Methoxymethyl complex

An equimolar quantity of sodium cyanoborohydride (Na+B CN ) in methanol reduces CpFe (CO) 3+BFi (1) over one hour to the known (36,37) methoxymethyl complex CpFe(C0)2(CH20Me) (2). 

The observed instability of CpFe(CO)2CH2OH (4) augments a growing body of evidence concerning thermal instability of a-hydroxyalkyl complexes (38,39,40,41,42). A similar hydroxymethyl complex CpRe(C0)N0(CH20H), the only fully characterized a-hy-droxymethyl complex to date (38 39), likewise converts to its methoxymethyl complex in methanol. 

Observations pertinent to Scheme 2 are as follows (61, 64-66) as followed by H NMR, CH3S03F reacts slowly with 39 at - 70°C but rapidly at -40°C. However, under no conditions (including inverse addition, use of other methylating agents, etc.) can the formation of 54 from 39 be directly observed. Rather, H NMR indicates that substantial quantities of methoxymethyl complex 57 build up. The carbonyl cation product 56 is also present. These data are readily explained if, subsequent to initial O-methylation (Scheme 2, step a), rapid hydride transfer from unreacted 39 to 54 occurs (step b). As shown in Eq. (25), this reaction (conducted with independently synthesized samples) is indeed rapid at -70°C. 

The iodomethyl complex [CpFe(CO)2CH2I] (4) has been prepared by two routes (32). The first route involves the reaction of complex 1 with HI gas, as shown in Eq. (2). The second route involves the reaction of complex 3 with Nal [Eq. (3)]. The methoxymethyl complex (1) was isolated as an air-sensitive oil, whereas the halogenoalkyl complexes were isolated as air-and light-sensitive solids. The bromomethyl complex (3) was found to be less stable than the chloromethyl complex (2) in all respects, and the iodomethyl complex (4) was found to be even less stable. 

Hydride abstraction with Ph C PFg from the methoxymethyl complex [(CjMe )(C0)2Fe(CH20Me)] affords the methoxycarbene complex [(C Me )(CO)2Pe =C(H)0Me ][PFg], which undergoes an addition... 

Recently, Aumann et al. reported that rhodium catalysts enhance the reactivity of 3-dialkylamino-substituted Fischer carbene complexes 72 to undergo insertion with enynes 73 and subsequent formation of 4-alkenyl-substituted 5-dialkylamino-2-ethoxycyclopentadienes 75 via the transmetallated carbene intermediate 74 (Scheme 15, Table 2) [73]. It is not obvious whether this transformation is also applicable to complexes of type 72 with substituents other than phenyl in the 3-position. One alkyne 73, with a methoxymethyl group instead of the alkenyl or phenyl, i.e., propargyl methyl ether, was also successfully applied [73]. 

As mentioned above (Scheme 3), condensation of triallylborane and 3-methoxybut-l-yne led, after treatment with methanol, to 7-(l-methoxymethyl)-3-methoxy-3-borabicyclo[3.3.1]non-6-ene. Hydroboration-isomerization of the latter with a THF solution of diborane gave a THF complex of 2-methyl-1-boraadamantane 15 in 85% yield. Treatment of the latter with (S)-(—)-phenylethylamine gave a mixture of diastereomeric complexes ( ) 57 isolated as white, well-shaped crystals (Scheme 19) <2003MC121, B-2003MI97>. 

The enantiomerically pure l-[(benzyl(dimethyl)silyl)methyl]pyrrolidine, obtained from ben-zyl(chloro)(dimethyl)silane and (5,)-2-(methoxymethyl)pyrrolidine , afforded after deprotonation and subsequent alkylation the diastereomerically pure (by NMR spectroscopy) (a-alkylben-zyl)silanes2. To obtain this high degree of diastereoselectivity, the alkylation had to be performed in the weakly complexing solvent diethyl ether. In THF a diastereomeric ratio of only 3 1 was found with iodomethane as alkylating agent. 

Chemoselective reduction of methyl ester 7 to aldehyde 2 is possible with DIB AH. The metallatcd hemiacetal that results from addition of DIBAII to the carbonyl group of ail ester usually decomposes rapidly in polar solvents like THF to an intermediate aldehyde This then competes with the ester and, as a result of its higher clcctrophilicity. js reduced by DIBAH to an alcohol. However, ester 7 bears a methoxymethyl residue in its a-position, which stabilizes the metallated hemiacetal by chelate formation. Chelate complex 22 is protolytically cleaved by way of the hemiacetal only in the course of aqueous workup, so in this case the DIBAH reaction produces only aldehyde 2, not the alcohol (see also Chapter 3), DIBAH, THF, -78 C 100. ... 

Quenching of the ion 36 with methanol at — 78° C gives the 2-methoxymethyl-butadiene complex (37). Alcohol 38, when protonated at low temperature, first forms cation 39, with an anti-methyl substituent, which rearranges to 40 on warming to room temperature [Eq. (23)]. Studies of trimethyl derivatives gave... 

Similarly, 56 reacts with sodium methoxide to give small quantities of the methoxymethyl analog of 56. In the same way, treatment of dichloro(2,2-dimethyl-3-buten-l-yl methyl sulfide)palladium(II) with CH2N2 gives the expected chloromethyl complex and a cyclopropanation product. 

Fischer recognized the first carbene complexes in 1964. They were formed by the attack of an alkyllithium on a metal carbonyl followed by methylation (equations 1 and 2). Resonance form (2), considered as the dominant one in the heteroatom stabilized Fischer carbenes, shows the multiple character of this carbon-heteroatom bond. This effect is responsible for the restricted rotation often observed for this bond in nuclear magnetic resonance (NMR) studies. For example cis and trans isomers (6) and (7) of methoxymethyl carbenes rapidly interconvert at room temperature, but can be frozen out in the proton NMR at -40 °C. By contrast, the M-C bond is close to single and often rotates freely. 

Elimination of a formaldehyde molecule is an important fragmentation channel for iV-methoxymethyl 43a and iV-ethoxymethyl 43b sultams. The process is mediated by an ion neutral complex and it yields a mixture of N- and O-alkylated ions in various ratios <2005JMP331>. Fragmentations of 43a are shown in Scheme 5. 

In [2,6-bis(methoxymethyl)phenyl]2Mg (38), the intramolecularly coordinating substituents force the complex into a distorted octahedral coordination mode. The complex is solvent free its central magnesium atom is... 

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