Acetylenic moiety

Benzyne, generated from diphenyliodonium 2-carboxyIate, reacts with various thiophenes by addition to the sulfur and /3-carbon to give, after loss of an acetylene moiety, benzo[Z)]thiophenes in low (<4%) yield (Scheme 52) (81CC124). 

Bis(tnfluoromethyl)-subsntuted hetero-1,3-dienes and acetylenes react to give open-chain tnfluoromethyl-substituted N-propargylic amides, 4//-1.3 oxazines, and 2-oxazohnes [42,144] The formation of 2-oxazolmes is an example of pathway 2 (equabon 25), where only one carbon atom of the acetylene moiety IS incorporated into the new nng system The selectivity of this reaction can be controlled efficiently in favor of the five-membered nng system by altering the reaction conditions In the presence of 4-dimethylaminopyndine, the five-mem-... 

To ascertain the possibility of inserting more than one acetylenic moiety into the pyrazole ring, the replacement of two and three iodine atoms in the appropriate halides by different alk-l-ynes was carried out. To increase the total rate, the cross-coupling of diiodopyrazoles and triiodopyrazole was performed with higher initial concentrations of the reactants than for the monoiodides. The reaction of diiodopyrazoles with the acetal was completed for the most part in 40 h, and in 64 h in the case of triiodopyrazole. The yields of the di- and triacetals reached 70-90% (Table XTTT). 

A mechanism for this reaction has been proposed [75], The first key intermediate in the reaction is the copper(I) acetylide 42. The additional ligand may be solvent or H2O. The acetylene moiety in 42 is activated for a 1,3-dipolar cycloaddition with the nitrone to give intermediate 43, with introduction of chirality in the product. A possible route to ris/traws-41 might be via intermediate 44. Finally, the cis isomer is isomerized into the thermally more stable trans-41. It should be mentioned that the mechanism outlined in Scheme 6.32 was originally proposed for a racemic version of the reaction to which water was added. 

The versatility of poly(phenylcne) chemistry can also be seen in that it constitutes a platform for the design of other conjugated polymers with aromatic building blocks. Thus, one can proceed from 1,4- to 1,3-, and 1,2-phenylene compounds, and the benzene block can also be replaced by other aromatic cores such as naphthalene or anthracene, helerocyclcs such as thiophene or pyridine as well as by their substituted or bridged derivatives. Conceptually, poly(pheny ene)s can also be regarded as the parent structure of a series of related polymers which arc obtained not by linking the phenylene units directly, but by incorporation of other conjugated, e.g. olefinic or acetylenic, moieties. 

The dipyrrylacelylenedicarbaldehyde 18, which already contains, along with the acetylene moiety, two of the two-carbon bridges of the final macrotetracycle, can be prepared by a Heck-type coupling of 5-iodopyrrole-2-carbaldehydes with acetylene.8a,b The main conjugation pathway with 22n electrons is already present in the acetylene-cumulate systems 19. The expected planarity of this chromophore has been confirmed by X-ray structure analysis.8a b... 

The addition of (TMS)3SiH to a number of monosubstituted acetylenes has also been studied in some detail. These reactions are highly regioselective (anti-Markovnikov) and give terminal (TMSlsSi-substituted alkenes in good yields. High cis or trans stereoselectivity is also observed, depending on the nature of the substituents at the acetylenic moiety. For example, the reaction of the alkynes 23 and 24 with (TMSlsSiH, initiated either by EtsB at room temperature (method or by thermal decomposition of di-ferf-butyl peroxide at 160 °C... 

The following is a comprehensive smwey of the chemistry of macrocycles comprised entirely of phenyl and acetylenic moieties. Although over fom" decades old, this area of research has come into its own just in the last few years. Widespread interest in the field has been spurred by recent discoveries utilizing these compoimds as ligands for organometallic chemistry, hosts for binding guest molecules, models of synthetic carbon allotropes, and precursors to fullerenes and other carbon-rich materials. This review will discuss the preparation of a tremendous variety of novel structm-es and detail the development of versatile synthetic methods for macro cycle construction. 

Over the last decade, the chemistry of the carbon-carbon triple bond has experienced a vigorous resurgence [1]. Whereas construction of alkyne-con-taining systems had previously been a laborious process, the advent of new synthetic methodology based on organotransition metal complexes has revolutionized the field [2]. Specifically, palladium-catalyzed cross-coupling reactions between alkyne sp-carbon atoms and sp -carbon atoms of arenes and alkenes have allowed for rapid assembly of relatively complex structures [3]. In particular, the preparation of alkyne-rich macrocycles, the subject of this report, has benefited enormously from these recent advances. For the purpose of this review, we Emit the discussion to cychc systems which contain benzene and acetylene moieties only, henceforth referred to as phenylacetylene and phenyldiacetylene macrocycles (PAMs and PDMs, respectively). Not only have a wide... 

