Exocyclic Si Research Articles

Stable Silapyramidanes

Starting from tetrakis(trimethylsilyl)cyclobutadiene and an amidinate-supported silylene of the Roesky-type, a sequence of addition and reduction cleanly gives the elusive silapyramidane via an isolable cyclobutene intermediate with an exocyclic Si═C bond. The silapyramidane features an unusually shielded 29Si NMR resonance at -448.3 ppm for the apex silicon atom. Treatment with Fe2(CO)9 results in the formation of the corresponding silapyramidane-iron complex. Silapyramidane also reacts with the cyclobutadiene starting material to cleanly afford a fluorescent spirobis(silole).

Selective Si-C(sp3) bond cleavage of a silyl-bridged amido alkyl ligand in an yttrium complex.

Compared with Si-C(sp2 and sp) bonds bearing neighboring π-bond hyperconjugative interactions, the activation of robust Si-C(sp3) bonds has proved to be a challenge. Herein, two distinct Si-C(sp3) bond cleavages have been realized by rare-earth-mediated and nucleophilic addition of unsaturated substrates. The reactions of TpMe2Y[κ2-(C,N)-CH(SiH2Ph)SiMe2NSiMe3](THF) (1) with CO or CS2 gave two endocyclic Si-C bond cleavage products, TpMe2Y[κ2-(O,N)-OCCH(SiH2Ph)SiMe2NSiMe3](THF) (2) and TpMe2Y[κ2-(S,N)-SSiMe2NSiMe3](THF) (3), respectively. However, 1 reacted with nitriles such as PhCN and p-R'C6H4CH2CN in a 1 : 1 molar ratio to yield the exocyclic Si-C bond products TpMe2Y[κ2-(N,N)-N(SiH2Ph)C(R)CHSiMe2NSiMe3](THF) (R = Ph (4); R = C6H5CH2 (6H); R = p-F-C6H4CH2 (6F); and R = p-MeO-C6H4CH2 (6MeO)), respectively. Moreover, complex 4 can continuously react with an excess of PhCN to form a TpMe2-supported yttrium complex with a novel pendant silylamido-substituted β-diketiminato ligand, TpMe2Y[κ3-(N,N,N)-N(SiH2Ph)C(Ph)CHC(Ph)N-SiMe2NSiMe3](PhCN) (5).

Synthesis and Unusual Reactivity of Acyl-Substituted 1,4-Disilacyclohexa-2,5-dienes.

In continuation of our recent studies on group 14 rings with exocyclic silicon-carbon double bonds, we report here on the synthesis and reactivity of previously unknown acyl-substituted 1,4-disilacyclohexa-2,5-dienes. 1,1,4,4-Tetrakistrimethylsilyl-1,4-disilacyclohexa-2,5-diene 1 cleanly afforded the silyl anion 1-K after addition of 1 equiv of KO t Bu. 1-K subsequently could be reacted with various electrophiles to the expected substitution products including compounds 4 and 5. When photolyzed with λ > 300 nm radiation, 4 and 5 undergo Brook-type 1,3-Si → O migration reactions to generate the corresponding 1,4-disilacyclohexadienes with exocyclic Si=C bonds as the primary products. These metastable silenes only could be characterized in form of appropriate quenching products. The reaction of compound 4 with KO t Bu followed by the addition of 1 equiv of PhMe2SiCl surprisingly gave the silylated 1,4-disilanorbornadiene cages 8 and 9 instead of the expected exocyclic silene. The responsible sila-Peterson-type mechanism could be elucidated by density functional theory calculations at the conductor-like polarizable continuum model (THF) B3LYP-GD3/6-31 + G(d) level and by the isolation and characterization of unstable intermediate products after proper derivatization.

Efficient Synthesis of a NHC-Coordinated Trisilacyclopropylidene and Its Coordination Behavior.

With a co-reduction procedure using 2 equivalents Mes2 SiCl2 and 1 equivalent NHCiPr2Me2 →SiCl4 an NHC-adduct of a trisilacyclopropylidene (SiMes2 )2 SiNHCiPr2Me2 1 was synthesized. Addition of NHCMe4 to 1 results in quantitative carbene exchange under release of NHCiPr2Me2 to yield (SiMes2 )2 SiNHCMe4 2. Both NHC-coordinated trisilacyclopropylidenes coordinate to BH3 . In reaction of 1 with SiMe3 N3 a NHC-coordinated trisilane with an exocyclic Si=N double bond is formed. With diphenylacetylene 1 undergoes a ring expansion at room temperature to afford a five-membered ring compound with an NHC-coordinated silylene functionality.

