Benzyne complex

The in i /V -generated CpeZ Bu Cl converts the arene into a zirconocene-benzyne complex which undergoes C-C bond formation with a nitrile to form an intermediary azazirconacycle (Equation (14)). The acidic hydrolysis of the latter species provides the corresponding 3-acyl-l-substituted benzene derivatives. 

Benzynes are highly strained molecules, which are recognized as useful intermediates in organic synthesis.44 They can be isolated by coordination to transition metals.45 Similar to the reaction of the cyclohexyne species 66, Ni-benzyne complex such as 85 reacted with C02 to give the corresponding five-membered oxanickelacyle complex 86 (Scheme 31).46... 

Thermolysis of the tantalum-phenyl/methyl complex 114 led to the formation of the Ta-benzyne complex 115 and the elimination of CH4.58 Similar to the reaction of the Ni-benzyne complex 85, one molecule of C02 could be incorporated into the carbon-tantalum bond to form the tantalumcycle 116 as shown in Scheme 40.59... 

Thermolysis of diphenylzirconocene (9) affords the zirconocene—benzyne complex (10), which can provide zirconaindene complexes (11) (Eq. 2.6) [17]. 

Some [MX]+ ions enter into reactions in which the ligand X and the reacting molecule become chemically bonded. Polymerization processes have been observed involving the [MC4H4]+ ions (147). The butadiene complex ions [MC4H4]+ of Co and Ni are unreactive to ethyne but the Fe, Ru, and Rh ions react to yield benzene and the bare metal ion. The [MC4H4]+ complex ions of Os+, Ir+, and Pt+ react with ethyne to form the MC4I I4 + ions that probably correspond to the benzyne complexes previously observed for platinum (126). 

Many stable metal complexes of arynes are known but in most of their reactions of synthetic interest, the yields are poor. For example, thermolysis of titanocene (Cp2TiPh2) at 80-100 C gives rise to a tita-nium-benzyne complex which reacts with molecular nitrogen to afford aniline with low efficiency.29 However, procedures are available for in situ generation of zirconium complexes (14) and for their coupling reactions to synthesize functionalized aromatic compounds in preparatively useful yields (Scheme I).30 Whether such complexes should be regarded as ir-bonded benzynes or o-bonded o-phenylenes, remains a debatable point.31... 

One other system shows potential for the production of ammonia and amines from dinitrogen. It was shown several years ago (155) that [(T 5-C5H5)2TiPh2] decomposes thermally via a benzyne intermediate formulated as 42. Such species fix nitrogen. The benzyne complex generated by thermal decomposition as above, or from the reaction of 43 with dinitrogen to form a species formulated as 44. A diphenylacety-... 

Zirconocene 1,9-anthracenediyl complex 69 presumably undergoes rearrangement to an isomeric benzyne complex prior to the insertion of external alkyne (Equation 26). The isomerization can be understood as a /3-hydrogen elimination/reductive elimination process, resulting in a formal reduction to Zr(ll), followed by a typical alkyne/ alkyne oxidative cyclization to the observed zirconacyclopentadiene product 70. The coordinated benzyne intermediate can be observed spectroscopically as a trimethylphosphine adduct <2000JA9880>. 

Finally, if the two-step mechanism is confirmed for benzyne formation in the case of 5, this may be the sole example of aryne formation by such a mechanism, although it is also a possibility for a rhenium-benzyne complex prepared by Arnold el al.30... 

With the exception of a nickel -benzyne complex (13) [Eq. (5)],32,33 which is probably formed by trapping the free intermediate with the coordinatively unsaturated metal,34 all complexes of nickel and platinum have been prepared by reduction of preformed (2-haloaryl)halometal complexes, which in turn have been synthesized by oxidative addition of M(0) to o-diha-loarenes.2 35-37 Typical of this procedure is preparation of the bis(triethyl-phosphine)nickel-benzyne complexes 19 shown in Eq. (6). 

Attempts to prepare titanaindanes and titanaindenes from thermolysis of 28a or 28b in the presence of alkenes and alkynes failed only the cyclometalated complexes 30a and 30b from intramolecular C-H activation in 29 were observed [Eq. (7)].42 A similar reaction has been observed for substituted Cp2Zr-benzyne complexes.2... 

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)]. 

Interestingly, cyclic allylic ethers do not give analogous coupling products with zirconium-benzyne complexes.47 Reaction of 52 with 2,5-dihydrofuran... 

