Carbene complexes Wittig reaction

The transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [617] their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(I) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(II) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(II) or palladium(II) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically. 

In traditional synthetic organic chemistry, the Wittig reaction plays an important role in carbon-carbon bond extension from the carbonyl group. CM is an attractive alternative for carbon-carbon extension from a terminal alkene. In fact, a pyrroh-dine ring of anthramycin derivative 55 has been constructed by RCM of 52, and the sidechain has been extended by CM of terminal alkene of 54 with ethyl acrylate. " In the CM, ruthenium carbene complex Ij, reported by Blechert, gives a good result since the ligand of the catalyst easily dissociated from the ruthenium metal at room temperature ... 

Wittig-type alkenation of the carbonyl group is possible with Ti carbene compounds [56], The reaction is explained by the formation of nucleophilic carbene complexes of Ti, although they are not isolated. In the carbonyl alkenation, the oxametallacyclo-butane intermediate 182 is formed by [2+2] cycloaddition of the carbene complex 181 with the carbonyl group. This intermediate is converted to the new alkene 183 and the Ti(IV) oxo species 184, which is a stable compound, and hence the carbonyl alkenation requires a stoichiometric amount of the Ti complex. Also, ester 185 is converted to the enol ether 187 via 186. 

The reactivity of early transition metal silylenoid complexes is still emerging. An example of the chemistry that these complexes can participate in is the sila-Wittig reaction [equation (7.4)].53 In this transformation, a metathesis occurred between the chromium silylenoid 24 and the dimethyl carbonate to afford a new Schrock carbene, 25, and the trimerized product 26. This methodology allowed access a new carbene complex that eluded previous synthetic efforts. 

WITTIG-TYPE REACTIONS Chromium car-bene complexes. Di-n-butyltelluronium carboethoxymethylide Zirconium carbene complexes. 

There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

The bis(methylenephosphonium ylide) platinum complex (61) reacts with 3-butyn-l-ol to give the vinylphosphonium salt (62) and evidence is presented for intermediate formation of the carbene complex (63). Phosphoranylidenephosphine complexes (64) have been synthesized and shown to undergo "phospha-Wittig" reactions with aldehydes to give the phosphaalkene complexes (65) which can be isolated or trapped. ... 

Systems 4 and 5 provide an interesting contrast. The thermodynamic stability of the cyclohepten-5-one derivative produced in system 4 effectively prevents any units of M2 from being incorporated in the polymer of Mi. The carbonyl group is evidently sufficiently protected to prevent it from undergoing a Wittig-like reaction with the metal carbene complex. 

What happens is a metathesis—an exchange of groups between the two arms of the molecule. But how The mechanism is not difficult, but is unlike any other you have met before, except, perhaps, the Wittig reaction, which also forms alkenes. First, the carbene complex adds to one of the alkenes in what can be drawn as a [2 + 2] cycloaddition (Chapter 34) to give a four-membered ring with the metal atom in the ring (a metallacyclobutane )-... 

Actually, terminal metal carbene and alkylidene complexes are ubiquitous throughout the transition elements. The nomenclatural distinction between "carbene" and "alkylidene" represents a fundamental difference in reactivity. Metal carbene complexes usually behave as electrophiles, with typical reactions including cycloadditions to un-saturabed bonds (e.g. cyclopropanation of olefins). On the other hand, metal alkylidene complexes are nucleophilic, undergoing Wittig-type alkylations and olefin metathesis. 

The carbonyl olefination is in fact a Wittig-like reaction of carbene complex. Some disputable, even unknown, reactions are also noticed in Chart 4 and that is why the three cycles are examined separately and some literature data supporting our hypothesis are quoted. 

Wittig like reactions with ketons (19) (Scheme 6) The heterogeneous carbene complex Si02/Np2WC(H)tBu is one of the most active heterogeneous metathesis catalyst (Table 1). 

Wittig reaction, it is called Wittig methylenation [75]. In carbonyl methylenation, the reagent which yields a carbene complex (Ti = CH2) in the reaction system is... 

Many reactions can be envisaged using this scheme. For some of them (OM (X = Y = Z = C) and carbonyl olefination via Wittig-like reaction of the transition metal carbene complexes (Z = O, X = Y C)) the vaUdity of Eq. 12 is well documented. For others (X = O, Y = Z = C) it is only suggested. The other combinations are still unknown reactions. 

The carbene mechanism of COER according to our hypothesis consists of forward (Z = O, X = Y = C) and backward (X = O, Y = Z = O) Wittig-like reactions. A transition metal-carbene complex reacts with a carbonyl compound generating an olefin and transition metal oxo-complex. Then the oxo-complex reacts with another olefin generating a new carbonyl compound and regenerating the transition metal-carbene complex. It is known that oxo-alkylidene complexes can be generated via oxidative addition reaction between some tungsten complexes with carbonyl compounds. 

The Wittig-type olefination of carbonyl compounds is one of the characteristic reactions of carbene complexes. High-valent carbene complexes of early transition metals show ylide-like reactivity to vards carbonyl compounds. In 1976, Schrock first demonstrated that niobium and tantalum neopentylidene complexes 1 and 2, the typical nucleophilic Schrock-type carbene complexes, olefinate various carbonyl compounds including carboxylic acid derivatives [4]. 

The mechanisms of these reactions bear marked similarities, in spite of the differences in their reactivities and selectivities. Thus, in certain cases, a four-membered intermediate similar to the 1,2-oxaphosphetane intermediate in the Wittig reaction appears in the Peterson reaction as a pentacoordinate 1,2-oxasiletanide. Reactions of transition metal carbene complexes with carbonyl compounds also proceed through the formation of a four-membered oxametallacycle, which was recently found to be an intermediate of some McMurry reactions. Carbonyl olefination utilizing dimetallic species of zinc or chromium is somewhat similar to the Julia reaction in that they both involve the process of ) -elimination. 

Extensive studies have shown that phosphonium ylides are readily generated in situ from triphenylphosphine and a-diazo carbonyl compounds through carbene transfer in the presence of a catalytic amount of a metal complex derived from Re [124-126], Ru [127-130], Ir [131], Fe [132-137], Cu [138, 139], or Co [140, 141] (Scheme 29). The conditions for carbene transfer are well compatible with aldehydes and ketones, and the metal catalyzed one-pot Wittig reaction of aldehydes (or ketones) with a-diazo carbonyl compounds proceeds smoothly to give electron-deficient alkenes with high E selectivity. 

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