Robert Belloli
California State College Fullerton, 92631
I II
Resolution and Stereochemistry of Asymmetric Silicon, Germanium, Tin, and Lead Compounds
Optical activity in organic derivatives of the following asymmetric atoms has been established (I): in Group IVA, C, Si, Ge, Sn; in Group VA, N, P, As, Sb; in Group VIA, S, Se, and Te. Because of the tremendous number of studies performed and amount of information accumulated on compounds with asymmetric carbon atoms, the resolution and stereochemistry of the other group IVA elements is of particular interest and importance. It is the purpose of this review to summarize the results of stereochemical studies, es~eciallythe significant achievements of the past decade, on compounds containing an asymmetric group IVA atom (with the obvious exclusion of carbon).
Three-step Walden cycles were observed for the hydrolysis and methanolysis of a-napthylphenylmethyl silane
(4) (-)RsSi80H -20"
LiAI&
L (-)&Si*H
-33"
Silicon Derivatives Synthesis, Resolution, and Relative and Absolute Configurotion
The first tetracovalent silicon compound with an asymmetric silicon and no other asymmetric centers (I) was resolved by Eaborn and Dill (2)by fractional crystallization of its quinine salts. Me
However, the extensive study of the stereochemistry and mechanism of reaction of optically active organosilicon compounds began with the synthesis and resolution (eqn. (1)) by Sommer and Frye (5) of a-napthylphenylmethylsilane (a-NpPhMeSiH).
(+)&SiaH
LiAI& t-LiAlHd
(-)R8Si*H
fractional ((i)arN PhMeSi crystaUizstion (-)8~enthol (1)
With the separated enantiomers available, the reactions of the silane and key derivatives were then studied to determine stereochemistry and stereospecificity. One of the first sequences studied is given in eqn. (2)
where rotations are [ a ]unless ~ otherwise noted. This represents the first Walden cycle for silicon and a similar cycle was found using bromine 640
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lournol of Chemicol Education
The fact that the enantiomer of opposite sign was obtained in these cycles with nearly equal magnitude of rotation indicates that all the steps in the cycle are highly stereospecific. However, as with the original Walden cycle, it is not known from this data alone which of the reactions ~roceedswith inversion and which with retention. The relative configurations of the silane and its key derivatives need to be known if the stereochemical path of each reaction in the cvcle is to be determined. T h e Fredga method (5) (method of quasi-racemates) can be used to correlate configurations of compounds having closely similar structures by noting differences in phase behavior. Sommer states (4), "Conclusions drawn on the basis of a differencein phase behavior have proved accurate without exception." These organosilanes have very closely similar structures. The case of interest here is the one in which pure optical isomers of two different substances that are isomorphous give solid solutions when they are of the same configuration and a eutectic mixture when they are of opposite configuration (6). Mixtures of the organosilane derivatives were analyzed by X-ray crystallography and the assignments made are listed in Table 1. These results were confirmed by optical , ... "'*--... .--*- . ~ . "-.".." ..... ."... " , : ..-..-.. ,; . * -','.i -.":' . -.-,. ." ;. ...-.. , ." , , ~
3 '
~~
'
-
"3.
Table 1.
"
(
--" .
.
Enantiorners Having the (+)R,SiH Configuration
rotary dispersion studies. The (-)SiF, (-)SiOH, (-)SiH, and (+)SiOCH3 derivatives all gave similar optical rotary dispersion curves with negative Cotton effects and seem to have the same relative configuration (7). Knowing the relative configurations, the stereochemical path for each step of eqns. (2)-(4) can be assigned. For example, the formation of the silyl chloride from the hydride proceeds with inversion. The absolute configuration of the silane itself was determined by rigorous X-ray methods (8) and is shown in Figure 1. The absolute configurations of the derivatives are then also known.
-
Figure 1. The absolute contlguration of (f )R&Si*H.
-
7
zmi
252
2.74
i
Figure 3.
Comporiron of R-R dirtonce as a fundion of bond angle.
Figdie 4.
