Alkene reaction with carbene

Before we close the discussion of carbene reactions with alkenes we mention tetrahedral boron hydride-substituted diazomethanes (8.11-8.13) which were obtained by the group of Jones (Li and Jones, 1992 Li et al., 1993) by manipulating substituted o-carboranes, as shown in (8-19). Reaction of 8.13 with ( )-but-2-ene yielded the pure (jE )-derivative, i.e., the product of singlet addition. With (Z)-but-2-ene 3 % triplet reaction product was observed. The percentage of triplet products was higher with 8.13 (22 and 18 7o with (E)- and (Z)-but-2-ene, respectively) (Huang et al., 1992). 

Reaction of ayfi-Unsaturated Fischer Carbene Complexes with Alkenes,... 

A common feature of these intermediates is that they are of high energy, compared to structures with completely filled valence shells. Their lifetimes are usually very short. Bond formation involving carbocations, carbenes, and radicals often occurs with low activation energies. This is particularly true for addition reactions with alkenes and other systems having it bonds. These reactions replace a tt bond with a ct bond and are usually exothermic. 

The reactive intermediates under some conditions may be the carbenoid a-haloalkyllithium compounds or carbene-lithium halide complexes.158 In the case of the trichloromethyllithium to dichlorocarbene conversion, the equilibrium lies heavily to the side of trichloromethyllithium at — 100°C.159 The addition reaction with alkenes seems to involve dichlorocarbene, however, since the pattern of reactivity toward different alkenes is identical to that observed for the free carbene in the gas phase.160... 

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

Carbene protonation has been amply demonstrated by product studies, time-resolved spectroscopy, and kinetic measurements. The ability of singlet carbenes to accept a proton is not adequately described by the traditional scale of carbene philicities, which is based on addition reactions with alkenes. In particular, aryl- and diarylcarbenes excel as proton acceptors but would traditionally be classified as electrophiles. 

Compared with the classical procedures, which employ chloroform and dry potassium /ert-butoxide, Makosza s method is several magnitudes superior, in spite of the normally recognized requirements that the dichlorocarbene should be produced under totally anhydrous conditions. Several early reports of the reactions of dichlorocarbene, generated by Makosza s procedure, led to suggestions that the activity of the carbene was considerably greater than that of the classically produced carbenes. This assumption was based on the overall higher yields of dichlorocyclopropanes derived from the reaction with alkenes, and upon the observation that weakly activated alkenes reacted with Makosza carbenes, but not with the classically produced carbenes. A consideration of the mechanism of formation of the carbenes under phase-transfer catalytic conditions exposes the fallacies in the assumptions. 

By analogy with the mechanism proposed for the reaction with alkenes, C—H insertion product formation can be explained in terms of a H abstraction-recombination process of triplet arylcarbenes. The observations that ground-state singlet carbenes, for example, chlorophenylcarbene (67), produce only O—H insertion... 

Section 14.13 Carbenes are species that contain a divalent carbon that is, a carbon with only two bonds. One of the characteristic reactions of carbenes is with alkenes to give cyclopropane derivatives. 

The reactions of these iron carbene reagents with alkenes to give cyclopropanes are stereospecific. They also exhibit high syn stereoselectivity in many cases. Optically active derivatives have been reported that have chiral ligands on iron or chiral alkoxy groups on the prospective caibene center and which have been resolved with the iron itself as a chiral center. Resulting from this work have been some highly enantioselective cyclopropanations. 

Two resonance-contributing structures (3a and 3b), in the formalism of ylide structures, can be used to describe metal carbene intermediates. The highly electrophilic character of those derived from Cu and Rh catalysts suggests that the contribution from the metal-stabilized carbocation 3b is important in the overall evaluation of the reactivities and selectivities of these metal carbene intermediates. Emphasis on the metal carbene structure 3a has led to the subsequently discounted proposal that cyclopropane formation from reactions with alkenes occurs through the intervention of a metallocyclobutane intermediate [18]. The metal-stabilized carbocation structure 3b is consistent with the cyclopropanation mechanism in which LnM dissociates from the carbene as bond-formation occurs between the carbene and the reacting alkene (Eq. 5.4) [7,15]. 

Although carbene type insertions have been discussed as displacements and their reactions with alkenes could be treated as cycloadditions, both could just as easily fit into this section. Typical of the latter is the addition (145). Apart from a primary syn stereospecificity, there is a... 

Despite the expected enhanced reactivity of trifluoromethyl-substituted carbenes as electrophilic species, yields in their [2+ l]-cycloaddition reactions with alkenes are highly dependent on the nature of the second substituent of the carbene. Furthermore, the method of carbene generation has a large influence on carbene reactivity. 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

Several heteroatom nucleophiles, for example, amines, alcohols, thiols, carboxylates, and dialkylphosphines, undergo Michael addition reactions with alkene- and alkyne-substituted carbene complexes. Reaction of alkyne-substituted chromium carbenes with urea affords products derived from Michael... 

---