Bis(cyclopentadienyl)bis(2,6-dimethylphenylamino)zirconium

[117939]  · C26H30N2Zr  · Bis(cyclopentadienyl)bis(2,6-dimethylphenylamino)zirconium  · (MW 461.76)

(hydroamination catalyst, precursor to the reactive imido intermediate Cp2Zr=NAr)

Alternate Name: bis(cyclopentadienyl)zirconium bis(2,6-dimethylphenyl)amide.

Solubility: insol n-hexane and ether; sol toluene and THF.

Preparative Methods: Cp2Zr(NHAr)2 (1) can be prepared by heating Cp2ZrMe2 with 2,6-dimethylaniline or by addition of Cp2ZrCl2 to a solution containing 2 equiv LiNHAr (eq 1).1

Purification: recrystallization from toluene layered with n-hexane.

Handling, Storage, and Precautions: Cp2Zr(NHAr)2 is water sensitive and should be handled and stored under a nitrogen atmosphere in a dry box or using standard Schlenk techniques. Solvents should be free of traces of air and moisture. Zirconium is believed to be of low toxicity; 2,6-dimethylaniline is toxic. Use in a fume hood.

Reactivity.

Thermolysis of (1) at 95-110 °C in toluene results in the a-elimination of 1 equiv of 2,6-dimethylaniline and generation of the transient intermediate Cp2Zr=NAr.1,2 This intermediate has been trapped as its ligand adduct Cp2Zr(=NAr)(L) (L = THF, OPPh3) with Lewis bases, and it reacts with alkynes to give azametallacyclobutenes (2) (eq 2). Cycloadditions with a variety of alkynes have been observed and were found to be reversible near 95 °C.

Catalytic Hydroamination of Alkynes and Allene.

(1) also serves as a catalyst for the hydroamination of alkynes.1,3 For example, heating a toluene solution containing equal amounts of 2,6-dimethylaniline and diphenylacetylene in the presence of 3 mol % of (1) to 95-110 °C results in the formation of the N-(2,6-dimethylphenyl)enamine (3) (eq 3). The rate of production of enamine is slow under these conditions; however, if the reaction is conducted with the exclusion of air and water, complete conversion to the enamine is observed. Additionally, heating allene and 2,6-dimethylaniline with 3 mol % of (1) to 90 °C for 6 days results in the formation of the anti-Markovnikov addition product, the 2,6-dimethylphenylimine of acetone.3 When (1) and 2,6-dimethylaniline are heated in the presence of ethylene, allylbenzene, or norbornene, no addition of the amine to the alkene occurs.

Regiochemistry of Additions.

Stoichiometric generation of the reactive imido intermediate Cp2Zr=NAr in the presence of unsymmetrical alkynes results in formation of two regioisomeric azametallacyclobutenes.3 Regioselectivity comparable to that of the hydroborating reagents disiamylborane and dicyclohexylborane has been observed.4 The larger substituent on the alkyne exhibits a strong preference to be a to the zirconium center leading primarily to the anti-Markovnikov products after hydrolysis of the metallacycles. The origin of this selectivity is believed to be dictated by the steric interactions between the large nitrogen bound group and the substituent on the alkyne. Generation of Cp2Zr=NAr from (1) under catalytic conditions results in an erosion in the regiospecificity of the hydroamination product. This is due to the reversibility of the cycloaddition and the difference in relative rates of protonation of the regioisomeric metallacycles by 2,6-dimethylaniline (which favors protonation of the minor regioisomer for steric reasons).

Formation of a,b-Unsaturated Imines.

(1) can be used in a two-step process for the synthesis of a,b-unsaturated imines as illustrated in eq 4.5 The first step involves synthesis of the azametallacyclobutene. Addition of benzaldehyde or isobutyraldehyde initially gives the addition product which, upon warming, undergoes a [2 + 4] cycloreversion to provide the a,b-unsaturated imine.

Related Reagents.

Chloro(cyclopentadienyl)dimethyltitanium.


1. Walsh, P. J.; Baranger, A. M.; Bergman, R. G. JACS 1992, 114, 1708.
2. Walsh, P. J.; Hollander, F. J.; Bergman, R. G. JACS 1988, 110, 8729.
3. Baranger, A. M.; Walsh, P. J.; Bergman, R. G. JACS 1993, 115, 2753.
4. Brown, H. C.; Campbell, J. B., Jr. Aldrichim. Acta 1981, 14, 3.
5. (a) Baranger, A. M. Ph.D. Thesis, University of California, Berkeley, 1993. (b) Hanna, T. A.; Baranger, A. M.; Walsh, P. J.; Bergman, R. G. JACS 1995, 117, submitted for publication.

Patrick J. Walsh

San Diego State University, CA, USA



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