Arynes amination reactions

Beller et al. have reported a transition metal-free amination reaction of an aryl chloride in the presence of KOt-Bu by intermediate aryne formation (Equation 12.8) [10]. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

Preparation of aryne derivatives in the presence of tertiary amines leads to betaines which are stabilized via various rearrangement reactions.214-217... 

In connection with the substituent effects, the kinetic stability of benzyne is suggested to be increased by electron withdrawal (-/) and decreased by electron release (+/).73 However, the inference cannot be extrapolated to selectivity of substituted arynes in general. For example, in additions involving competition between phenyllithium and lithium piperidide, the methyl substituents (+/) on benzyne increase its selectivity, whereas methoxy groups (-/) decrease it (Scheme 6). On the other hand, in reactions of car-banions derived from acetonitrile in alkylamine solvents both +/ and -/ benzyne substituents lower selectivity and cause predominant amination. Thus, the method was found unsuitable for preparation of many substituted benzyl nitriles.74 In symmetrically disubstituted arynes there is partial cancellation of polarization, and in fact acceptable yields of acetonitrile adducts could be obtained from 3,6-dimethoxy-benzyne.75 The selectivity of substituted arynes varies with the set of nucleophiles in the competition and no comprehensive theory or simple generalization is available on this point. 

There has been a summary of the use of insertion reactions of arynes into a-bonds to prepare nrt/io-disubs tituted arenes. A key to the success of these processes is the ability to generate benzyne under mild conditions by the reaction of readily available o-(trimethylsilyl)phenyl triflate with fluoride ions.61 Reaction of amines and their derivatives with benzynes generated in this way has been shown to be an efficient method for the production of N-arylated derivatives, as illustrated in Scheme 8. The method also works well in the O-arylation reactions of phenols and carboxylic acids 62... 

Aryl halides in which electron-withdrawing groups activate the halogen atom to nucleophilic displacement react directly with ammonia to produce amines. Non-activated aryl halides yield amines on reaction with sodamide through the intermediacy of an aryne intermediate (see Chapter 9). 

Thus, reaction of 6-bromo-4-phenylpyrimidine (316) with lithium isopropylamide in isopropylamine proceeds via 317 to the ring-opened product 318 to eventually give 6-isopropylamino-4-phenylpyrimidine (319) in 70% yield. Since these particular aminations have been reviewed (78ACR462), they will not be discussed further. Nor will nucleophilic aminations proceeding via aryne-type intermediates (65AG557, 65AHC121) be a topic of this review. 

Observation of Scheme 54 shows a close analogy between these results and the ones obtained in arynic condensations. Thus we observe the formation of normal ketones 138, transposed ketones 139 or 140, and methylene cyclobutanols 141. However, we met large difficulties with the mechanism. Indeed, starting from the hypothesis of cyclohexyne being the intermediary (what is consistent with our works on amine condensations) and by analogy with arynic condensations, the reaction should be written as in Scheme 55. 

The use of sodium amide or potassium amide in liquid ammonia with bromo- or chlorobenzene leads inevitably to the capture of benzyne by its reaction with ammonia. However, the utility of bromo- or chlorobenzene as a benzyne precursor is extended to ethereal solvent systems by employing the conjugate base of a hindered secondary amine (diisopropylamine, 2,2,6,6-tetramethylpiperidine) which can be formed in situ from the amine and alkyllithium. Alternatively, butyllithium itself is used with the halogeno-benzene, and pentafluorobenzene and butyllithium are the usual source of tetrafluorobenzyne. In all of these reactions the aryne is generated by decomposition of an o-halogenoaryl anion at temperatures below 0°C. 

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]. 

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