Dihalocarbenes insertion

Although the C=C bond of allyl alcohols is frequently less susceptible to reaction with dihalocarbenes, insertion of the carbene into the C=C bond invariably occurs (Table 7.4) to the exclusion of reaction at the hydroxyl group (see Section 7.5) [98]. A complex mixture of products is obtained from the reaction of dichlorocarbene with allyl alcohol, but the cyclopropane can be obtained in high overall yield (>70%) via... 

Details of reactions with carbenes are also rare. Carbene, derived photolytically from diazomethane, attacks 1,3-dioxolane at the 2- and the 4-positions. Together with 2- and 4-methyl-l,3-dioxolane, there are obtained lesser amounts of ring-expanded products (69CB1087). Dihalocarbenes insert into the C(2)—H bond of 1,3-dioxolanes in a stereospecific manner and without racemization or inversion. The highest yields are obtained when there is substitution at the 4- and 5-positions and (84) is converted into (85) in over 80% yield (80JCR(S)95). 

Complexation of dienes with iron carbonyls protects the double bonds against addition of dihalocarbenes thus, when such complexes are exposed to the PTC-generated dihalocarbenes, insertion into saturated C-H bond takes place (eq. 195). 

The first example of dihalocarbene insertion into a saturated C—H bond of an organometallic complex has been reported using a phase-transfer catalyst The first example of phase-transfer catalysts for electrophilic substitution involved an azo-coupling reaction in water-dichloromethane using sodium 4-dodecylbenzene-sulphonate giving a rate acceleration of ca. 200-fold. ... 

The only known reaction of a furan with a dihalocarbene is that recently reported between benzofuran and dichlorocarbene in hexane at 0°. The initial adduct (7) could not be isolated but on hydrolysis gave the ring-expanded product 8, possibly via 9, in 15% yield. Benzothiophene was recovered in 92% yield under the same conditions. 2,5-Dihydrofuran reacted with dichloro- and dibromo-carbene to give the products of allylic insertion, 2-dihalogenomethyl-2,5-dihydrofuran, as well as the normal addition products. ... 

Dihalocarbene ligands, like other neutral 2-e donor carbon ligands, are expected to participate in migratory-insertion reactions when bound adjacent to a rx-bound alkyl or hydride ligand. An example is provided by the following reaction (119) ... 

The advantages of this method of carbene synthesis are that reaction can be carried out in neutral solution, and that reaction yields are often dramatically improved. Thus, although reactions of dihalocarbenes generally do not give rise to products corresponding to single bond insertion, Seyferth has reported insertion of phenyl (trihalomethyl) mercury-generated carbenes into... 

Selected examples of insertion reactions of dihalocarbenes into C-H bonds... 

As noted with the reactions between terpenes and dihalocarbenes, mono-insertion adducts at the more electron-rich sites can be isolated from the reaction of non-conju-gated acyclic and cyclic dienes although, depending on the reaction conditions, the bis-adducts may also be formed. Norbomadiene produces both 1,2-endo and 1,2-exo mono-insertion adducts with dichlorocarbene, as well as a 1,4-addition product (Scheme 7.4) [67]. The mono adduct produced from the reaction with dimethylvinylidene carbene rearranges thermally to yield the ring-expanded product (Scheme 7.4) [157] a similar ring-expanded product is produced with cyclo-hexylidene carbene [149]. 

Acyclic and cyclic allylic ethers and acetals react normally with dihalocarbenes at the C=C bond [e.g. 77, 85, 108,114,121,122], Carbene insertion into the C=C bond of allylic ketones, which can be complicated by competitive reaction by the carbonyl group, can also be effected via the initial formation of the acetal and has been used in the synthesis of cyclonona-3,4- and -4,5-dienones from cyclooctenones [125],... 

Addition of carbenes to Jt-electron excessive aromatic compounds, or those which possess a high degree of bond fixation, is well established. Dihalocarbenes react with naphthalenes with ring expansion to produce benztropylium systems (Scheme 7.8). Loss of hydrogen halide from the initially formed product leads to an alkene which reacts with a second equivalent of the carbene to yield the spirocyclopropyl derivatives in high yield (>95%) [14, 50]. Insertion into the alkyl side chain (see Section 7.2) also occurs, but to a lesser extent [14]. Not unexpectedly, dichlorocarbene adds to phenanthrenes across the 9,10-bond [9, 10, 14], but it is remarkable that the three possible isomeric spiro compounds could be isolated (in an overall yield of 0.05% ) from the corresponding reaction with toluene [14]. 

Predictably, the reaction of A V-disubstituted enamines [26-29] and non-conju-gated unsaturated amines with dihalocarbenes results in the exclusive formation of the dihalocyclopropane derivatives (see Section 7.3). Dichlorocarbene inserts into the a-CH bond of Af-alkyldibenzo[6,/]azepines [16], in addition to the expected electrophilic addition to the C=C bond (see Sections 7.2 and 7.3). 

The formation of dihalocarbene is one of the most useful phase-transfer processes developed in organic chemistry. The dihalocarbenes, once generated from haloform and base, can undergo addition or insertion reactions (3, 11, 12). If the double bond is complexed to a metal, addition by the carbene will not occur, but rather insertion into a saturated carbon-hydrogen bond will take place. 

It is believed that, because of the presence of the halogen atoms, the singlet form, with the electrons paired, is the.more stable form of dichlorocarbene, and is the one adding to the double bond. (Stabilization by the halogen atoms is presumably one reason why dihalocarbenes do not generally undergo the insertion reaction that is so characteristic of unsubstituted singlet methylene.)... 

Dihalocarbenes, like CCI2 or CBr2, generated in situ by phase transfer catalysis are inserted into a / C—H bond of the five- and six-membered heterocycles 40b,c to give the functionalized metallacycles 63 and 64 (Scheme 23). A precondition for this reaction is a trans-M—C—C—H... 

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