Aromatization of triafulvene and its exocyclic Si, Ge, and Sn derivations by complexation with halogen atoms

GRAPHICAL ABSTRACT

Selective Silylation of Nitriles with an NHC-Stabilized Silylene to 1,2-Disilylimines and Subsequent Synthesis of Silaaziridines

The highly regioselective synthesis of trans-1,2-disilylimines have been achieved by the bis-silylation of nitriles with the NHC-stabilized silylene NHC-Si(NArSiMe3)Cl (1; Ar = 2,6-iPr2C6H3, NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). The bis-silylation involves the migration of the SiMe3 group on the nitrogen atom in the silylene to the carbon atom of the nitrile functionality. The 1,2-disilylimine products feature an NHC-stabilized silaimine moiety and could undergo nucleophilic attack by phenyllithium reagents to yield novel silaaziridines with an NHC-stabilized exocyclic Si═N double bond.

Promotion of exocyclic bond cleavages in the decomposition of 1,3-disilacyclobutane in the presence of a metal filament.

The primary decomposition of 1,3-disilacyclobutane (DSCB) on a tungsten filament and its secondary gas-phase reactions in a hot-wire chemical vapor deposition (CVD) reactor have been studied using laser ionization mass spectrometry. Under the collision-free conditions, DSCB decomposes on the W filament to produce H2 molecules with an activation energy of 43.6 ± 4.1 kJ·mol(-1). With the help of the isotope labeling and chemical trapping methods, the mechanistic details in the secondary gas-phase reactions important in the hot-wire CVD reactor setup have been examined. The dominant pathway has been demonstrated to be the insertion of the cyclic 1,3-disilacyclobut-1-ylidene, generated by exocyclic Si-H bond rupture, into the Si-H bond in DSCB to form 1,1'-bis(1,3-disilacyclobutane) (174 amu). The successful trapping of 1,3-disilacyclobut-1-ylidene by both 1,3-butadiene and trimethylsilane provides compelling evidence for the existence of this cyclic silylene species in the hot-wire CVD reactor with DSCB. Other reactions operating in the reactor include the DSCB cycloreversion to form silene and the ring opening of DSCB via 1,2-H shift to produce silene/methylsilylene and 1-methylsilene/silylene. The introduction of an additional Si atom in the four-membered ring monosilacyclobutane molecule has caused two major changes in the reaction chemistry assumed by DSCB: (1) The endocyclic cycloreversion reactions that dominate in the decomposition of monosilacyclobutane molecules only play a much less important role in the dissociation of DSCB; and (2) the exocyclic bond cleavages are promoted in DSCB due to the ring stabilization caused by the introduction of one additional Si atom.

Photoinduced Brook-Type Rearrangement of Acylcyclopolysilanes

Previously unknown 1,1,4-tris(trimethylsilyl)-4-acyldodecamethylcyclohexasilanes (Me3Si)2Si6Me12(Me3Si)COR (16a, R = tert-butyl; 16b, R = 1-adamantyl) have been synthesized by the reaction of the potassium silanides (Me3Si)2Si6Me12(Me3Si)K with acid chlorides ClCOR, and their photochemical rearrangement reactions have been studied. The molecular structures of 16a,b as determined by single-crystal X-ray diffraction analysis exhibit an unusual twist-boat conformation of the cyclohexasilane ring. When 16a,b were photolyzed with λ >300 nm radiation, they underwent Brook type 1,3-Si → O migration reactions to generate the cyclohexasilanes 17a,b with exocyclic Si=C bonds along with smaller amounts of the ring-enlarged species 19a,b with endocyclic Si=C double bonds. While 17a,b were stable enough to allow characterization by NMR and UV absorption spectroscopy, the less stable products 19a,b could only be observed in the form of their methanol adducts.

