Catalytic syntheses of polycyclic compounds based on norbornadiene-2,5 in the presence of nickel complexes: II. cyclic d
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Catalytic Syntheses of Polycyclic Compounds Based on Norbornadiene-2,5 in the Presence of Nickel Complexes: II. Cyclic Dimerization of Norbornadiene-2,5. Kinetics and Mechanism of the Process V. R. Flid, O. S. Manulik, A. A. Grigor'ev, and A. P. Belov Lomonosov State Academy of Fine Chemical Technology, Moscow, 117571 Russia
ReceivedDecember17, 1999 Abstract--Kinetics of the cyclodimerization of norbornadiene-2,5 (NBD) is studied in the presence of a catalytic system based on bis(rl3-allyl)nickel. The forms of the rate laws characterized by different reaction orders with respect to NBD are determined. The influence of temperature and solvent nature on the process is studied. The thermodynamic parameters are determined. The structure of the products is shown to be determined by the structure of intermediates. The mechanism of the process, consisting of the following main steps, is proposed: (1) the formation of Ni(NBD)2, which is the true catalyst; (2) the reversible addition of NBD to the indicated complex, resulting in the formation of Ni(NBD) 3 and Ni(NBD)4 ~-complexes and accompanied by a change in ligand coordination; (3) the oxidative addition of coordinated NBD molecules to a nickel atom that gives fiveand six-membered metallacyclic intermediates; and (4) the reductive elimination of nickel from them to form cyclic dimers. The conditions for the selective formation of individual isomers are proposed.
INTRODUCTION
EXPERIMENTAL
Numerous papers published on norbornadiene-2,5 (NBD) cyclodimerization do not contain the kinetic data necessary to understand the mechanism and development of selective catalytic systems. We showed in a previous study that the catalytic system generated in situ from bis(rl3-allyl)nickel is suitable for the study of NBD cyclodimerization kinetics [1]. In the presence of this system, exo-trans-exo (I) and exo-trans-endo (II) isomers of pentacyclo[8.2.1.14,7.02,9.03,8]tetradeca-5,11-diene, as well as traces endo-endo (III) and exo-endo (IV) isomers of hexacyclo[7.2.1.13,715,13.01,8.04,6]tetradeca- 10-ene are formed.
Experimental kinetic studies of NBD cyclodimerization were carried out at constant temperatures in 25-ml vacuum reactors with a sampler in solutions of nnonane, tetrahydrofuran (THF), benzene, and metaxylene. The course of the process was monitored by the accumulation of dimers, using GLC analysis of the reaction solutions. The concentrations of reactants and experimental conditions were varied within the following limits:
~ III
'
n
+
IV
(I)
NBD concentration Ni(C3Hs)2 concentration Temperature
From 0.1 to 9.8 mol/l From 0.005 to 0.20 mol/l From 10 to 120~
Bis(rl3-allyl)nickel was prepared by the reaction of anhydrous NiBr2 with C3HsMgC1 in diethyl ether (molar ratio NiBr 2 : C3HsMgCI = 1 : 2) at -5 to -10~ under a pressure of 0.1 Pa. After the reaction solution became yellowish orange, it was concentrated in a lowtemperature Vigreaux column and cooled to -80~ Bis(rl3-allyl)nickel was crystallized as yellow needles. The complex was sublimed in a high vacuum and re
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