Syntheses of Variations of Stereogenic-at-Metal Imido Alkylidene Complexes of Molybdenum

Abstract

In this paper we describe the syntheses of several new stereogenic-at-metal imido alkylidene complexes of molybdenum, Mo(NR)(CHR′)(X)(Y), many of which had to be prepared through selective nucleophilic displacement reactions in imido alkylidene complexes. H 3 (Ar, 6a), 2,6-Me 2 C 6 H 3 (Ar′, 6b), 2-iPrC 6 H 4 (Ar iPr , 6c), Ad (6d)), Mo(NR)(CHCMe 2 Ph)(OR F6 )[N(H)HMT] (7a (R = Ar′) and 7b (R = Ar iPr )), and Mo(NAd)(CHCMe 2 Ph)(OR F6 )(HMT) (8). X-ray structural studies were carried out on 1b, 2a−c, 3(PMe 3 ), 4, 5, 6d, 7b, and 8. Compound 1b is an octamer in which two η 1 -pyrrolides are trans to one another at each metal center and cyano groups bind from neighboring Mo centers trans to the alkylidene and imido ligands. ■ INTRODUCTIONRecent advances in olefin metathesis by molybdenum, 1 tungsten, 2 and ruthenium 3 alkylidene complexes have focused on initiators in which the metal is a stereogenic center. For example, the chemistry of high-oxidation-state molybdenum or tungsten complexes of the type M(NR)(CHR′)(OR″)(Pyr), where Pyr is a pyrrolide or a substituted pyrrolide (MAP complexes) were first prepared in 2007 4,5 in the process of demonstrating that a M(NR)(CHR′)(Pyr) 2 complex 6,7 could act as a precursor to a bisalkoxide, a biphenolate, or a binaphtholate imido alkylidene catalyst through addition of the monoalcohol or diol to it. Recent examples of selective metathesis applications by MAP catalysts that contain a sterically demanding HIPTO ligand (HIPTO − = hexaisopropylterphenoxide = O-2,6-(2,4,6-iPr 3 C 6 H 2 ) 2 C 6 H 3 -) or HMTO ligand (HMTO − = hexamethylterphenoxide = O-2,6-(mesityl) 2 C 6 H 3 -) include Z-selective metathesis homocoupling of terminal olefins, 8 Z-selective ringopening metathesis polymerization (ROMP) of 2,3-disubstituted norbornenes and norbornadienes, 9 Z-selective ring-opening/ cross-metathesis reactions, 10 ethenolysis reactions 11 (including Z-selective ethenolysis 12 ), and Z-selective cross-metathesis 1c or ring-closing metathesis. 2a Theoretical calculations by Eisenstein and co-workers 13 help explain why olefin metatheses by MAP species are relatively efficient. Isolation of 4-coordinate 14-electron methylidene species when the aryloxide is sterically demanding is consistent with the relatively long lived nature of MAP catalysts under some circumstances. 14 Syntheses of MAP species via the "protonolysis" route is proposed to involve binding of ROH to the metal through the oxygen atom before a proton migrates to the pyrrolide, probably to the α carbon atom first to yield an intermediate pyrrolenine complex. 15 Therefore, syntheses of MAP species via the protonolysis route strongly depend upon steric factors associated with the size of the pyrrolides (so far usually parent pyrrolide or a 2,5-dimethylpyrrolide), the imido, the alkylidene, and the added ROH. Another persistent synthetic problem is that intermediate MAP species can react with a another 1 equiv of alcohol to form a bisalkoxide. In short, in our experience, many potentially useful MAP species cannot be made i...

Mono‐ and Bisionic Mo‐ and W‐Based Schrock Catalysts for Biphasic Olefin Metathesis Reactions in Ionic Liquids

“…Precipitation of this anionic complex prevents the formation of a product mixture. With few exceptions, Mo‐imido alkylidene monotriflate/monoalkoxides are not common products of this reaction. According to 19 F NMR spectroscopy, both triflates in 4 are equivalent and bound to the metal ( δ triflate =−77.09 ppm, CDCl 3 ), which clearly points toward an anionic molybdenum center with a cationic aryloxide ligand.…”

High‐Oxidation State Molybdenum and Tungsten Complexes Relevant to Olefin Metathesis

“…If a donor functionality is present in the pyrrolide, then it can bind to another metal to form oligomers. For example, Mo(NAd)(CHCMe 2 Ph)(2-CNPyr) 2 is an octamer in which two η 1 -pyrrolides are trans to one another at each metal center, and the cyano groups from neighboring Mo centers bind trans to the alkylidene and imido ligands [17]. Bis-η 1 -pyrrolides are also found as adducts, as in W(NAr)(CHCMe 2 Ph)(η 1 -MesPyr) 2 (dme) [37], Mo(NAd)(CHCMe 2 Ph)(η 1 -NC 4 H 4 ) 2 (PMe 3 ) [12], or one of the metals shown in Figure 1.1.…”

“…The deprotonation of the alkylidene by the incoming pyrrolide nucleophile is especially problematic when one or two triflates is (are) present, and the pyrrolide is relatively sterically demanding [17]. Nevertheless, several bispyrrolides have been prepared in which the pyrrolides are relatively sterically demanding, for example, 2-mesityl [15][16][17], 2,5-diisopropyl [12], 2,5-diphenyl [12], or 2,3,4,5-tetramethyl [12].…”

“…The deprotonation of the alkylidene by the incoming pyrrolide nucleophile is especially problematic when one or two triflates is (are) present, and the pyrrolide is relatively sterically demanding [17]. Nevertheless, several bispyrrolides have been prepared in which the pyrrolides are relatively sterically demanding, for example, 2-mesityl [15][16][17], 2,5-diisopropyl [12], 2,5-diphenyl [12], or 2,3,4,5-tetramethyl [12]. The chemistry of complexes that contain one or two sterically demanding pyrrolides is underdeveloped with respect to the chemistry of the parent pyrrolide or 2,5-dimethylpyrrolide complexes, but the steric differences between various substituted pyrrolides could have significant consequences worthy of study.…”

“…In contrast to approaches to MAP species that involve the protonation of a pyrrolide ligand, several SAM complexes have now been prepared employing approaches that involve nucleophilic displacement at the metal [17]. [70] or M(NAr)(CHt-Bu)(CH 2 -t-Bu)(OR ′ ) [71,72].…”