Pure Blue Emitting Poly(3,6-dimethoxy-9,9-dialkylsilafluorenes) Prepared via Nickel-Catalyzed Cross-Coupling of Diarylmagnesate Monomers

Polysilafluorenes (PSFs) are an important class of light-emitting conjugate polymers noted for their characteristic brilliant solid state blue luminescence, high quantum efficiency, excellent solubility, and improved thermal stability. These polymers are also reported to have superior electron conductivity to polyfluorenes. The higher electron affinity and conductivity, which are particularly promising for OLEDs, originate from σ*−π* conjugation between the σ* antibonding orbital of the exocyclic Si–C bond and the π* antibonding orbital of the butadiene fragment. In this paper, we present the synthesis and thorough characterization of several new derivatives of 2,7-dibromo-3,6-dimethoxy-9,9-dialkylsilafluorene monomers and demonstrate an efficient room temperature route to their polymerization. In addition to silafluorene monomers with simple alkyl side chains, we have increased the functionality of several of our monomers by incorporating vinyl, cyclohexenyl, and norbornenyl moieties into their side chai...

Isolablep- andm-[(tBu2MeSi)2Si]2C6H4: Disilaquinodimethane vs Triplet Bis(silyl radical)

Isomeric p- and m-disilaquinodimethanes 2 and 4 were synthesized by the reductive dehalogenation of the corresponding p- and m-bis(halosilyl)benzenes 1 and 3, respectively, and were isolated and structurally characterized. The X-ray diffraction and solid-state NMR studies of 2 revealed its singlet quinodimethane structure featuring two exocyclic Si═C double bonds with some singlet biradical contribution. In contrast, the X-ray crystallography and EPR measurements of 4 disclosed its biradical nature, described as a triplet ground state bis(silyl radical).

Palladium-Catalyzed Cross-Coupling Reactions of Dithienosilole with Indium Reagents: Synthesis and Characterization of Dithienosilole Derivatives and Their Application to Organic Light-Emitting Diodes

Three dithienosilole (DTS) derivatives bearing naphthyl segments, 5,5′-dinaphthyl-1,1-dimethyldithienosilol (1), 5,5′-dinaphthyl-1-methyl-1-phenyldithienosilole (2), and 5,5′-dinaphthyl-1,1-diphenyldithienosilol (3), are prepared through Pd(II)-catalyzed cross-coupling using an organoindium reagent as a nucleophile. The molecular structure of 3 is confirmed by single-crystal X-ray analysis. In addition, thermal, photophysical, and electrochemical properties for all three compounds are systematically investigated. The introduction of naphthyl segments into the DTS framework leads to an excellent enhancement of thermal stability with relatively high glass transition (Tg: 87 °C) and decomposition temperatures (Td: 320−380 °C). A red shift in both absorption and emission is observed in the DTS series as the 1,1-substituents on the ring silicon atom become more electronegative. On the basis of absorption spectra and DFT/TDDFT calculations, intense green photoluminescence observed for all compounds can be attributed to the effective π−π* transition of the DTS and naphthyl group with a small contribution of the σ* orbital of the exocyclic Si−C bond. The electron-transporting properties of 1, 2, and 3 were evaluated by the performance of organic light-emitting diodes (OLEDs), comprising 4,4′,4′′-tris(N-(2-naphthyl)-N-phenylamino)triphenylamine (2-TNATA) as hole-injection layer, 4,4′-bis(N-phenyl-1-naphthylamino)biphenyl (NPB) as hole-transport layer, 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]pyrano[678-ij]quinolizin-11-one(C545T)/tris(8-quinolinato)aluminum (Alq3) as emitting layer, and the DTS series as electron-transporting layer, respectively. The device shows intense green emission with a high efficiency of 8.0 cd/A at 1000 cd/m2.

1,1,1,3,3,3-Hexamethyltrisilanylene-bridged bis(cyclopentadienyl)tetracarbonyldiiron and its derivatives: Synthesis, properties and molecular structures

The title ring-bridged bis(cyclopentadienyl)diiron complexes [η 5,η 5-C 5H 4–Si(SiMe 3) 2–C 5H 4]Fe 2(CO)L(μ-CO) 2 [L = CO ( 1), P(OPh) 3 ( 2), P(OMe) 3 ( 3), PPh 3 ( 4), PMe 3 ( 5)] that contain exocyclic Si–Si bonds attached to the bridging silicon atom have been synthesized. The Si–Si bonds were found to be stable to the intramolecular iron centers under both thermal and photochemical conditions, in sharp contrast to the facile cleavage of the Si–Si bond in 1,1,2,2-tetramethyldisilanylene-bridged analogous complexes. The stability of the Si–Si bonds in the present cases may be attributed to the fact that these Si–Si bonds are spatially unapproachable by the intramolecular coordinatively unsaturated iron centers. Molecular structures of 1 and 2 have been determined by X-ray diffraction methods. An obvious conformational change due to substitution of CO for P(OPh) 3 was observed.

