Cyclic alkynes from elimination reactions

Several thorough reviews of this topic have been published [1-3]. Here we will focus on new methods, typical examples, and particularly interesting molecules which show considerable ring strain. Molecules whose existence has been postulated only after isolation of trapping products [3 c] are not included. 

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In a reaction similar to the (>-alkoxide elimination reactions seen with zir-conocenes, catalytic Rh(OH)(cod)2 and 2 eq. of arylboronic acids gave cyclic products 165 from enynes 166 (Scheme 35) [100]. In this reaction, transmet-allation of Rh - OR with B - Ph gave Rh - Ph species 167, which inserted into the alkyne, cyclized to 168, and finally underwent [>-alkoxidc elimination to provide Rh-OCH3. This reaction is limited to the formation of five-membered rings, but it can also undergo cascade type reactions of enediynes to give multicyclic products [100]. 

To probe the reaction mechanism of the silane-mediated reaction, EtjSiD was substituted for PMHS in the cyclization of 1,6-enyne 34a.5 The mono-deuterated reductive cyclization product 34b was obtained as a single diastereomer. This result is consistent with entry of palladium into the catalytic cycle as the hydride derived from its reaction with acetic acid. Alkyne hydrometallation provides intermediate A-7, which upon cw-carbopalladation gives rise to cyclic intermediate B-6. Delivery of deuterium to the palladium center provides C-2, which upon reductive elimination provides the mono-deuterated product 34b, along with palladium(O) to close the catalytic cycle. The relative stereochemistry of 34b was not determined but was inferred on the basis of the aforementioned mechanism (Scheme 24). 

The combination of probably the oldest synthetic procedure for formation of a triple bond, i.e., the dehydrobromination of a vinyl bromide, with modern crown ether chemistry has resulted in one of the simplest yet very powerful methods for making highly strained cycloal-kynes. Thus, 1,5-cyclooctadiyne (56) can be made by treating l,5-dibromo-l,5-cyclooctadiene (55) with potassium rerf-butanolate in nonpolar solvents in the presence of 18-crown-6 [3 b, 24]. The nonpolar solvent protects the bent triple bond from nucleophilic attack by tert-butanol (Scheme 8-5). 1,5-Cyclooctadiyne had previously been made in very low yield by dimerization of butatriene (57), which is not a readily accessible compound [25]. Other important 1,2-elimination reactions generating cyclic alkynes are the oxidative degradation of... 

Other important cycloelimination procedures correspond to an elimination of H2O from cyclic ketones. Thus, the a-hydrogen atoms of semicarbazones of cyclic ketones are removed by oxidative cyclization with thionyl chloride or selenium dioxide (Scheme 8-7). The 1,2,3-thiadiazoles (71) or 1,2,3-selenadiazoles (72) which result from these reactions can be cleaved in a second step to yield cyclic alkynes (Scheme 8-8) [28]. Several fragmentation conditions are known, among them thermal decomposition and base-induced cleavage. The mechanism of these reactions has been studied in detaU [29]. It has been noted that the crucial step is the cleavage of the carbon-sulfur or carbon-selenium bond, as in this step the geometrical strain is introduced into the system. Clearly, due to the weakness of the C —Se... 

What reactions of halides do you know besides displacement The El and E2 reactions compete with displacement reactions (Chapter 7). If HCl were lost from chlorobenzene, a cyclic alkyne, called benzyne (or dehydrobenzene), would be formed. This symmetrical bent alkyne might be reactive enough to undergo an addition reaction with the amide ion. If this were the case, the labeled material must produce two differently labeled products of addition (Fig. 14.113), because the NH2 can attack either carbon of the alkyne. This mechanism is the elimination-addition version of the SnAt reaction. 

The cyclic enol ether 255 from the functionalized 3-alkynoI 254 was converted into the furans 256 by the reaction of allyl chloride, and 257 by elimination of MeOH[132], The alkynes 258 and 260, which have two hydroxy groups at suitable positions, are converted into the cyclic acetals 259 and 261. Carcogran and frontalin have been prepared by this reaction[124]. 

