1.3- Enynes alkynes

Enynes. Alkyne-bromoalkene couplings are useful for synthesizing enynes. Thus. P-bromoalkenyl phenyl tellurides give coupled products that retain the tellurium group." ... 

Both 2-alkynylindoles and 2-allenylindoles have been shown to participate as dienes in Diels-Alder reactions. While attempting to promote the cycUzatiOTi reactions of indol-2-ylacetylenes 224a-b with dienophiles, Passarella and coworkers observed the formation of dimerization products 225a-b (Scheme 49) [88]. These products were formed via an enyne-alkyne [4+2] cycloaddition reaction between two molecules of the indol-2-ylacetylene species. Interestingly, such... 

Passarella D, Giardini A et al (2001) Cyclodimerization of indol-2-ylacetylenes. An example of intermolecular enyne-alkyne cycloaddition. J Chem Soc, Perkin Trans 1 127-129... 

Reaction of Primary Alcohols with Enynes, Alkynes and Allenes... 

Terminal alkynes undergo the above-mentioned substitution reaction with aryl and alkenyl groups to form arylalkynes and enynes in the presence of Cul as described in Section 1.1.2.1. In addition, the insertion of terminal alkynes also takes place in the absence of Cul, and the alkenylpalladium complex 362 is formed as an intermediate, which cannot terminate by itself and must undergo further reactions such as alkene insertion or anion capture. These reactions of terminal alkynes are also treated in this section. 

In the coupling of the allenyl ester 7 with a terminal alkyne, an electron-deficient phosphine (Ph3P) gave the enyne-conjugated ester 8 as the major product, while an electron-rich phosphine (TDMPP or TTMPP) yielded the non-conjugated enyne esters ( )- and (Z)-9[4],... 

The Pd(0)-catalyzed addition of trimethylsilyl iodide to an alkyne, followed by capture with alkynylstannane, affords the stereo-defined enyne 186. The reaction is explained by the oxidative addition of iodosilane, the insertion of an alkyne to generate the vinylpalladium 185, and the capture of 185 with the alkynylstannane 184[102]. 

Recently, Aumann et al. reported that rhodium catalysts enhance the reactivity of 3-dialkylamino-substituted Fischer carbene complexes 72 to undergo insertion with enynes 73 and subsequent formation of 4-alkenyl-substituted 5-dialkylamino-2-ethoxycyclopentadienes 75 via the transmetallated carbene intermediate 74 (Scheme 15, Table 2) [73]. It is not obvious whether this transformation is also applicable to complexes of type 72 with substituents other than phenyl in the 3-position. One alkyne 73, with a methoxymethyl group instead of the alkenyl or phenyl, i.e., propargyl methyl ether, was also successfully applied [73]. 

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

The fact that pentacarbonyl carbene complexes react with enynes in a chemo-selective and regiospecific way at the alkyne functionality was successfully applied in the total synthesis of vitamins of the Kj and K2 series [58]. Oxidation of the intermediate tricarbonyl(dihydrovitamin K) chromium complexes with silver oxide afforded the desired naphthoquinone-based vitamin K compounds 65. Compared to customary strategies, the benzannulation reaction proved to be superior as it avoids conditions favouring (E)/(Z)-isomerisation within the allylic side chain. The basic representative vitamin K3 (menadione) 66 was synthesised in a straightforward manner from pentacarbonyl carbene complex 1 and propyne (Scheme 38). 

Hexacarbonyldicobalt complexes of alkynes have served as substrates in a variety of olefin metathesis reactions. There are several reasons for complex-ing an alkyne functionality prior to the metathesis step [ 125] (a) the alkyne may chelate the ruthenium center, leading to inhibition of the catalytically active species [125d] (b) the alkyne may participate in the metathesis reaction, giving undesired enyne metathesis products [125f] (c) the linear structure of the alkyne may prevent cyclization reactions due to steric reasons [125a-d] and (d) the hexacarbonylcobalt moiety can be used for further transformations [125c,f]. 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

Enynes 71 react with aldehydes 61 in the presence of the [Ni(COD)J/SIPr catalytic system to afford two distinct products 72 and 73 (Scheme 5.20) [20b], The enone 72 is derived from aldehyde addition with the alkyne moiety while the adduct 73 arises from the aldehyde addition with the alkene moiety. The product distribution is dependent on the substituent on either the alkyne or alkene moieties. The reaction between 71 and ketones 74 led to the unprecedented formation of pyrans 75 (Scheme 5.20). The reaction showed to be highly regioselective in aU the cases, the carbonyl carbon was bound to the olefin. 

Although the path (a) has been verified by a stoichiometric reaction [23], the details of exact reaction mechanism remain unsettled. Triggered by this publication [and the Pd-catalyzed doublethiolation of alkynes described in Eq. (7.7) in Section 7-3], a number of transition metal-catalyzed additions of S-X or Se-X bonds to C-C unsaturated organic compounds started to be published. In 1994, BackvaU et al. applied the Pd(OAc)2-catalyzed hydrothiolation to conjugated enynes and obtained 17,... 

It was described that the use of the combination of Pd(OAc)2/dppp /THE resulted in the highest yield. It is not clear whether such a combination of catalyst, ligand and solvent is restricted to the hydrothiocarboxylation of conjugated enynes or also may be applied to other alkynes. Furthermore, it remains to be explored whether, in the Pt-catalyzed reaction performed in CHjCN [see Eq. (7.18)], conjugated enynes can be used. 

AUylzirconium complexes are conveniently obtained by the regio- and stereoselective hydrozirconation of allene [127-133] and can be, for example, used subsequently for the MAO-catalyzed allylzirconation of alkynes to prepare enyne derivatives [132]. Alternatively, the preparation of (E)-l,3-dienes from aldehydes and the appropriate allylstannane zirconocene derivative (R = SnBu,) is accomplished (Scheme 8-17) [131], Note that addition of [Cp2Zr(H)Cl[n (1) on the aUenyl reagent with the... 

As predicted from the comparative rates for C=C over C=C hydrozirconation cited earlier, a (poly)enyne is selectively hydrozirconated at the alkyne moiety, whatever the position of the alkene function [138, 210] in the molecule. It can be exempUfied by the chemoselective hydrozirconation of 1,3-butenyne. One exception to this chemoselectivity has been reported, which showed the terminal alkene to react with 1 but leaving the TMS-substituted alkyne function intact (Scheme 8-25). 

It is interesting to note that a copper(II)-mediated coupling reaction of alkenyldialkyl- or trialkylboranes with alkynylcopper compounds, generated in situ, in the presence of various solvents and a small amount of water, gives (E)-l,3-enynes (or disubstituted alkynes) with various functional groups in reasonable yields (Eq. 4.3).12... 

Sugihara et al. in 1997.106 They utilized aqueous ammonium hydroxide as a reaction medium, which provided ammonia as a hard ligand to labilize the CO ligands and therefore enhance the rate of the PKR. The reaction of dicobalthexacarbonyl complexes of enynes and alkynes provided expected cyclopentenones via intramolecular and intermolecular modes respectively (Scheme 4.10). 

Similar reactivity is observed in the cyclization of enynes in the presence of the yttrium-based catalyst 70 and a silane reductant [53,54]. The 1,6- and 1,7-enynes 90 and 91 provide -E-alkylidene-cyclopentancs 92 and -cyclohexanes 93 in very good yield (Eq. 15, Scheme 20) [55]. These transformations likely proceed by syn hydrometallation of the 7r-basic alkyne, followed by insertion of the alkene and a-bond metathesis. The reaction of 1,6-enynes tolerated... 

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