Coupling Reactions of Zirconate Complexes Induced by Carbonyl Compounds

Abstract

During the past few decades the chemistry of 16-electron organozirconocene complexes has been extensively explored and a tremendous number of applications in synthetic chemistry have been found.[1] However, organozirconate complexes, which usually have an 18-electron configuration with two cyclopentadienyl (Cp) rings, three Zr À C bonds, and alkali counter ions, have rarely been explored in spite of being the supposed intermediates in a number of stoichiometric and catalytic reactions, and able to form CÀC bonds through a successful 1,2-migration of organozirconates. [2][3][4] Our research group has an interest in the above mentioned subjects, and we have recently reported [5] that by adding quinone as an oxidant to a mixture of alkynylzirconates then zirconocenendiyne compounds are afforded: these compounds can be converted into various endiynes through coupling reaction with electrophiles. These results prompted us to further study the chemistry of organozirconate complexes. Herein, we report the coupling reactions of three-membered alkynylzirconates induced by carbonyl compounds. The unique features of this reaction involve the carbonyl compound acting as an electrophile, participating in the reaction with zirconates, and also inducing the coupling of an alkynyl group and an sp 2 -hybridized carbon atom of zirconacyclopropene. To the best of our knowledge, this type of reaction has not been reported. When acyl compounds were used as electrophiles, then functionalized allenes were formed. When aldehydes were used as electrophiles, then dienols were formed. These outcomes are illustrated in Scheme 1.The reaction of zirconate 1 a, which was generated by the treatment of nBuLi with bis(phenylethynyl)zirconocene at room temperature, [6] with chloroformate at 0 8C for five hours afforded allene 3 a in 73 % yield after hydrolysis. Quenching the reaction mixture with DCl in D 2 O provided allene 5 a in a comparable yield with 91 % deuterium incorporation. This result indicates that the zirconium-containing complex 6 a is formed (Scheme 2).Oxazirconacycle 6 a was formed in 74 % yield (based on NMR spectroscopy). The 1 H NMR spectrum of 6 a showed a singlet resonance at d = 5.96 ppm, which was assigned to the protons on the Cp ring. In the 13 C NMR spectrum, the signal for the carbon atoms in the Cp ring appeared at d = 104.5 ppm, and the signal for the allenic-type sp-hybridized carbon atom appeared at d = 198.5 ppm. The carbon signals in Zr À C(Ph) and À C(OEt)O appeared at d = 149.5 and 169.6 ppm, respectively. The allene group is a versatile functionality because it is useful as either a nucleophile or an electrophile as well as a substrate for cycloaddition reactions. This multiple reactivity make allenes as excellent candidates for synthetic manipulation.[7] The reaction described here allowed the efficient synthesis of various substituted allenes. A variety of acyl compounds were subjected to this novel reaction and all reactions afforded analogous products in good to high yields after hydrolysis. Notably, the ac...

How easy are the syntheses of allenes?

“…Alkynylzirconate complexes 239, formed in situ from terminal alkynes, alkyllithium reagents, and zirconocene dichloride, reacted with carbonyl compounds, such as ethyl chloroformate, ethyl acetate, and diethylcarbamic chloride, afforded highly substituted 2,3-allenyl ketones 240 (Scheme 83). 170 Conjugated bis(allene)s 242 could be obtained from zirconium-promoted linear coupling of alkynes 241 in high yields (Scheme 84). 171…”

Zirconoarylation of alkynes through p-chloranil-promoted reductive elimination of arylzirconates

“…Recently, we have reported a p -chloranil (TCQ)-promoted reductive elimination reaction of the zirconate complex Li[Cp 2 Zr(C≡CR) 3 ] toward geminal enediynes [ 44 ]. As part of our ongoing project on organozirconate chemistry [ 45 – 48 ], we envisioned that the use of an aryl ligand instead of one of the alkynyl ligands would provide an arylzirconation product of the alkyne. Herein we describe the TCQ-promoted reductive elimination of arylzirconate to afford an arylzirconation product of the alkyne, which can be converted to multi-substituted olefins through coupling with electrophiles ( Scheme 3 ).…”

Toward an integrated route to the vernonia allenes and related sesquiterpenoids

“…These bicyclic germacranolides are the only known endocyclic allene-containing natural products. Interestingly, no synthetic studies of these complex natural products have been reported, perhaps in part due to the limited number of routes for cyclic allene construction that are sufficiently mild to effect allene formation in complex settings [ 9 – 16 ].…”