The reaction proceeds with isolated double bonds and electron-rich alkynes. Electron-withdrawing groups in the acetylene moiety decelerated the reaction. A plausible mechanism implies the activation of the olefin by coordination of the metal triflate followed by nucleophilic attack of the acetylene or acetylide (Scheme 31). 

When colorless crystals of rac-s-trans-3,8-di-tert-butyl-l,5,6,10-tetraphenyl-deca-3,4,6,7-tetraene-l,9-diyne (123) were heated at 140 °C for 2 h, the ben-zodicylobutadiene derivative (126) was produced as green crystals. As shown in the sequence (Scheme 20), 123 is first isomerized to its s-ds-isomer (124), and intramolecular thermal reaction of the two allene moieties through a [2+2] conrotatory cyclization gives the intermediate 125, which upon further thermal reaction between acetylene moieties gives the final product 126 [19,22].This is another example of the crystal-to-crystal reaction. 

The cyclopropane diester (800) bearing a vicinal acetylenic moiety, when treated with Co2(CO)s, affords the formation of the dicobalt hexacarbonyl complex (801). It undergoes a smooth cycloaddition with a,N -diphenylnitrone, in the presence of Sc(OTf)3, to form the corresponding dicobalt hexacarbonyl complex of tetraydro-l,2-oxazine (802). De-complexation of adduct (802) gives 6-ethynyl-tetrahydro-l,2-oxazine (803) (Scheme 2.332) (856). 

Next, the cycloisomerization of l, -enynes involving a vinylmetal species originating from the hydro-, hetero-, or carbometallation of the acetylene moiety in the first step is summarized. 

Danheiser et al. [133] reported a variety of intramolecular [4 + 2]-cycloadditions of a butenyne subunit with a remote acetylene moiety by thermolysis of the substrates, with the best yields being obtained in the presence of phenolic additives. Two examples are presented in Scheme 6.49. Of particular significance with regard to synthetic utility is the observation that protic and Lewis acids were powerful promoters of these reactions. The intermediacy of 1,2,4-cyclohexatriene derivatives, as shown in Scheme 6.49, is highly likely, at least in the non-catalyzed cases. 

Bispropargyl ether 222 isomerized on treatment with tBuOK into the naphthalene 223 via the intramolecular [4+2]-cydoaddition of the phenylallene with the acetylene moiety. Similar reactions of enynyl propargyl ether 224 took place at room temperature to give two isomeric isobenzofurans, 225 and 226. The major product 226 presumably arises from the intramolecular [4 + 2]-cycloaddition of the bisallenyl ether, whereas the minor product 225 is formed by the [4 + 2]-cycloaddition of the monoallenyl ether [180]. 

A few examples are chosen in order to illustrate the potentialities of this remarkable methodology. In Reaction (6.6) the sequence is initiated by the removal of the PhSe group and the formation of a carbamoyl radical. It is worth mentioning that the stereochemical outcome of these cascade reactions is controlled by the stereochemistry of the oxygen-bearing asymmetric carbon in 29. Indeed, Reactions (6.6) and (6.7) show clearly the stereochemical control. On the other hand, Reactions (6.7) and (6.8) illustrate the role of R which is carried as a terminal group in the acetylenic moiety. For R = Ph the last step is the hydrogen abstraction, whereas for R = SnBus, the last step is the ejection of BusSn radical (cf. Scheme 6.7). 

We have explored two types of carbon-carbon bond forming reactions operated under almost neutral conditions. Both reactions are initiated by the formation of an H-Rh-Si species through oxidative addition of a hydrosilane to a low-valence rhodium complex. Aldol-type three-component couphngs are followed by the insertion of an a,yS-unsatu-rated carbonyl compound into a Rh-H bond, whereas silylformylation is accomplished by the insertion of an acetylenic moiety into a Rh-Si bond. 

Although silylformylation of 3-butyn-I-ol 84 gives normal product 85 preferentially in the absence of EtsN, an appreciable amount (38%) of 7-lactone 86 is formed concomitantly." Protection of the hydroxy group in 84 leads to selective silylformylation of the acetylenic moiety as shown in Scheme 3. Hydrolysis of the silyl ether in 88 gives 85 as a single product. 4-Pentyn-I-ol 89 reacts with Mc2PhSiH under CO pressure to give a mixture of silylformylation product 90 (20%) and (5-lactone 91 (38%) after a short reaction time (0.5 h) (Equation (16)). The unusual lactone formation is not observed in the reaction of 5-hexyn-l-ol 92 in the presence of EtsN (Equation (17)). ... 

More complicated results are observed in the reaction of 4,4-dimethyl-l,6-heptadiyn-3-ol 269 with Bu Me2SiH, giving a mtKture of 270 and 271 (Equation (44)). Discrimination of the two acetylenic moieties in 269 is hard at the silylrhodation, and thus a complex mixture of regioisomers and diastereomers results. In any event, use of rhodium species as a catalyst is essential for smooth construction of bicyclo[3.3.0]-octenone frameworks. 

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