Vinyl ether complexes

Anionic alkoxy Fischer-type carbene complexes were shown to react with (Ph3P)AuGl to give a unique vinyl ether complex, which is bound in a quasi-771 fashion to the Cr/Mo centers. Upon treatment with triphenylphosphine, the gold-vinyl ether ligand can be liberated and isolated. This reaction thus gives access for the first time to aurated vinyl ethers (Scheme 57).38... 

It is important to stir the product continually to insure effective removal of tetrahydrofuran. If appreciable solvent remains, the vinyl ether complex 1 may not crystallize readily. 

Alkyne complexes of Os(II) are observed to undergo addition of water and methanol across the alkyne bond, resulting in stable vinyl alcohol and vinyl ether complexes, respectively (168). When an aqueous solution of [Os(NH3)5(CH3C=CCH3)]2+ is allowed to stand, the initial product observed is the cis-2-hydroxy-2-butene complex. Over a period of several days, this species equilibrates with its trans stereoisomer, the latter being slightly favored in aqueous solution (Keq = 1.5). 

Treatment of some iron-acyl complexes with trifluoromethanesul-phonic anhydride (TfzO) affords vinylidene derivatives directly (5 7,38). The reaction is envisaged as a nucleophilic attack on TfzO by the acyl, followed by deprotonation to the vinyl ether complex. A combination of an excellent leaving group (TfO-) with a good electron-releasing substituent on the same carbon atom facilitates the subsequent formation of the vinylidene ... 

The carbonyls Fe(CO)5 and [CpFe(CO)2]+ (2) form stable cationic complexes with alkenes, which are used for both protection and activation of alkenes [1]. [CpFe(CO)2]+ (2 abbreviated as Fp+) is prepared by the reaction of cyclopentadienyl anion (1) with Fe(CO)5, followed by oxidative cleavage with bromine, and used for the protection of alkenes. The electron density of the double bond is decreased by the coordination of [CpFe(CO)2]+ and hence this bond is activated to nucleophilic attacks. Introduction of nucleophiles, such as the carbon nucleophile of malonate, to cyclopentene becomes possible via the formation of the complex 3, and the stable tftmv-er-alkyliron complex 4 of cyclopentane is prepared. The vinyl ether complex 6 is obtained easily from the a-bromoacetal 5, and reacts with an enolate of ketone 7 as an... 

When the anisolium complex generated by the addition of MVK to the 2-methoxytetra-hydronaphthalene complex 59 is utilized in the above cyclization sequence, the tricyclic oxonium complex 61 is generated (Scheme 6). Deprotonation of 59 with pyridine forms the extended vinyl ether complex 60, which cydizes and eliminates water to form 61 when exposed to TBSOTf. Hydrolysis of 61, followed by oxidation of the metal fragment, yields the dienone 62 in 15 % yield (based on 59). 

The chemistry of iron vinylidene complexes is dominated by the electrophilicity of the carbon atom adjacent to the iron organometallic unit. While addition of water leads to an acyl complex (i.e., the reverse of the dehydration shown in equation 10), addition of an alcohol leads to a vinyl ether complex. Similarly, other iron vinyl complexes can be prepared by the addition of thiolate, hydride, or an organocuprate (Scheme 33). " The nucleophilic addition of imines gave enaminoiron intermediates that could be further elaborated into cyclic aminocarbenes. This methodology has been used to provide access to /3-lactams and ultimately penicillin analogs, and good diastereoselectivities were observed (6 1-15 1) (Scheme 34). 04 Iso, vinylidene complexes are intermediates in cyclizations of alkynyl irons with substituted ketenes, acid chlorides, and related electrophiles an example is shown (equation 11). These cyclizations led to the formation of a series of isolable and characterizable cyclic vinyl iron complexes. 

Work in this area has also focused on optically active complexes. Exchange of vinyl ether complex (94) with optically active alcohols gives unequal mixtures of diastereomeric vinyl ether complexes in the best case, (-)-menthol gives a 4 1 mixture emiched in (99). Optically active epoxides are capable of affording enantiomerically pure complexes allylic alcohol substituents allow reversible conversion to the related lactone complexes (100) (equation 21). 

Wakatsuki and co-workers 287) have prepared vinyl ether complexes of Pd(II) and Pt(II) of composition [CI2ML] (M = Pd or Pt L = vinyl ethers, propenyl ethers, 1,2-dimethoxyethylene) by displacement of PhCN from Pd(PhCN)2Cl2- They are fairly stable at 25°C. Since these ethers would be good a donors but poor tt acceptors, the stability of these complexes suggest that a donation is most important with tt complexes of Pd(II) and Pt(II). This conclusion is in agreement with earlier IR studies, as well as a recent NMR study of Pt olefin and acetylene complexes 35). Furthermore, a study of complex formation... 

Alkyne addition reactions involving [Os(NH3)5] proceed via 7r-enol and 7r-vinyl ether complexes, " while treatment of [Os(NH3)5(i7 -C6H6)] with hydrogen in methanol in the presence of Pd/C catalyst gives, selectively, [Os(NH3)5(i7 -hexene)]"-". ... 

Figure 11.14 Energy profile of the Rh-catalyzed Claisen rearrangement of propargyl vinyl ethers initiated by coordination at the alkyne. Red and gray depict the cis and the trans orientations, respectively, of CO relative to substrate at the Rh center. Energies were calculated relative to the most stable Rh(l)-vinyl ether complex at the M05-2X/LANL2DZ level.

Cationic platinum(II) acetylene complexes react with a variety of nucleophiles. Chisholm and Clark showed that the reaction of methanol with coordinated disubsti-tuted acetylenes (generated in situ) affords trans u-bonded vinyl ether complexes (Equation 11.35). The trans stereochemistry of the cr-vinyl group of the product suggests that the nucleophilic attack occurs external to the metal and does not involve prior coordination of methanol, Reger has shown that stable cationic iron-alkyne complexes undergo reaction with a wide variety of nucleophiles to give stable cr-alkenyl complexes resulting from trans attack of the nucleophile on the coordinated acetylene (Equation 11.36). A variety... 

Greatly improved regioselectivity is observed when the alkene ligand has electron-withdrawing substituents. Vinyl ether complexes, prepared from a-bromoacetals, are attacked exclusively at the carbon atom which bears the... 

Vinylation of enolates have been performed using Fp (alkyl vinyl ether) complexes [Fp = CsH5Fe(CO)2] as vinyl cation equivalents (Scheme 51). ... 

Several examples of enolstannanes serving as nucleophiles towards allylic acetates under the influence of Pd° catalyst to give suitably alkylated products have been reported. Enolate anions also react with cyclopentadienyldicar-bonyliron alkyl vinyl ether complexes to afford eventually a net vinylation. The complexes are therefore behaving as vinyl cation equivalents (Scheme 11). 

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