Carbenes, from bromoform

When each stereoisomeric reactant forms a different stereoisomeric product the reaction is known as stereospecific reaction. For example, the addition of CBr2 (dibromo-carbene, prepared from bromoform and base) to ds-2-butene gives cis-2,3-dimethyl-l,l-dibromocyclopropane (1.32), whereas addition of CBr2 to the trans-isomer exclusively yields the trans-cyclopropane 1.33. 

Dibromocarbene is one of the most reactive (the least selective) carbenes (Houben-Weyl, Vol. E19b, p 1598). Its selectivity index 0.65, if generated from bromoform and potassium... 

Historically, potassium /er/-butoxide was the first base used for the preparation of 1,1-dibromo-cyclopropanes from bromoform and an alkene. Since generation and cycloaddition of dibromo-carbene to alkenes proceeds rapidly, this method is still in laboratory practice. 

Carbenes generated the more traditional way react similarly. In Trost s synthesis of (-)-anatoxin,297 the alkene unit in 366 reacted with dibromocarbene, generated from bromoform and base, to give an 85% yield of 367 and 368 as a 3.5 1 mixture of exo and endo isomers. 

The olefinic selectivity of dibromocarbene generated from bromoform has been reinvestigated and, by employing 3-ethyl-3-heptoxide as the base (to suppress alkene formation), the results reveal minimal kinetically effective carbenoid involvement. This same group of workers have shown that MeSCCl is a free carbene while PhCF is in fact a carbenoid when generated by oxygenated base. A careful study of the relative reactivity of the vinylcarbene (48) by addition to a series of para-substituted styrenes has revealed a Hammett correlation with p = —0.75 treatment of the vinyl triflate with potassium t-butoxide results in the free carbene. ... 

No matter how they are generated, carbenes and carbenoids undergo four typical reactions. The most widely used reaction is cyclopropanation, or addition to a TT bond. The mechanism is a concerted [2 + 1] cycloaddition (see Chapter 4). The carbenes derived from chloroform and bromoform can be used to add CX2 to a 7T bond to give a dihalocyclopropane, while the Simmons-Smith reagent adds CH2. Carbenoids generated from diazoalkanes with catalytic Rh(II) or Cu(II) also undergo cyclopropanations. 

---