Arynes nucleophiles

Arynes. Nucleophiles readily attack the reactive intermediate 2,3-pyridyne (104) entirely at C-2.93 A calculation 94 shows that the coefficient in the LUMO of the pyridyne... 

Nucleophilic substitution of benzene itself is not possible but the halogeno derivatives undergo nucleophilic displacement or elimination reactions (see arynes). Substituents located in the 1,2 positions are called ortho- 1,3 meta- and 1,4 para-. 

These reactions follow first-order kinetics and proceed with racemisalion if the reaction site is an optically active centre. For alkyl halides nucleophilic substitution proceeds easily primary halides favour Sn2 mechanisms and tertiary halides favour S 1 mechanisms. Aryl halides undergo nucleophilic substitution with difficulty and sometimes involve aryne intermediates. 

Nucleophilic aromatic substitution can also occur by an elimination-addition mechanism This pathway is followed when the nucleophile is an exceptionally strong base such as amide ion m the form of sodium amide (NaNH2) or potassium amide (KNH2) Benzyne and related arynes are intermediates m nucleophilic aromatic substitutions that pro ceed by the elimination-addition mechanism... 

The first benzazetidine (243) was isolated from the photolysis of 3-phenyldihydroben-zotriazine (242) (66JA1580). Another route to benzazetidines involving formation of the N to aryl-C bond utilizes intramolecular nucleophilic substitution via aryne (287). It is not general, however, and is only satisfactory when R and/or are alkoxy groups. The reaction also fails for iV-alkylamines (78LA608). 

Elimination-addition mechanism (Section 23.8) Two-stage mechanism for nucleophilic aromatic substitution. In the first stage, an aryl halide undergoes elimination to form an aryne intermediate. In the second stage, nucleophilic addition to the aryne yields the product of the reaction. 

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. 

Heterocyclic compounds may show a higher tendency than carbocycles to react with nucleophiles according to the addition-elimination mechanism than via arynes. 

Androcymbine, O-benzyl-, synthesis 282 Anilinium ion, as nucleophilic partner 49 f. Anthracene, trapping of arynes 187f. Antimony-de-diazoniation 275 Arenediazonium ions... 

In such cases, the more acidic hydrogen is removed. Since acidity is related to the field effect of Z, it can be stated that an electron-attracting Z favors removal of the ortho hydrogen while an electron-donating Z favors removal of the para hydrogen. The second factor is that the aryne, once formed, can be attacked at two positions. The favored position for nucleophilic attack is the one that leads to the more stable carbanion intermediate, and this in turn also... 

The chemistry of dehydrobenzene, the parent aryne, has become well established during the past almost twenty years 4>. It is essentially the chemistry of a short lived (half-life ca. 10-4 sec.), and highly electrophilic intermediate. It reacts with a large number of nucleophiles, and undergoes cyclo-addition reactions with a wide variety of compounds. A number of observations have led us, and others, to concentrate our efforts on the tetrahalogenobenzynes. It seemed reasonable to predict that the presence of four electron withdrawing substituents on the aryne (1) would result in a significant increase in the electrophilicity compared with that of benzyne. 

Overlap between these orbitals will, on spatial grounds, be very poor, and the resultant bonding correspondingly weak arynes are thus likely to be highly reactive towards nucleophiles (and electrophiles), though they are found not to be entirely unselective in this. 

The nucleophilic addition of the mesoionic compound 41 was further investigated upon addition to arynes <20030BC978> (Scheme 8). In this case the process stops at a single addition of the anion to the aryne to form 45 and workup under aqueous conditions led to the formation of the tetrazolium-5-olate 46. 

The reactive zwitterions arising from the nucleophilic attack of imines 479 on the benzyne generated in situ from 2-(trimethylsilyl)phenyl triflate 478 proved to be an appropriate molecular scaffold for the capture of CO2 with sufficient electrophilicity to yield 2-aryl-3,l-benzoxazin+-ones 480 (Equation 53). Both substituents of the C=N bond affected the course of the reaction considerably the best yields were achieved by using imines with electron-rich or neutral aryl groups on the carbon, and benzyl or nonbranched chain alkyl substituents on the nitrogen atom. With substituted derivatives of 478, the unsymmetrically substituted arynes led to regioisomeric products <2006JA9308>. 

Markl, Lieb and Martin were also able to add arynes 112 to 2.4,6-tiiphenyl-X -phosphorins the yields are better with 2.4.6-tri-tert-butyl-X -phosphorin. Here again 1,4 addition takes place with the formation of the 1-phosphabarfelenesiii. The arynes were generated either from 2-fluorophenylmagnesium bromide or penta-chlorophenyl-lithium. The reaction of the more nucleophilic 2.4.6-tri-tert-butyl-X -phosphorin with benzene-diazonium carboxylate also leads to 1,4 addition. The structure of the benzo-phosphabarrelenes 113a-d is supported by analytical and spectroscopic data (Table 16). 

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