Methylidene group

Examine the charge on the methylidene group, as well as the magnitude and direction of the molecule s dipole moment. Are they consistent with representation of the ylide as a hypervalent molecule or as a zwitterion ... 

Other interesting examples of highly distorted dienes are 29 and 30. In these cases there is a further difficulty. For 3039, for instance, two low-energy conformations in equilibrium are possible These two conformations are differentiated essentially by the interaction between the methylidene groups and the equatorial and axial hydrogens on the ring. 

As mentioned above, Eisch and co-workers have synthesized new methylidene-group IV metal complexes, such as the methylidene zirconium complex 88 from zirconium(iv) chloride and 2 equiv. of methyllithium at low temperature (Scheme 36).53 By using this reagent, benzophenone was easily converted at low temperature into the desired 1,1-disubstituted alkene in quantitative yield. 

As mentioned already, new methylidene-group IV metal complexes have been prepared and were subsequently used in nucleophilic additions to carbonyl electrophiles (Scheme 43).53 In contrast to titanium and zirconium, the reaction of methylidene hafnium dichloride 97 benzophenone stopped at the first stage (i.e., addition). The tertiary alcohol was obtained in 73% yield, while the corresponding alkene was formed only as minor product. 

Tri- and tetrasubstituted oxepanes 47 can be obtained in 50-60% overall yield (Scheme 15) using vinyl radical cyclization of homopropargyl and arylhomopropargyl derivatives 48 of Baylis-Hillman adducts 49, followed by methylidene group deprotection using pyridinium/>-toluenesulfonate (PPTS) <2005TL3369>. 

Many organometallic complexes carry ligands such as cyclopentadienyl, phenyl, methylene and methylidene groups, all of which are capable of... 

Under more forcing conditions, that is, enhanced CO pressme, the complexed alkyne can combine with excess CO, giving lactones that are bound to the Co2(CO)6 moiety via a bridging methylidene group (Scheme 9). 

The first facially selective hydroboration of a 5-methyhdene[2.2.1]bicychc intermediate has also recently been reported. The rhodium-catalyzed hydroboration of the methylidene group with HBcat proved superior to stoichiometric borane or dialkylborane reagents, in terms of higher diastereomeric excess and chemical yields (equation 15). For instance, while borane gave 89% of the desired 5-endo product along with 11% of the 5-exo product, the catalyzed variant with HBcat gave 95% for the desired 5-endo product when the reaction was run at room temperature. No improvement in diastereoselec-tivity was observed when reactions were run at lower temperatures. 

At the methylidene group carbon atom of 3 a relatively high electron density NMRCCDCls) 5 = 8S.5 ppm) is located which could be caused by the mesomeric electron-releasing effect of the silicon-substituted enamine nitrogen atom. Therefore this methylidene group carbon atom is able to pick up a proton from some acids. Even weak acids react with 3 if a stable Si-0 bond can be formed with the corresponding anion. 1,4-Addition of methanesulfonic acid, picric acid, benzoic acid and hydroquinone to 3 (solution in THE) led to hexacoordinate salen-silicon complexes by precipitation of compounds 4, 5, 6 and 7 respectively (Scheme 1). The Si chemical shifts of the solids obtained (CP/MAS-NMR) are presented in Table 2. 

This mechanistic tour de force was concluded with one final, stereochemical probe on degenerate propylene metathesis (Scheme 10.15). A mixture of (Z)-propylene-l-(ij and propylene-d5 was subjected to the MoOj catalyst. Propylene-dg was formed with 80% initial retention of stereochemical information (where retention here means that the deuterium of the methylidene group ended up on the same side as the methyl group, as in (Z)-propylene-l-dj). While the stereochemical purity of this product eventually degraded due to secondary metathesis, it is mechanistically significant that the stereochemistry of the methylidene moiety was initially preserved. Again, this reaction could proceed through either a metal-methylidene or ethylidene intermediate. 

Quinomethanes (formerly known under incorrect names quinone methides or quinomethides) are methylidenecyclohexadienones and dimethyhdenecy-clohexadienes, formally derived from quinones by replacement of one or both of the quinone oxygen atoms by a methylidene group. 

From the isolated products, one cannot determine which level of substitution on silicon is decisive for a transmetallation reaction to take place. However, if the formation of the methylidene group is completed by elimination of HMgCl, subsequent methylation of the remaining SiCl group will be so rapid that formation of SiH groups will not occur. Through the influence of the C = C double bond on the Si atoms, an increased reactivity of the Si—Cl bond becomes apparent. 

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