Bis(oxazolinylmethyl)pyrrole Derivatives and Their Coordination as Chiral “Pincer” Ligands to Rhodium

Abstract

Bis(oxazolinylmethyl)pyrrole derivatives (R)L(N)H (4a-e), which were designed as protioligands for meridionally coordinating "pincer" ligands, were synthesized by cyclization of pyrrole-2,5-diethylacetate with a series of chiral amino alcohols. Deprotonation of (R)L(N)H (4a-d) with tBuLi and subsequent reaction with [RhCl(CO)(2)](2) gave the corresponding rhodium(I) complexes [Rh((R)L(N))(CO)] (R = iPr: 5a, Ph: 5b, Bn: 5c, Ind: 5d), which were also prepared by reaction of (R)L(N)H with one molar equivalent of [Rh(acac)(CO)(2)]. Upon heating the compounds at 100 degrees C in toluene over a period of 2-5 h, complete rearrangement via a 1,3-H shift between the pyrrole ring and the bridging methylene groups took place to yield the corresponding isomeric complexes [Rh(iso-(R)L(N))(CO)] (6a-d). The transformation induced a planarization of the tridentate ligand system, resulting from the formation of a series of conjugated double bonds. Stirring the rhodium(I) complex 5a with an excess of CH(3)I in dichloromethane at ambient temperature(14) gave the octahedrally coordinated product of an oxidative addition [Rh((iPr)L(N))(CH(3))I(CO)] (7), while reaction of complex 5a with one molar equiv of CsBr(3) as a mild brominating reagent in toluene at 80 degrees C led to complete conversion of the rhodium(I) species to the dibromorhodium(III) complex [Rh(iso-(iPr)L(N))Br(2)(CO)] (8).

Bis(oxazolinylmethyl) Derivatives of C4H4E Heterocycles (E = NH, O, S) as C2‐Chiral Meridionally Coordinating Ligands for Nickel and Chromium

“…We previously noted that pyrrmebox ligands have the tendency to undergo isomerization by a formal 1,3‐H shift when coordinated to palladium(II) or rhodium(I) 7,8. As will be discussed below, similar behaviour may be observed for the nickel complexes reported in this work.…”

“…The preparation of the key intermediates in the subsequent oxazoline synthesis, pyrrole‐2,5‐bisacetamides 3a – c , was achieved by melting 2a – c with ( S )‐valinol in the presence of catalytic amounts of NaH,10 giving the reaction products in almost quantitative yield and sufficient purity for the following cyclization step. The synthesis of i PrL N H ( 4a ) was already reported by the route depicted in Scheme 8. In the same way, conversion of 3b and 3c into the bis(oxazolinylmethyl) derivatives i PrL O ( 4b ) and i PrL S ( 4c ), respectively, was conveniently achieved by reaction with the tetranuclear zinc complex [Zn 4 O(O 2 CCF 3 ) 6 ], first employed by Oshima et al for this purpose 11…”

“…The synthesis of pyrrole derivative 2a by Paal–Knorr‐type cyclization was reported previously by us8 and is based on an earlier protocol by Brooker et al9a Compounds 2b and 2c are reported in the literature,9d,9e albeit by different synthetic routes and by using different precursor materials, which gave both in lower yield. In the case at hand, the acid‐catalyzed cyclization of 1 in refluxing acetic anhydride in the presence of H 2 SO 4 and subsequent workup gave 2b in 75 % yield, whereas treatment of 1 with Lawessons reagent and subsequent thermal cyclization in refluxing toluene provided a convenient route to 2c (91 % yield).…”

“…We recently reported the synthesis of 2,5‐bis(2‐oxazolinylmethyl)pyrroles IIa and their coordination to palladium(II)7 and rhodium(I/III) 8. They are readily obtained by condensation of pyrrole‐2,5‐diethyl acetate with two molar equivalents of a chiral amino alcohol.…”

New Chemistry with Anionic NNN Pincer Ligands

“…This could be prepared starting from the 2,5-bis(cyanomethyl) pyrrole and reacting this with chiral alcohols in chlorobenzene under reflux with catalytic amounts of ZnCl 2 , following the same synthetic methodology just described. However, for this reaction, the cyclization proved to be difficult, and in 2009, an alternative method was reported to synthesize this ligand [23]. The new method used diethyl pyrrole-2,5-diacetate which was condensed with an amino alcohol in the presence of catalytic NaH to yield the intermediate pyrrole-2,5-bis (acetamides) which could be cyclized in the presence of Zn 4 O(O 2 CCF 3 ) 6 (Scheme 2).…”