Frustrated Lewis Pairs beyond the Main Group: Synthesis, Reactivity, and Small Molecule Activation with Cationic Zirconocene–Phosphinoaryloxide Complexes

Abstract

The extension of the frustrated Lewis pair (FLP) concept to the transition series with cationic zirconocene-phosphinoaryloxide complexes is demonstrated. Such complexes mimic the reactivity of main group FLPs in reactions such as heterolytic hydrogen cleavage, CO(2) activation, olefin and alkyne addition, and ring-opening of tetrahydrofuran. The interplay between sterics and electronics is shown to have an important role in determining the reactivity of these compounds with hydrogen in particular. The Zr-H species generated from the heterolytic activation of hydrogen is shown to undergo insertion reactions with both CO(2) and CO. Crucially, these transition metal FLPs are markedly more reactive than main group systems in many cases, and in addition to the usual array of reactions they demonstrate unprecedented reactivity in the activation of small molecules. This includes S(N)2 and E2 reactions with alkyl chlorides and fluorides, enolate formation from acetone, and the cleavage of C-O bonds to facilitate S(N)2 type reactions with noncyclic dialkyl ethers.

Frustrated Lewis Pair Chemistry: Development and Perspectives

“…In a similar sense, an exchange reaction of t Bu 3 P(CO 2 )B-(C 6 F 5 ) 2 4 ] (211; Scheme 72). [93] While the above reactivity suggested the possibility, it was Wass and co-workers [140] and subsequently the Erker group, [141] who demonstrated the generation and reactivity of FLPs incorporating transition-metal Lewis acids directly. In the original work of Wass et al…”

“…[109] Similarly, the species t Bu 3 P(N 2 O)B(C 6 H 4 F) 3 (136) was prepared and used in Lewis acid exchange reactions to give a variety of analogues including the exchange of the weak Lewis acid for other more electrophilic boranes. [109] For example, treatment with a variety of boranes gave (140). [109] This exchange route is also effective beyond B-based Lewis acids affording access to the trityl derivative [ …”

Frustrated Lewis Pairs

“…179 This situation is highly reminiscent of bifunctional transition metal catalysts. In this regard it is also important to draw attention to transition metal based FLPs in which metal centers with pendant donors activate a range of small molecule substrates, which has been explored by the groups of Wass [126][127][180][181][182][183] and Erker 128-129,184-188 among others. 130 The concept of FLPs has also been exploited to describe a variety of systems that activate small molecules.…”

Stoichiometric and Catalytic Inter‐ and Intramolecular Hydroamination of Terminal Alkynes by Frustrated Lewis Pairs

“…[16] Beyond this main-group chemistry,t he FLP paradigmh as been appliedt ot ransition-metal systemsi nc ombination with alkynes, affording routes to cationic zirconocene phosphinoaryloxide-hydride complexes throughs elective deprotonation of terminala lkynes (Scheme 1c). [17] In ar ecentp aper,w eh ave shownt hat Ru-acetylides act as carbonn ucleophilesi nc ombinationw ith Lewis acids to effect FLP addition to alkynes yielding trans-addition products.…”