Styrene carbene addition

Finally, addition reactions of the isolable phosphasilylcarbenes (13) to such electron-poor substrates as methyl acrylate, C4F9CH=CH2, and styrene afford cyclopropanes. The additions of 13a to (E)- or (Z)-p-deuteriostyrene are stereospecific, and the competitive additions of 13b to ring-substituted styrenes exhibit nucleophilic selectivity, consistent with singlet, nucleophilic carbene addition (Fig. 7.8). ... 

Overall, the transformations are equivalent to carbene additions to the styrenes. However, a carbene mechanism can be ruled out since the only alkenes which are successful are those carrying anion-stabilising groups. 

Several instructive experiments describing singlet carbene addition reactions to styrene and styrene derivatives (a-, p-methyl styrene and 4-methyl styrene) were submitted by... 

One can also characterize the reactivity and selectivity of carbenes using the venerable Hammett ap linear free energy methodology. [5,39] Additions of CXY could be examined with a series of ring-substituted styrenes, ArCH=CH2. Relative reactivities could be obtained, with styrene itself as the standard olefin, and then log could be plotted against the appropriate a constant for each substituted styrene substrate. The slope of the resulting correlation is the Hammett p value for the carbene addition. 

Hammett p values have been similarly determined for many other carbene additions to styrenes. [40] Thus, p = -0.62 (vs a+) for CCI2 at 80 °C. [41] CCI2 is also electrophilic to styrenes. Note, however, that mQ fmQQ = 1.48/1.00 = 1.48,... 

What about a nucleophilic carbene, for which negative charge should build up on the olefinic carbon atoms during the carbene addition cf. 5 With ArCH=CH2 substrates, electron-withdrawing aryl substituents would stabilize such a transition state and the p value should be positive. There are several examples of this phenomenon. For example, cycloheptatrienylidene, 10, adds to / -substituted styrenes with p = -t-1.02 - 1.05 (vs. a) consistent with a nucleophilic selectivity that seems to implicate the aromatic resonance form 10a as an important feature of the carbene. [45] It is satisfying to compare this result with p = -0.76 (vs. a) or -0.46 (vs. a" ") for additions to styrenes of cyclopentadienylidene, 11, where contributions of the cyclopentadienide form (11a) would render the carbene electrophilic. [46] However, these conclusions are too facile. There is reason to believe that the chemistry attributed to 10 might in fact be due to its allenic isomer 12. [47] And the electronic structure of 11 is also more complicated than the simple depiction above. [48]... 

Carbene 42 adds to a variety of electron deficient olefins, and even to styrene " with -substituted styrenes, the nucleophilicity of these additions can be observed (for 42 with cyclohexyl replacing isopropyl). [117] Moreover, additions of 42 to Z- and ii-2-deuteriostyrene are stereospecific with respect to the styrene substituents, as anticipated for singlet carbene additions. [117] Thus, the isolable phosphinosilylcarbenes are nucleophiles, but retain the essential reactivity expected of such carbenes. 

Factors affecting the stereoselectivity of carbene addition to styrene and phenyl vinyl ether have also been discussed. ... 

The reaction of carbenes with olefins to form cyclopropyl derivatives has been used to modify elastomers. Pinazzi and Levesque and Berentsvich et al. found that carbene addition had a significant influence on the properties of polydienes. Thermogravimetric analysis (TGA), flammability and oil resistance in NR and dichlorocarbene modified styrene butadiene rubber (DCSBR) blends were investigated by thermogravimetrie analysis as a funetion of different composition. The TGA plots confirmed the better thermal stability and flame resistance of DCSBR as well as its blends with NR. The amount of DCSBR in the blend significantly affected the properties of blends. 

The obtained stereoselectivities are in the same range as those of other ruthenium- or rhodium-based catalysts. As the carbene addition occurs with trans (exo) diastereoselectivity, the more thermodynamically stable trans (exo) isomer is preferentially formed. It was observed that two steric effects have influence on the stereoselectivity the steric bulk of the 4-position of the styrene derivatives and the steric bulk of the diazoacetate used. In both cases the cisitrans ratio decreases with the... 

Aryldiazomethane can also be used for iron porphyrin-catalyzed alkene cyclopropanation [55]. For example, the treatment of p-tolyldiazomethane with styrene in the presence of [Fe(TTP)] afforded the corresponding arylcyclopropapane in 79% yield with a high transicis ratio of 14 1 (eq. 1 in Scheme 11). Interestingly, when bulkier mesityldiazomethane was used as carbene source, ds-selectivity was observed (cisitrans = 2.0 1). Additionally, mesityldiazomethane was found to react with frans-p-styrene, the latter was found not to react with EDA or trimethyl-silyldiazomethane under the similar reaction conditions, to give l-mesityl-2-methyl-3-phenylcyclopropane in 35% yield. Trimethylsilyldiazomethane is also an active carbene source for [Fe(TTP)]-catalyzed cyclopropanation of styrene, affording l-phenyl-2-trimethylsilylcyclopropane in 89% yield with transicis ratio of 10 1 (eq. 2 in Scheme 11). 

