Carbenes addition

Facial selectivity in electrophilic additions (carbene addition, mercuration, epoxi-dation, and hydroboration) to 4-substituted 9-methylenenorsnoutanes (1) as model alkenes has been elucidated and the observed preference for yyn-attack (Table 1)... 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

The predominant formation of five-membered carbocydes or heterocycles 122 (Scheme 50) via a sequential conjugate addition-carbene insertion pathway is generally observed in the reactions of the appropriate alkynyliodonium salts 119 (R = long alkyl chain or other group with C-H bond available at C5) with various relatively hard nucleophiles. Typical nucleophiles used to initiate these selective cyclizations are enolate, azide, sulfinate, tosylamide, thioamide and some other anions. 

Reaction of recently synthesized 1 -al kvnyl />-( trifluoromethyl )phenyl]( tetrafluoro-borato)-X3-bromanes with 2-mercapto- benzimidazole or benzothiazole in dichloro-methane at 0°C under argon resulted in a domino Michael addition-carbene rearrangement-cyclization reaction to produce directly tricyclic heterocycles in high yields (Scheme 67).103... 

A variety of soft nucleophiles have since been shown to react with alkynyliodonium salts via the Michael addition-carbene (MC) pathway (equation 45). If the tropophilicities of R... 

The benzannulation reaction tolerates phenyl carbene complexes bearing both electron-donating [35a] and -withdrawing [35b] substituents. Additionally, carbene complexes containing condensed systems such as naphthalene and other carbocyclic hydrocarbons as well as heterocydes have also been successfully submitted to the reaction (see Section 8.5). 

In addition, carben-like terminal groups make such clusters extremely reactive. All of these factors result in a significantly lower value of cohesive energy. A comparison of the calculated energies for C ), Csg, and C59 (Table 6) indicates that the more energetically favorable defragmentation of Ceo is, in this situation, the simultaneous removal of a pair of atoms. 

Scheme 3-5 Cyclopentene annulations via a tandem Michael addition-carbene insertion reaction [51].

As they are prepared from diazo compounds [6, 37, 38], the methodology is mostly of interest when no direct reaction pathway is available. Such ylides also transfer the carbene to unsaturated centers or insert into various bonds. Ylides appear therefore as additional "carbene relay" from the diazocompound to the substrate. 

The study of boron-centered radicals dates back several decades with early reports describing the s)mthesis and characterization of anionic boryl radicals/ The use of carbenes to generate stable boron-centered radicals has brought recent attention to these paramagnetic species as promising new reagents for radical initiation and related reactions/ Whereas early reports primarily described anionic boron radicals, recent developments involving carbenes have led to the discovery of a variety of species which can be cationic or neutral. Typically, these radicals contain a boron(ii) center, and are obtained by reduction or oxidation of boron(iii) or boron(i) precursors, respectively. Additionally, carbene-stabilized diborenes may be oxidized to afford radical cations which feature B-B one-electron it bonds. 

The seemingly limited number of available methods to access cyclopropanes by standard transformations in the laboratory are a result of the inherent ring strain of these systems. The fact that carbenes are intrinsically high-energy species renders them unique in the construction of numerous natural and nonnatural products that include cyclopropanes (Sections 15.2-15.5). However, there have been recent developments in methods that provide access to cyclopropanes via alternative intermediates and mechanistic pathways that are strikingly efficient (Section 15.6). Additionally, carbene and carbenoid intermediates have been reported to partake in a variety of stereoselective C-H insertions (Sections 15.7-15.8). 

---