Reactions of Neutral and Cationic Diamide-Supported Imido Complexes with CO₂ and Other Heterocumulenes:  Issues of Site Selectivity

Abstract

Experimental and DFT computational studies of the reactions of the cationic diamidepyridine-supported methyl tungsten complex [W(NPh)(N 2 N py )Me] + (2 + ) with the heterocumulenes CO 2 , CS 2 , RNCO, and RNCS (R ) tert-butyl or aryl) are described, together with comparative studies of the group 5 compounds M(N t Bu)(N 2 N py )Cl(py) (M ) Nb (13) or Ta ( 14)) and Ta(N t Bu)(N 2 N py )Me (N 2 N py ) MeC(2-C 5 H 4 N)(CH 2 NSiMe 3 ) 2 ). In all of the reactions of 2 + the heterocumulene inserted exclusively into a W-N amide bond to give unstable intermediates (isolated for certain isocyanate substrates), which subsequently underwent rearrangement via a 1,3-migration of a SiMe 3 group. DFT studies of the reaction of 2 + with CO 2 showed that insertion into the W-Me bond is in fact thermodynamically preferred, but that insertion into W-N amide is kinetically more facile. Cycloaddition to the WdNPh bond was neither kinetically nor thermodynamically viable. The reactions between Ta(N t Bu)-(N 2 N py )Me and CO 2 or RNCO in contrast gave complex mixtures, as did those between 13 and 14 and CO 2 . DFT studies showed that in a neutral but otherwise identical tantalum analogue of 2 + the preference for substrate attack at the M-N amide bond is much less pronounced. Reactions of 13 and 14 with the more sterically discriminating RNCO (R ) t Bu or p-tolyl) did, however, exclusively afford M-N amide insertion/1,3-SiMe 3 migration products, and Nb(N t Bu){MeC(2-C 5 H 4 N)(CH 2 NSiMe 3 )(CH 2 NC(OSiMe 3 )N t Bu)}Cl was structurally characterized. The previously reported reaction of Ti(N t Bu)(N 2 N py )(py) with ArNCO (Ar ) 2,6-C 6 H 3 i Pr 2 ) was re-evaluated and shown to involve an analogous Ti-N amide insertion/1,3-SiMe 3 migration reaction, ultimately forming Ti(N t Bu){MeC(2-C 5 H 4 N)(CH 2 NSiMe 3 )(CH 2 NC(N(Ar)-SiMe 3 )O)}(py).

Reaction Site Diversity in the Reactions of Titanium Hydrazides with Organic Nitriles, Isonitriles and Isocyanates: TiNα Cycloaddition, TiNα Insertion and NαNβ Bond Cleavage

“…Unfortunately both reactions led to a complex mixture of products, possibly as a result of competition with insertion into the Ti À N amide bonds. [35] The solid-state structures of 21 a and 23 are shown in Figure 6 and selected distances and angles are given in Table 5. The coordination geometry at Ti (1) and a large number have been structurally authenticated.…”

“…[35] Mixtures were also obtained with 4 and various isocyanates and isothiocyanates. However, reaction of 5 with the bulky 2,6-diisopropylphenyl cyanates Ar'NCO, Ar'NCS and Ar'NCSe in Et 2 O at room temperature gave the [2 + 2] cycloaddition products [Ti- (26), respectively, in 51-65 % isolated yield (Scheme 11; Ar' = 2,6-C 6 H 3 iPr 2 ).…”

Scandium Complexes Bearing Bis(oxazolinylphenyl)amide Ligands: An Analysis of Their Reactivity, Solution‐State Structures and Photophysical Properties

“…However there have been cases where there are more p π donors than d π acceptors (“π‐loaded” complexes), thus making nitrogen‐based p π electrons effectively non‐bonding . Such a situation has been shown to have a pronounced effect on their subsequent reactivity, rendering the M–NR 2 moiety subject to protonation and insertion reactions . Although these BOPA complexes are not π‐loaded, the calculations suggest that there is little, if any, p π → d π bonding with the scandium‐bound amides.…”

The Progression of Synthetic Strategies to Assemble Titanium Complexes Bearing the Terminal Imide Group

“…[3][4][5][6][7] For example, this type of functionality is responsible for fundamentally important reactions such as the intermolecular activation of aliphatic (including methane) [8,9] as well as aromatic C À H bonds. [7,[10][11][12][13] The ubiquitous imide group can also participate in other processes such as cycloaddition reactions with substrates like carbon dioxide, [14][15][16][17][18][19][20][21] carbon disulfide, [14-16, 19, 22] ke-tones, [16,22,23] alkenes (intra-and intermolecularly), [24,25] A C H T U N G T R E N N U N G alkynes, [22,[24][25][26] nitriles, [23,27] isonitriles, [16,28] imines, [16,23,29,30] carbodiimides, [4,16,[30][31][32] allenes, [4,24,33] allylic alcohols, [34] and carboxamides…”