Allyl isocyanides

The chemistry of the isocyanides began when, in 1859 Lieke formed allyl isocyanide from allyl iodide and silver cyanide, and when, in 1866 Meyer ° produced in the same way 1-isocyano-l-desoxy-glucose. In 1867, Gautier used this procedure to prepare alkylisocyanides, and Hofmann introduced the formation of isocyanides from primary amines, chloroform, and potassium hydro-xyde. Gautier also tried to prepare an isocyanide by dehydrating an amine formiate via its formylamine using phosphorus pentoxide, but this process produced no isocyanide. Gautier had not yet realized that acidic media destroyed the isocyanides. 

The reaction of an isocyanide containing an acidic hydrogen with copper(I) oxide and an activated olefin or a ketone [Eq. (123)] provides a synthesis of either pyrrolines or oxazolines, respectively (251,252). Addition of allyl bromide gave the coupling product with the allyl carbanion derived from allyl isocyanide. Oxazolines are obtained in yields as high as 957o> not pyrrolines because of competing dimerization... 

Addition of an electrophile (see Electrophile) to metal-bound cyanides will often form an isocyanide ligand (see Electrophile), -CsN-R. For example, the compound [Fe P(OMe)3 (NO)2(j7 -C3H4R)], which is a source of the allyl cation ( -C3H4R)+, reacts with trans-[Mn(CN)(CO)(dppm)2] to alkylate the cyanide, giving an allyl isocyanide ligand (equation 8). The tungsten alkyne... 

The first isocyanide, allyl isocyanide, was prepared by Lieke in 1859 from allyl iodide and silver cyanide it tainted the air in the room for days and provoked continuing complaints in the neighborhood about the vile odor . At that time it was, however, not noticed that a new distinct class of chemical compounds had been discovered. 

Preparation of cyanides.1 An attractive route to cyanides involves as the first step dehydration of alkylformamides with phosgene-tricthylamine (1,857) or phosphoryl chloride-diisopropylamine (13,249) to form isocyanidcs followed by isocyanidc-cyanide rearrangement. This rearrangement traditionally was conducted in the gas phase, but proceeds in almost quantitative yield when carried out by flash pyrolysis at 600°. This route to cyanides is attractive because allyl isocyanides rearrange without allylic rearrangement. Moreover, optically active carboxylic acids can be obtained from optically active amines without raccmization. 

The overtone excitation tends to excite molecules in local modes (chapter 4). Thus, the excitation would appear to be highly localized. Furthermore, by the appropriate choice of laser wavelength, it is possible to excite different parts of a molecule. For instance, in the isomerization of allyl isocyanide to allyl cyanide. 

Figure 5.8 Overtone spectrum of allyl isocyanide in the region of the sixth overtone. The three peaks from low to high energies correspond to C—H overtones at the following positions the methylenic CH2, nonterminal olefinic CH, and the terminal olefinic CH groups, respectively. Adapted with permission from Segall and Zare (1988).

Thus, a plot of (k pp) versus the gas pressure [M] yields a straight line with intercept ( a[/iv]) > and slope k /(k [hv]k(E)). Since the photoactivation rate, k [hv] is known from the intercept, the slope permits the determination of the ratio kJk(E). An example of such a Stem-Volmer plot is shown in Figure 5.17 for the isomerization of the previously mentioned allyl isocyanide reaction, C3H5NC —C3H5CN, in which k [hv] k pp is plotted. This results in an intercept of 1.0 and a slope of kJk(E). The quantity of interest, k(E), can be extracted if we know the deactivation rate, k. This is generally taken to be equal to the gas kinetic collision rate constant (strong collision assumption), which is typically about 10 cm3/(molec sec). 

Figure 5.17 Stern-Volmer plots for the isomerization reaction of allyl isocyanide at several laser energies. The slope [/cj//c(E)) is inversely proportional to the reaction rate constant. Adapted with permission from Reddy and Berry (1979a).

The copper-catalysed reaction of allyl isocyanide with ketones and aldehydes gives 4-vinyl-2-oxazoline derivatives (465) in good yields. With activated olefins, as in a -unsaturated nitriles and esters, 5-vinyl-l-pyrroline derivatives are obtained (466). A copper complex is proposed as intermediate. 

Equimolar mixtures of allyl isocyanide and benzaldehyde in toluene heated 12 hrs. at 80° under Ng in the presence of a catalytic amount of CU2O -> product. Y 95%. F. e., also J -pyrrolines from ethylene derivs. and isonitriles, s. T. Sae-gusa, I. Murase, and Y. Ito, Bull. Chem. Soc. Japan 45, 830 (1972). 

The experiments that seem to show the most promise in studying unimolecular dynamics are the ones in which a molecule is vibration-ally excited by one photon absorption. In very elegant experiments Berry and coworkers have studied the intramolecular and unimolecular dynamics of benzene,methyl isocyanide,and allyl isocyanide containing 4-7 quanta of C-H stretch excitation. For the isomerization of allyl isocyanide nonstatistical state-selected effects were observed. These one photon absorption experiments suggest that state-selected behavior may be prevalent in many unimolecular reactions. However, it is apparent that the sensitivity of the experiments should be improved so that excitation and unimolecular reaction occur in a collision-free environment, thus ensuring the elimination of intermolecular effects that may wash out some of the state-selective characteristics. 

Though unimolecular fall-off curves are important historically, their general insensitivity to intramolecular dynamics suggests that other types of experiments are more useful for detecting intrinsic non-RRKM behavior. Such an experiment is the one by Reddy and Berry on allyl isocyanide isomerization ... 

Here one-photon absorption is used to prepare allyl isocyanide with 5, 6, and 7 quanta in the terminal olefinic (H2C=), nonterminal ole-finic (=CH-), or methylene (-CH2 ) CH stretches. Because of their different frequencies each of these CH stretches can be excited selectively with very little spectral impurity. For a particular overtone the methylene CH stretch contains the least energy and the terminal olefinic CH stretch the most. Unimolecular rate constants were measured by Stern-Vollmer plots, and were found not to agree with RRKM predictions. Though the terminal olefinic CH stretch is most excited for a particular overtone, excitation at this site gives a smaller unimolecular rate constant than does excitation at the nonterminal olefinic CH stretch. This result unambiguously characterizes allyl isocyanide isomerization as intrinsically non-RRKM. 

The experimental evidence for the presence of long-lived vibrational states in highly excited polyatomic molecules is fragmentary. One of the more definitive results is that of Reddy and Berry for allyl isocyanide.Another example is SF0 containing about 27 kcal/mol of vibrational energy, for which intramolecular vibrational energy redistribution is incomplete on a microsecond time scale.Photophysical measurements of the nonradiative lifetimes of molecules excited to their first electronic state have shown vibrational state selective effects on a 10 - 10 s time scale. 

Intramolecular relaxation rates have also been determined from the linewidths of CH stretch overtones.The linewidths are about 100 cm which corresponds to a lifetime for the initially prepared states of approximately 5 x 10 s. What is not known is whether this lifetime results from dephasing or true intramolecular vibrational energy relaxation. In allyl isocyanide,for which... 

K. V. Reddy and M. J. Berry, A nonstatistical unimolecular chemical reaction Isomerization of state-selected allyl isocyanide, Chem. Phys. Lett. 66 223 (1979). 

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