Carbon reactions with arynes

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

Criado et al. (2013) have investigated the functionalization of single wall carbon nanotubes (SWCNT) by cycloaddition reaction with arynes under microwave... 

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... 

Progresses of diaryhodonium salts in organic reactions 13CJ02119. Reaction of arynes with carbon—heteroatom double bonds to give heterocycles 12H(85)515. 

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. 

A final example of aryne formation involves the reaction of o-phenylene carbonate (90) with phosphines. An analogous reaction with thiocarbonates (91) fails as does generation of the anion of 2-phenyl-1,3-benzodioxole (92), which had been expected to lose benzoate ion to give benzyne. ... 

As noted in Section I.2.C, this review will cover five-membered hetarynium species such as 10 and 11 which are isoelectronic with arynes but in which the dehydro bond is exclusively between a carbon and a heteroatom and not between two carbon atoms. Such species may be cationic (10) or neutral (11) depending on the nature of the heteroatom. Hetarynium ions were first defined,proposed,and, in some cases, rejected as possible reaction intermediates in the six-membered pyridine system (9). The observed chemistry of 9 is that expected for either the a-pyridyl cation structure 9b or, preferably, the cyclic nitrilium ion form 9a. No uniquely arynelike properties have been reported, which accounts for the absence of hetarynium species from Hoffmann s monograph. ... 

K. Okuma, Reactions of arynes with carbon-heteroatom double bonds. Heterocycles 85 (2012) 515-544. 

Catalytic carbonylation of arynes was first reported by Chatani et al. in 2001 [18], As depicted in Scheme 28.14, the cobalt-catalyzed reaction of arynes with CO provided anthraquinones 37 in 80% yield. Furthermore, the three-component coupling of arynes, CO, and allyl acetates 38 was promoted efficiently in the presence of a palladium catalyst, giving 2-methyleneindanones 39 in good yield. Direct construction of 2-methyleneindanone skeletons (39 and 41) was also achieved by employing methyl allyl carbonates 40 instead of allyl acetates (Scheme 28.15), where the regios-electivities were totally dependent on the ligands [PPhs or P(o-tol)3] [19]. 

Mono- or di-arylaied products may be formed by a-arylation of -dicarbonyl compounds, such as malonamide esters, with arynes generated by reaction of fluoride with ortho-silylaryltriflates. A similar method of benzyne generation has been used in the a-arylation of -ketoamides in a procedure which may be modified to yield asymmettic products. The reaction of 2-haloacetanilides with arynes in the presence of a palladium catalyst may produceiV-acylcarbazoles such as (26). Possible pathways are initial formation of a palladacycle with aryne followed by oxidative addition of the haloacetanilide, or direct insertion of the palladium into the carbon-halogen bond of the acetanilide followed by carbopalladation of the aryne. ... 

Some palladium-catalyzed reactions of organotins, such as carbostannylations, are not related to the Stille cross-coupling. The history of the transition-metal-cata-lyzed carbostarmylation [148] began with alkynylstannylation of alkynes catalyzed by a palladium-iminophosphine complex [149]. Thus, alkynylstannanes added to a carbon-carbon triple bond of various acetylenes, conjugated ynoates and propar-gyl amines and ethers in the presence of a catalytic amount of a palladium-iminophosphine complex [150]. The reaction also proceeded with arynes to afford ortho-substituted arylstannanes, which could further be converted into 1,2-substituted arenes via carbon-carbon bond-forming reactions [151]. 

The first clue to the existence of the SrnI mechanism came from product studies both in aliphatic and aromatic cases. It was noticed that in the reaction of benzyl and substituted benzyl chlorides with the 2-nitropropane anion, oxygen alkylation, yielding the oxime and then the aldehyde, occurs exclusively in the case of benzyl chloride and 3-nitrobenzyl chloride, whereas, with 4-nitrobenzyl chloride, the yield of aldehyde is only 6% and the carbon-alkylated (104) product is obtained in 92% yield (Kornblum, 1975). This was interpreted as the result of a competition between 8, 2 (O-alkylation) and S l (C-alkylation) reactions. In the aromatic case, it was observed that the reaction of 5- and 6-halopseudocumenes with KNHj in liquid ammonia (Kim and Bunnett, 1970) forms the 5- and 6-pseudocumi-dines in a ratio which is the same whether the starting compound is the 5- or 6-isomer in the case of the chloro- and bromo-derivatives, as expected from an aryne mechanism (Scheme 9), whereas much more non-rearranged... 

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>. 

From the addition reactions of phthalic anhydride with pyridine, naphthalene was formed in much greater quantity than quinoline isoquinoline was either totally absent or present in only minute amounts. These facts indicated (a) if there were much 1,2-addition of benzyne, it occurred predominantly at the 1,2 and 3,4 atoms in pyridine (b) 1,4-addition took place at carbon atoms in preference to a nitrogen and a carbon atom. Studies with other systems indicate that arynes have a decided preference for 1,4- over 1,2-addition. 

Tetrachlorophthalic anhydride gave a relatively low yield of products derived from tetrachlorobenzyne. The pyrolysis tube was badly carbonized evidently extensive decomposition of the anhydride, the aryne, or the chlorinated products had occurred. Tetrabromophthalic anhydride gave a still lower yield of products. These are not listed in the table because none of them retained all four bromine atoms and could be definitely ascribed to reactions of tetrabromobenzyne, although tri-bromo- and dibromonaphthalene were present in appreciable amounts. Tetraphenylphthalic anhydride also gave low yields of products of the reaction of tetraphenylbenzyne with pyridine. This was not because of the stability of the anhydride and its reluctance to form the aryne, but rather because the aryne preferred to stabilize itself intramolecularly. The behavior of tetraphenylphthalic anhydride is discussed in another Section. 

The aryne intermediate is usually written with a triple bond and a delocalized aromatic system, as shown in 3-44. The anion in the side chain reacts as a nucleophile with the electrophilic aryne. The resulting anion, 3-45, can remove a proton from ammonia to give 3-46. Because the product has been reached, we usually stop writing the reaction mechanism at this point. However, in the reaction mixture, amide will remove a proton from the carbon a to the cyano group of 3-46. Only during workup will the anion be protonated to give back 3-46. 

They are highly strained and have a singlet ground state.73,74 Because such intermediates have two reactive carbon centers they are attractive substrates for cycloaddition reactions. However, little consideration will be given to this important use of arynes and hetarynes because it is outside the scope of this article. Information dealing with the generation of new intermediates is included, although attempts have not yet been made to exploit them in syntheses. 

In this connection it is perhaps worthwhile to reconsider the synthesis of benzothiazoline-2-thiones. These substances can, as already mentioned, be made from o-nitrochlorobenzene with sodium hydrogen sulfide and carbon disulfide (method C). Starting from 0- or w-halo anilines, benzothiazoline-2-thiones are also obtainable with carbon disulfide in strong alkali (55). The intermediate in this reaction is an aryne (CXXXIII). 

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