2- Butene carbene addition

The differences in deuterium and carbon isotope effects indicate the asymmetric transition state with more advanced carbon-carbon bond formation to the terminal Cl atom. The difference between deuterium isotope effects for HCI-S and Htmns hydrogens probably originates from experimental uncertainty. Theoretical calculations (B3LYP/6-31G, B3LYP/6-311+G ) for carbene addition to 1-butene were carried out for two modes with carbene approaching carbon atom Cl or C2. The best agreement for experimental isotope effects is for carbene attack on terminal carbon atom and the carbene-alkene separation in the transition state of 2.5 A. 

The dilution technique makes use of the different concentration dependence of the S—T conversion (a) and the carbene addition to the olefin (/S). The decay of the metastable singlet state is unimolecular, while the stereospecific addition rate is first-order in olefin concentration 78). The dilution technique has not yielded a common ratio in the experiments with cis- or iraws-butene and 2c (see Table 11). Extrapolation of the data to infinite dilution gives a product ratio of 0.16, suggesting that... 

Finally, what of my early idea of reacting a series of arylcarbenes (X-CgFl4CH) with a fixed pair of olefins, and correlating log (kjk ) vs. Gx (see Section 1.1) I actually carried out such experiments during my doctoral research. For example with carbene substituents X = p-MeO, p-Me, FI, p-C, and m-C, and the olefin pair isobutene/iraws-butene, the log ( 1) data is correlated by a constants to give p = +0.3. [7] We can interpret this result as the difference of the p values for carbene addition to isobutene vs. traws-butene, where the p for isobutene is more positive than that for traws-butene. It is difficult to interpret this data further. A clearer picture emerges with the use of absolute rate constants for carbene/aUcene addition reactions. Those experiments will be described below in Section 2.2. 

Xylylene dibromide and c/j-l,4-dichloro-2-butene react similarly with ethanedithiol <64ZC302>, while alkylation of ethanedithiol with tetrabromides under high dilution conditions afford bis(dithiocin) (185) (Equation (57)) and propellane (186) (Equation (58)) (also preparable by carbene addition to (185)) in 57% and 15% yields, respectively <85M229>. 

In laser-impulse experiments with chlorophenyldiazirine the carbene could be observed by UV spectroscopy. On addition of defined amounts of alkene the rate of cyclopropanation was measured directly. The rate constants with various alkenes were (lO moF s ) 1-hexene, 1.3 ( )-2-pentene, 34 2-methyl-2-butene, 77 2,3-dimethyl-2-butene, 130 (80JA7576>. 

Woodworth, based on the common reaction of addition of carbenes to double bonds to form cyclopropane derivatives (15-50). If the singlet species adds to cis-2-butene, the resulting cyclopropane should be the cis isomer since the movements of the two pairs of... 

Carbene itself is extremely reactive and gives many side reactions, especially insertion reactions (12-19), which greatly reduce yields. When it is desired to add CH2 for preparative purposes, free carbene is not used, but the Simmons-Smith procedure (p. 1088) or some other method that does not involve free carbenes is employed instead. Halocarbenes are less active than carbenes, and this reaction proceeds quite well, since insertion reactions do not interfere.The absolute rate constant for addition of selected alkoxychlorocarbene to butenes has been measured to range from 330 to 1 x 10 A few of the many ways in... 

No products resulting from C—H insertion of the carbene were found. Also, ethylidene failed to undergo addition to 2-butene, indicating this species to be considerably more selective than methylene.<23)... 

A singlet carbene was proposed to account for this stereoselectivity. Attempts to produce triplet carbene by collisional deactivation with octafluorocyclo-butane were unsuccessful and stereospecific addition to olefin still occurred. However, nonstereospecific addition to olefins and larger amounts of olefinic (insertion) products result from irradiation of the phenyldiazomethane in a frozen m-butene matrix at — 196°C ... 

The addition of the methylmercuriacetylcarbene to cis- and //ms-butene was found to be completely stereospecific, suggesting that this carbene has a singlet ground state (with the heavy mercury atom, relaxation to the ground state should be rapid). 

Additional evidence for the contention that metathesis carbenes are nucleophilic was offered by Gassman in an interesting series of trapping experiments utilizing Michael acceptors as carbene traps (15, 17). Thus, an ethylidene carbene generated from 2-butene was trapped by ethyl acrylate to yield the expected ethylcyclopropyl ester, although yields were quite low. 

