Trimethylenemethane complexes bonding

The reaction of methylenecyclopropanes with transition metal complexes is well known to promote a catalytic a-ir cycloaddition reaction with unsaturated compounds, in which a trimethylenemethane complex might exist71-76. Recently, much interest has been focused on the interaction of strained silicon-carbon bonds with transition metal complexes. In particular, the reaction of siliranes with acetylene in the presence of transition metal catalysts was extensively investigated by Seyferth s and Ishikawa s groups77-79. In the course of our studies on alkylidenesilirane, we found that palladium catalyzed reaction of Z-79 and E-79 with unsaturated compounds displayed ring expansion reaction modes that depend on the (Z) and (E) regiochemistry of 79 as well as the... 

Except for the trimethylenemethane complex, complexes of the types shown in Figure 4.8 can be treated as arachno mixed metal-carbon MC clusters, formally containing (n -1- 4) skeletal bond pairs to hold their (n + 1) skeletal atoms together. ... 

Scheme 4.30 summarizes the proposed reaction mechanism. The substrate would react with palladium(O) species to form a palladacyclobutane intermediate A in equilibrium with the tautomeric trimethylenemethane complex A. Insertion of CO2 into the palladium-carbon bond affords a palladium carboxylate, which can release y-lactone 27 by reductive elimination. The latter species can isomerize to the thermodynamically more stable furanone 28. a,p-Unsaturated lactone 28 contains acidic protons and, in principle, can react with intermediates A and/or A (Scheme 4.30) to give cooligomers, which have been observed as by-products. 

Farmgia LJ, Camtaxrn E, Tegel M (2006) Chemical bonds without chemical bonding a combined expraimental and theoretical charge density study on an iron trimethylenemethane complex. J Phys Chem A 110 7952—7961... 

Similar to the first syntheses of cyclobutadiene complexes [2, 9], the first synthesis of a trimethylenemethane complex started from dichloride 2, which was treated with diironenneacarbonyl to give tricarbonyl(trimethylenemethane) iron(O) (3) in 30% yield In addition to iron(II) chloride (Scheme 10.1) [10]. The r -coordination has been confirmed by crystal structure analyses [11, 12]. Very recently, Frenking et al. published a detailed theoretical bonding analysis of some late transition metal sandwich trimethylenemethane complexes [13]. 

While the q -bonding mode in the iron complex 3 has earlier been established crystallographically [11, 12], it is remarkable that this coordination mode is observed in a 17 electron iron(III) complex as well. Dixneuf et al. treated iron(III) chloride with magnesium and dichloride 2 in the presence of phosphane ligands and obtained the respective trimethylenemethane iron(II) complexes, which were then oxidized with silver triflate resulting in the formation of cationic trimethylenemethane complexes with the ligand still being q -coordinated [30]. Bazan et al. reported a distorted coordination to zirconium [31], and... 

The TT-allylpalladium complexes 241 formed from the ally carbonates 240 bearing an anion-stabilizing EWG are converted into the Pd complexes of TMM (trimethylenemethane) as reactive, dipolar intermediates 242 by intramolecular deprotonation with the alkoxide anion, and undergo [3 + 2] cycloaddition to give five-membered ring compounds 244 by Michael addition to an electron-deficient double bond and subsequent intramolecular allylation of the generated carbanion 243. This cycloaddition proceeds under neutral conditions, yielding the functionalized methylenecyclopentanes 244[148], The syn-... 

Theoretical studies 225>226> as well as preparative work strongly indicate that the reactive palladium organic intermediate in Reaction 115b and 115c is an unsym-metrical, zwitterionic trimethylenemethane-palladium (TMM-Pd) complex, as formulated in Eq. 117. Moreover, cycloaddition with a cyclic TMM-Pd-precursor revealed that the electron-deficient olefin attacks the TMM-Pd unit from the side away from the metal. This demonstrates that complexation of the olefin with the metal does not occur prior to C—C bond formation 183>. 

Figure 4.9. Frontier orbitals available for metal-carbon bonding in complexes of acyclic unsaturated systems C I I 2- (a) Allyl ligand H2CCHCH2 (b) buta-l,3-diene H2C=CHCH=CH2 (c) trimethylenemethane 6(012)3.

Cyclopropenes and mcthylcnccyclopropanes serve as multifunctional reagents in transition metal catalyzed reactions22. Methylenecyclopropanes, via C-C bond cleavage, are also used as trimethylenemethane precursors in transition metal catalyzed [3 + 2] cycloadditions for selective five-membered-ring formation. Low-valent nickel and palladium complexes are used as catalysts. This method has been extensively reviewed 22 and stereoselective applications are fully described in Section D.1.6.1.2.3. 

The success of this carboxylative trimethylenemethane cycloaddition extends to the addition to cyclohexenone. In contrast to the poorly yielding process involving the unsubstituted TMM -Pd complex, a respectable yield of 49 % is obtained here. This is explained by the reduced basicity of the silylated complex, thus leading to fewer side reactions. The reaction is also considered to have a greater degree of concertedness and this becomes apparent in the discussion of chiral Z- and f-olefins in Section 1.6.1.2.3.2. The failure of in situ derived palladium complexes to yield the desired product is attributed to the basic conditions employed which result in double-bond migrations to the endocyclic, conjugated system. 

In this latter reaction mode, which is observed much more rarely than /3-dehydropal-ladation, a wide variety of ligands can be coupled to each other with the formation of new C—C, C—H, C— N, C—O, and C—Hal bonds. This section does not cover the numerous cascade couplings in which a number of successive intramolecular additions of 2 onto double bonds is eventually completed by /3-dehydropalladationt as well as the numerous [2 -I- 2 -I- 2] and [4 + 2] cyclotri- and cyclodimerizations of alkynes, enynes, and related compounds. " The Pd(0)-catalyzed Cope rearrangement also will not be considered here, as it proceeds via bis(i7 -allyl)palladium(ll) intermediates. The carbopalladation reactions of aUenes, which have been reviewed recently, are covered in Sect. IV.7. (For new examples see also refs. [10]-[12]). On the other hand, the numerous Pd-catalyzed formal [3 + 2] cycloadditions of trimethylenemethane (TMM) complexes may be classified as carbopalladations of alkenes without subsequent dehydropalladation. As the subject of this section has partially been covered in several newly published reviews, " the attention here will be on the most recent and interesting communications. 

Nucleophilic attack on coordinated unsaturated hydrocarbons is one of the fundamental and particularly well studied reactions in Organometallic Chemistry. The addition of carbonylme-tallates instead of common nucleophiles provides a directed synthesis of hydrocarbon bridged complexes. Carbonylmetallates (particularly Re(CO)5", Os(CO)4 ") add to 7C-bonded olefin, acetylene, allyl, diene, trimethylenemethane, dienyl, benzene, triene and cycloheptatrienyl ligands in cationic complexes and give hydrocarbon bridged bi- and trimetallic, homo- or he-teronuclear complexes [1]. 

Cyclization of an (T -trimethylenemethane)iron complex with a double bond of a diene in an appropriate distance leads to bis(T -allyl)iron complexes (Scheme 4—100), which are presumably also intermediates in the reactions presented in Scheme 4—99. ... 

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