Synthesis and NMR study of intramolecular silacyclobutane complexes: 8-aza-5,11-dioxa-4-silaspiro[3,7]undecane and 8-methyl-8-aza-5,11-dioxa-4-silaspiro[3,7]undecane

The intramolecular silacyclobutane complexes with pentaacoordination at silicon, 8-aza-5,11-dioxa-4-silaspiro[3,7]undecane and 8-methyl-8-aza-5,11-dioxa-4-silaspiro[3,7]undecane, were synthesized by reacting 1,1-dimethoxy-or 1,1-bis(diethylamino)silacyclobutanes with the corresponding diethanolamines. The existence of (Si N) pentaacoordination at silicon was deduced from the pronounced upfield 29 Si and downfield 13 C NMR shifts as well as from the low temperature 13 C NMR study of their dynamic behavior in solution that proved complexes to appear as two isomers with coalescence barriers of 11.5 kcal/mol and 12.9 kcal/mol for 1 and 2 , respectively.


Introduction
Known since 1954, 1 silacyclobutanes, the four-membered cycles containing silicon and three carbon atoms, continue to be a subject of great interest due to the peculiar features of their structure and a variety of chemical properties owing to both the dipolar nature of the endocyclic Si-C bond and the four-membered ring strain. 2 Thus, in liquid phase the ring opening polymerization and some other reactions can occur. 3Silacyclobutanes readily cyclorevert generating transient silenes, the silicon-carbon doubly bonded compounds containing the sp 2hybridizied silicon. 46][7][8] In particular, the X-ray and NMR data indicate the peri interaction between silicon and the dimethylamino group in bis[ (8'-(dimethylamino)naphth-1'-yl)-1-silacyclobut-1-yl]ether to be considerably stronger than in bis[ (8'-(dimethylamino)naphth-1'-yl)-dimethylsilyl]ether, thus evidencing a greater Lewis acidity of silacyclobutane derivatives. 5The gas-phase reactions of 1,1-dimethysilacyclobutane with fluorine and allyl anions give rises to 1-silacyclobutane pentacoordinate silicon anions.

Scheme 1
Treatment of 1,1-dichlorosilacyclobutane with lithium phosphinomethanide leads to a stable intramolecular silacyclobutane complex with hexacoordination at silicon center:

Scheme 3
The reactions were performed in CHCl 3 or CH 2 Cl 2 at very mild conditions (20 o C, 1 hour) followed by the evaporation of the solvent and a volatile product (ethanol or diethylamine) and drying of a colorless solid residue in vacuum.‡ The products 1 and 2 melted at 20 o C and 45 o C, respectively.Notice that the rise of the reaction temperature resulted in the formation of a nonvolatile residue.Our attempts to involve 1,1-dichlorosilacyclobutane (X = Cl) and O,O-bis-TMS derivatives of diethanolamines in the above reactions failed.At room temperature reactions would not proceed, whereas at 60 o C the cleavage of silacyclobutane Si-C bond in the starting compounds and products suppressed the formation of the target 1 and 2.
NMR data of 1 and 2 in solution are given in the Table .The upfield 29 Si NMR shifts (by 36 and 29 ppm, respectively) relative to that of 1,1-diethoxysilacyclobutane (lit., 13 δ Si -17.1 ppm), the tetracoordinate organosilicon compound with the same surroundings at silicon atom, was associated to the pentacoordination at silicon.A higher 29 Si shielding and smaller temperature coefficient (cf.0.04 ppm/degree for 1 and 0.06 ppm/degree for 2) indicate the ‡ However, we failed to obtain a reliable crystal for X-ray study Si N bonding in 1 to be stronger than in 2. Indeed, the upfield 29 Si NMR shifts are common for 2,8-dioxa-6-aza-2-silacyclooctanes R 2 Si(OCH 2 CH 2 ) 2 NR' in polar solvents 11 .These change similarly on going from R' = H to R' = Me (cf.for R = Ph, R ' = H, δ si .-44.7 ppm in CDCl 3 , δ si . -56.1 ppm in CD 3 CN and for R = Ph, R ' = Me δ si .-43.9 ppm in CDCl 3 , δ si .-47.8 ppm in CD 3 CN) indicating a decrease of coordination interaction Si N. Presumably, this effect is due to both the steric factors and higher nucleophilicity of nitrogen when the attached hydrogen atom forms intermolecular Н-bond with the basic centers of the dissolved substance and solvent. 14At ambient temperature, a silicon pentacoordination of 1 and 2 manifests itself in greater downfield 13 C shift of the carbon atoms attached to silicon (6.5 and 4.9 ppm, respectively) as compared to that of the model tetracoordinate silicon compound, 1,1-diethoxysilacyclobutane (lit., 13   This mechanism was proposed for an explanation of splitted signals in the NMR spectra of RR'Si(OCH 2 CH 2 ) 2 NR". 11Also, the turnstile mechanism 15 of the ligands exchange at silicon atom may be taken into account (Scheme 6).The rearrangement barrier, ∆G c *, of 11.5 kcal/mol was estimated for silacyclobutane 1.For silacyclobutane 2 it rised to 12.9 kcal/mol.Taking into consideration that the Si←N bond in 1 is stronger than that in 2, the opposite ratio between ∆G c * values is apparently due to an essential contribution of the conformational rigidity of the eight-membered ring to the ∆G c * value which increases with N-substitution.The ∆G c * value for 2 is higher than those determined for the related N-methylaminoethoxy derivatives of dimethylsilane (9.3 kcal/mol), 1silacyclohexane (10.0 kcal/mol) and 1-silacyclopentane (11.5 kcal/mol) in (CD 3 ) 2 CO solution. 11his is in favor of the Si N coordination bonding in silacyclobutane 2 to be stronger than that in the related medium-sized silacycles and acyclyc dialkylsilyl analogs.The result is in accord with the enhanced F -affinity of silacyclobutanes in the gas-phase reactions 6c and could be explained by an essential energetic gain upon silicon pentacoordination due to some release of the four-membered ring strain when carbons adjacent to silicon span one equatorial and one apical position.