(Amidomethyl)dimethylsilanol hydrohalides: Synthesis, NMR and IR studies. Characteristic features of the electronic structure from high-resolution X-ray study and quantum chemical calculation

(C,O)-chelate silanol hydrohalides RC(O)NHCH 2SiMe 2OH · HHal ( 2a, b and 5b), and their precursors, (C,O)-chelate chlorosilanes RC(O)NHCH 2SiMe 2Cl ( 6a, b) and disiloxanes [RC(O)NHCH 2SiMe 2] 2O ( 8a, b) (R = Me ( a), Ph ( b); Hal = Cl ( 2), Br ( 5)), were obtained by several routes. The original scheme of hydrolysis of the above chlorides was discussed in detail. X-ray analysis has shown that the silanol hydrohalogenides PhC(O)NHCH 2SiMe 2OH · HX ( 2b and 5b) in the crystal exist in the form of cation–anion pairs [PhC(O)NHCH 2SiMe 2(OH 2)] + · X − ( 14b · Cl − and 14b · Br −) assembled by H-bonds in a 3D framework. The Si atom in the cation has a trigonal bipyramidal configuration with the oxygen atom of the carbonyl group and protonated hydroxyl exo-substituent in axial positions. The endocyclic Si–O bonds are equal with an average of 1.905 Å while the exocyclic Si–O bonds are 1.979 and 2.009 Å, for Hal = Cl and Br, respectively. Quantum chemical calculations have shown that the cation [PhC(O)NHCH 2SiMe 2(OH 2)] + ( 14b) is stable only in the crystal. Based on a high-resolution X-ray study and a quantum chemical calculation, it was found that the chemical bonding pattern in the OSiO axial fragment of the cation 14b corresponds to a three-centred four electron interaction. The cation 14b should be considered as a silylium cation stabilized by coordinated H 2O molecules rather than a silyloxonium ion.

Tetrasilatetrahedranide: a silicon cage anion.

The first silicon cage anion, tris{bis[bis(trimethylsilyl)methyl](methyl)silyl}tetrasilatetrahedranide (6-), has been synthesized by the reaction of tetrakis{bis[bis(trimethylsilylmethyl](methyl)silyl}tetrasilatetrahedrane with potassium graphite in diethyl ether by reductive cleavage of exocyclic Si-Si bond. The structural characterization of K+(18-crown-6).6- has been achieved by X-ray crystallography, showing that 6- is a separated ion pair and the tetrasilatetrahedranide moiety has a significantly distorted tetrahedrane skeleton containing one inverted tetrahedral (umbrella type) silicon atom. The four silicon atoms in the Si4 skeleton are equivalent on the NMR time scale due to the migration of the Dis2MeSi substituent.

Structure and oxidation of octakis( tert-butyldimethylsilyl)octasilacubane

The molecular structure of the silyl-substituted octasilacubane [( t-BuMe 2Si)Si] 8 ( 1) was analyzed by X-ray crystallography, and the structural parameters (bond length and angles) were compared with those of the alkyl- and aryl-substituted octasilacubanes. A thermogravimetric analysis revealed that 1 remained stable under argon up to 300 °C, indicating that 1 is much more thermally stable than alkyl-substituted (ThexSi) 8 (Thex=1,1,2-trimethylpropyl) ( 2), which decomposed at 200 °C. Although compound 1 is stable in the air, 1 reacted with m-chloroperoxybenzoic acid ( mCPBA). Thus, treatment of 1 with fourteen equivalents of mCPBA led to the formation of octakis( tert-butyldimethylsilylsilsesquioxane) ( 3) in 98% yield. The structure of 3 was also established by X-ray crystallography, indicating that all the oxygen atoms inserted into the SiSi bonds of the Si 8 framework and the exocyclic SiSi bonds remained intact under the conditions employed. Compound 3 is the first example of silsesquioxane with bulky silyl substituents, and shows relatively intense absorptions in the UV region.

