Alkylidene dimerization

When Group VI carbonyl complexes were reacted with alane or Cp2NbH3 reduction of CO to ethylene was noted.- - Ethylene was the primary product of the Cp2NbH3 reduction although it subsequently was hydrogenated to ethane. Masters and co-workers suggested that ethylene was formed through an alkylidene dimerization as shown below. 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

The most reactive Michael acceptors, such as alkylidene malonates, gem-dicyanoalkenes and nitroalkenes, react with a-halozinc esters in a conjugate fashion. Beautiful examples were offered by two stereocontrolled conjugate additions to piperidinone 102 and pyrro-lidinone 104 leading to optically active bicyclic lactams 103147 (equation 60) and 105 (equation 61)148. With these electron-poor alkenes a Grignard two-step protocol is to be adopted in order to avoid the single electron transfer reactions from the metal to the Michael acceptor, which should afford olefin dimers. The best solvent is found to be a... 

Butadiene d4 complexes were obtained (i) from [NbCU(dmpe)2] and magnesium butadiene (equation 87) j706 (ii) by dimerization of ethylene using alkylidenes (equation 88) 707 or (iii) by metal vapor techniques (equation 89), which yielded sublimable methylallyl derivatives.70 Compound (62) could not be prepared by Na/Hg reduction of (22) in the presence of butadiene. Compound (63) is also accessible from [TaH2ClL4] (Scheme 9). 

Intramolecular carbene insertion has been utilized as a route to thiepins and benzothiepins but is seriously complicated by C—H insertion side reactions leading to alkylidene thiopyrans (Scheme 17) (78TL3567, 78CL723), while the generation of the 9-carbene from thioxanthene (via the diazo species) results in dimerization to the bisthioxanthylene compound. 

Thermolysis of the complexes R2M[N(SiMe3)2]2 (M = Zr, Hf) was found to proceed with double activation of the y-CH bonds leading to a dimeric complex containing a bridging alkylidene function (equation 98).252... 

Thioketenes can be prepared in several ways, from carboxylic acid chlorides by thionation with phosphorus pentasulfide [1314-80-3], P2S5, from ketene dithioacetals by p-elimination, from 1,2,3-thiadiazoles with flash pyrolysis, and from alkynyl sulfides (thioacetylenes). The dimerization of thioketenes to 2,4-bis(alkylidene)-l,3-dithietane compounds occurs quickly. They can be cleaved back pyrolytically (63). For a review see Reference 18. 

Rhodium(i)-catalyzed ene-allene carbocyclization strategy is suggested for the formation of seven-membered heterocycles, azepines and oxepines. In particular, treatment of an allenyl allyl ether with a catalytic quantity of chlorodi(carbonyl)rhodium dimer affords 4-alkylidene-5-alkyl-2,3,4,5-tetrahydrooxepines (Equation 28) in 40-55% yields <20040L2161>. 

Reductive dimerization of propargyl chlorides.2 Treatment of propargyl chlorides with this nickel(O) complex effects cyclodimerization to 3,4-bis(alkylidene)-cyclobutenes. 

These carbene (or alkylidene) complexes are used as either stoichiometric reagents or catalysts for various transformations which are different from those of free carbenes. Reactions involving the carbene complexes of W, Mo, Cr, Re, Ru, Rh, Pd, Ti and Zr are known. Carbene complexes undergo the following transformations (i) alkene metathesis (ii) alkene cyclopropanation (iii) carbonyl alkenation (iv) insertion to C—H, N—H and O—H bonds (v) ylide formation and (vi) dimerization. Their chemoselectivity depends mainly on the metal species and ligands, as discussed in the following sections. 

The yields from aldehyde alkylidenation is somewhat lower due to the reductive dimerization of aldehydes with low-valent Ti. Alkylidenation of esters is possible by the reaction of 1,1 -dibromoalkane. TiCU and Zn in the presence of TMEDA to give (Z) vinyl ethers [60], Cyclic vinyl ethers are prepared from unsaturated esters in two steps. The first step is formation of the acyclic enol ethers using a stoichiometric amount of the Ti reagent, and the second step is ring-closing alkene metathesis catalysed by Mo complex 19. Thus the benzofiiran moiety of sophora compound I (199, R = H) was synthesized by the carbonyl alkenation of ester in 197 with the Ti reagent prepared in situ, and the subsequent catalytic RCM of the resulting enol ether 198 catalysed by 19 [61]. 

Alkylidene borane 360 undergoes reaction with BH3-THF 361 to yield a dimerized cycloaddition product 362. BH3 hydroborates the B—C bond in two molecules of methylidene borane 360. The regioselectivity of hydroboration is governed by the electronic factors which facilitate the attack of boron (from BH3) on the two terminal carbon atoms, thus generating the B-C-B-C-B chain. The final product is obtained by the binding of two boryl ends via two B-H-B three-center, two-electron bridged bonds (Equation 20) <2004ZFA508>. 

As mentioned in the introduction [lc], no selectivity was observed in early dimerization experiments of 1. But when other partners were offered, the corresponding crossdimerizations were quite selective. Probably methylene metallacyclopentenes 2 [4], which could be isolated, are intermediates that then react with the other partners. Generally, the related 1,3-dienes are less reactive than 1 with its reactive allenic double-bond and do not react in a similar manner [4a]. Rh-catalyzed [4+1] cycloadditions with CO as a second reaction partner led to alkylidene cyclopentenones 3 and 4 [4, 5], while in Pd-catalyzed reactions where 1 was generated in situ and a base was present, only 4 [6] was formed. When Pt(0) was used instead of Rh(I) in the carbonylation reaction, both in the presence of the (R,R)-DuPHOS-ligand, opposite enantiomers of 3 were obtained [5b], This observation still needs a precise explanation. [Fe(CO)5]-mediated reactions of diallenes form dialkylidene cyclopentenones 7 (Scheme 2, here 10 mol-% of catalyst are needed) [7],... 

The even more strained alkylidene cycloproparenes gave rise to the same kind of G-complex intermediate with silver ion. In the presence of alcohol, trapping of this intermediate occurred, leading to alkoxystyrene derivatives. Water could also act in the same way, yielding arylmethylketones after keto-enol equilibration. However, if a proton was present on the alkylidene moiety, H shift occurred, leading to an arylalkyne. No dimerization was observed in this case, probably due to steric constraints in such a process (Scheme 3.17).31... 

Dimethylbutadiene)HfCp (Cl)] (74c) reacts with one molar equivalent of acetylene to yield the unusual product 87. This is probably formed by a conventional butadiene/alkyne coupling at the Group 4 metal center, followed by an intramolecular alkene insertion into the adjacent hafnium to carbon cr-bond. The resulting alkylidene complex (86) then rapidly dimerizes to yield the observed final product (see Scheme 28), that was characterized by X-ray diffraction.96... 

An alternative method to form the ditellurole ring employs the dimerization of ethynyltellurolates. The products are 2/7-2-alkylidene-l,3-ditelluroles2. Telluroketenes are likely intermediates in these reactions3. When phenylethynetellurolate was treated with trifluoroacetic add, cis- and trans-2-benzylidene-4-phenyl-2H-l, 3-ditelluroles were isolated in low yields2. 

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