Alkenes reactions with arynes

Reactions of phosphines and phosphites have received some attention but their preparative value is limited. The zwitterion formed from diphenylmethylphosphine and benzyne rearranges to ylide (124) which can be captured by Wittig alkenation, with cyclohexanone, in about 20% yield.159 Some synthetically useful reactions of tellurium and selenium compounds with arynes have been reported. For example, heating diphenyl iodonium carboxylate and bis(p-ethoxyphenyl) ditelluride in dichlorobenzene affords the compound (125).160 The corresponding reactions with diphenyl selenide and diphenyl sulfide... 

Zirconium-benzyne complexes have been used rather extensively in organic synthesis.8 45 For this purpose, one particularly important characteristic of zirconium-aryne complexes is that olefin insertion into the Zr—C bond occurs stereospecifically. Thus, when generated in situ, the zirconium-benzyne complex (45) reacts with cyclic alkenes to give exclusively the cis-zirconaindanes (46), which upon treatment with electrophiles provide access to a variety of m-difunctionalized cycloalkanes (47-49) (Scheme 5).46 For example, carbonylation of intermediate 46 affords tricyclic ketone 49, reaction with sulfur dichloride gives thiophene 48, and reaction of 46 with tert-butylisocyanide followed by I2 gives 47 via 50 and, presumably, intermediate 51 [Eq. (12)]. 

Four-membered carbocyclic ring systems are commonly formed by cycloaddition of electrophilic alkenes, ketenes and arynes to enamines. Since cycloaddition reactions of enamines are dealt with in Chapter 18 these reactions will only be mentioned briefly here. 

Aryne complexes of late transition metals are very reactive towards both nucleophiles (amines, alcohols, water) and electrophiles (iodine). They also undergo insertion reactions with CO, alkenes and alkynes,but while the behaviour of ruthenium complexes is somewhat similar to that of titanium or zirconium complexes, the reactivity of nickel complexes is rather different [6,8]. Examples of these reactions that are particularly interesting for the purposes of this chapter are shown in Schemes 8 and 9. Ruthenium complex 33 undergoes insertion of a molecule of benzonitrile,benzaldehyde or di(p-tolyl)acetylene to yield met-allacycles 40,41 and 42, respectively (Scheme 8). Further insertion of a second unsaturated molecule into these metallacycles has not been observed [25,27]. 

Arynes react readily with simple alkenes to give either benzocyclobutenes or substituted benzenes (Scheme 7.31). The formation of benzocyclobutenes by [2+2] cycloaddition reaction of the aryne to the alkene proceeds best for strained and electron-rich carbon-carbon (C=C) double bonds. For example, dicyclopentadiene reacts to give the ex o-isomer of the corresponding four-membered ring in good yield. The addition to cyanoethene (acrylonitrile) and the reaction with the electron-rich ethoxyethene (ethyl vinyl ether) gives the cyano- and ethoxy-benzocyclobutenes in 20% and 40% yields, respectively. The latter reaction almost certainly involves nucleophilic addition of the enol ether to the electrophilic aryne followed by coUapse... 

P-Enamino esters and ketones react with arynes in an interesting manner. Aryla-tion occurs at the a-carbon to form the C-arylation product, instead of the expected N-arylation product (Equation 12.13) [15]. In addition, a variety of functional groups that are known to react with arynes, such as amino and hydroxyl groups, as well as alkenes and furans, are all well tolerated in this reaction. 

Another reaction with alkenes which arynes readily undergo is the ene reaction [equation (32)]. Although this process can sometimes compete... 

Finally, transition metal-catalyzed reactions of arynes have been explored as a useful method for the construction of a wide variety of carbo- and heteocycles. These reactions include cyclotrimerization of arynes, cocyclization of arynes with alkynes or alkenes, and carbopaUadation of arynes with Pd-complexes. Moreover, some insertion reactions of arynes into a-bonds are also catalyzed by metal complexes. 

Coupling of 2,3-dihydrofuran with alkene-zirconocene <2004AGE3932> or aryne-zirconocene <2005SL2513> complexes and subsequent addition of an electrophile provided rA-disubstituted homoallylic alcohols, as illustrated in Equation (130). An insertion//3-elimination pathway that involved the formation of an oxazirconacyclooctene intermediate was proposed for the reaction mechanism. 

This chapter reviews thermal [2 + 2] cycloadditions of alkenes to form cyclobutanes, extending the coverage provided by earlier summaries of the literature.It docs not deal with [2 + 2] cycloadditions that are achieved photochemically, or that utilize allenes, arynes or ketenes, or that are deliberately catalyzed, or that result in heterocyclic four-membered rings, even though these other varieties of [2 + 2] cycloaddition reactions are important and of substantial interest. 

The reaction, initiated by carbopalladation of the aryne by stable ir-allylpalladium intermediates, is followed by the insertion of a second molecule of aryne into the arylpalladium bond and subsequent intramolecular carbopalladation of the alkene by the newly formed biarylpalladium species. When the second equivalent of aryne is replaced with an internal alkyne, naphthalene 236 is formed in a 47% yield (Equation 12.61). 

Until this reaction can be studied further with a variety of alkenes and aryne generation methods, its generality and possible significance as a probe of the structure of the thiophyne (4) must remain an open question. 

Cheng et al. have demonstrated the [2+2+2] cocyclization of two arynes with bicyclic alkenes giving the corresponding norbomane anellated 9,10-dihydrophenanthrene derivatives 105. The reaction probably proceeds by initial formation of a palladacycle from one aryne molecule and the... 

In this field, Cheng et al. have also developed the carbocyclization reaction of aromatic iodides, bicyclic alkenes, and arynes to afford various armulated 9,10-dihydrophenanthrene derivatives 117 (Scheme 12.58) [102]. A related eross-coupling of arynes with aryl iodides but using alkynes as the third component has been described by Larock and Liu, furnishing substituted phenanthrenes 118 (Scheme 12.58) [103]. 

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