Patterns of Benzyne Reaction

The reactions of benzyne are all additions to the formal triple bond of structure 1. The variety of such reactions can be clarified in terms of open-chain additions and cycloadditions, although the isolated products are in many cases derived from secondary decomposition of such primary adducts. The isolation or postulation of cycloadducts in very few cases provides any 

Benzyne also reacts with compounds containing nucleophilic carbon atoms such as enolates and aryl anions. Intramolecular nucleophilic addition to an aryne by the ortho ring carbon atom of another benzene ring substituted 

The formation of biphenyl by the formal insertion of benzyne into a C—H bond of benzene occurs in competition with cycloaddition processes, particularly when benzyne is generated at high temperatures in the vapor phase.17 

The ene reaction (Eq. 2) provides a nonpolar mechanism for open-chain addition of benzyne to compounds containing an allylic hydrogen atom. The implied migration of a double bond is not always apparent from the structure of the adduct. The ene reaction frequently occurs in competition with 1, 2-cycloaddition some examples are described in Refs. 1 and 2. 

Benzyne can be trapped in 1,3-dipolar cycloaddition reactions provided that the 1,3-dipolar species is sufficiently stable under the conditions necessary for benzyne generation. As an illustration of this, benzyne reacts with the nitrile oxide group in preference to the furan ring of compound 19, whereby adduct 20 is obtained.27 Benzyne and the nitrone 21 give adduct 22,28 and 

3-Dipolar cycloaddition reactions lead to heterocyclic products, but our concern will be only with those in which a hetero-ring is already present in reactions involving benzyne. Heterocyclic A(-oxides (Section IX) and meso-ionic heterocycles (Section VI,B) provide most examples of this type, although there are some cases of apparent 1,3-cycloaddition of benzyne to heterocycles in which no formal 1,3-dipole is identifiable. 

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The neat resin preparation for PPS is quite compHcated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been pubUshed in an attempt to describe the basic reaction of a sodium sulfide equivalent and -dichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Buimett s (22) Sj l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and /)-dichlorohenzene proceeds via the S Ar mechanism is fairly recent (1991) (26). Eurther complexity in the polymerization is the incorporation of comonomers that alter the polymer stmcture, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

C—F bond and the C—Li bond. On the starting material side of the reaction coordinate, which ought to be important, since it is an exothermic reaction, the C—F bond is a Z-substituent on the benzyne triple bond. Following the device we used earlier (see pages 60-65) to deduce the pattern of coefficients in substituted alkenes, we can argue that the C—F bond has some of the character of a cation on carbon 4.97, in which the empty p orbital will be conjugated to the in-plane bent n bond. The LUMO will resemble that of an allyl cation, and will therefore have the larger coefficient on C-3 4.98. 

The dibenzodihydrothiepin 231 reacts in the same way as 229 with benzyne to give phenanthrene in high yield.115 The same pattern of reaction has been used to extrude sulfur atoms, usually two at a time, from larger rings (e.g., 232) for the synthesis of cyclophanes.116 However, in such cases elimination of thiophenol does not follow spontaneously after the Stevens rearrangement, and indirect methods for subsequent removal of the SPh groups (e.g., from 233) are necessary. 

Palladium catalysts have been used for cycloaddition of dimethylacetylene di-carboxylate (DMAD) to polycyclic arynes 3, 77 and 79 (Schemes 34-36). All these reactions exhibit the same reactivity pattern as is observed in the [2+2+2] cycloaddition of benzyne to DMAD (see Sect. 3.1) Pd2(dba)3 leads selectively to the cocycloaddition of one molecule of aryne and two molecules of DMAD, while Pd(PPh3)4 induces the reaction of two molecules of aryne with one molecule of DMAD. Both reactions afford the corresponding polycyclic aromatic hydrocarbons in good yields and with high chemoselectivity, constituting a novel and versatile method for the synthesis of functionalized PAHs under mild reaction conditions [70-72] (Scheme 34). 

T. Hoye et al. were understandably surprised when a seemingly trivial oxidation of 7 took an unexpected course (Scheme 7.28). Instead of ketone 8 the tricyclic product 9 was obtained in 53% yield. The course of the events could be rationalized readily and pointed to the unprecedented cyclization of ketone 8 to give the benzyne derivative 10 in a [4-1-2] cycloaddition between a diyne and an yne moiety. In fact, this intramolecular cycloaddition turned out to be a viable route to generate arynes bearing electron-withdrawing substituents, a substitution pattern that is not always amenable to the standard methods for generating arynes. Thus, this surprising reaction broadens the scope of benzyne chemistry. 

On the other hand, carboryne, l,2-dehydro-t -carborane, is a three-dimensional relative of benzyne (Fig. 7.1) [6]. It can react with alkenes, dienes, and alkynes in [2-1-2], [4-1-2] cycloaddition and ene-reaction patterns [7], similar to those of benzyne [8], Although these reactions show the potential for the preparation of functionalized carboranes in a single operation, they are complex and do not proceed in a controlled manner. In view of the spectacular role of transition metals in synthetic chemistry, we envisage that the aforementioned reactions may work efficiently and in a controlled way with the help of transition metals. In this connection, we initiated a research program to develop transition-metal-mediated/catalyzed synthetic methodologies for the functionalization of carboranes. This chapter summarizes the recent progress in this research area. 

In these reactions, a er-bond is formed at the expense of two re-bonds and, thus, the process leads to a net loss of one chemical bond that is intrinsically unfavorable thermodynamically. Formation of the new er-bond leads to ring closure, whereas the net loss of a bond leads to the formation of two radical centers, which can be either inside (the endo pattern in Scheme 1) or outside of the newly formed cycle (the exo pattern). Note that er-radicals are formed through the endo path, while exo-closures may produce either a er-radical when a triple bond is involved or a conjugated re-radical when the new bond is formed at the central carbon of an allene. The parent version of this process is the transformation of enediyne 1 into p-benzyne diradical2 (the Bergman cyclization), shown in Scheme 2. 

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