Tungsten enolates

At the time the chemistry of main group enolates flourished already for a while, that of late transition metals had a shadowy existence in synthetic organic chemistry. Their stoichiometric preparation and the sluggish reactivity - tungsten enolates, for example, required irradiation to undergo an aldol addition [24a] - did not seem to predestine them to become versatile tools in asymmetric syntheses [27]. The breakthrough however came when palladium and rhodium enolates were discovered as key intermediates in enantioselective catalyses. After aldol reactions of silyl enol ethers or silyl ketene acetals under rhodium catalysis were shown to occur via enolates of the transition metal [8] and after the first steps toward enantioselective variants were attempted [28], palladium catalysis enabled indeed aldol additions with substantial enantioselectivity... 

The reaction of anionic metal complexes 162 with a-chloro ketones, esters, or tertiary amides occurring under chloro-metal exchange is an appropriate method for the preparation of molybdenum and tungsten enolates. The reaction fails with the corresponding a-bromo compounds. In order to obtain tungsten enolates, which are substituted in a-position, a-methanesulfonyloxy-substituted esters are suitable starting materials. The tungsten and molybdenum complexes 163 were fully characterized as C-bound // -enolates (Scheme 2.47) [162]. 

Scheme 2.47 Preparation of molybdenum and tungsten enolates 163 through halogen-metal exchange in various a-chloro carbonyl compounds.

Another example of a [4S+1C] cycloaddition process is found in the reaction of alkenylcarbene complexes and lithium enolates derived from alkynyl methyl ketones. In Sect. 2.6.4.9 it was described how, in general, lithium enolates react with alkenylcarbene complexes to produce [3C+2S] cycloadducts. However, when the reaction is performed using lithium enolates derived from alkynyl methyl ketones and the temperature is raised to 65 °C, a new formal [4s+lcj cy-clopentenone derivative is formed [79] (Scheme 38). The mechanism proposed for this transformation supposes the formation of the [3C+2S] cycloadducts as depicted in Scheme 32 (see Sect. 2.6.4.9). This intermediate evolves through a retro-aldol-type reaction followed by an intramolecular Michael addition of the allyllithium to the ynone moiety to give the final cyclopentenone derivatives after hydrolysis. The role of the pentacarbonyltungsten fragment seems to be crucial for the outcome of this reaction, as experiments carried out with isolated intermediates in the absence of tungsten complexes do not afford the [4S+1C] cycloadducts (Scheme 38). 

Strong evidence exists for the intermediacy of a tungsten ethoxycarbonyl carbene 425 in cyclopropanation of various enol ethers, 1,3-dienes and cyclohexene with ethyl diazoacetate in the presence of catalytic amounts of (CO)5W = C(OMe)Ph 413). The following equations could account for the obtained products ... 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

A complementary route to carbohydrate-based oxepines was developed by the McDonald group.67 It is based on the endo-selective cycloisomerization of alkynyl alcohols in the presence of molybdenum or tungsten catalysts to give the cyclic enol... 

Halogen shifts have been found for tungsten, with assumed formation of iodovinylidenes in reactions of 1-iodo-l-alkynes with W(CO)5(thf) en route to cyclization of 2-(iodoethynyl)styrenes to naphthalenes and of iodo-alkynyl silyl enol ethers [147], while more substantial confirmation is found in Mn =C=C(I)CH (OR)2 (CO)2Cp [R = Me, Et (OR)2 = 0(CH2)30], of which the XRD structure of Mn =C=C(I)CH(OMe)2 (CO)2Cp was determined [148]. 

The original Fischer-type caibene complexes react with electron deficient alkenes such as a,f3-unsatu-rated esters102 and especially electron rich alkenes such as enol ethers103 and enamines,104 but reactions with simple alkenes have been limited in their usefulness. Furthermore, because these particular caibene complexes most commonly contain heterosubstituted caibene units, they formally fall outside the scope of this chapter. A few nonheterosubstituted examples that have proven to be useful are benzylidene105 and diphenylcaibene106 complexes of tungsten. 

Vinyl ethers have also been prepared by addition of alkoxides to acetylene,6 7 6 elimination from halo ethers and related precursors,6 8 and vinyl exchange reactions.6 Reaction of an electrophilic tungsten carbenoid with methylene phosphorane or diazomethane also produces vinyl ethers.9 Enol ethers have resulted from the reaction of some tantalum and niobium carbenoids with esters,10 and the reaction of phosphoranes with electrophilic esters.4... 

CASSCF calculations.7 Treatment of substituted 5-triethylsilyloxyhexa-l,2,5-triene derivatives with catalytic amounts of W(CO)6 was found to give the products of formal Cope rearrangement 2-triethylsilyloxyhex-l-en-5-ynes. The mechanism is believed to involve 6-endo attack by the silyl enol ether on the tungsten-activated allene, followed by ring opening with simultaneous loss of W(CO)s.8... 

Cycloadditions of alkynyl carbenes. Alkynyl carbene complexes of chromium and tungsten undergo [2 + 2]cycloaddition at room temperature with a wide range of enol ethers. 

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