Dichlorocarbene stereoselectivity

Raising the temperature above —30° noticeably reduces the stereoselectivity of the addition of dichlorocarbene to the central double bond of isotetralin, whereas lowering the temperature causes the yield of 1 1-adducts to drop due to partial crystallization of isotetralin. 

Acylation of limonene at the disubstituted double bond is favoured by a factor of 2.3 over reaction at the trisubstituted double bond using acetyl hexachloroan-timonate. Mixed alkylcuprate alkylation of tricarbonylcyclohexadienyliron salts has been used to synthesize the a-phellandrene tricarbonyliron complex. Dichlorocarbene addition to limonene in the presence of 1,4-diazabicy-clo[2,2,2]octane is almost 100% stereoselective at the trisubstituted double bond (no yield given) (cf. Vol. 6, p. 31) in contrast to dibromocarbene addition to carvone (Vol. 7, p. 34), dichlorocarbene addition to the carveols is not regio-specific. ... 

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... 

Reaction of the 6-endo deuterium-labeled silabicyclo[2.2.1]heptane derivative 70 with dichlorocarbene gave the product 71 by insertion exclusively into the C-H bond ann -periplanar to the C-Si bond (Scheme 7).67 The stereoselectivity in this case was rationalized in terms of stabilization of the partial positive charge in the exo direction in the transition state, due to a-p conjugation with the strained C-Si a bond, in addition to a steric effect favoring approach to the exo C-H bond. 

Thus, the p-hydroxyalkyl selenides (35a) and (35b) react with dichlorocarbene and stereoselective-ly lead to trans- and cir-9,10-epoxyoctadecene (37a and 37b Scheme 196). Under similar conditions the p-hydroxyalkyl selenide (36a) regio- and stereoselectively leads to the cycloheptanone (38a) resulting from the migration of the most-substituted carbon, and its diastereoisomer (36b) produces a mixture of the regioisomeric cycloheptanones (38b) and (39a) in which the latter, arising from the migration of the least-substituted ca n, prevails (Scheme 196, c and e). In all cases the reactions take place apparently in a concerted manner with inversion of the configuration at the substituted caibon, and in the... 

Chlorofluorocarbene is an electrophilic species whose reactivity and selectivity is between those of difluoro- and dichlorocarbene. It reacts with alkenes to give 1-chloro-l-fluorocyclopropanes, often in good yield these reactions are stereospecific and stereoselective (the isomers with the chlorine cis or endo to the alkyl group or the second ring, respectively, predominate, see Houben-Weyl, Vol. E19b, p 1493). 

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. ... 

The complete stereoselectivity of the first reaction can be explained by the hydroxy-directed addition of dichlorocarbene to the / e-face of 5 via the conformation 10. 

Formation of a bond a to a cyclopropane ring at the expense of a C H bond in 7 was achieved under phase-transfer conditions by ultrasound-mediated dihalocarbene C-H insertion adjacent to the ring."" The yields of 8 were considerably better with dichlorocarbene (81%) than dibromocarbene (27%), but the stereoselectivity was almost the same transjcis -80 20).""... 

Dichlorocarbene can be prepared by treating chloroform with potassium tert-butoxide, removing the elements of HCl. The resulting carbene reacts cleanly with alkenes to give dichlorocyclopropanes. Addition of a dihalocarbene to an alkene shows syn stereoselectivity. 

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