Publications

A series of η²-[Os(NH₃)₅(vinyl ether)]²⁺ complexes have been prepared by three independent methods that involve direct coordination of a vinyl ether, alcohol addition to an η²-alkyne complex, or nucleophilic substitution of an η²-vinyl ether species. In the presence of an acid catalyst, the vinyl ether ligand undergoes a novel acid-catalyzed substitution reaction at the α-carbon with a broad range of nucleophiles that includes alcohols, amines, carboxylates, hydrides, silylated enols, nitriles, phosphines, and dialkyl sulfides. These reactions appear to proceed through an elimination−addition process where the first step is loss of an alcohol to form an η²-vinyl cation intermediate. In cases where the α-carbon bears an alkyl group, an η²-vinyl cation species can be isolated and characterized. For example, protonation of [Os(NH₃)₅(η²-2-methoxypropene)]²⁺ (3) in neat HOTf allows the characterization of the substitution reaction intermediate η²-[Os(NH₃)₅(C₃H₅)]³⁺ (32), formally a metallocyclopropene that behaves chemically like a vinyl cation. In contrast, when the α-carbon of the vinyl ether bears a hydrogen such as with [Os(NH₃)₅(η²-ethoxyethene)]²⁺ (1), the hypothetical vinyl cation intermediate, in absence of a suitable nucleophile, undergoes an intramolecular 1,2-hydrogen shift to yield the Fischer carbyne [(NH₃)₅Os⋮CCH₃]³⁺ (33). Examples of nucleophilic substitution reactions for other types of η²-[Os(NH₃)₅(olefin)]ⁿ⁺ complexes are also demonstrated.

In summary, it is clear that the photochemistry and photo-physics of metal carbyne chromophores, especially in coordina-tion spheres of “simple” ligands such as ammines, have a widelandscape still waiting to be explored. Current work involvingthe base-induced quenching mechanism of the excited state isunderway as well as molecular orbital calculations that will more clearly delineate excited state structure and reactivity.

Summary: A series of complexes of the type [OsfNH&s- (4,5-rf2-L)](OTf)2 (where L= thiophene (la), 2-methyl-thiophene (lb), 3-methylthiophene (lc), 2,5-dimethyl-thiophene (Id), 2-methoxythiophene (le), 3-methoxythio-phene (If), and benzofbjthiophene (lg))were prepared and characterized by NMR and cyclic voltammetry (CV). Treatment of la-d with 1 equiv of HOTf yields novel isolable 2H-thiophenium complexes with the osmium bound to C(5) and sulfur.

The pentaammineosmium(II) complex of 5-methylfuran (2) undergoes stereoselective aldol

reactions with various aldehydes in the presence of a Lewis acid (Sn(OTf)2 or BF3OEt2) to

give 4-acetylated-4,5-dihydrofuran complexes where the aldehyde carbonyl group has been

incorporated into the dihydrofuran nucleus. A detailed analysis of the substitution pattern

and stereochemistry of these products reveals a reaction sequence involvingan aldol reaction

at C(4) of the ?/2-furan followed by nucleophilic displacement at C(2) of the furan oxygen by

the aldol-derived alkoxide. When the parent furan complex is subjected to otherwise identical

reaction conditions, this rearrangement does not occur. Instead, two diastereomers of a

novel bicyclic diacetal are formed.

The reactivity of a series of pyrrole complexes of the form [Os(NH3)5(4,5-?72-L)]2+(OTf)2 (L= pyrrole

and alkylated pyrroles) is surveyed with various electrophiles. The pyrrole ligand undergoes

alkylationor acylation with a wide variety of electrophiles (e.g., acids, alkyl triflates, anhydrides,

aldehydes, ketones, and Michael acceptors) predominately at the /3-position. Depending on reaction

conditions, the resulting products are either /3-substituted lif-pyrroleor 3if-pyrrolium complexes,

the latter of which resist rearomatization due to the electron-donating properties of the metal. In

all cases observed, the initial addition of the electrophile occurs on the ring face anti to osmium

coordination. The osmium(II)—4,5-?/2-pyrrole complexes are each in dynamic equilibrium with a

minor isomer where the metal binds across C(3) and C(4). In this form, the uncoordinated portion

of the pyrrole ring resembles an azomethine ylide, which can undergo a 1,3-dipolar cycloaddition

reaction with certain electrophiles. The resulting 7-azanorbomene complexes may be ring-opened

with Lewis acids to generate -substituted 2//-pyrrolium complexes. As with the 3H-pyrrolium

species, the 2H-pyrrolium complexes are stabilized by metal coordination and thereby resist

rearomatization. The selectivity between Michael addition and dipolar cycloaddition depends on

the pyrrole, electrophile, solvent, temperature, the presence of Lewis acids, and in some cases,

concentration. The iminium carbon of both 2H- and 3/f-pyrrolium tautomers is considerably less

electrophilic than its organic analogs, but readily undergoes borohydride reduction to form complexes

of 3- and 2-pyrrolines, respectively. When pyrrole complexes are combined with alkyne Michael

acceptors, the intermediate enolate can be trapped by the iminium carbon of the 3i/-pyrrolium

species in DMSO to form a metalated cyclobutene derivative. Decomplexation of most pyrrole and

3-pyrroline derivatives can be accomplished in good yield either by heatingor by oxidation of the

metal (Ce™ or DDQ). Complexes of 2-pyrrolines are considerably more difficult to remove from

the metal; however, quaternizationor acylation of the nitrogen facilitates their decomplexation.

