Hexanes reaction 4- carbenes

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. ... 

Analytical thin layer chromatography (TLC) was conducted on 10 x 2.5-cm precoated glass plates (silica gel GF, 0.25-mm thickness, Analtech), eluted with 10% ethyl acetate in hexane, and visualized with both UV (254 nm) and aqueous 50% sulfuric add spray/heating. The carbene complex moves as an orange spot on TLC the reaction is complete when this spot is no longer visible. 

Of course carbene C-H insertion reactions are well known absolute kinetics have been reported for the insertions of ArCCl into isooctane, cyclohexane, and n-hexane,67 and of PhCCl into Si-H, Sn-H, and C-H bonds.68 More recently, detailed studies have appeared of PhCCl insertions into a variety of substrates bearing tertiary C-H bonds, especially adamantane derivatives.69 Nevertheless, because QMT is considered important in the low temperature solution reactions of MeCCl,60,63 and is almost certainly involved in the cryogenic matrix reactions of benzylchlorocarbene,59 its possible intervention in the low temperature solution reactions of the latter is a real possibility. We are therefore faced with two alternative explanations for the Arrhenius curvature exhibited by benzylchlorocarbene in solution at temperatures < 0°C either other classical reactions (besides 1,2-H shift) become competitive (e.g., solvent insertion, azine formation), or QMT becomes significant.7,59,66... 

At 62% yield, the main product of the reaction of 42b with MeLi (molar ratio 1 5.5) was l,2-dichloro-2-methylbicyclo[2.1.1]hexane (66). Again, the most probable mechanism leading to 66 is addition of MeLi to carbene 54 (X=H, X=C1), followed by lithium chlorine exchange of the intermediate tertiary alkyllithium base 67 with the trichloride 42b. An alternative mechanism, addition of LiCl to carbene 54 and methylation of the intermediate carbenoid by MeLi, formed during the reaction from MeLi and 42b, is less probable.24... 

Bicyclic cyclopropanes.1 Reaction of the Fisher carbene 1 with the 1,6-enyne 2 results in the bicyclic cyclopropane 3 (a bicyclo[3.1.0]hexane) in 69% yield. The 1,7-enyne homolog of 2 reacts with 1 in the same way to form a bicyclo[4.1.0]-... 

Preparation of spiro[2.3]hexanes is possible by the addition of methylene or a methylene equivalent to an exocyclic C —C double bond. Generally, the Simmons Smith reaction is used (see Section l.A.1.2.1.), but formation of carbenes from nitroso compounds can also be used.73-75,333 The 1,1-dihalo derivative can be prepared by phase-transfer-catalyzed reactions.48 For example, reaction of a mixture of cis- and trans-1,2-dimethyl-3-methylenecyclobutane with dibromo-carbene gave a mixture of cis- and lrans-1,1-dibromo-4,5-dimcthylspiro[2.3]hexanc (2).48... 

Butenyl)-, (2,3-dimethyl-3-butenyl)- and (4-pentenyl)-dimethylsilyl)]carbene have been generated by treatment of the corresponding chloromethylsilanes with sodium. Intramolecular [1 + 2] cycloaddition of the carbenic carbon atom to the double bond leads to l-silabicyclo[3.1.0]hexanes and l-silabicyclo[4.1.0]heptanes, respectively, usually in competition with intramolecular C,H insertion (equation 24)56. In contrast, no carbene-derived product could be obtained from (allyldimethylsilyl)carbene. Finally, reaction of chloromethyldimethylvinylsilane with sodium provided, besides the typical products of a Wurtz reaction (103 and 104), a small amount of cyclopropane 106 (equation 26)56. It has been suggested that (dimethylvinylsilyl)carbene (102) isomerizes to silabicyclo[1.1.0]butane 105 by intramolecular cyclopropanation, and nucleophilic ringopening finally leads to 106. 

Alternatively, a mixture of 2.0 g (6.4 mmole) of the carbene complex, acetylene (2.6 eq.) and acetic acid in tetrahydrofuran is heated at 65°C for 2 hours under argon atmosphere. After cooling the reaction solution is concentrated and the black residue is loaded on a silica gel (200 g) column for a flash chromatography. Elution by 10% ether in n-hexane gives 895 mg (51.4%) of the title product. 

The photolysis of diazoalkanes both in the gas phase and in solution is a carbenoid reaction. Moreover, the results of EPR-spectroscopic investigations (Section IIB) demonstrate that triplet carbenes can be generated by irradiation of diazoalkanes. That the reactive intermediates in carbenoid reactions are free carbenes is usually taken as self-evident. While such an assumption is probably wholly justified in most cases, it is worth remembering that both in the gas phase and in solvents such as n-hexane, the electronic absorption spectra of simple diazoalkanes show definite fine structure (Bradley etal., 1964a). This implies that the photo-excited state is bonding (Hoffmann, 1966) and consequently may have a life-time long enough to enable it to react directly with another molecule... 

The formation of side products depends on the choice of substituents and solvent [21]. The role of the solvent is illustrated by the reaction of phenyl carbene complex 1 with diphenylethyne (Scheme 7). An ethereal solvent such as THF leads exclusively to the benzannulation product isolated as quinone 7 after oxidative work-up, while use of the noncoordinating solvent hexane results in comparable amounts of cyclobenzannulation and cyclopentannulation products 7 and 8a. Strongly coordinating acetonitrile suppresses benzannulation product 7 in favor of the cyclobutenone 9, which is accompanied by minor amounts of cyclopentannulation products 8a and 8b. Indene 8a is obtained exclusively if the polar solvent DMF is employed. 

Intramolecular cyclopropanations of pendant alkenes are more favorable. Heteroatom-substituted 2-aza- and 2-oxabicyclo[3.1.0]hexanes, together with 2-oxabicyclo[4.1.0] heptanes, can be prepared from chromium and tungsten Fischer carbenes having a tethered alkene chain. An interesting carbene formation via a cationic alkylidene intermediate, nucleophilic addition (see Nucleophilic Addition Rules for Predicting Direction), and intramolecular cyclopropanation is shown in Scheme 59. An intramolecular cyclopropanation via reaction of alkenyl Fischer carbene complex (28) andpropyne was used in a formal synthesis of carabrone (Scheme 60). 

---