Sulfur and selenium heterocycles have also been prepared by reaction of zirconium-benzyne complexes with the elemental chalcogens. In these reactions, chalcogens insert into both of the zirconium-carbon bonds (Scheme 7).53-55 Interestingly, neither para-bromo norpara-dimethylamino substituents in 65 interfere with their conversion to 66 (60-80% yields).54 Complexes 66 react further with mono- and bifunctionalized electrophiles (Scheme 7) to yield ort/io-dichalcogenated benzenic compounds 67-69. 

Thermolysis of para-substituted diphenylzirconocene 70a in the presence of gray selenium powder gives, in addition to the expected complex 71a, the rearranged product 72a in a ratio of 60 40 55 whereas 70b gives only the rearranged product 72b [Eq. (15)]. Formation of the rearranged product was rationalized by proposing an equilibrium between two isomeric benzyne complexes [Eq. (16)] prior to reaction with selenium. 

Support for this mechanism is found in the reactions of 74 (R = H) (which can be isolated) with 3-hexyne, acetone, acetonitrile, and selenium to give products expected of an aryne complex.55 Cyclometallation of other zirco-nium-benzyne complexes with alkyl-substituted Cps has been observed in a few other cases.2... 

The zirconium-benzyne complex 78 reacts with 2 equiv trimethylalumi-num57 or trimethylgallium58 to give 79 and 80, respectively [Eq. (17)]. As shown by X-ray analysis, these complexes contain a planar-tetracoordinate carbon center (C2) at the bridgehead position. Triethylaluminum and diiso-butylaluminum hydride react similarly.57... 

A paramagnetic vanadium-benzyne complex (112) has been synthesized by thermolysis of CpVPh2(PMe3)2 (111) at 50°C tia = 3-4 h) [Eq. (18)].64 An X-ray crystal structure was reported for 112. Bond lengths for the benzyne ligand are given in Table III. The C1-C2 distance [1.368(5) A] is similar to those in the other structurally characterized benzyne complexes. A band in the IR spectrum of 112 at 1547 cm"1 was assigned to the coordinated triple bond. 

A mononuclear tantalum-benzyne complex (121) has been prepared by thermolysis of 120 [Eq. (20)].14 An X-ray crystal structure was reported for 121. Bond lengths for the benzyne unit are given in Table III. Complex 121 exhibits a rich insertion chemistry similar to that of Ti, Zr, and Ru benzyne complexes. Insertion reactions of 121 with ethylene, 2-butyne, acetonitrile, and carbon dioxide give 122, 123, 124, and 125, respectively (Scheme 15). Diphenylacetylene does not couple with 121, presumably because of steric constraints. Reagents with acidic protons such as methanol or terminal alkynes cleave the Ta—C bond to give butyl isocyanide and carbon monoxide, but... 

A tantalum-benzyne complex possessing a mdo-carborane as a ligand (130) has been prepared66 by reaction of 128 with phenylmagnesium bromide followed by refluxing in toluene in the presence of PMe3 (Scheme 16). An X-ray crystal structure was reported for 130. Bond lengths for the benzyne unit are recorded in Table III. 

The tantalum-benzyne complex (130) is much less reactive than other early transition-metal aryne complexes. It shows no reaction with acetone, benzophenone, benzaldehyde, acetonitrile, 3-hexyne, or methanol. The lack of reactivity of 130 was attributed to nonlability of the PMe3 ligand. Indeed, no phosphine exchange was observed when 130 was mixed with an excess of PMe3-d9. Refluxing 129 in a mixture of methanol and toluene (3 10 v/v) leads to clean formation of 131. This presumably results from reaction of a 16-electron benzyne complex with the alcohol. 

Thermolysis of 6 at 60°C gives the benzyne complex 7. This reaction is significantly retarded by excess trimethylphosphine, which indicates that benzyne formation occurs from a 16-e intermediate. An X-ray crystal structure of 7 was reported. Bond lengths for the benzyne unit are given in Table III. No reactions of 7 have been reported. 

The rich addition and insertion chemistry exhibited by 5 is unusual for late transition-metal complexes and, as mentioned, more closely resembles the reactivity of benzyne complexes of electrophilic d° metals. 

New nickel-benzyne complexes (143-147) have been prepared by reaction of o-dihaloarenes with Ni(COD)2 in the presence of a trialkylphosphine followed by reduction of the oxidative addition product with either Li or 1% Na/Hg in ether [e.g., Eq. (23)]. The oxidative addition reaction depends on the nature of substituents on the arene and fails to occur when strong electron-donating groups are present. Based on NMR and mass spectrometry (MS) data, the new complexes were formulated as monomeric. It had been... 

Comparison of C—C Bond Lengths in Metal-Benzyne Complexes with Those in... 

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