ARKAT
The higher solubility of compound 2 made it possible to measure the values of the onebond 1 J( 29 Si-13 C) coupling constant for the apical and equatorial carbons.These values were found to be -54.2Hz and -58.6 Hz, respectively.The coupling constant ( 29 Si-13 C α ) in tetracoordinate 1,1-diethoxysilacyclohexane is the intermediate value (J Si-C = 55 Hz). 13 Such change of the coupling constant of silicon atom with subsituents well agree with the change of scharacter of the bond with axial and equatorial substituents in TBP. 16RKAT Experimental Section 1 H, 13 C, 29 Si NMR spectra of 20% solutions (CDCl 3 ) of compounds 1 and 2 were recorded on a JEOL 90Q spectrometer.1,1-Dimethoxy-and 1,1-bis(diethylamino)silacyclobutanes were synthesized as described in ref. 13.The value of ∆G was calculated using the equation taken from ref. 17.
Preparation of 8-aza-5,11-dioxa-4-silaspiro [3,7]undecane (1)   a. Diethanolamine 0.52 g (5 mmol) in 10 ml of CHCl 3 was added dropwise to a solution of 0.66 g (5 mmol) of 1,1-dimethoxysilacyclobutane in 15 ml of dry CHCl 3 at 5 0 C.After warming up to room temperature and evaporating the solvent and formed methanol in vacuum, a cream-colored solid residue was filtrated and washed with pentane.After the evaporation of pentane, a colorless solid of 1 (0.45 g; 51% a. N-Methyldiethanolamine 0.59 g (5 mmol) in 5 ml of CHCl 3 was added dropwise to a solution of 0.66 g (5 mmol) of 1,1-dimethoxysilacyclobutane in 5 ml of dry CHCl 3 and was stirred for 1 h at room temperature.The solvent and methanol were evaporated and viscous residue was dried in vacuum.The product was purified then by low temperature sublimation in vacuum (20 o C, 10 -3 mm Hg) to yield 0.38 g (40 %) of 2, melting point 20 o C. Elemental analysis (Found: C, 49.67; H, 8.75; N, 7.95; Si 15.87.Calc.for C 8 H 17 NO 2 Si: C, 51.30; H, 9.15; N, 7.48; Si 14.99%).b.N-Methyldiethanolamine 0.59 g (5 mmol) in 10 ml of CHCl 3 was added dropwise to a solution of 1.07 g (5 mmol) of 1,1-bis(diethylamino)silacyclobutane in 20 ml of dry CHCl 3 at 5 0 C.After slow warming up to room temperature and evaporation of diethylamine and a part (4/5) of the solvent in vacuum, an oily product was decanted and washed with pentane.After evaporation of the pentane, a colorless solid of 20.8 ppm).A single resonance was observed for the Si-CH 2 carbons at this temperature.Upon cooling the signals of the Si-CH 2 carbons broaden, coalesce (temperature of coalescence at ca -50 o C for 1 and at ca -20 o C for 2) and giving rise to two signals (Figure).The occurrence of C and C* signals most likely indicate the rearrangement resulting in a positional exchange between apical and equatorial carbon atoms (see Scheme 4).