Structure of a Syn Aldol Addition Product of Benzaldehyde and A Prolinol-Derived O-Silacyclopentyl Ketene N,O-Acetal

The compound (5S,6S,11 aS)-hexahydro-6-methyl-5- phenylspiro[1H,7H-pyrrolo[2,1-e][1,3,6,2]dioxazasilo-nine-3,1'-silacyclopentan]-7-one (1) results from the silicon-directed condensation of the prolinol-derived t Contribution No. 8633

The title compound was obtained from free singlet dimethylgermylene, which is generated by thermolysis of a 7-germanorbornadiene (Neumann, 1991), and methyl (E)-2-methyl-2,4pentadienoate at 343 K in benzene (Wienken & Neumann, 1992) and is separated from the isomeric c/s-3,4-dialkenyl-1-germacyclopentane by repeated crystallization.The crystals were obtained from methanol at room temperature, m.p. 384 K (yield 16%).The lattice parameters were determined from a syrmnetry-constrained least-squares fit.Refinement was based on full-matrix least-squares methods with H atoms in calculated positions ( C m H 0.96 A,).One common isotropic temperature 0108-2701/93/010185-04506.00 factor was refined for the H atoms of the methyl groups and another was refined for the remaining H atoms.
Lists of structure factors, anisotropic thermal parameters, H-atom coordinates and complete geometry have been deposited with the British Library Document Supply Centre as Supplementary Publication No. SUP 55576 (8 pp.).Copies may be obtained through The Technical Editor, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England.[CIF reference: ALl027] O-silacyclopentyl ketene N,O-acetal (2) with benzaldehyde.The five-membered pyrrolidine ring has an envelope conformation and the silacyclopentyl ring has an en-type conformation with C17 and C18 on opposite sides of the CI6--Si--C19 plane.Both of these, rings appear to be disordered.

C o m m e n t
The non-catalyzed condensation of O-silyl ketene N,O-acetals with aldehydes has been proposed to proceed via a pentavalent organosilicon intermediate (Myers & Widdowson, 1990;Myers, Widdowson & Kukkola, 1992).In further mechanistic studies of this reaction, the effect of constraining the alkyl groups bound to silicon in a ring was investigated.The reaction of the O-silyl ketene N,O-acetal (2) with benzaldehyde at 296 K forms both syn and anti aldol addition products; syn product (1) forms X-ray quality crystals (m.p. 430-434 K) upon slow evaporation of a benzene solution.The structural analysis of ( 1) is described herein.
This compound closely resembles the aldol condensation product (3) (Schaefer, Widdowson & Myers, 1991); the only difference is in the substituents of the Si atoms: a cyclic tetramethylene chain in ( 1 ) a n d two methyl groups in (3).The unit-cell dimensions are comparable, and the space group, packing and conformation of the nine-membered ring are the same. (2) ..c c., (1) 0,.,,.,-.Si~

( " o\o_
N . ( The molecule is shown in Fig. 1.The pyrrolidine ring has the common envelope conformation (Marsh & Donohue, 1967, and references therein) with 'flap' atom C12 lying 0.52/~ out of the (nearly exact) plane of C11, N, C13 and C14.We note that the large Uu's of C l l , C12 and C13 suggest possible disorder involving inversion of the ring, as first found in crystals of L-leucyl-L-prolylglycine (Leung & Marsh, 1958;Marsh, 1980).Similar disorder in the fivemembered ring containing Si may explain the surprisingly short C17-C18 distance [1.442 (9)A.].The large anisotropic displacement parameters of these two atoms show that their apparent motion is primarily perpendicular to the bond between them.Correcting the observed distance for this apparent motion gives a value of 1.50 or 1.51 A, as expected for a --CH2mCH2 -single bond.We have not attempted to model these possible disorders.The goodness-of-fit for merging (point group 2/m) 1599 reflections with multiple observations was 1.01.The coordinates of the Si atom were found from a Patterson map and those of the C, N and O atoms from subsequent structure factor-Fourier cycles.All reflections, F 2 positive and negative, were used in the refinement.H atoms were placed at calculated postions (C--H 0.95 A) and re-positioned once near the end of the refinement.The variances (tr2/) were derived from counting statistics pins an additional term (0.014/)2; variances of the merged data were obtained by propagation of error pins another additional term (0.014~ 2. Atomic scattering factors and values for Af' were taken from Cromer & Waber (1974) and Cromer (1974).
The programs used were CRYM (Duchamp, 1964) and ORTEP (Johnson, 1976).The chirality of the molecule was known from its synthesis.Near the conclusion of the refinement, the intensities were re-averaged in point group 2 and refinements carded out for both chiralities; the correct one gave slightly smaller residuals (0.0642 versus 0.0645 for R, 1.52 versus 1.53 for S).
There were no significant differences between the three sets of coordinates (Af'~i = 0.071 e-).

Comment
Synthesis of 2-[4-(dimethylamino)phenylimino]-3-oxo-N-phenylbutanamide (DPA) (Fig. 1) from acetanilide and N,N-dimethyl-4-nitroso-benzenamine (NDA) was undertaken as part of studies on charge-transfer complexes formed by 1-oxa-4-azabutadienes.The reaction mixture yielded the desired product but dark-green crystals, which had semiconductive properties, were also precipitated.On the basis of spectroscopic investigations, the structure of a polymeric quinhydrone-type charge-transfer complex of DPA and NDA was proposed (Moskal, Moskal & Milart, 1984).To verify this model we determined the crystal structure using X-ray diffraction methods.

$ 6 Fig. 1 .
Fig. 1.An ORTEP drawing of the molecule with 50% probability ellipsoids showing the numbering system.H atoms are shown as circles of arbitrary small diameter.

Fig. 2 .
Fig. 2.An ORTEP drawing of the unit-cell contents (one unit cell outlined).The projection is roughly perpendicular to the bc plane, with the b axis horizontal; H atoms are not shown.
O F LEWINSra, WoJciEci-i Nrr~ AND Pto'm Mm~T Jagiellonian University, Department of Chemistry, uL Karasia 3, 30-060 Cracow, Poland (Received 29 April 1992; accepted 5 October 1992) Abstract 2-[4-(Dimethylamino)phenylimino]-3-oxo-N-phenylbutanamide and N,N-dimethyl-4-nitrosoaniline form a molecular complex in which they are linked by a hydrogen bond and n'-Tr* interactions.Intermolecular contacts shorter than 3.2 ,~ are also observed between adjacent complexes arranged in double layers.The geometry of the dimethylaminophenyl moiety in both molecules indicates a contribution from the para-qulnonoid structure.

Table 1 .
Fractional atomic coordinates and equivalent isotropic thermal parameters(A 2)