Metal allylation reactions characterized

The deprotonation of alkenes by organometallic reagents affords allyl species. As the simplest example of delocalized organometallic systems, the alkali metal allyl system has been studied in solution and the solid state in quite some detail this work has been further supported by theoretical studies. Allyl species are usually very reactive undergoing complex rearrangement reactions, and often, the reaction products cannot be directly characterized. Instead, they are often identified by their reaction products. 

Rhodium complexes were generally found to be more effective than iridium, but on the whole they show moderate activity in alkene oxygenation reactions. Significantly, epoxides, a typical product of the oxidation of olefins catalyzed by the middle transition metals, have rarely been evoked as products [18-22]. Although allylic alcohols [23] or ethers [24] have sometimes been described as products, the above cited rhodium and iridiiun complexes are characterized by an excellent selectivity in the oxygenation of terminal alkenes to methyl ketones. 

Among the characterized metal homoenolates, only zinc homoenolate of alkyl propionate undergoes substitution reactions with electrophiles under suitable conditions. Two types of metal catalysts, copper(I) and metals of the nickel triad (e.g. Pd), have successfully been used to effect allylation, arylation, and vinylation reactions. 

Reduction of the cyclic allene 72 by sodium metal affords the polycyclic allyl anion 37, which was crystallized and characterized. Reaction with methyl iodide resulted in the formation of the methylated compounds 38 and 39 (Scheme 2) <1996OM3480>. 

The behavior of allene (1,2-propadiene) and its derivatives as ligands in transition metal complexes has been investigated for more than 20 years (197,198). Allene may be coordinated by only one C=C double bond to a metal (197,199), or it may link to two metal centers as a 2 2 or 3 1 electron donor. Dinuclear /i-f/2l2-allene complexes are known for Mo (200,201), W (201), Mn (202-203), and Rh (199,204). In [Fe2(CO)7(/ -f/3 1-C3H4)] a bonding mode (205,206). In the course of the reaction, often two or more allene molecules become connected with transition metal complexes. Two allenes linked by C-2 yield tetramethyl-eneethane (207,208), which is stabilized by coordination on Fe (205,208), Ni (270), or Pd (210,211). 

Green has also used the method of H+ addition to a -CHO-substituted (allyl)metal complex for the preparation of (s-trans-diene) complex derivatives of a Group 8 metal. The Ru complex 35 was synthesized in that way.33 Stable [(s-tra ,s-r 4-conjugated diene) Ru(acac)2] isomers have been described34 and related [(s-tra s-r 4-diene)Ru(trispyrazolylbo-rate)Cl] systems are stable and isolable.35 Eventually, Mashima et al. have described the reaction of all-tra ,s-l,8-diphenyloctatetraene with [Ru(acac)3] under reducing conditions. A dimetallic complex (36) was isolated and characterized by X-ray diffraction, that contained a linear array of two (s-tra ,s-r 4-diene)Ru subunits36 (see Scheme 10). 

A versatile application of Co(allyl)3 has been found in its reaction with butadiene, which effects catalytic dimerization under very mild conditions. Individual reaction steps are reproduced in Scheme 19. These reactions lead to the isolated and structurally characterized complex (13) as the true catalyst. Scheme 19 is in fact a collection of all the classical steps involved in homogeneous transition metal bond-forming reactions. The first step consists of replacement of two allyl groups by butadiene, which is reductively eliminated as diallyl, similar to what has been found with cod and with other two-or four-electron ligands. 

The preparation of tropidinyl transition metal complex 53 was achieved by reaction of 4-trimethylstannyltropidine 52 with M(CO)sBr, M = Mn, Re <2003ZFAC2408>. In complex 53, the tropidinyl ligand is coordinated through the nitrogen and the three-carbon allylic system which serve as a 2o /47t-electron donor. It has been characterized by single-crystal X-ray diffraction analysis (Equation II). 

The chain reaction is initiated by abstraction of an allylic hydrogen to give an allylic radical stabilized by delocalization over three or more carbons. The initiator is a free radical, most probably produced by decomposition of hydroperoxides already present or produced by photooxidation. The decomposition may be thermal, but it is more likely promoted by traces of variable redox state metal ions. Autoxidation is characterized by an induction period during which the concentration of free radicals increases until the autocatalytic propagation steps become dominant. During the induction period, there is little increase in oxidation products. 

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