Stereospecific Synthesis of α-Amino Allylsilane Derivatives through a [3,3]-Allyl Cyanate Rearrangement. Mild Formation of Functionalized Disiloxanes

Abstract

An efficient asymmetric synthesis of α-amino allylsilane derivatives is reported. The strategy is based on a [3,3]-allyl cyanate sigmatropic rearrangement from enantioenriched γ-hydroxy alkenylsilyl compounds. The isocyanate intermediate can be trapped by several nucleophiles, opening the way for the preparation of unknown chiral functionalized compounds such as the α-ureido allylsilanes as well as carbamate derivatives. A computational study was conducted to rationalize the complete 1,3-chirality transfer of this kind of rearrangement. Moreover, starting from products bearing a phenyldimethyl silyl substituent, the α-amino silane derivatives or the corresponding disiloxanes can be obtained under hydrogenation conditions in an exclusive way according to the used catalyst.

[3,3]‐Sigmatropic Rearrangement of Cyclopropenylcarbinyl Cyanates: Access to Alkylidene(aminocyclopropane) Derivatives

“…It is also noteworthy that ureas 6 and 6′ possess opposite configuration at C2 (Scheme ). Although the [3,3]‐sigmatropic rearrangement of allylic cyanates is concerted and proceeds with a high level of chirality transfer, a highly asynchronous process is involved in which dissociation of the allylic carbon–oxygen bond is more advanced in the transition state than formation of the carbon–nitrogen bond . Due to the peculiar structure and bonding properties of the cyclopropene ring, cyclopropenylcarbinols should rather be compared to propargylic rather than to allylic alcohols and it was of interest to check whether the [3,3]‐sigmatropic rearrangement of cyclopropenylcarbinyl cyanates would proceed with chirality transfer.…”

“…The two diastereomers 6 and 6' were separated by preparative thin-layer chromatography on silica gel and isolated in 51 and6 %y ield, respectively.T he configuration of the exocyclic alkene in alkylidene cyclopropanes 6 and 6' was assigned as E and Z,r espectively,b yN MR spectroscopy (NOESY) on the basis of the observed nuclear Overhauser effects (NOE). In agreement with ar ecent computationali nvestigation, [35] the rearrangement of 4a would proceed throughac yclic transition state in which the oxygen, carbon and nitrogena toms of the cyanate moiety would form an angle of 1738,w hereas the dihedral angle value between the C(2)=C(1) and C(4)ÀOb onds would be close to 1208.O f the two possible transition-state models ET and ET',t he latter would lead to ah ighera ctivation barrierb ecause of 1,3strain [36] with the substituent at the allylic position, [35] thereby accounting for the formation of alkylidene(isocyanatocyclopropane) 5a with E configuration as the major stereoisomer and urea 6 after addition of morpholine. It is also noteworthy that ureas 6 and 6' possess opposite configurationa tC 2 (Scheme 4).…”

“…Although the [3,3]-sigmatropicr earrangement of allylic cyanates is concerted and proceeds with ah igh level of chirality transfer,ahighly asynchronous process is involved in which dissociation of the allylic carbon-oxygen bond is more advanced in the transition state than formation of the carbon-nitrogen bond. [35] Due to the peculiar structure and bonding properties of the cyclopropene ring, [37] cyclopropenylcarbinols should rather be compared to propargylic rather than to allylic alcohols and it was of interest to check whether the [3,3]-sigmatropic rearrangement of cyclopropenylcarbinyl cyanates would proceed with chirality transfer.C arbamate (R)-3a (ee = 88 %), prepared from the optically enrichedcyclopropenylcarbinol (R)-1a, [28,38] wase ngaged in the sequence devised previously (TFAA/Et 3 N, CH 2 Cl 2 , À78 8C, 1h,t hen morpholine, À78 8C to RT) and the optically enriched urea( À)-6 possessing an enantiomeric excess of 86 %w as isolated (75 %). [39] Thisl atter result demonstrates that ah igh level of chirality transfer can be achieved in the allylic transposition of cyanate 4a (Scheme 4).…”

“…sults are in agreement with the fact that ap artial positive charge at the carbon bearingt he oxygen atom is generated in the transition state of the concerted but asynchronous [3,3]sigmatropic rearrangemento fa llylic cyanates. [35] The replacement of the methylg roups at C3 on the cyclopropene ring by benzyloxy substituents did not affect the efficiency of the sigmatropic rearrangemento ft he cyanate generated from substrate 16,w hich led to allyl carbamate 27 in 86 %y ield ( Table 2, entry 8).…”

The Allyl Cyanate/Isocyanate Rearrangement: An Efficient Tool for the Stereocontrolled Formation of Allylic C–N Bonds

“…The asymmetric synthesis of (α‐aminoallyl)silane derivatives was achieved by a similar strategy (Scheme ) . The optically active allylic alcohol 19 was first prepared with an excellent enantioselectivity by lipase‐catalyzed resolution of the racemic 4‐(trimethylsilyl)but‐3‐yn‐2‐ol, followed by a stereoselective partial hydrogenation of the triple bond.…”