The compound [Ren(terpy)(PPh3)2Cl]+ (with counterion Cl- (I) or OTf” (II)) has proven to be a useful synthon

in the synthesis of a series of Re(I) and Re(III) compounds. Compound I can be oxidized in the presence of Cl~

to form [Rera(terpy)(PPh3)Cl2]+ (IV)or Reffl(terpy)Cl3 (V). Compound II can be reduced in the presence of

cyclohexenone to form ReI(terpy)(PPh3)Cl(>/* 12-cyclohexenone) (VI). The chloride on compound VI undergoes

facile substitution to form a series of Re(I)—terpyridyl derivatives (VII— ). The electrochemistry of these

compounds has been studied and compared to that of other known rhenium compounds using an electrochemical

parametrization model. A correlation is drawn between the number and strength of -acidic ligands and the

deviations of the observed II/I reduction potentials from the predicted values. An X-ray crystal structure

determination for II is also reported. [Ren(terpy)(PPh3)2Cl]OTf (II) crystallizes in the triclinic space group PI

(No. 2) with Z= 2 and lattice parameters a= 14.552(7) k,b= 15.726(7) Á,c= 12.215(3) k,a= 109.09(2)°,

ß= 96.98(3)°, and y= 98.42(4)° at -80 °C.

Abstract: A series of 7-azabicyclo[2.2.1]hept-5-ene complexes are prepared from [Os(NH3)s(?72-L)]2+ (L= pyrrole,

1-methylpyrrole, 2,5-dimethylpyrrole, 1,2,5-trimethylpyrrole, or l-(trimethylsilyl)pyrrole) and various dipolarophiles

(e.g., acrylonitrile, methyl acrylate, -methylene-y-butyrolactone, dimethyl maleate, dimethyl fumarate, W-phenyl

maleimide, cyclopentene-1,2-dicarboxylic acid anhydride, and (E)- and (Z)- methyl 3-(3'-pyridyl)acrylate). The

cycloaddition is promoted by coordination of the pyrrole with [Os(NH3)s]2+ across C3 and C4, transforming the

uncoordinated portion of the pyrrole nucleus into an azomethine ylide capable of undergoing 1,3-dipolar cycloadditions.

The metal serves not only to activate the pyrrole ring but also to stabilize the resulting 7-azabicyclo[2.2.1 ]heptene

ligands. A number of organic 7-azabicyclo[2.2.1]heptanes, including analogs of the alkaloid epibatidine, have been

synthesized by this methodology. For the cases examined, the cycloaddition favors exo stereochemistry of the electron-

withdrawing substituent when the pyrrole nitrogen is unsubstituted. Crystal structures have been determined for the

complexes obtained from the reactions of pyrrole with W-phenylmaleimide (8a), 2,5-dimethylpyrrole with dimethyl

maleate (13a), and 2,5-dimethylpyrrole with -methylene-y-butyrolactone (22a).

At present, the pentaammineosmium(II) moiety is unparalleled

in its ability to form ^-coordinated complexes with unsaturated

organic ligands such as aldehydes, ketones,1 arenes,2 and aromatic

heterocycles.3 For aromatic ligands in particular, these complexes

have proven to be valuable synthons for organic systems.4 The

combination of an electron-rich heavy metal and an octahedral

ligand environment lacking strong -acids provides a system

showing properties common to both classical coordination and

organometallic chemistry. It is our expectation that isoelectronic

Re(I) in a suitable octahedral amine ligand environment may

show properties similar to those of the pentaammineosmium

system.......

A useful precursor dj-[0s(NH3)4(CF3S03)2](CF3S03) (1) has been synthesized by oxidation of the dinitrogen

complex cis-[Os(NH3)4(N2)2]Cl2 in CF3SO3H. While the reduction of 1 in aqueous solution leads to the * 12-

dihydrogen complexes1 in high yield, in acetone, and at room temperature, however, when reduced by Mg or Zn/Hg,

1 reacts rapidly with a variety of unsaturated ligands. In the presence of norbornadiene and 1,5-cyclooctadiene,

the corresponding ?/-diene tetraammine complexes were obtained in recovered yields of 30% and 80%, respectively.

Traces of acid left in 1 lead to much reduced yields of these and of other species we have prepared, owing to the

formation of the diacetone alcohol complex as a side product in which the ligand bonds to Os(II) center in rj’-fHO-)

and t?2-(0=C-) fashion. In the absence of a competing ligand, this chelate complex is the major product of the

reaction. In the presence of the arenes, benzene, phenol and naphthalene, reduction of 1 was found to lead to the

?;6-arene triammine complexes, the reaction yields as determined by NMR being 64%, 68%, and 95%.

Summary: Anisóle, when complexed by pentaammin-

eosmium(II), undergoes a [4 + 2] cycloaddition with

N-methylmaleimide which is shown to be nonconcerted.