Bridgehead fluorides

Bridgehead iodides, namely bicyclo[n.l.l]alkyl iodides, react with xenon difluoride in dichlo-romethane at ambient temperature to form the corresponding bridgehead fluorides in high yields.1617 The process apparently involves a cationic intermediate forming as a result of heterolytic cleavage of the appropriate difluoroiodoalkane and has been named "fluorode-iodination . 

Bridgehead Fluorides from the Corresponding Bromides General Procedure 90... 

Transmission of Polar Substituent Effects in Bridgehead Fluorides. There is a considerable literature, mainly from the group of Adcock, dealing with the transmission of polar substituent effects with... 

Scheme 3.13 Proton and carbon data for bridgehead fluorides...

Only little information is available on carbene formation by rearrangement of bridgehead olefins, generated in thermal reactions. A prominent example is the rearrangement of bridgehead olefin 2, obtained as short-lived intermediate from bromosilane 1 by reaction with potassium fluoride in DMSO at 110°C. Carbene... 

Adamantanecarboxylic acids with a carboxylic group at a bridgehead position, e.g. adaman-tane-1-carboxylic acid (8),111 react with sulfur tetrafluoride in the conventional way giving high yields of the corresponding trifluoromethyl-substituted adamantanes. l-(Trifluoromethyl)-adamantane (9) can also be formed in one step from adamantane by treatment with a mixture of sulfur tetrafluoride, hydrogen fluoride and formic acid the latter serves as a source of carbon monoxide. The reaction is believed to proceed via carbonylation of an intermediate carbocat-... 

Treatment of adamantane (2) with sulfur tetrafluoride results in the replacement of one, two or three bridgehead hydrogen atoms by fluorine the degree of fluorination depends on the reaction temperature. When the reaction is conducted in hydrogen fluoride solution, substitution of all four bridgehead hydrogens occurs at relatively low temperature.185... 

Adamantanecarboxylic acids, on treatment with sulfur tetrafluoride, react in a conventional manner to give (triiluoromethyl)adamantanes (see Section 8.2.4.1.). However, in the presence of anhydrous hydrogen fluoride in large excess, substitution of bridgehead hydrogen atoms by fluorine occurs in addition to fluorination of carboxylic acid groups di- or trifluoro(tri-fluoromethyl)adamantanes 7 and 8 arc obtained in one step.8 119186... 

Scheme 2.4 Substituent effects on chemical shifts of bridgehead bicyclic fluorides...

The bridgehead hydrogens in adamantane are susceptible to fluorination with sulfur tetra-fluoride with the number of hydrogens substituted dependent on the reaction temperature. The replacement of all bridgehead hydrogens in adamantane and (trifluoromethyl)adamantanes occurs with sulfur lc(rafluoride in hydrogen fluoride (see Vol, ElOa. pp 380, 381... 

This difference is obviously due to the difficulty in producing a double bond at a bridgehead position. Nevertheless, elimination of lithium fluoride does occur, especially at reflux temperatures the bridgehead alkene 10.8A, which probably has only a transitory existence or may even be more appropriately described as a diradical, may be trapped with furan. 

When bicyclic l-acetoxy-2,2-difluorocyclopropanes, e.g. 24,28, were reacted with lithium or sodium hydroxide in methanol the carbon-carbon bond between the bridgehead atoms was cleaved rather than the donor-acceptor-substituted bond. Two or three different products were obtained, formation of which can be rationalized by the occurrence of the cyclo-propanolate anion followed by ring opening and stabilization by protonation or elimination of fluoride. 

Bridgehead fluoroadamantanes. 1-Bromoadamantane is converted by anhydrous zinc fluoride in refluxing cyclohexane into 1-fluoroadamantane in 61% yield. However, the yield is sensitive to the heat treatment required to obtain anhydrous salt from the tetrahydrate and the reaction becomes increasingly slow as the number of bridgehead bromine atoms is increased. In this case the more expensive silver fluoride is used. Even 1,3,5,7-tetrabromoadamantane can be... 

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