Transition %totesfor reduction of RSi'OR' by RMgX and LiAlHc
eNp*iMMe
H Stereochemistry and Mechanism of Reactions
SNi-SiMechanism. Many reactions of the type generalized in eqns. (5) and (6) were thenstudied by Sommer and coworkers (9). LiAI&
&Si*OR'
----t
or
RMsX
RsSi'H
(5)
KOHW
R,SiLOR'+RnSi*OH
(6)
R1 = H, CH,, cyolo C6H,,, (-)menthyl, and Si*Rs
Some representative results of these studies are given in Table 2.
Stereochemistry of Reactions of R3Si*OR
Table 2.
Reaat,ant
Reaeent
Product Solvent
Stereospecificity
These reactions all have a common preferred retention stereochemistry. In the compound RaSi*OR', --OR' is a poor leaving group and hard to displace as a negative ion. A "poor" leaving group is one whose conjugate acid has a pK. > 10 (10). Also the unshared pairs of electrons on the oxygen of the -OR' group are available for donation to the electrophilic part of the attacking reagent, especially if nucleophilic attack on silicon occurs simultaneously. Sommer proposed auasi-cyclic, SN~-Si transition states for these retention r&tions (Fig. 2). The tetragonal pyramid and trigonal hipyramid shown Fieure 2 have the s ~ a t i a confinuration l ehar~ - in ~ acteristic of dsp8 bonding. silicon has-a vacant 3d orbital, and pentacovalent or hexacovalent transition states could conceivably have &orbital participation. The need to compress the R-Si-R angles from 109' to 100" or 90" does not involve prohibitive non-bonded R-R interactions due to the larger size of silicon compared to carbon (9)(Fig. 3). Therefore, for certain reactions given in Table 2, the
-
transition states shown in Figure 4 seem reasonable (9) and are specific examples of the general SN~-Si transition states. The SNi-Sitransition state provides for a minimum of charge separation and also the needed electrophilic assistance (pull on X by E, as Y attacks Si) for substitution of these poor leaving groups. SN~-S-S~ Mechanism. The reactions of carboxylate, tosylate, and halide derivatives of the optically active orNpPhMeSiH were then studied (11). Some representative results are given in Tables 3 and 4. The reactions summarized in Tables 3 and 4 again show a common preferred stereochemistry. Predominant inversion of configuration was found for replacement of chloride from R&*c~ with the following reagents: LiAlH4, NaBH,, c ~ c ~ o - C ~ H U N HHg(OCOCH&, ~F, HzO, CH80H, t-C4H,0K, etc. (12, 13). Similar results Table 3.
Stereochemistry of Reactions of R3Si*OCORf
Resctant
Reagent
(+)RIS~*OCOCHI LiAIH, (+)RIS~*OCOCIHI LiAIH. +)RaSi*OCOCHa KOH(S) l+m,si*ocoaa, KoH(s) (+)RaSikOCOCHaMeOH
Product
Xylene specificity
(-)SiH (-)SH )SiOH
EhO
(-)SiOMe
90% inven@n EhO SO% lovars!on Xylena 85% /nveralon xylen.75% yverslon Xylene 87% mverslon
Stereochemistry of Reactions of (+)R3Si*CI
Toble 4.
Reagent
Product
Solvent
Stereochemistry
Hz0 KOH(S) MeOH Hg(OCOMe)z LiAIH, NaBH, none
(+)%OH (-)SiOK (+)SiOMe (+)SiOCOMe (+)SiH (+WH (+)Sic1
Et10 Xylene Pentane Benzene Ether Uiglyme CHaNOl
Inversion Inversion Inversion Inversion Inversion Inversion Raoemization
Volume 46, Number 10, October 1969
/
641
were obtained with bromide, tosylate, or carboxylate leaving groups, i.e., inversion was the preferred stereochemistry with attacking groups of widely differing basicity and steric requirements and groups for which d,-p, dative bonds are not possible. These "good" leaving groups (pK. of conjugate acid less than 6) undergo displacement from silicon with inversion with a wide variety of nucleophiles, regardless of solvent, as long as the entering group is more basic than the group displaced (12). This is termed the SNZ-S~ stereochemistry rule, and two possible transition states are depicted in Figure 5.
Figure 6. Determinotion of absolute conflgurdion of RsSi'H metric rvnthesir
Figure 5.
s ~ 2 - S transition i stmter.
