Silyl cyanides, structure

The structure of silyl cyanides was a matter of dispute for many years. Emeleus, Maddock and Reid (1941) prepared silyl cyanide by the reaction of SiH3I with solid AgCN. The normal cyanide structure was written for this product without comment. However, on the basis of analogy with carbon chemistry, MacDiarmid (1956) suggested that the compound should be formulated as an isocyanide, H3SiNC. The problem Was finally resolved by careful infrared and microwave spectral studies which showed that the normal cyanide, H3SiCN, is correct (Linton and Nixon, 1958 a Mutter and Bracken, 1960 Sheridan and Turner, 1960). 

Experiments described by Corey constitute a noteworthy example of double asymmetric induction where neither participant in the reaction is chiral [95] As illustrated in Figure 4.18 two different catalysts are necessary to achieve the best results. Control experiments indicated that the nucleophile is probably free cyanide, introduced by hydrolysis of the trimethylsilylcyanide by adventitious water, and continuously regenerated by silylation of the alkoxide product. Note that the 82.5% enantioselectivity in the presence of the magnesium complex shown in Figure 4.18a is improved to 97% upon addition of the bisoxazoline illustrated Figure 4.18b as a cocatalyst. Note also that the bisoxazoline 4.18b alone affords almost no enantioselectivity, and that the enantioselectivity is much less when the enantiomer of the bisoxazoline (Figure 4.18b) when used as the cocatalyst. Thus 4.18a and 4.18b constitute a matched pair of co-catalysts and 4.18a and ent-A. %h are a mismatched pair (see Chapter 1 for definitions). The proposed transition structure... 

Ketimines and aldimines proved to be good substrates [73]. Based on the structure modification, nuclear magnetic resonance (NMR) study, kinetic study, and computational study, they proposed a double hydrogen bond structure between the acidic N-H protons and the imine lone pair to activate the electrophile toward the attack by the cyanide ion (Figure 2.15) [74]. The catalyst system is applicable to the Mannich-type reaction of N-Boc-aldimine with ketene silyl acetals to give corresponding P-amino esters in 86-98% ee (Scheme 2.25) [75]. Hydrophospho-nylation of aldimines with bis(2-nitrobenzyl)phosphite by means of (4d) furnished a-amino phosphonates in good enantioselectivity (Scheme 2.26) [76]. 

Nucleophilic addition to this complex salt occurs mostly at the 5-position. Addition of cyanides affords also a minor amount of an T), n -structure by addition to the 2-position. The tricarbonyl(Ti -3-methoxycyclohexa-2,4-dien-l-yl)iron cation reacts with a variety of nucleophiles, comprising organolithium reagents, ketones, silyl enol ethers, and allylsilanes, to give after oxidative demetalation 5-substituted cyclohex-2-enones (Scheme 4-175). 

---