Under photochemical conditions several 1,4-dipoles reacted with alkenes, alkynes, phenyl isothiocyanate and carbon disulphide to afford, after iodobenzene expulsion, cyclic adducts in moderate to low yields. This drawback is offset by the fact that it is difficult to obtain some of these compounds through alternative methods. Table 10.6 are listed selected examples of cyclic compounds formed by this methodology. Formally, these reactions can be considered as 1,3-dipolar cycloadditions, in which the 1,3-dipole comes from the iodonium precursor after elimination of iodobenzene actually, they arise most probably through 6-membered intermediate iodanes. 

The cyclic oxo esters are very much a feature of osmium(VI) chemistry but analogous rings are rare with other elements they have been detected for ruthenium(VI) and manganese(V), but only for chromium(V) has any substantial oxo ester chemistry been uncovered. The osmium species are formed by two main routes (a) by reaction of 0s04 with alkenes, dienes or alkynes, in which case two electrons are transferred from the multiple bond to osmium(VIII) which is thus reduced to osmium(VI), or (b) by reaction of m-glycols with fra/w-[0s02(02 R)2] (R = H, Me) with elimination of water or methanol (see p. 582). 

Internal alkynes will also readily undergo palladium-catalyzed annulation by functionally substituted aromatic or vinylic halides to afford a wide range of heterocycles and carbocycles. However, the mechanism here appears to be quite different from the mechanism for the annulation of terminal alkynes. In this case, it appears that the reaction usually involves (1) oxidative addition of the organic halide to Pd(0) to produce an organopalladium(II) intermediate, (2) subsequent insertion of the alkyne to produce a vinylic palladium intermediate, (3) cyclization to afford a palladacycle, and (4) reductive elimination to produce the cyclic product and regenerate the Pd(0) catalyst (Eq. 28). 

It is accepted that this reaction involves the formation of the alkynedicobalt hexacarbonyl complex from an alkyne and Co2(CO)s by the evolving of two CO ligands, followed by the alkene coordination at one of the two enantiotopic Co atoms with concomitant CO insertion, and final reductive elimination of the metal to an a,(3-unsaturated cyclopentenone. In the traditional protocol, the reaction mixture is heated in toluene at 110°C, or tertiary amine A-oxides are added to promote the reaction at ambient temperature. For the purpose of stereochemical control, many Pauson-Khand reactions are designed as intramolecular reactions P " or using cyclic alkenes, such as norbornene. It has been found that the reactivity of cyclic alkenes is in the order of cyclohexene < cyclopentene < norbornene. For the intermolecular Pauson-Khand reaction, alkene is positioned adjacent to the less bulky acetylenic substituent during coordination because of steric hindrance, and a subsequent C-C bond forms between an alkenic... 

C-C cleavage of strained rings and ketones has been used to develop useful catalytic reactions. For example, vinylcyclopropanes and vinylcyclobutanes react with alkynes (Equation 6.66) to generate products from 5+2 and 6+2 addition processes that form seven- and eight-membered ring products by overall transformations that are homologs of the Diels-Alder reaction. " The mechanism of these catalytic reactions continues to be studied, but these reactions most likely occur by coordination of the olefin to rhodium and insertion of the metal into the cyclopropene or cyclobutane. Decarbonylation of dialkyl ketones, including relatively unstrained cyclic ketones, has been reported and most likely occurs by oxidative addition into the acyl-alkyl C-C bond, subsequent de-insertion of CO, and C-C reductive elimination. 

Desilylbromination. -Silyl ketones are desilylbrominatedto a,/)-unsaturated ketones with CuBr2 in DMF. This occurs spontaneously in cyclic ketones, while with open-chain ketones sodium bicarbonate is required to eliminate HBr from the -bromo ketone thus formed. The carbon-silicon bond in organopentafluorosU-icates prepared from alkenes and alkynes is cleaved with cop-per(II) bromide to give the corresponding alkyl and alkenyl bromides (eq 19). The reaction is stereoselective thus ( )-alkenyl bromides are obtained from ( )-alkenylsilicates. 

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