoro [37], chloro [38], and alkoxy-disilanes [39,40] and those with cyclic structure (Scheme 4) [41-44]. The bis-silylation reactions could be successfully applied to terminal alkynes and acetylenedicarboxylates to give (Z)-l,2-bis(silyl)alkenes, which are otherwise difficult to synthesize. 

Types of reactions we meet elsewhere in the book include the reduction of a,p-unsaturated carbonyl compounds prepared by the aldol or Wittig reactions. The cyclic enone 14, prepared by a Robinson annelation is reduced regio- and stereo-selectively by LiAlH4 to the allylic alcohol2 15. The reduction of acetylenic alcohols 16, prepared by addition of metallo-alkynes to aldehydes, also with LiAlH4 is -selective (chapter 15) giving the allylic alcohol3 -17. 

Carbocarbonylation reactions involve dicarborative alkene or alkyne addition reactions with the introduction ol at least one carbonyl group. In this way, linear and cyclic carbonyl and carboxyl systems can be generated1,2. 

Alkenes lead to 1,4-dihydropyridazines (347) or the spiro derivatives (348). Cyclic alkenes, such as cyclopropenes give rise to diazanorcaradienes (349) or higher homologues, and reaction with alkynes yields pyridazines (350). Hetero-27r-systems such as nitriles, imines, thioketones, and N-sulflnylamines open the way to heterocyclic six-membered systems (351), (353), (357), and (356). (4-1-1) Addition reactions of isocyanides and carbenes make isopyrazoles (354) and (355) available. Reduction processes lead to 1,4- or 1,6-dihydrotetrazines (340) or (352). The reactions of 1,4-dihydrotetrazines (340) were briefly collected in Scheme 57. 

The first step in the mercuric-ion-catalyzed hydration of an alkyne is formation of a cyclic mercurinium ion. (Two of the electrons in mercury s filled 5d atomic orbital are shown.) This should remind you of the cyclic bromonium and mercurinium ions formed as intermediates in electrophilic addition reactions of alkenes (Sections 4.7 and 4.8). In the second step of the reaction, water attacks the most substituted carbon of the cyclic intermediate (Section 4.8). Oxygen loses a proton to form a mercuric enol, which immediately rearranges to a mercuric ketone. Loss of the mercuric ion forms an enol, which rearranges to a ketone. Notice that the overall addition of water follows both the general rule for electrophilic addition reactions and Markovnikov s rule The electrophile (H in the case of Markovnikov s rule) adds to the sp carbon bonded to the greater number of hydrogens. 

A characteristic feature of the reactions of cyclic alkynes is the tendency to release ring strain by changing the hybridization at the alkyne carbons from sp to sp. This is achieved either by rearrangement reactions or by intra- or intermolecular addition reactions. 

The triple bonds in cyclic alkynes can, of course, be subjected to all known addition reactions of acetylenes. Here, we will discuss examples which either lead to particularly interesting addition products or demonstrate unusual reactivity of bent triple bonds. 

In contrast, when thiophenes react with triosmium carbonyl clusters it is C-H bond activation that occurs in preference to C-S bond cleavage. The direct reaction of [Os3(CO)i2], or the -acetonitrile complex [Os3(CO)io(NCMe)2], with thio-phene results in the oxidative addition of a C-H bond to give the exo- and endo isomers of the n, -thienyl hydrido cluster [Os3(//-H)(/z-C4H3S)(CO)io] (21a, 22a) (Scheme 7). These isomers are in rapid equilibrium at room temperature. The complex [Os3(/i-H)2(/X3-C4H2S)(CO)9] (23a) also results from his reaction, presumably by additional thermal C-H bond activation and decarbonylation of the thienyl complex. This cluster contains a triply bridging cyclic alkyne ligand related to /i3-benzyne. " ... 

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