Hydrosilylation of Enamines

Discuss the main advantages of the organocatalytic approach to amine syn thesis, the environmental issues, and the current limitations of the hydro silylation method. 

Discuss the structure of the organocatalysts for hydrosilylation of imines with ClaSiH and identify the key structural features. Suggest other functional groups that can be predicted to act in a similar way. 

1 For a general overview of the reduction of imines, see the following (a) Morrison, J.D. (1983) Asymmetric Synthesis, vol. 2, Academic, New York (b) Noyori, R. (1994) Asymmetric Catalysis in Organie Synthesis, John WUey Sons, New York (c) Ojima, I. (2000) Catalytic Asymmetric Synthesis, 2nd edn, John WUey Sons, New York  

4 For an overview of transfer hydrogenation of imines, see Samec. J.S.M., Backvall, 

7 Activation of CljSiH (a) Benkeser, R.A. and Snyder, D., (1982) J. Organomet. Chem., 225,107 (b) Kobayashi, S., Yasuda, M., and Hachiya, 1. (1996) Chem. Lett., 407 (c) Iwasaki, F., Onomura, O., Mishima, K., Maki, T., and Matsumura, Y. (1999) Tetrahedron Lett., 40, 75O7 (d) Brook, M.A. (2000) Silicon in Organic, Organometallic and Polymeric Chemistry, John Wiley Sons, New York, pp. 

---

Scheme 2.114 DKR hydrosilylation of enamines catalysed by L-valine-derived formamide.

In 2008, Kocovsky et al. reported a novel methodology based on the organocatalytic asymmetric hydrosilylation of enamines that allowed a direct access to a range of and p -amino acid derivatives for some of which other methods proved less satisfactory. This method relied on the fast equilibration... 

The hydrosilylation of unsaturated carbon-rhodium-catalyzed silylcarbocyclizations. In the presence of Rli4(CO)i2 and triethoxysilane, a rigid triyne backbone can undergo a silylcarbotricyclization cascade reaction to yield [5,6,5]-tricycles (eq 16). Similar to the results observed by Sieburth for the hydrosilylation of enamines, the alkoxysilane functionality provides significant rate enhancement in comparison to silylcarbocyclizations using alkyl- and arylsilane reagents. The incorporation of carbonyl functionality as terminal electrophiles into these cyclizations has also been successful. Rhodium-catalyzed carbonylative silylcarbocyclizations proceed in the presence of carbon monoxide (10 atm) to incorporate a carbonyl unit, usually as the aldehyde. Both of these tandem ad-dition/cyclization strategies produce functionalized carbocycles with simultaneous incorporation of sUyl functionality as aryl- and vinylsilanes. These alkenylsilanes can then be exploited for further synthetic manipulations as discussed above. "" ... 

The catalytic hydrosilylation of 1-aza- and 1,4-diaza-1,3-dienes R N=CH-CH=CHR [R = Me,Ph R = McjCH, McjC, (Me2CH)2CH] and R N=CH-CH=NR were performed with rhodium(I) complexes. Under mild conditions N-silylated enamines, such as R N(SiR )CH=CHCH2R or R NCH=CHN(SiR )RS were formed as the main products. ... 

In another context, an L-valine-derived formamide was applied by Kocov-sky s group as an organocatalyst to the asymmetric hydrosilylation of enam-ines, allowing a direct access to a range of jS-amino add derivatives. This method relied on the fast equilibration between the enamine and the imine... 

Cobalt carbonyl-catalyzed hydrosilylation of aromatic nitriles with HSiMes proceeds smoothly at 60 °C and ambient pressure of carbon monoxide in toluene to give N, Af-bis-TMS-amines 191 in moderate to excellent yields (equation 78) . Reactions of aliphatic nitriles require 100 °C, except for cyclopropanenitrile, to give 191 in 87-100% yields. The reaction of butanenitrile gives a mixture of 191 (R = n-Bu) (44%) and N, Af-bis-TMS-enanime CH3CH2CH=CHN(SiMe3)2 (20%). The reactions of a, yS-unsaturated nitriles bearing substituent(s) at the a- and/or -position(s) give N, Af-bis-TMS-enamines 192 (Z/E mixture) as the major products accompanied by iV, Af-bis-TMS-allylic amines 193 and/or saturated N, Af-bis-TMS-amines 194 (equation 19). ... 

Transition metal-catalyzed hydrosilylation, an established route for transformations of unsaturated carbon substrates, is also well known for reactions of nitriles,183 imines,184 and oximes.185 Work in this area includes the Co2(CO)8-catalyzed addition of trialkylsilanes to aromatic, aliphatic, and a,/3-unsaturated nitriles, giving /V,/V-bis(silyl)amines and/or -enamines in fair to good yields [Eqs. (73) and (74)].186... 

Finally, enamines and silanes in the presence of a catalyst such as PdCl2 afford aminosilanes (equation 192)216. This reaction is an analogy to hydrosilylation (vide supra). 

---