Alkylidene Methylene

Alkylidene-methylene coupling affording a coordinated olefin... 

The proofs to justify defining real pjt-pjt-double bonds within these compounds are found in the parameters of the chemical structure as well as in properties that show the evident affinity to the classical CC double bond, thus confirming the designation phosphaalkenes not only for formal reasons. Ocassionally the term alkyliden(methylene)phos-phanes is used for these types of compounds. 

Alkylidene-2,3-dihydropyridazines (124) are synthesized by coupling of 3-thiomethyl-pyridazinium salts with active methylene compounds in the presence of potassium carbonate in DMF (Scheme 39) (79TL4837). 

Tertiary pyrrolines (49, = 1) and piperideines (49, = 2) (if R = H and the enamine can exist in the monomeric form or if R = aryl) evidently possess an endocyclic -double bond (79,155,156). The stretching frequency of the double bond can be lowered to 1620-1635 cm by conjugation with an aromatic substituent. The double bond of an analogous compound with aliphatic substituents in position 2 may occupy either the endo or the exo position. Lukes and co-workers (157) have shown that the majority of the five-membered-ring compounds, traditionally formulated with the double bond in a position, possess the structure of 2-alkylidene derivatives (50) with an exocyclic double bond, infrared absorption at 1627 cm . Only the 1,2-dimethyl derivative (51) is actually a J -pyrroline, absorbing at 1632 cm . For comparison, l,3,3-trimethyl-2-methylene pyrrolidine (52) with an unambiguous exocyclic double bond has been prepared (54). 

Alkyl-Alkylidene Tautomerism. Some 2- or 3-(substituted alkyl)quinoxalines, like 3-ethoxycarbonylmethyl-2(177)-quinoxalinone (133), have long been known to exist in equilibrium with their (substituted methylene) tautomers, for example 3-ethoxycarbonylmethylene-3,4-dihydro-2( 1 /7)-quinoxalinone (133a).The effects of solvent change, protonation, and the like on such tautomeric systems have been examined as well as the kinetics thereof. In... 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

The characteristic reactivity of neutral dg alkylidene complexes of Ru, Os, and Ir is with electrophilic reagents. The osmium methylene 47 reacts with the widest range of electrophiles, the most significant reactions being summarized in Scheme 2. 

The ruthenium and iridium methylene complexes and the osmium alkylidenes 48-50 also react with S02 to afford sulfene complexes (104). 

The two articles in this current volume describe recent developments with small ring compounds which have not teen compiled in such a context before. T. Hirao discusses selective transformations initiated by transition derivatives in the construction of functionally substituted five-, six- and seven-membered rings as well as open-chair compounds. Cycloadditions onto methylene- and alkylidene-cyclopropane derivatives, described by A. Goti, F. M. Cordero and A. Brandi, not only yield products with spirocyclopropane moieties which can be desirable as such or as potential mimics of gem-dimethyl groupings, but also intermediates which can undergo further transformations with ring-opening of the cyclopropane units. 

Goti, A., Cordero, F. M., and Brandi, A. Cycloadditions Onto Methylene- and Alkylidene-cyclopropane Derivatives. 178,1-99(1996). 

Table 4.10. Optimized bond lengths Rue and Rmh and angles hmc and hmh of transition-metal alkylidenes H ,M=CH2 in Fig. 4.14. (The subscripts i and "o distinguish H—M—H angles that are respectively in plane and out of plane with respect to the methylene group.)...

Very recently, Eisch and co-workers have developed new alkylidene-group IV metal complexes such as methylidene titanium dichloride 67, readily accessible from titanium(iv) chloride and an excess of methyllithium at low temperature (Scheme 24).53 The new methylenating agent 67 can easily convert benzophenone at low temperature into 1,1-diphenylethylene in quantitative yield. 

Removal of a product (e.g., ethylene, Eq. 2) from the system could dramatically alter the course and/or rate of a desired metathesis reaction, since ethylene reacts with an alkylidene complex to form a methylene (M=CH2) complex, which is the most reactive (and also the least stable) of the alkylidene complexes. Of... 

Alkylidene phenylaminomethylenemalonates (444) were reacted with sodium hydride or lithium hydride in acetonitrile at 40°C for 2 hr. The reaction mixtures were then cooled to 0-20°C, and they were treated with a solution of dialkyl peroxydicarbonate in methylene chloride for 24 hr to give 5-alkoxycarbonyloxy-5-phenyliminomethyl-1,3-dioxane-4,6-diones (1616) in 54-75% yields (80CB2630). 

Metathesis of enynes is another intriguing application in the laboratory. It would seem from the outcome, that it is a completely intramolecular reaction, but if the mechanism involves a metal alkylidene this is not true and the alkylidene group moves on from one substrate molecule to the next, see Figure 16.25. The methylene group moves to the next ring-closed diene. This is a useful tool in organic synthesis [47],... 

Cycloadditions Onto Methylene- and Alkylidene-cyclopropane Derivatives... 

The intramolecular 2 - - 2 - - 1-cycloadditions of allene, alkyne (106), and carbon monoxide yield a -methylene-(107) or 4-alkylidene-cyclopentenones (108) depending on the allene structure or the reaction conditions (Scheme 4i).i59.i6o The cobalt-catalysed 4 - - 2 - - 2-cycloaddition of norbornadienes (109) with buta-1,3-dienes readily produces cycloadducts (110) when a bimetal system is used (Scheme A2) ... 

Goti A, Cordero FM, Brandi A (1996) Cycloadditions Onto Methylene- and Alkylidene-cyclo-propane Derivatives. 178 1 - 99... 

Perfluoro(3-isothiocyanato-2-methyl-2-pentene) reacts with N-nucleophiles to produce a series of fluoroalkyl-substituted 6/f-l,3-thiazines <1997RJO720>. The acid-catalyzed cyclization of thioureas immobilized on Wang (X = O) or Rink resin (X = NH) provides a convenient route to a wide range of 2-amino-4/7-benzothiazine derivatives 212 (Scheme 23) <20000L3667>. The thioureas are obtained in four steps from 2-nitrocinnamic acids. A general synthesis of 2-alkylidene-4-imino-l,4-dihydrobenzo-l,3-thiazines 213-215 involves treatment of 2-isothiocyanato-benzonitrile with acidic methylene compounds under basic conditions <2003SL1503>. The ( )-isomers are the predominant isomers formed. 

The reaction of nucleophiles with 1,3-dioxanes containing three double bonds is mainly confined to the group of 5-alkylidene-l,3-dioxane-4,6-diones. The parent compound, 5-methylene-l,3-dioxane-4,6-dione, is, however, quite unstable. Two reagents 170 and 171 have been developed to prepare 5-methylene-l,3-dioxane-4,6-dione in situ (Figure 8) <19968215, 2002SC2009>. 

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