In pentane, the distribution of 1,3-insertion product 25 to 1,2-Me shift product 26 is 91 9. Upon addition of 2-methyl-1-butene, the yield of 25 smoothly decreases (to 19% with 4 M alkene), but the yield of 26 is unaffected 1 Moreover, correlation of addn/l,3-CH insertion (to 25) for 18 is nicely linear. The simplest interpretation is that 25 comes directly from carbene 18, whereas the 1,2-Me shift product 26 comes from the excited diazirine.27 Interestingly, thermolysis of 24 at 79°C produces 90% of 25 and 10% of 26, but now the yields of both products smoothly decrease in the presence of an alkene. In thermolysis the (electronically) excited diazirine is unavailable, both 25 and 26 stem from the carbene, and their formation is suppressed by the alkene s interception of the carbene. A pyridine ylide kinetic study gave the 1,3-CH insertion rate constant (18 - 25) as 9.3 x 10s s"1.27-47... 

Dimethyl-4-silacyclohexadienylidene (lv) is of interest as a potential source of silaxylene 24, however, all attempts to convert the carbene into an aromatic compound failed.107 The only isolated product from gas phase reactions is the dimer 25. In solution, carbene lv was found to add stereospecifically to cis-2-butene. With butadiene as trapping reagent both the products of the 1,2- and 1,4-addition 26 and 27, respectively, are observed (Scheme 21).107 In addition, silacyclopentene 28 is formed, which is the trapping product of cyclo-... 

The addition of carbenes is also a stereospecific reaction, each geometrical isomer forms the cis addition product. Thus carbene generated from the photolysis of diazomethane adds in a cis manner to both cis and trans butene-2. 

The two nonbonded electrons of a carbene can be either paired or unpaired. If they are paired, the species is spectrally a singlet, while, as we have seen (p. 193), two unpaired electrons appear as a triplet. An ingenious method of distinguishing between the two possibilities was developed by Skell,202 based on the common reaction of addition of carbenes to double bonds to form cyclopropane derivatives (5-50), If the singlet species adds to cis-2-butene, the resulting cyclopropane should be the cis isomer since the movements of... 

Table 5.10 Stereochemistry of Addition of Carbenes to cis- and trans-2- Butene... 

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. 

To draw each product, add the carbene carbon from either side of the alkene, and keep all substituents in their original orientations. The cis methyl groups in c/s-2-butene become cis substituents in the cyclopropane. Addition from either side of the alkene yields the same compound—an achiral meso compound that contains two stereogenic centers. 

The addition of dichlorocarbene to the double bond of cw-2-butene goes with 100% stereospecificity, that is, the only product is cw-l,2-dimethyl-3,3-dichlo-rocyclopropane. The addition of dichlorocarbene to tra i-2-butene gives only the corresponding trans isomer. The stereospecificity of the reaction has been interpreted to mean that dichlorocarbene is a singlet and that both ends of the double bond react simultaneously, or nearly so, with the carbene. 

The photolysis and pyrolysis of difluorodiazirine has been shown to involve difluoromethylene and stereospecific addition to 2-butene indicates that it is probably in a singlet state. In argon or nitrogen matrices, Cp2 has been detected by spectroscopic measurements . Products arising from the thermolysis or photolysis of fluoromethoxydiazirine, cyanofluorodiazirine, difluoroaminodiazirine and chlorofluorodiazirine have been reported the main feature of these reactions is that the intermediate carbene in all cases can be trapped by olefin reagents, whereas fluorocarbenes from other sources are quite unreactive. 

The view that electronic states of different multiplicity need not be considered cannot easily be ruled out, since both deactivation of vibrationally excited carbenes and intersystem crossing between singlet and triplet states are brought about by collision with other molecules. The difficulty is not restricted to reactions in the gas phase in solution, collisional deactivation and collision-induced intersystem crossing can still be expected to compete with collisions leading to chemical reaction. However, the parallelism between the variation in stereospecificity in the gas-phase addition of methylene to the 2-butenes with the pressure of inert gas (Frey, 1959, I960 Anet et al., 1960 Bader and Generosa,... 

The same acidic chloroaluminate ionic liquids have been used as solvent for tungsten aryl oxide complexes for the metathesis of alkenes [24]. Slightly acidic chloroaluminates also dissolve the [Cl2W=NPh(PMe3)3] complex which catalyze ethene oligomerization without the addition of co-catalysts [25]. In a similar way, Ni-catalyzed 1-butene dimerization into linear octenes was carried out in acidic chloroaluminates buffered with small amount of weak bases [26]. Neutral chloroaluminates (l-ethyl-3-methylimidazolium chloride/AlCl3 = 1) were employed to immobilize ruthenium carbene complexes for biphasic ADMET (acyclic diene metathesis) polymerization of an acyclic diene ester [27]. 

---