Other Halocarbenes

Dibromocarbene has been utilized in place of dichlorocarbene in the phase transfer Hofmann carbylamine reaction (see Sect. 3.5). Methyl and ethyl isocyanide are low boiling and separation of these products from residual chloroform is difficult. The purification can be facilitated by use of bromoform in these two cases (Eq. 4.8) [16]. 

Reaction of dibromocarbene with several adamantyl alcohols has been reported to give the corresponding bromides in low yield [17]. Three such reactions are illustrated in equations 4.9—4.11 [17]. The reaction is analogous to the more thoroughly studied reaction of alcohols with dichlorocarbene (see Sect. 3.3). 

In addition to the phase transfer catalyzed generation of dichlorocarbene (Chaps. 2 and 3) and dibromocarbene (Sect. 4.2-4.6), most of the other possible dihalocarbenes have been formed by this method. Fluorodichloromethane (Freon 21) reacts with 50% aqueous base under phase transfer catalysis to give fluorochlorocarbene ( CFC1) 

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The syntheses, structures, spectroscopy, bonding, and reactivity of difluoromethylidene and other halocarbene complexes have been reviewed recently (10). Only some typical structures and more recent results are discussed briefly here. 

The thermochemistry of CI2 and its spectral properties, as for a range of other halocarbenes studied already, have been characterized by quantum chemical methods using a... 

Carbene itself is extremely reactive and gives many side reactions, especially insertion reactions (12-19), which greatly reduce yields. When it is desired to add CH2 for preparative purposes, free carbene is not used, but the Simmons-Smith procedure (p. 1088) or some other method that does not involve free carbenes is employed instead. Halocarbenes are less active than carbenes, and this reaction proceeds quite well, since insertion reactions do not interfere.The absolute rate constant for addition of selected alkoxychlorocarbene to butenes has been measured to range from 330 to 1 x 10 A few of the many ways in... 

As shown in Figure 9.2(a), an X substituent, which has a p orbital, or other suitable doubly occupied orbital that will interact with the n bond, raises the 2p orbital of the carbene, thereby increasing the separation of the 2p and ip" (a) orbitals. The ground state of an X -substituted carbene becomes a singlet, and many carbenes in this class are known. The most familiar of these are the halocarbenes. 

CHi generated by photolysis of CH2N2 in the liquid phase is indiscriminate—totally non-selective—in its reactivity (p. 199). CH, generated in other ways and other carbenes are less reactive and insert in the order tertiary > secondary > primary.232 Halocarbenes insert much less readily, though a number of instances have been reported.233 Nevertheless, even for less reactive carbenes, the insertion reaction has seldom been used for synthetic purposes.234 The carbenes can be generated in any of the ways mentioned in Chapter 5 (p. 198). For the similar insertion of nitrenes, see 2-12. 

IS the most popular, one-step method for m ag fluorinated cyclopropanes and cyclopropenes a-Fluorocarbenes are particularly well behaved, because they all have singlet ground states [/, 2] and therefore usually add stereospecifically to alkenes and do not insert into C-H bonds competitively with addition Moreover, quantitative competition studies of carbene additions to alkenes near room temperature show that a-fluorocarbenes are more selective than other a-halocarbenes, with difluorocarbene being the most selective electrophihc carbene known [3, 4] The relative selectivities, however, can be quite temperature dependent [5, d] The numerous preparations and cycloaddmons of fluorocarbenes have been reviewed thoroughly [7, 8 9,10 ... 

The same behaviour is observed for the other important group of reactions involving the generation of halocarbenes via halocarbanions (Eqs. (19)-(21)). 

Others have reported analogous photocatalyzed polymerizations when W(CO)6 is irradiated in halocarbon solvents in the presence of alkynes and halocarbene complexes have been suggested as the key intermdiates [110]. Halocarbene intermediates have also been implicated in the photocatalysis of olefin metathesis reactions by the group VI carbonyls M(CO)6 in carbon tetrachloride [111,112]. 

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