Allylic dichlorocarbene addition

Dramatically differing effects of phase-transfer catalysts on the cyclopropanation of cw,trans,trans-cyclododecatriene (61) and a series of dienes have been reported. Addition of dichlorocarbene to (61) results in tris-cyclopropanation when cetyltri-methylammonium bromide (i) is employed, whereas with benzyl-P-hydroxyethyl-dimethylammonium ion (ii) as catalyst only monocyclopropanation (of the more strained bond) is observed (Scheme 7). From the extensive study it may be concluded that, for dichlorocarbene addition, the P-hydroxyethyl catalyst restricts potential polycyclopropanation to monocyclopropanation at the most highly substituted (or strained) double bond. With dibromocarbene a different situation results. Catalyst (i) does not effect the addition of dibromocarbene to styrene, cyclohexene, or allyl bromide while catalyst (ii), with the P-hydroxyethyl function, effects dibromocyclo-propanation, in yields of up to 80 %. 

Dichlorocarbene Addition to Allylic Alcohols A Cyclopentenone Synthon... 

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. ... 

Tochtermann reported the addition of dichlorocarbene to the racemic glycal 52, whose cyclopropyl-allyl rearrangement leads to the 2//-pyran. The synthesis of the optically pure (+)-(2S,3R,7S) and (-)-(2f ,3S,7f )-glycal precursors has also been achieved. As pointed out, optically pure glycals are versatile precursors for carbohydrate synthesis <00EJO1741>. 

This dominance of sulfur in the reactions with electrophiles is well brought out in the addition of carbenes to the-two monocycles. Tire allylic sulfide (5,6-dihydro-2jF/- thiopyran) only affords the products of reaction at sulfur, while the vinylic sulfide (3,4-dihydro-2f/-thiopyran), in which the alkene is a little more nucleophilic due to the small interaction with the heteroatom, shows dichotomous behaviour. Dichlorocarbene affords the cyclopropane product (78) in 70% yield, but the stabilized ylide (76) is produced from bismethoxycar-bonylmethylide and (75). In fact it is possible that the initial reaction with dichlorocarbene is reaction at sulfur and subsequent rearrangement of this less stabilized ylide. Schemes 6 and 7 illustrate the results and proposed mechanisms (77JOC3365,64JOC2211). 

Predictably, with enynes dihalocarbene addition occurs preferentially with the double bond (equation 17).63 64 The reaction of dichlorocarbene with allenes usually gives rise to spiro-linked dichlorocyclopro-panes (equation 18),65 but rearranged products are obtained in some cases (equation 19).65 66 Somewhat surprisingly, a measure of stereoselectivity has been observed in the formation of dichlorocyclopro-pylcarbinols from secondary allylic alcohols and dichlorocarbene (equation 20).67... 

Chelotropic addition of dichlorocarbene to bornadiene gave (200 X = C1) whereas with difluorocarbene the, yyn-adduct was favoured over the anh-adduct (200 X = F) (cf. Vol. 3, p. 59). A thio-Claisen rearrangement has been reported with the allylic enethiolic ether of thiocamphor. Flash thermolysis of allyl exo-2-bornyl sulphide to thiocamphor and propene has been examined. 

Unsaturated compounds possessing an allylic C-H bond(s) are prone to insertion of dichlorocarbene, aside from the addition of dichlorocarbene to the double bond. This reaction occurs, in particular, more frequently if the dichlorocarbene is generated from chloroform by phase-transfer catalysis or from dichlorohalomethyl(phenyl)mercury. Thus, careful investigation reveals that, in contrast to earlier findings, 9,10-octalin forms both carbene addition 1 and insertion 2 products in comparable yields." ... 

It is often difficult to make a comparison between the various results obtained for the same polyenes as different reaction conditions (ratio of reactants, temperature, time) were used in each case. The addition of dichlorocarbene (chloroform/base/phase-transfer catalysis) to straight chain and cyclic unconjugated di- and trienes, carried out under identical conditions but varying the catalysts, showed the peculiar properties of tetramethylammonium chloride. Under precisely tailored conditions, either highly selective mono- or polyaddition of dichlorocarbene to the polyenes is possible tetramethylammonium chloride was the most efficient catalyst for monocyclopropanation. (For the unusual properties of tetramethylammonium salts on the phase-transfer catalyzed reaction of chloroform with electrophilic alkenes see Section 1.2.1.4.2.1.8.2. and likewise for the reaction of bromoform with allylic halides, see Section 1.2.1.4.3.1.5.1.). For example, cyclopropanation of 2 with various phase-transfer catalysts to give mixtures of 3, 4, and 5, ° of 6 to give 7 and 8, ° and of 9 to give 10 and 11. °... 

The double bonds in tetraphenylallene are inert to dichlorocarbene therefore, it underwent addition to the phenyl ring in low yield. The product 34 thus formed underwent cyclopropyl-allyl rearrangement to give 35. ... 

Esters of allylic and homoallylic alcohols undergo addition of dichlorocarbene, the reactions are conveniently carried out by the chloroform/base/phase-transfer catalyst method. Some products of this type are specified in Table 17. 

The stereoselective addition of dichlorocarbene to straight chain allylic alcohols is frequently observed the most stable rotamer reacts, predominantly from the side of the hydroxy group. ... 

Five-membered, cyclic allylic sulfones, except the parent 2,5-dihydrothiophene 1,1 -dioxide, underwent smooth addition of dichlorocarbene generated from chloroform/base/phase-transfer catalyst giving 4. ... 

Reaction of e J<9-5-hydroxymethyl-2-norbornene with dichlorocarbene generated under phase transfer conditions leads to 3-chloro-5-oxatricyclo-[5.2.1.0 ]-dec-2-ene as the major product (see Eq. 2.21). Formation of this product probably involves initial addition of dichlorocarbene to the carbon-carbon double bond to yield a 1,1-dichlorocyclopropane which ionizes and ring-opens to form a chloro-substituted allylic carbonium ion. This cation is then trapped by the intramolecular nucleophilic alcohol [40]. 

The addition of dichlorocarbene to hexamethyl Dewar benzene has been studied (see Eq. 2.22) [37, 41]. The initial mono-dichlorocarbene adduct undergoes ionization and opening of the cyclopropane ring to yield an allylic carbonium ion which loses a proton from a methyl group to yield adduct 1. 

---