Synthesis of cis,syndiotactic ROMP polymers containing alternating enantiomers.

Ring-opening metathesis polymerization (ROMP) of rac-endo,exo-5,6-dicarbomethoxynorbornene (inter alia) yields a cis,syndio,alt-polymer, one in which the sequential units in the cis,syndiotactic polymer consist of alternating enantiomers. Cis selectivity arises through addition of the monomer to produce an all-cis-metallacyclobutane intermediate, while syndioselectivity and alternating enantiomer structures arise as a consequence of inversion of configuration at the metal center with each metathesis step.

Z-Selective and Syndioselective Ring-Opening Metathesis Polymerization (ROMP) Initiated by MonoAryloxidePyrrolide (MAP) Catalysts.

We report the Z-selective and syndioselective polymerization of 2,3-bis(trifluoromethyl)bicyclo[2.2.1]hepta-2,5-diene (NBDF6) and 3-methyl-3-phenylcyclopropene (MPCP) by monoaryloxide monopyrrolide imido alkylidene (MAP) catalysts of Mo. The mechanism of polymerization with syn-Mo(NAd)(CHCMe(2)Ph)(Pyr)(OHIPT) (1; Ad = 1-adamantyl, OHIPT = O-2,6-(2,4,6-i-Pr(3)C(6)H(2))(2)C(6)H(3)) as the initiator is proposed to consist of addition of monomer to the syn initiator to yield a syn first insertion product and propagation via syn insertion products. In contrast, the mechanism of polymerization with syn-Mo(NAr)(CHCMe(2)Ph)(Pyr)(OTPP) (4; Ar = 2,6-i-Pr(2)C(6)H(3), OTPP = 2,3,5,6-Ph(4)C(6)H) as the initiator at -78 °C consists of addition of monomer to the syn initiator to yield an anti first insertion product and propagation via anti insertion products. Polymerizations of NBDF6 and MPCP at room temperature initiated by 4 led to polymers without a regular structure. We propose that the syndiotacticity of cis polymers is the consequence of the required inversion at the metal center with each insertion of monomer, i.e., stereogenic metal control of the polymer structure. We also propose that the two mechanisms for forming cis,syndiotactic polymers arise as a consequence of the relative steric bulk of the imido and phenoxide ligands.

Iron-promoted ortho- and/or ipso-hydroxylation of benzoic acids with H(2)O(2).

Regioselective hydroxylation of aromatic acids with hydrogen peroxide proceeds readily in the presence of iron(II) complexes with tetradentate aminopyridine ligands [Fe(II)(BPMEN)(CH(3)CN)(2)](ClO(4))(2) (1) and [Fe(II)(TPA)(CH(3)CN)(2)](OTf)(2) (2), where BPMEN=N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-ethylenediamine, TPA=tris-(2-pyridylmethyl)amine. Two cis-sites, which are occupied by labile acetonitrile molecules in 1 and 2, are available for coordination of H(2)O(2) and substituted benzoic acids. The hydroxylation of the aromatic ring occurs exclusively in the vicinity of the anchoring carboxylate functional group: ortho-hydroxylation affords salicylates, whereas ipso-hydroxylation with concomitant decarboxylation yields phenolates. The outcome of the substituent-directed hydroxylation depends on the electronic properties and the position of substituents in the molecules of substrates: 3-substituted benzoic acids are preferentially ortho-hydroxylated, whereas 2- and, to a lesser extent, 4-substituted substrates tend to undergo ipso-hydroxylation/decarboxylation. These two pathways are not mutually exclusive and likely proceed via a common intermediate. Electron-withdrawing substituents on the aromatic ring of the carboxylic acids disfavor hydroxylation, indicating an electrophilic nature for the active oxidant. Complexes 1 and 2 exhibit similar reactivity patterns, but 1 generates a more powerful oxidant than 2. Spectroscopic and labeling studies exclude acylperoxoiron(III) and Fe(IV)=O species as potential reaction intermediates, but strongly indicate the involvement of an Fe(III)--OOH intermediate that undergoes intramolecular acid-promoted heterolytic O-O bond cleavage, producing a transient iron(V) oxidant.

Z-selective olefin metathesis processes catalyzed by a molybdenum hexaisopropylterphenoxide monopyrrolide complex.

The molybdenum-based monoaryloxide monopyrrolide (MAP) species, Mo(NAd)(CHCMe(2)Ph)(C(4)H(4)N)(HIPTO) (2a), which contains "small" imido (Ad = 1-adamantyl) and "large" aryloxide (HIPTO = O-2,6(2,4,6-i-Pr(3)C(6)H(2))C(6)H(3)) ligands, catalyzes Z-selective metathesis reactions as a consequence of intermediate metallacyclobutane species not being able to have an (anti) substituent pointing toward the HIPTO group. Ring-opening metathesis polymerization (ROMP) of dicarbomethoxynorbornadiene (DCMNBD) with 2% 2a in toluene leads to >99% cis and >99% syndiotactic poly(DCMNBD), while ROMP of cyclooctene and 1,5-cyclooctadiene (300 equiv) with initiator 2a leads to poly(cyclooctene) and poly(cyclooctadiene) that have cis contents of >99%; all are previously unknown microstructures. Z-Selectivity is also observed in the metathesis of cis-4-octene and cis-3-hexene by initiator 2a to give cis-3-heptene.

Ortho-hydroxylation of benzoic acids with hydrogen peroxide at a non-heme iron center.

The iron-assisted hydroxylation of benzoic acid to salicylic acid by 1/H2O2 has been achieved in good yield under mild conditions (where is [Fe(II)(BPMEN)(CH3CN)2](ClO4)2 and BPMEN =N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine); the product of this reaction is a novel mononuclear iron(III) complex with a chelating salicylate.