The bonds to X and Y are not "full" bonds, and the d-orbital participation may change from reaction to reaction and may be zero in some cases. There is no real experimental basis for choosing between the models in Figure 5 (12). The stereochemical path of the displacements or substitutions a t asymmetric silicon are evidently almost entirely dependent on the nature of the leaving group. S N I - S ~ Mechanism. The racemization of (+)R3Si*Cl in nitromethane-chloroform solvent is thought to proceed by a SNI-Simechanism involving a siliconium ion (14). Halide-halide exchange studies at asymmetric silicon showed that the rate of racemization and chloride radiochloride exchange are the same within experimental error. IbSi'C1
+ cyclo-CaHnNH~C1"
RaSiCI"
+ eyclo-CsHIINHICI (7)
This result was interpreted as indicating rate controlling silieonium ion pair formation followed by equally rapid exchanges with retention or inversion on the ion pairs. R3Si*C1is optically stable in non-polar solvents in the absence of added salts. However, based on studies of rates of racemization as a function of solvent nucleophilicity, Corriu, et al., prefer a racemization mechanism that proceeds via an extension of silicon atom coordination, rather than a siliconium ion or ion pair (16a, b). Other Mechanisms. Mechanisms termed SN2*-Siand SN2**-Si have been invoked by Sommer to explain the changes in stereochemistry for reactions of RaSi*F with different organolithium reagents. These mechanisms use Si-5 intermediates (5 full bonds to silicon) and arguments based on apical versus equatorial entry of the RLi reagent (16). Other Studies with Optically Active Organosilicon Compounds
Absolute Configuration of a-Napthylphenylmethylsilane by Asymmetric Synthesis. The absolute configuration of a-NpPhMeSiH was independently determined by Brook and Limhurg using an asymmetric synthesis (Fig. 6) (17). The absolute configuration of the carbinol obtained in the last reaction of Figure 6 is known and the assignments of retention a t Si[R(Si)l or retention a t C 642
/
by asym-
Journal of Chemical Education
[R(C)] in the other reactions are based on earlier work of Brook, Sommer (18), and Cram (19). Invoking Cram's rule in the first reaction permits the assignment of the configuration of the original asymmetric atom from the configuration of the new asymmetric center formed. The absolute configuration determined for (-)R3Si*H was in agreement with the result of Sommer. Asymmetric Silicon-Sulfur and -Nitrogen Compounds. A number of compounds with asymmetric silicon to sulfur and to nitrogen bonds have been prepared by substitution reactions on R3Si*C1 with HzS and RzNLi reagents (20, 21). Enantiomers with the (+)RsSiH /configuration are (+) R~S~*-N,-], (+)R3Si*NHBu, (-)RaSi*NH2, (-)RaSi*NHSiR3, (-)RaSi*SCH3,
+
(-)RaSi*SSi*Rl, (-)RsSi*S-Et2NHz. Stereochemistry of reactions of these compounds is accurately predicted on the basis of the rules given above. Dihalomrbene Insertions into the Si-H Bond. Dihalocarbene generated from phenyltrihalomethyl mercurial~inserts into the Si-H bond of optically active (+)RaSi*H with very high retention of configuration (22). Sommer proposed a three-center mechanism with direct electrophilic attack on the Si-H bond. Transition-Metal Catalyzed Reactions. The platinum catalyzed hydrosilation of 1-octene with optically active (+)RaSi*H occurs with a high degree (89-100%) of retention of configuration for three catalyst systems; 5% platinum on charcoal, chloroplatinic acid (HzPtCls. 6Hz0), and Pt(I1)-olefin complex, [(C2H3zPtC1z12(28). However, 10% palladium on charcoal or a Raney nickel catalyst system gives predominant inversion of configuration for the conversion of R3Si*H to R3Si*OR1 (24). RsSi*H R'
-
+ R'OH Catalyst RaSiaOR' =
H, alkyl, aryl, aeyl
(8)
Optically Active Disilanes. Compounds of the type PhsSiSi*PhMeX (X = H, CI, Br, OH, OMe) have been prepared, transformations of the X group studied, and absolute configurations assigned. The replacement of an R group in RIRzRIS~X with the triphenylsilyl group does not result in any significant changes in the stereochemistry of reaction at the asymmetric silicon (25).