The common by-products obtained in the transition-metal catalyzed reactions are the formal carbene dimers, diethyl maleate and diethyl fumarate. In accordance with the assumption that they owe their formation to the competition of olefin and excess diazo ester for an intermediate metal carbene, they can be widely suppressed by keeping the actual concentration of diazo compound as low as possible. Usually, one attempts to verify this condition by slow addition of the diazo compound to an excess (usually five- to tenfold) of olefin. This means that the addition rate will be crucial for the yields of cyclopropanes and carbene dimers. For example, Rh6(CO)16-catalyzed cyclopropanation of -butyl vinyl ether with ethyl diazoacetate proceeds in 69% yield when EDA is added during 30 minutes, but it increases to 87 % for a 6 h period. For styrene, the same differences were observed 65). 

By contrast, much of the work performed using ruthenium-based catalysts has employed well-defined complexes. These have mostly been studied in the ATRP of MMA, and include complexes (158)-(165).400-405 Recent studies with (158) have shown the importance of amine additives which afford faster, more controlled polymerization.406 A fast polymerization has also been reported with a dimethylaminoindenyl analog of (161).407 The Grubbs-type metathesis initiator (165) polymerizes MMA without the need for an organic initiator, and may therefore be used to prepare block copolymers of MMA and 1,5-cyclooctadiene.405 Hydrogenation of this product yields PE-b-PMMA. N-heterocyclic carbene analogs of (164) have also been used to catalyze the free radical polymerization of both MMA and styrene.408... 

The additional reactive intermediate responsible for the curvature was postulated17,33 to be a CAC.30 The mechanism of Scheme 2 was proposed, in which carbene 10a was in equilibrium with the CAC. Thus, styrenes 11a and 12a can be formed by two pathways from the free carbene (kj) and from the CAC (k-). A steady-state kinetic analysis of Scheme 2 affords Eq. 11, which predicts that a correlation of rearr/addn with l/[alkene] should be linear the behavior actually observed by Tomioka and Liu.17,33 The CAC mechanism also accounts for the observation that the lla/12a product ratio depends upon the identity and concentration of the added alkene both k[ and k2, which define the Y-intercept of Eq. 11, depend on the added alkene. The dependence has been observed,19,33-37 albeit with only small variations in the Y-intercepts. 

Table 9. Stereochemistry of addition of diphenyl-carbene to styrene as a function of added hexa-flurobenzene...

Prompted by W. M. Jones work on the formation of heptafulvene from phenyl-carbene in the gas phase,which implied a seven-membered ring intermediate, and by the implications of the possible reversibility of the process, Baron et al. rediscovered Vander Stouw s cryptic work and showed that all three possible tolyl-carbenes gave benzocyclobutene and styrene, albeit in different ratio from the ortho isomer than from the meta or para species.Baron et al. ° proposed a mechanism in which a methylcycloheptatrienylidene (CH3-7I) was the active seven-membered species, and which additionally feamred the intermediacy of methylbicyclo[4.1.0]-heptatrienes 73 and 73 (Scheme 7.35). 

The bimolecular rate constant for the reaction of DPC with butadiene is determined to be 6.5 X 10 M s . Isoprene can be employed as a selective trap for triplet carbenes. Styrene is also shown to be an efficient trap for triplet carbene. (E)-p-Deutero-a-methylstyrene (89) is a very convenient reagent to diagnose the mult-plicity of the reacting carbene because it reacts with both singlet and triplet carbenes with different stereochemical outcomes. The stereochemistry of the adduct cyclopropane (90) can be easily judged by NMR (Scheme 9.28). For example, BA (22) reacts with styrene with total loss of stereochemistry, while in the reaction with dimethoxy FL (23a), the expected cyclopropane is obtained with complete retention of stereochemistry. The rate constants for the additions are (1.2 0.2) x 10 and... 

Synthetic uses came from two directions, neither based on fundamentals but both moving the field from uncertainty to the effective control of parameters. Electrophilic metal carbenes were recognized from their ability to undergo addition to electron-rich aUcenes such as vinyl ethers or styrene (Eq. 4), but not electron-poor... 

Catalytic methods are suitable for nitrene transfer," and many of those found to be effective for carbene transfer are also effective for these reactions. However, 5- to 10-times more catalyst is commonly required to take these reactions to completion, and catalysts that are sluggish in metal carbene reactions are unreactive in nitrene transfer reactions. An exception is the copper(ll) complex of a 1,4,7-triaza-cyclononane for which aziridination of styrene occurred in high yield, even with 0.5 mol% of catalyst. Both addition and insertion reactions have been developed. 

Dioximato-cobalt(II) catalysts are unusual in their ability to catalyze cyclopropanation reactions that occur with conjugated olefins (e.g., styrene, 1,3-butadiene, and 1-phenyl-1,3-butadiene) and, also, certain a, 3-unsaturated esters (e.g., methyl a-phenylacrylate, Eq. 5.13), but not with simple olefins and vinyl ethers. In this regard they do not behave like metal carbenes formed with Cu or Rh catalysts that are characteristically electrophilic in their reactions towards alkenes (vinyl ethers > dienes > simple olefins a,p-unsaturated esters) [7], and this divergence has not been adequately explained. However, despite their ability to attain high enantioselectivities in cyclopropanation reactions with ethyl diazoacetate and other diazo esters, no additional details concerning these Co(II) catalysts have been published since the initial reports by Nakamura and Otsuka. 

Non-stereospecific cyclopropanation reactions of the diazafluorenylidene (10), generated by photolysis of the diazo compound, indicated a triplet carbene.18 Competition experiments suggested a singlet-triplet equilibrium at room temperature and a Hammett study of additions to substituted styrenes indicated that the carbene reacts as an electrophile (p = —0.65). 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

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