Nucleus independent chemical shift evaluation of the aromaticity of pentafulvene and its exocyclic Si, Ge, and Sn derivatives

Nucleus independent chemical shift (NICS) values, the negative isotropic shielding calculated at the ring centers, were used to evaluate the aromatic character of a series of substituted pentafulvene analogs. NICSs were calculated for pentafulvene 3a, its exocyclic Si, Ge, and Sn analogs 3b– d, and their 7,8-difluoro derivatives 4a– d. Calculations on cyclopentadiene, 1, and the aromatic cyclopentadienyl anion, 2, were also carried out for comparison. Geometries were optimized using the B3-LYP method while NICS values were calculated at both the HF and B3-LYP levels using the gauge invariant atomic orbital (GIAO) approach. All calculations used a triple split basis set with two sets of polarization functions (tsV+2P). Calculated NICS values suggest 3a– d and 4a exhibit little or no aromaticity. However, the 7,8-difluorosilyl- and 7,8-difluorogermyl derivatives ( 4b and 4c) appear to exhibit a degree of aromaticity, which might be due to π-donation from fluorine to silicon (germanium) permitting some net π-donation from Si and Ge to the cyclopentadiene ring. The SnF 2 derivative 4d, in contrast, does not appear to exhibit aromaticity.

Aromatic Character of Tria- and Pentafulvene and Their Exocyclic Si, Ge, and Sn Derivatives. An ab initio Study

The structures and dipole moments have been calculated for both methylenecyclopropene (triafulvene) and pentafulvene and their exocyclic Si, Ge, and Sn analogues 1a−d and 2a−d, respectively. Ab initio calculations employing the HF, DFT, and MP2 methods, each using split valence plus polarization and triple split valence with two sets of polarization functions basis sets, have been performed. The results of these six levels of theory on each of the eight molecules were compared, and the aromatic character of these systems was examined. The structures 1a−d exhibited pronounced bond alternation in the ring suggesting the existence of only weak cyclopropenyl cation-like resonance contribution. However, very large dipole moments were predicted in the direction that would be expected from an aromaticity contribution. Since Si, Ge, and Sn are less electronegative than C, the large dipole moments in the direction of the exocyclic heteroatom were surprising. The most striking finding was that, while 1a is planar, 4-sila-, 4-germa-, and 4-stannatriafulvene were nonplanar with trans-bent structures in which the exocyclic double bond is bent slightly out of the cyclopropene ring plane and out of the HXH plane (X = Si, Ge, Sn) by large angles. Thus, the Si, Ge, and Sn atoms exhibit pyramidal geometries. The dipole moment of pentafulvene, 2a, (oriented toward the ring) was substantially smaller than that of its Si, Ge, and Sn analogues, 2b−d, The large dipoles of 2b−d were due to greater charge separation resulting from the lower electronegativities of Si, Ge, and Sn versus that of C. The cyclopentadienyl rings exhibited strong bond alternation indicating only a modest cyclopentadienyl anion-like contribution in 2b−d.

Generation of the 1,3-Phosphasilolene Skeleton from Ortho-Silylated Biarylphosphonates

Treatment of diethyl 2-(3-trimethylsilyl)biphenylphosphonate 15 with excess methyllithium affords a mixture of the 2H-benzo[d]-1,3-phosphasilolen-1-one 16 together with the silyl transfer product 17. Deuterium incorporation studies show that the mechanism involves the conversion from 15 to 19 and 20, followed by formation of a siliconate intermediate 21. Cleavage of an exocyclic Si−C bond affords 16 while endocyclic C−Si cleavage leads to 17. The phosphasilolene structure 16 is confirmed by X-ray crystallography.

Synthese und Kristallstrukturen von Cyclosilazanen mit ungewo¨hnlichen Konformationen