The cleavage of the disilane (-)neoC5HlI PhMeSi*SiMePh, with lithium metal results in an optically active lithium reagent, neoC5HllPhMeSiXLi. The total reaction sequence showed that the lithium cleavage must occur with retention and a quasi-cyclic four-center mechanism was proposed (86). New Optically Active Organosilicon Systems. By the general reaction scheme outlined in eqn. (9), new optically active organosilicon compounds were synthesized with R = neopentyl, benzhydryl, and ethyl (27). a-NpPhMeSi'Cl
RLi
da-NpPhMeSi'R
I
The replacement of a-napthyl by diierent R groups results in no changes in predominant stereochemistry of reaction for some 21 different reactions. In addition, Corriu, et al., have recently reported the preparation of an optically active vinyl silane, ~NpPhVinylSiOMe. Retention of configuration is observed upon reduction to rrNpPhEtSiH with L i 1 H 4 ($8). These facts show that the generalizations about the stereochemistry of R3Si*Xare not artifacts of the ornapthylphenylmethyl system.
derivatives have also been studied (38). The preferred stereochemical path of reaction, inversion or retention, for these optically active germanes is the same as for their silicon analogues and Sommer's silicon stereochemistry rules successfully predict the stereochemistry observed for the corresponding - reactions of germanium . compounds. a-Napthylphenylmethylchlorogermane is optically stable in hydrocarbons, chloroform, and carbon tetrrtr chloride but solvents with a nucleo~hilicatom cause racemization (15~). Tin Derivatives
The resolution of an organotin compound was first described by Pope and Peachey in 1900 (33). Methylethyl-n-propyltin iodide was prepared and resolved using silver-d-camphorsulfonate. Attempts by Kipping (34) to use this method to resolve higher molecular weight mixed alkyl-aryltin iodides failed since no crystalline products formed with the various resolving agents. Moreover, later attempts to resolve methylethyl-mpropyltin iodide itself as described by Pope and Peachey were unsuccessful (35,36). A recent nmr study on B,B-dimethylphenethylmethylphenyltin chloride, I1 C1
Me
Germanium Derivatives
In 1963, Bott, et al., reported the first reliable resolution of an organogermanium compound with an asymmetric germanium atom, ornapthylphenylethylgermane (89). Very shortly thereafter, using the resolution method of Sommer (eqn. (I)), Brook and Peddle r e ported the synthesis and resolution of a-NpPhMeGeH, R3Ge*H(SO). The method of "quasi-racemates" was used to establish the relative configurations of the corresponding germane and silane, R3M*H. Thus, (+)R8Ge*H and (+)RaSi*H behave as a solid solution having a t most a one degree melting point range over a range of concentrations, but mixtures of (-)Ge*H and (+)Si*H give melting point ranges of 10-25 degrees over the same range of concentrations. The relative and therefore the absolute stereochemistry of (+)RsGeXH can be assigned (Fig. 7). The stereochemistry of many of the same reactions studied by Sommer on RaSi*Hwas then determined by Brook and Peddle on the germanium analogs (eqns. (10) and (11)) (31).
suggests that the failure to isolate optically active organotin halides is due to rapid inversion of confignration about the asymmetric tin atom (37). The doublet observed for the methyl peaks was shown to be due to diastereotopic shielding (58) of the neighboring asymmetric tin center and not to restricted rotation and collapsed to a singlet upon increasing the solvent polarity or concentration of tin compound and upon addition of small amounts of good ligands or other organotin compounds.
Figure 7. Abwlute sonflguration of (+)RsGe*H.
.-~p+e
!
Lead Derivatives
The only reported instance of an organolead compound with an asymmetric lead atom rests on indirect evidence and the data presented is inconclusive. AUStin in 1933 was unable to separate the diastereomers he claimed to obtain in an attempted resolution of an organolead compound (39). No organolead compound with an asymmetric lead atom has been resolved to date. Correlations and Summary Brewster's Rules
R
=
retention, I
=
inversion
Similar reactions of ornapthylphenylethylgermanium
Brewster's rules of atomic asymmetry (40) are applicable to the optically active silanes and germanes and their derivatives. Correct predictions of the sign of rotation can be made (assuming C1 > a-Np > Ph > Me > H in order of polarizability). This is demonstrated in Figure 8. Volume 46, Number 10, October 1969
/
643
t'h
Bh
orNp+-Me
o~p-$i-Me
k Figure 8.
