A-ferrocenylalkyl carbocations

Reaction of (284) with an aldehyde, ketone, or enol ether in the presence of acid results in an electrophilic substitution that produces a -ferrocenylalkyl carbocations that may be trapped by nucleophiles (azides, amines, thiols). This chemistry may be used to prepare enantiomerically pure ferrocene derivatives in a maimer that avoids resolution procedures (Scheme 86)." For example, the enol ether from (-)-menthone affords a kinetic carbocation (302) that may be trapped or allowed to rearrange to the more thermodynamically stable cation (303) and then trapped, thus offering a means of controlling the configuration of the stereocenter adjacent to the ferrocene unit. Use of an enantiomerically pure aldehyde derived from Q -pinene (304) affords a 1 1 carbocationic mixture that similarly isomerizes to a single cation. 

One fascinating aspect of ferrocene chemistry is its extraordinary ability to stabilize carbocations that formally should have their positive charge in a position adjacent to the cyclopentadienyl ring (a-ferrocenylalkyl carbocations). Such cations are so stable that they form quantitatively from appropriate precursors (e.g., alcohols) on treatment with acid and many of them remain unchanged in solution for days and... 

As the enantiomeric a-ferrocenylalkyl carbocations are stable species, a stereochemical nomenclature is desirable. We have made a suggestion that extends the planar approach for 1,2-disubstituted ferrocenes to the adjacent cationic center. 

Fig. 4-5. xo-type stereochemistry in the formation and reaction of a-ferrocenylalkyl carbocations. 

As resolution procedures are often tedious, and asymmetric synthesis provides chiral products with only limited enantiomeric excess, it seems an obvious strategy to use an enantiomerically pure material from the chiral pool to construct chiral ferrocenes by incorporating these compounds in the final product. As such chiral materials, cheap terpenes (menthone, a- and -pinene, and camphor) were chosen. The reaction of ferrocene with carbonyl compounds under acidic conditions is a very convenient way to obtain directly a-ferrocenylalkyl carbocations. The starting materials were therefore converted to aldehydes or their enol ethers (menthone and camphor are too sterically hindered and do not react with ferrocene). Joint dissolution of the aldehydes and ferrocene in trifluoroacetic acid or in the trichloroacetic acid/ fluorosulfonic acid system gives a-ferrocenylalkyl carbocations, which can either... 

The nitrogen atom in a-ferrocenylalkylamines generally shows the same reaction pattern as that in other amines alkylation and acylation do not provide synthetic problems. Due to the high stability of the a-ferrocenylalkyl carbocations, ammonium salts readily lose amine and are, therefore, important synthetic intermediates. Acylation of primary amines with esters of formic acid gives the formamides, which can be dehydrated to isocyanides by the standard POClj/diisopropylamine technique (Fig. 4-16) [92]. Chiral isocyanides are obtained from chiral amines without any racemization during the reaction sequence. The isocyanides undergo normal a-addition at the isocyanide carbon, but could not be deprotonated at the a-carbon by even strong bases. This deviation from the normal reactivity of isocyanides prompted us to study the electrochemistry of these compounds, but no abnormal redox behaviour, compared with that of other ferrocene derivatives, was detected [93]. The isocyanides form chromium pentacarbonyl complexes on treatment with Cr(CO)s(THF) (Fig. 4-16) and electrochemistry demonstrated that there is no electronic interaction between the two metal centres. 

CSj, with subsequent alkylation with methyl iodide, led to (i )-l-ferrocenylethyl-5-methyltrithiocarbonate, which involves a rearrangement with retention (Fig. 4-17, top) [100]. Sulfides [101], tertiary phosphines [101], and tertiary amines [102] are also reasonably good nucleophiles and form reactive ionic products, e.g., pyridinium salts [103], but this has only been verified for achiral or racemic substrates. Pyridinium salts may be considered as a storage form of a-ferrocenylalkyl carbocations, and show almost the same behaviour towards nucleophiles [103, 104]. Primary and... 

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