Oxidative Addition Reactions of [Rh^I(Br)(Tpy*)] (Tpy* = 4‘-(4-tert-Butylphenyl)-2,2‘:6‘,2‘ ‘-terpyridine) with Alkyl Bromides

Abstract

Oxidative addition reactions with the highly nucleophilic complex [Rh I (Cl)(4′-(4-tertbutylphenyl)-2,2′:6′,2′′-terpyridine)] (1) and alkyl halides containing chlorine and bromine atoms gave mixtures of uncharacterized products. In contrast to this, oxidative addition reactions with the complex [Rh I (Br)(4′-(4-tert-butylphenyl)-2,2′:6′,2′′-terpyridine)] (2) and various alkyl bromides, containing (mainly terminal) carbon-bromine bonds, gave clean single products in almost quantitative yield. The corresponding air-stable yellow Rh(III)terpyridine complexes are, in contrast to the parent Rh(I)-terpyridine complex, soluble in various organic solvents. Consequently, these products have been further characterized by means of 1 H, 13 C, 15 N, 103 Rh and 2D NMR techniques. NMR techniques were also used to determine the geometry of the studied Rh(III)-terpyridine complexes, which were found to be octahedral with the alkyl moiety axially positioned with respect to the Rh-terpyridine plane (except for complexes 3 and 4). The oxidative addition reaction of complex 2 with 1-bromodecane in ethanol was monitored by UV/vis spectrometry. The recorded electronic absorption spectrum of this reaction showed two distinct isosbestic points in the visible region. It is most likely that first the carbon-bromine bond of 1-bromodecane is activated by the Rh metal followed by the arrangement of the alkyl moiety in an axial position. Unfortunately, on the basis of our findings, no single mechanism for this oxidative addition reaction can be suggested.

Transition metal-alkenyl complexes [M–(CH2)nCHCH2, n≥2]: Synthesis, structure, reactivity and applications

“…This product (27) has also been reported to form when the iron-butenyl complex is heated in CH 3 NO 2 . This sort of reactivity is not observed with the longer chain iron-pentenyl complex.…”

“…Thus far, the most frequently reported metal-alkenyl complexes are those containing the metals such as Fe [13][14][15][16][17]32,33], Pt [18][19][20] and Ti [21][22][23][24], with only a few reports of complexes based on other metals such as Ni [25], Hf [26], Rh [27], Mo [28,29], Co [30,31], Pd [34], Ru [35] and Zr [36] (Scheme 2 ). The main methods used for the synthesis of these complexes are (i) the anion route, (ii) the Grignard route, with only a few reports on synthesis, (iii) from dienes and (iv) by oxidative addition of a haloalkene to a transition metal.…”

“…Frühauf and co-workers [27] recently reported the synthesis of a series of rhodium-alkenyl complexes of type (13) …”

“…The iron-alkenyl complexes (n = 2, 3) were also reacted with synthesis gas (CO:H 2 ; 1:1) in the presence of the hydroformylation catalyst HRh(CO)(PPh 3 ) 3 , to give a mixture of the -hydroxyalkyl (26) complex and the aldehyde (25). It is also reported that the ironbutenyl complex reacts with TiCl 4 to undergo carbonyl insertion to form complex (27) where the pendant alkene is coordinated to the iron in an 1 , 2 -fashion [17].…”

“…The irradiation of the iron-butenyl complex resulted in the evolution of carbon monoxide and formation of the 3 -methylallyl derivative (29) whereas an analogue of compound (27) is obtained if the original alkenyl ligand is disubstituted at the ␤-carbon. In contrast, thermolysis of the iron-butenyl complex results in the formation of (27), as shown in Scheme 9 [13].…”