Predistiml of sign of rotation by Brewrter'r ruler.
Isoconfigurotionol Series
Brook. defined an "isoconfigurational series" as a series of enantiomers, R1R2R1R4M,of identical structure and configuration differing only in the nature of the asymmetric atom M (41). a-Napthylphenylmethylmethane, -silane, and -germane constitute such a series (Fig. 10). Brook obtained a linear plot of [ale versus electronegativity of M (using Rochow's values (42)C, 2.60; Si, 1.90; Ge, 2.00). Since a property of the molecule is being measured and the [ a ] ~ values do not take into account the different molecular weights of the methane, silane, and germane, the molecular rotation (at the ORD maximum, [Mia,,) should he used. A plot of [MI versus electronegativity is also linear. The resolution of the corresponding tin and lead enantiomers would complete the isoconfigurational series but the finding of rapid inversion for asymmetric tin ($7) (and therefore probably also for lead) is a serious obstacle to achieving this objective. The studies on the resolution and the stereochemistry and mechanisms of reactions of asymmetric organosilicon and germanium compounds described above have shown the applicability, generality, and limitations of the stereochemical principles derived from the study of asymmetric carbon. These studies should aid greatly the understanding of the stereochemistry of other asymmetric atoms. Literature Cited ( 1 ) Soaomv, Y. I..AND REUTOY, 0. A,, Russ. Chem.Reos.. 1, l (1965). ( 2 ) Emona, C., A N D PIT%C.. Chem. andind.. 8-30 (1958). ( 3 ) Souren, L. II., A N D Far&,C. L., J. Am. Chem.Soc., 81, 1013 (1959). L. H.. Fnw. C. L., P*nxe~,G. A . . *no MICHAEL, K. W., ( 4 ) SOMMEB. J . Am. Chsm. Soc., 86,3271 (1964). ( 5 ) Fmoan, A., "The Svedberg." Aimquist and Wikeaells, Uppaala, 1944, P. 261.
644 / Journol of Chemical Education
A
m.p.
54-5592
63-64'C
74-75°C
[UID MI,,
+7.5O
+34.99
f26.7'
193'
623'
513'
Figure 9.
Therefore with Brewster's rules it would have been predicted that (+)SiH would give (-)Sic1 if the reaction proceeded with retention of configuration. The same accurate predictions can he made for the corresponding germanium compounds (51).
yh Np+-~e
Irosonflgurotiondseries.
M l s ~ o W K.. , &No HEFOLER, M., J . A m . Chem. Soc.. 74,3668 (1952). SoMr%n,L. H., Anseta. Chem.Int'1. Ed.. 1 , 143 (1962). ABHIDA,T., PEPINBET, R.. A N D OKAT*, Y., Abstraota. Intarnotianal Union of Cwatolloora~hvCon~resa,Rome. Italy. Sept. 1963. SOMMER, L. H.. F ~ Y EC.. L..A N D PARXBR. G. A., J . A m . Chom. Sac., 86, 5276 11(16*1. S o r w m , L. H., Fnrs. C. L., MUBOLP.M. C.. PARXER. G. A,. ROD* R., 3. w * m . P. G.. Mrm*e=. K. W . . Oa*r*, T.. A N D PEPINBHY, Am. O h m . Soc., 83,2210 (1881). SOMMBB, L. H.. PARKER, G. A,. A N D FRY=, C. L., J . Am. Chom. SOC.,86,
,-~~-..
