Carbene addition to alkenes

At that time, however, it was not possible to measure the absolute rates of carbene additions to alkenes, which were too rapid. Accordingly, relative reactivities... 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

What was evident for macrocyclization involving metal carbene addition to alkenes is even more so for addition to alkynes [110]. However, here the chiral dirhodium(II) catalyst Rh2(4.S -IBAZ)4 exhibits the highest degree of enantiocontrol, superior even to Cu(MeCN)4PF6 (Eq. 5.22). 

Because 1,1-dihalocyclopropanes are so readily available by carbene addition to alkenes, their dehydrohalogenation to 1-halocyclopropenes provides, in principle, one of the most attractive routes to functionalised cyclopropenes. However, most early studies of the reaction did not lead to the cyclopropenes themselves, but to products of their further reaction. The main problems arise when the 1-halocyclopropene (9) can undergo prototropic shifts by removal of a proton from C 2 or C3, or when the base used is also a good nucleophile and addition to the cyclopropene can occur ... 

In a very different area of organic chemistry Ken produced a series of landmark theoretical papers on carbene reactions. He developed a general theory, showing how orbital interactions influence reactivity and selectivity in carbene additions to alkenes. Ken also showed how entropy control of reactivity and negative activation barriers in carbene addition reactions could both be explained by a new, unified model. 

IS the most popular, one-step method for m ag fluorinated cyclopropanes and cyclopropenes a-Fluorocarbenes are particularly well behaved, because they all have singlet ground states [/, 2] and therefore usually add stereospecifically to alkenes and do not insert into C-H bonds competitively with addition Moreover, quantitative competition studies of carbene additions to alkenes near room temperature show that a-fluorocarbenes are more selective than other a-halocarbenes, with difluorocarbene being the most selective electrophihc carbene known [3, 4] The relative selectivities, however, can be quite temperature dependent [5, d] The numerous preparations and cycloaddmons of fluorocarbenes have been reviewed thoroughly [7, 8 9,10 ... 

Among the methods at hand to synthesize cyclopropane derivatives, carbene addition to alkenes plays a prominent role 63). As a source of vinylcarbenes, cyclopropenes might be useful in this kind of approach. In 1963, Stechl was the first to observe a transition metal catalyzed cyclopropene-vinylcarbene rearrangement64). When treating 1,3,3-trimethylcyclopropene with copper salts, dimerization occurred to give 2,3,6,7-tetramethyl-octa-2,4,6-triene (9), the product from a formal recombination of the corresponding vinylcarbene (Eq. 8). 

A wide range of cyclopropane derivatives can be prepared by this carbene addition to alkenes.294 There is a general preference for the less congested exo product in reactions with cyclic alkenes. Initial addition of the... 

Scheme 10.11 Electron pushing for four different carbene reactions. A. Singlet carbene addition to alkenes, B. Triplet carbene addition to alkenes, and C. Singlet CR, ) a. R R -A...

FIGURE 10.48 Triplet carbene addition to alkenes.The formation of an intermediate 1,3-diradical induces scrambling of stereochemistry. Unlike singlets, which add in a single step to generate cyclopropanes stereospecilically, triplets give both cis and trans cyclopropanes. 

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