Die 1,3-Disilazane, (Me 3C) 2SiNH 2 NHSiFR 2, R = Me ( 1), CHMe 2 ( 2), und das 1,3,5-Trisilazan, (Me 3C) 2Si(NHSiFMe 2) 2 ( 3), werden in der Reaktion des lithiierten Diamino-di- tert-butylsilan mit F 2SiMe 2 und F 2Si(CHMe 2) 2 erhalten. 1 und 2 reagieren mit BuLi zu den Cyclodisilazanen 4 und 5, (Me 3C) 2Si(NH) 2SiR 2, R = Me ( 4), CHMe 2 ( 5) und dem Cyclotetrasilazan, [(Me 3C) 2SiNH SiMe 2 NH) 2 ( 6). Die Lithiumderivate von 4 und 5 reagieren mit Fluorsilanen zu den silylsubstituierten Cyclodisilazanen 7, 8, 10 und 11: 7, (Me 3C) 2SiNH(NSiFMe 2)SiMe 2; 8, (Me 3C) 2Si(NSiFMe 2) 2SiMe 2; 10 (Me 3C) 2SiNH(NSiF 2CMe 3)Si(CHMe 2) 2; 11, (Me 3C)Si(NSiF 2C 6H 5)(NSiF 2CMe 3)Si(CHMe 2) 2. Die Reaktion des dilithiierten Diamino- tert-butylmethylsilans mit Dichlor- tert-butylmethylsilan fu¨hrt zur Bildung des Cyclotetrasilazans 9, (Me 3CMeSi NH) 4. Die Kristallstrukturen von 4, 6 und 11 wurden untersucht. 4 bildet monokline Kristalle der Raumgruppe P2 1/m, Z = 2. Das Vierring-Geru¨st ist planar. Die NSiN-Winkel sind kleimer und die SiNSi-Winkel entsprechend gro¨βer als 90°. 6 kristallisierim triklinen Kristallsystem mit der Raumgruppe P1, Z = 2. Der Ring besitzt Sattelkonformation mit den Si-Atomen etwa in einer Ebene und den N-Atomen abwechselnd daru¨ber und darunter. Die N-Atome haben eine weitgehend planare Umgebung (Winkelsumme 357.4–359.0°) mit SiNSi-Winkeln zwischen 139.8–142.8°. 11 bildet monokline Kristalle mit der RaumgruppeC2/c, Z = 8. Der Winkel zwischen den Linien N1-N2/N2-Si4 betra¨gt 166.5° und zwischen N2-N1/N1-Si3 176.4°. Das N2-Stickstoffatom hat eine trigonal pyramidale Umgebung (Winkelsumme 355.1°), einea¨uβerst seltene Geometrie fu¨r Stickstoffatome, die an drei Siliciumatome gebunden sind. Die exocyclischen Silylgruppen haben eine cis-Position zueinander. Der elektronenziehende Effekt der Fluoratome fu¨rht zu einer Verku¨rzung der exocyclischen Si N-Absta¨nde.

Thermolysis of [tris(trimethylsilyl)methyl](diphenyl)fluorosilane. Isomerization of a sila-olefin intermediate

The compound (Me 3Si) 3CSiPh 2F loses Me 3SiF under reflux or on passage through a tube at 450°C to give three products, A, B, and C, in approximately 20/20/60 ratio. Products A and B, which are solids, were shown by X-ray crystallographic analysis to be the diastereoisomeric forms of 1-dimethylsila-2-trimethylsilyl-3-[(methyl)(phenyl)sila]indane. From its mass and 1H NMR spectra, C (a liquid) was tentatively identified as 1,3-bis(dimethylsila)-2-[(dimethyl)(phenyl)silyl]indane. All three products are isomers of the sila-olefin (Me 3Si) 2CSiPh 2, and it is suggested that the latter is first formed by loss of Me 3SiF from (MeSi) 3CSiPh 2F, and the equilibrium (Me 3Si) 2CSiPh 2 ⇌ (Me 3Si)(Ph 2MeSi)CSiMe 2 ⇌ (Me 3Si)(PhMe 2Si)CSiMePh ⇌ (Me 2PhSi) 2CSiMe 2 is then rapidly established; internal cyclizations involving addition of aryl CH bonds across SiC bonds then occur to give the observed products. Consistent with this is the observation that a mixture of silicon alkoxides, thought to be (Me 3Si) 2CHSiPh 2OMe and its isomers (which would be formed by addition of methanol across the SiC bonds of the four sila-olefins) is produced when methanol is passed through the hot tube with the (Me 3Si) 3CSiPh 2F. Full structural details are given for compounds A and B. Some features of interest are: (a) the conformation of the 5-membered ring is different in the two diastereoisomers; (b) the exocyclic SiCSiMe 3 bond angles, of ca. 120° are unusually large; and (c) there is a little distortion of the fused benzene ring, which is attributed to the effect of silicon substituents on the hybridization of carbon atoms to which they are attached.