r17M I1ORd> ------.,. ~
S o ~ r s s L. . H.. PARKZR. G. A,, LLOID. N . C.. FRIE, C. L., AND MIonmL, K. W., J. Am. Chern. Soc.. 89,857 (1967). SOMMER, L. H.. STARK. F. 0.. AND MICXAEL. K. W.. J . Am. Chem. Soc.. 86,5683 (19641. SoMMr;n, L. H.. "Stereoehemistry, Mechanism and Silicon," MoGrsw Hill Book Co... New York. -~ 1965. 17R~ ~ o. ~ .. . . ~ (a) C u m , F. R., Conmu, R. J. P.. Awn T x o m ~ s s r ~ R.. B.. Cham. Conm., 10, 560 (1968). (b) Conruv. R.. L m m . M.. nwo M ~ s a r . J., Bull. Soo. Chim. Flonce, to be published. Sorarsn. L. H.. K o n r ~ W. . D.. A N D R O D E W AP. ~ .G.. J. Am. Chem. Soc.. 89, 862 (1967). B ~ o o aA.G., , AND L ~ ~ a a nW a .. W . , J. Am. Chem.Soo..85,832 (1963). S o ~ ~ sL.n H.. . AND Fnre, C. L.. J . A m , Chern. Soc.. 82,4118 (18601. Bnoon, A. G., Wnnmn. C. M., A N D L~Msuna.W . W., Con. J . Chen.. 45, 1231 (1967). Sorrm. L. H., CITnoN, J. D.. AND Fsre, C. L.. J. Am. Chcm. Soc., 86. 5684 (1964). SoMMsn. L. H.. A N D MCLICR.I.. J . Am. Chem. Soo.. 88,5361 (1966). SOMMER. L. H.. UCLARD, L. A., A N D RITTER.A,, I . A m . Chcm. SOC.,90, 4486 -~~~11068>. , ~~~,~ S o ~ ~ eL. n .H.. MrcnAen, K. W.. A N D Funuo~o.H., J. Am. Chem. Sac.. 89, 1519 (1967). S o ~ u s nL. , H., A N D Lmws, J . E.. J. Am. Chcm. SDE.,89. 1521 (1907). SOMMER. L. H.. A N D ROSBOROU~H. K. T., J . Am. Chcm. Soc.. 89. 1756 ~
~
~....,.
114R1>
(26) SOMMER, L. H., A N D MABON.R . . J . Am. Chsm. Sac.. 87. 1619 (1965). L.~H., , M ~ c u e K. ~ .W . , AND KORTE,W . D.. J . Am. Chcm. (27) S O M M D Sm., 89. 808 (1907). Roro, , O., C.R. Acnd. SeC. Paris. Scr. C. 264, (28) Conmu. R.. M u s e , I. 987 (1967); C.A., 67.327254. (29) BOTT. R. W., EABORN, C.. AND YARN*, I. D.. Chem. o n d l n d . (London). 614 (1963). (30) Bnoon, A. G., m n PEDDLE, G. J. D., J. Am. Chem. SOE.,8s. 1870 (l9RS1 ~-...,.
(31) B n o o ~ A. , G., A N D Peoom, G. J. D., J . A m . Chem. Soo.. 8s. 2338 (1903). (32) E ~ s o n n .C.. S r ~ ~ s o P.. w . AND VABM*. I. D., J . Chen. Soc. ( A ) . 1133 (1906). W. J.. * N O P E A C ~ I 8..J.. Proe. C h m . Sac.. London, 16. 42. 110 (331 POPE. (1900). (341 KIPPIN..F. B., J. Cham. Sm.. 2365 (1928). (35) N ~ u r oS. ~ N. . . A ~ M * N U L K IZ.M.. N, J. Gen. Chim. (U.S.S.R.),S, 281
..""..
IlV60).
(36) RED&,G.. Ph.D. Thesis, University of Leioeatu. 1963. (37) P E ~ D LG. E .J. D., AND REDL,G.. Chem. Conm., 11, 626 (1968). (38) For example, VAN Gonxox. M . , AND H & m , G. E., Quart. Reus.. 22, 14 (1968). (39) A n a ~ mP. . R.. J . Am. Chern. Soc.. 55,2948 (1933). n . N..J. Am. Chcm. SDE.,81. 5475 (1959). (40) B n ~ r v a ~ s J. (41) Bno0x.A. G.. J.Am. Chem. Soc.. 85,3051 (1903). (421 ALLRGD, A. L., &ND ROCHOW, E. G., J.Inovo. Nud. Chcm., 5.269 (1958).
