Synthesis of molybdenum olefin metatheses catalysts through protonation reactions

Abstract

The attempted syntheses of molybdenum imido alkylidene complexes of the type Mo(NArc,)(CH-t-Bu)[Biphen] and Mo(N-2-CF3C6H4)(CHCMe2Ph)[Biphen] (Biphen2 = 3,3'-di-t-butyl-5,5',6,6'-tetramethyl- 1,1'-biphenyl-2,2'-diolate) from Mo(NArcl)(CH-t-Bu)(OTf)2(dme) and [Biphen]K2 have sporadically afforded mixtures containing the desired products along with the corresponding amido alkylidyne complexes, Mo(NHArcl)(C-t-Bu)[Biphen] and Mo(NH-2-CF3C6H4)(CCMe2Ph)[Biphen], respectively. The reaction of [Biphen]K2 with Mo(NArc,)(CH-t-Bu)(OTf)2(dme) and 10 equivalents of triethylamine reproducibly gave Mo(NHArc,)(C-t-Bu)[Biphen] in 40% yield. An X-ray crystal structure of a related complex, Mo(NHArc,)(CCMe2Ph)[S-Biphen] confirmed the proposed structure and also revealed that one ortho chloride approaches within 2.93 A of the metal approximately trans to the alkylidyne ligand. Attempts to prepare three other amido alkylidyne complexes in an analogous manner from Mo(NR")(CH-t-Bu)(OTf)2(dme) (NR" = N-2-CF3C6H4, N-2,6-i-Pr2C6H5, N-2,6-Me2C6H5) with [Biphen]K2 in the presence of 10-20 equivalents of triethylamine failed.

(cont.) Chapter 2 The reaction between Mo(NAr)(CH-t-Bu)(CH2-t-Bu)2 (Ar = 2,6-i-Pr2C6H3) and various alcohols (1-AdamantylOH, t-BuOH, ArOH, (CF3)2CHOH, (CF3)2MeCOH, (CF3)3COH, C6F5OH) in pentane or toluene yielded either complexes of the type Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) through direct addition of ROH across a Mo-C bond, or complexes of the type Mo(NAr)(CH2-t-Bu)3(OR) through direct addition of ROH across a Mo=C bond. The trineopentyl species appear to be formed when the alcohol has a relatively low pKa. The outcome also can depend upon whether the alcohol is employed neat, or in benzene, and mixtures are observed in some circumstances. The conversion of Mo(NAr)(CH2-t-Bu)3(OR) into Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) was shown to be unimolecular in several examples. Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) complexes have been found to be surprisingly active catalysts for various metathesis reactions. In contrast, M(NAr)(CH-t-Bu)(CH2-t-Bu)2 species are virtually inactive for metathesis. X-ray structures are reported for Mo(NAr)(CH2-t-Bu)3(OC6F5), Mo(NAr)(CH-t-Bu)(CH2-t-Bu)IOSi(O-t-Bu)3], [Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F)12, and Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5)(PMe3).

(cont.) Chapter 3. Complexes of the type Mo(NR")(CHR')(N(R)3,5-C6H3Me2)2 (NR" = N-2,6-i-Pr2C6H, N-2,6-Me2C6Hs; R' = t-Bu, CMe2Ph; R' = i-Pr, t-Bu) and Mo(NR")(CHR')(NR2)2 (NR" = N-2,6-i-Pr2C6H,, N-2,6-Me2C6H5; R' = t-Bu, CMe2Ph; R = Me, Ph) can be isolated as orange-red solids in 30-35% yields or oils by reacting Mo(NR")(CHR')(OTf)2(dme) with LiN(R')(3,5-C6H3Me2)(ether) or with LiNR2. The synthesis of Mo(NR")(CHCMe2Ph)(NPh2)2 can be improved to 70-90% isolated yields when Mo(NR")(CHCMe2Ph)[OCMe(CF3)212 is used with LiNPh2(ether). Mo(NAr)(CHCMe2Ph)(NPh2)2 has been crystallographically characterized. Mo(NR")(CHR')(N(R)3,5-C6H3Me2)2 species reacted with t-BuOH and Me(CF3)2COH in benzene to give Mo(NR")(CHR')(OR)2 (OR = O-t-Bu, OCMe(CF3)2) in situ. However, no reactions of Mo(NR")(CHR')(N(R')3,5-C6H3Me2)2 were observed with enantiomerically pure diols such as [R-TRIP]H2 (3,3'-2,4,6-i-Pr3C6H2-binaphthol), [R-Ph]H2 (3,3'-C6H5-binaphthol), [rac-Mesitylbinap]H2 (3,3'-2,4,6-Me3C6H2-binaphthol) and [R-TMSbinapJH2 (3,3'-SiMe3-binaphthol).

(cont.) Bisamido complexes of the type Mo(NR")(CHR')(NPh2)2 were found to react with the aforementioned alcohols and diols to give Mo(NR")(CHR')(diolate) species in situ, which were further reacted in a catalytic fashion with two substrates to give the corresponding ring-closed products. Preliminary :results of the in situ catalysis demonstrated here compare fairly well with the analogous catalytic reactions reported with isolated catalysts. Appendix A. Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) (Ar = 2,6-i-Pr2C6H3) can be reacted with 5-10 equivalents of trans-3-hexene to give a crystallographically characterized dimeric complex, [Mo(NAr)(CH2-t-Bu)(OC6F5)]2 that contains an unbridged Mo=Mo bond (2.410(8) A) in high yields. The above complex can also be prepared by treating Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) with 0.5 equivalents of divinylbenzene. IMo(NAr)(CH2-t-Bu)(OC6F5)]2 will slowly catalyze the metathesis reactions of simple substrates, although less than 5% of the catalyst seems to be activated in such reactions.

(cont.) It was observed that catalytically active species for metathesis reactions can be generated by another Mo (d2) species, Mo(NArcl)(Biphen)(H2C=CH2)(ether) (NArc, = N-2,6-C12C6H3, Biphen2 = 3,3'-di-t-butyl-5,5',6,6'-tetramethyl-1,1'-biphenyl-2,2'-diolate) that could effect the ring-opening metathesis polymerization of norbornene. A mixture of Mo(NArcl)(Biphen)(H2C=CH2)(ether) and 20 equivalents of diallylether in benzene-d6 when treated with 10 equivalents of norbornene gives 54% conversion to dihydrofuran in 10 days.