Benzyne hydride elimination

A tetrakis(trimethylphosphine)ruthenium complex of benzyne has been prepared6 26 by a reaction similar to that used for the Group 4 and 5 metals thermally induced /3-hydride elimination of methane or benzene from 1 or 2, respectively [Eq. (3)]. A careful study of the kinetics of the elimination of methane from 1 revealed that dissociation of trimethylphosphine pre-... 

The palladium-catalyzed reaction of allyl chloride 11 with the benzyne precursor 104 to produces phenanthrene derivatives 131 is also known [83]. A plausible mechanism for this inter molecular benzyne-benzyne-alkene insertion reaction is shown in Scheme 38. Initially tt-allyl palladium chloride la is formed from Pd(0) and 11. Benzyne 106, which is generated from the reaction of CsF and 104, inserted into la to afford the aryl palladium intermediate 132. A second benzyne insertion into 132 produce 133 and subsequent carbopalladation to the alkene afford the cyclized intermediate 134. -Hydride elimination from 134 followed by isomerization gave 9-methylphenanthrene 131. 

Eisenberg et al have reported studies on the reaction of the benzyne-hydride complex (Cp 2Ta(CgH4)H) with p-Hj [24]. At 273 K, the complex reacted to form a new phenyl dihydride complex, Cp 2Ta(CgH5)H2, which yielded enhanced hydride resonances with p-Hj. This complex went on to eliminate benzene and react with H2 to form the trihydride complex Cp 2TaH3- The central component of the proton resonance due to the central hydride ligand was again absent under PHIP conditions. This report is especially noteworthy because it represented the first example of PHIP at an early transition metal centre. 

According to the proposed reaction mechanism (Scheme 44), carbopalla-dation of benzyne with the 7i-allyl palladium complex gives intermediate 157. Insertion of CO in the palladium-aryl bond then leads to the acyl palladium complex 158, which undergoes intramolecular cyclization and subsequent P-hydride elimination, affording indanone 156 [78]. 

Carbopalladation of Arynes by n-Allylpalladium and Related Complexes. Pioneering work from Yamamoto established that ji-allylpalladium species are particularly effective for the carbopalladation of arynes leading to an arylpalladium intermediate 109, which can be employed in three-component couplings with a second aryne to give phenanthrenes such as 110, or with alkynes to produce naphthalenes such as 111. In the first case, 109 evolves through insertion of another benzyne (1) molecule (carbopalladation of 1 by an arylpalladium) and further carbopalladation of the pendant alkene to afford a tricyclic species that undergoes p-hydride elimination (Scheme 12.55) [96],... 

As to the final oxidation (aromatization) step that follows these cycloadditions in the same reaction mixture and prior to workup, some results suggest a stepwise process. Treatment of either Z-694 or jE -694 (or a mixture of the two) gives 697 in approximately identical yield (52%). Since only one of the two initial cycloadducts, cz -695, can undergo a benzyne-promoted 1,4syn elimination of hydrogen, it is suggested that aromatization proceeds by hydride abstraction by benzyne to generate carbocation 696, which then loses a proton. 

---