Unusual Mechanism for the Reaction of a Niobocene Hydride Complex with Activated Alkynes. Experimental and DFT Studies

Abstract

The reactivity of the activated alkynes RCCR (R = CO 2 Me, CO 2 t Bu) with the Nb−H bond of the 18e − niobocene hydride trimethyl phosphite complex [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)(P(OMe) 3 )] (1) has been studied, and the new (E)-niobocene alkenyl trimethyl phosphite complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (C(CO 2 R)C(CO 2 R)H)(P(OMe) 3 )] (R = Me (2a), t Bu (2b)) have been obtained. DFT studies were carried out to identify the lowest energy mechanism and the possible intermediates in the process. An unusual nucleophilic d 2 Nb(III) center was identified as a σ-donor entity responsible for the nucleophilic attack on a carbon atom of the activated (electrophilic) alkyne molecules. A subsequent hydrogen transfer to the other carbon atom produces the final (E)-niobocene alkenyl trimethyl phosphite complexes, the stereochemistries of which were elucidated by NMR spectroscopy.T he elementometalation process, a term that was coined by Negishi several years ago, 1 is formally a 1,2-insertion that corresponds to the addition of the two fragments of the E−M system (E = H, C, heteroatom (X), and M = metal) to unsaturated moieties such as those present in alkyne or alkene substrates. The reaction mechanism for the insertion into alkynes or alkenes has been studied by several groups 2 and involves a four-center transition state.In broad terms, two types of insertion reactions can be distinguished: namely, migratory and nonmigratory insertions. Migratory insertions, which are better known, require a vacant site at the metal center or a labile ligand that can be easily replaced by the incoming reagent, and these should be cis stereospecific processes. 2 In contrast, complexes that are coordinatively saturated or are inert toward substitution cannot undergo migratory insertion, and these produce new complexes of varying stereochemistry through cis or trans stereospecific processes. These reactions have been called nonmigratory insertion reactions 3 (Scheme 1).Some of us previously reported the nonmigratory insertion reaction of [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)(L)] (L= CO, CNR) with activated alkynes 4 in a process that allowed the preparation of new niobocene isocyanide or carbonyl (E)-alkenyl and (Z)-alkenyl complexes (Scheme 1).However, an additional route that involves nucleophilic attack on alkenes or alkynes is well-known to produce the same final product, albeit through a different mechanism. In fact, a few years ago it was shown that, when an electron-rich ligand with one lone pair is present in some niobocene complexes, this type of attack was responsible for the interaction with the activated alkyne species, thus allowing the isolation of phosphorus alkenyl complexes (Scheme 2). 5 As a continuation of our studies on the complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)(L)] (L = π-acid ligand), we report here the reaction of [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)(P(OMe) 3 )] with two activated alkynes. The results of DFT studies established that

Stereoselectivity and nonmigratory insertion mechanism of dimethylacetylene dicarboxylate into metallocene‐hydride of Cp2M(L)H [Cp =η5‐C5H5; M = Nb, V; L = CO, P (OMe)3]: A DFT study

“…In TS3, another five‐membered ring structure, Nb–C2–C5–O2–C4, appears again. The topological structure changes indicate that the E ‐isomer forms via a five‐centered transition state rather than a four‐centered transition state as proposed in Scheme . The four‐membered ring structure is an intermediate, not transition state.…”

“…Transition metal hydrides play a critical role in stoichiometric and catalytic transformations . They play important roles in various catalytic reactions, such as hydrogen exchange, hydrogen evolution and hydrogenation .…”

Mechanism and Stereoselectivity of the Elementometalation Process of Activated Alkyne RCCR(RCO2Me) by Cp2TaH3

“…[16,17] Recently, the synthesis and characterization of metal alkyne complexes have been reported by some groups. [18][19][20][21][22][23] The reactivity, [24][25][26] stereochemistry, [27][28][29] and mechanism [23,30] of alkyne or alkene insertion into M E bonds have also received increasingly more attention. The reaction mechanism studies show that the insertion of alkynes into metal hydrides can be classified into two types, namely, migratory and nonmigratory insertions.…”

“…The reactivity of M III -hydride compounds toward unsaturated molecules has been extensively studied. [22,23,30,34,35] The M Vhydride compounds, such as niobium/tantalum metallocene trihydride complexes (Cp 2 MH 3 , M Nb, Ta) [36] and their derivatives, being classical trihydrides, often show the characteristics of proton-proton exchange couplings. [37][38][39][40] The first example of alkyne insertion into the M H bond in experiment is the reaction of Cp 2 TaH 3 with the esters RO 2 CC CCO 2 R (R Me, SiMe 3 ), which are activated alkynes containing two electron-withdrawing substituents that inserted into the Ta H bond in trans form to give an alkenyl-dihydride intermediate and following a subsequent rearrangement to give the (E)-type hydride complex Cp 2 Ta(H)(E RO 2 CCH CHCO 2 R).…”

“…The group 5 metals in most of their metal hydrides are M III (in monohydride compounds) or M V (in trihydride compounds). The reactivity of M III ‐hydride compounds toward unsaturated molecules has been extensively studied . The M V ‐hydride compounds, such as niobium/tantalum metallocene trihydride complexes (Cp 2 MH 3 , MNb, Ta) and their derivatives, being classical trihydrides, often show the characteristics of proton–proton exchange couplings .…”