Synthetic, Mechanistic, and Computational Investigations of Nitrile-Alkyne Cross-Metathesis

Abstract

The terminal nitride complexes NW(OC(CF 3) 2Me) 3(DME) ( 1-DME), [Li(DME) 2][NW(OC(CF 3) 2Me) 4] ( 2), and [NW(OCMe 2CF 3) 3] 3 ( 3) were prepared in good yield by salt elimination from [NWCl 3] 4. X-ray structures revealed that 1-DME and 2 are monomeric in the solid state. All three complexes catalyze the cross-metathesis of 3-hexyne with assorted nitriles to form propionitrile and the corresponding alkyne. Propylidyne and substituted benzylidyne complexes RCW(OC(CF 3) 2Me) 3 were isolated in good yield upon reaction of 1-DME with 3-hexyne or 1-aryl-1-butyne. The corresponding reactions failed for 3. Instead, EtCW(OC(CF 3)Me 2) 3 ( 6) was prepared via the reaction of W 2(OC(CF 3)Me 2) 6 with 3-hexyne at 95 degrees C. Benzylidyne complexes of the form ArCW(OC(CF 3)Me 2) 3 (Ar = aryl) then were prepared by treatment of 6 with the appropriate symmetrical alkyne ArCCAr. Three coupled cycles for the interconversion of 1-DME with the corresponding propylidyne and benzylidyne complexes via [2 + 2] cycloaddition-cycloreversion were examined for reversibility. Stoichiometric reactions revealed that both nitrile-alkyne cross-metathesis (NACM) cycles as well as the alkyne cross-metathesis (ACM) cycle operated reversibly in this system. With catalyst 3, depending on the aryl group used, at least one step in one of the NACM cycles was irreversible. In general, catalyst 1-DME afforded more rapid reaction than did 3 under comparable conditions. However, 3 displayed a slightly improved tolerance of polar functional groups than did 1-DME. For both 1-DME and 3, ACM is more rapid than NACM under typical conditions. Alkyne polymerization (AP) is a competing reaction with both 1-DME and 3. It can be suppressed but not entirely eliminated via manipulation of the catalyst concentration. As AP selectively removes 3-hexyne from the system, tandem NACM-ACM-AP can be used to prepare symmetrically substituted alkynes with good selectivity, including an arylene-ethynylene macrocycle. Alternatively, unsymmetrical alkynes of the form EtCCR (R variable) can be prepared with good selectivity via the reaction of RCN with excess 3-hexyne under conditions that suppress AP. DFT calculations support a [2 + 2] cycloaddition-cycloreversion mechanism analogous to that of alkyne metathesis. The barrier to azametalacyclobutadiene ring formation/breakup is greater than that for the corresponding metalacyclobutadiene. Two distinct high-energy azametalacyclobutadiene intermediates were found. These adopted a distorted square pyramidal geometry with significant bond localization.

Recent advances in the development of alkyne metathesis catalysts

“…Inspired both by a report of Johnson and co-workers, who found that molybdenum and tungsten nitride complexes 24 with fluorinated alkoxide ligands react with alkynes to generate the corresponding metal alkylidynes 25 in situ (Scheme 6) [80–81], and by the work of Chiu et al on the preparation of a silanolate-supported molybdenum-nitride complex [82], Fürstner’s group introduced a novel user-friendly catalyst system for alkyne metathesis by employing triphenylsilanol (Ph 3 SiOH) [83–84]. Two synthetic routes were developed, which are shown in Scheme 7.…”

Synthesis and characterization of a trianionic pincer supported Mo-alkylidene anion and alkyne insertion into a Mo(IV)-C bond to form metallocyclopropene(η2-vinyl) complexes

“…Recently, we proposed that a trianionic pincer ligand may provide the appropriate ligand arrangement for a reactive AM or NACM catalyst [25]. Trianionic pincer ligands [26e28] bind in the meridional plane of a metal ion [26,29,30], and thus eliminate any thermal barrier to obtaining the T-shaped pendant ligand arrangement proposed for metallocyclobutadiene transition states [12,31]. The difficulty thus far is metalating a trianionic pincer ligand while retaining the MhCR fragment.…”

Catalytic alkyne metathesis and stoichiometric metal–alkylidyne formation from N Mo(OR)3 complexes promoted by Lewis acids

“…Bis(4-methoxyphenyl)acetylene [24,25], VCl 3 (thf) 3 [26], N^Mo(OCMe 3 ) 3 (6) [27], N^Mo(OCMe(CF 3 ) 2 ) 3 (1) [28], and N^Mo(OC(CF 3 ) 3 ) 3 (NCMe) (2) [28], were prepared according to literature procedures. Mesitylene, diphenylacetylene, and chlorotitanium tri-isopropoxide were obtained from Acros.…”