RESEARCH
Stereo Comes to Silicon Optically active organosilicons and their reactions can be tailored stereochemical!/ Ptriiiiav i\ ctiiia Slate- Univ t*iiii\, ^_aii Oc
ACS NATIONAL MEETING
135
Organic Chemistry Organosilicon chemistry enters a new phase—a stereochemistry based on MUcoii. And these are the developments that are starting it off: • Practical synthetic routes to optically active organosilicon compounds; these have reactive groups hooked on to asymmetric silicon. « Substitution réactions at asymmetric silicon that either retain or invert configuration. • An example of a Walden cycle (a reaction sequence that inverts the original asymmetric center—like turning an umbrella inside out) in substitution reactions at a silicon atom. Such stereochemistry, according to Leo H. Sommer and Cecil L. Five of
as significant for silicon as it's been for carbon. Sommer told the Division of Organic Chemistry that he now anticipates rapid advances in silicon chemistry, and feels that tin· silicone industry will benefit as a result. Optically active organosilicons were first made in lUOT. But, Sommer explains, their synthesis was lengthy and difficult. And the amounts obtained were small. Also, the rotations ([a]t>) of polarized light were so feeble that it was impossible to study the stereochemistry of substitution reactions at silicon. The Pennsylvania State work, supported by Dow Corning, will make such studies feasible, claims Sommer. • Pure Isomers Came First, The first step was to get optical isomers of organosilicon compounds (C&EW March 9, page 3S). Sommer and F rye got pure isomers of cx-naphtm/lphenylmethylsilane by a simple, two-stage process. First, fractional ciystalliza-
By choosing your reaction, you can invert an organosilicon... LiAIH,
tion separates the corresponding diastereomeric alkoxysilanes. Then stereospecific reduction with lithium aluminum hydride gives pure optical isomers. To demonstrate inversion, separation was followed by stereospecific reduction with lithium aluminum hydride in this manner: Chlorinating in rnrhon t e r r a f h l o r i d e c o n v e r t e d t h e ΓΙΡΥ-
trorotatory ( D ) isomer to its optical isomer, the levorotatory ( L ) . Result was a pure L-chlorosilane (90'','( yield). This ehloroderh ative, says Sommer, was then reduced with lithium alumi num hydride to give optically pure L-c^-naphthylphenylmethylsilane. The L- compound, he claims, is identical with the D- isomer in every respect ex cept for the direction of rotation, [C*]D. This is —32° for the L- isomer and -f-32 c for the D- isomer. The reaction sequence actually takes in both inversion of configuration and retention. But although the results definitely show pure retention for one reaction and pure inversion for the other, the decision that chlorination in verts is tentative. Future work should settle the question, says Sommer. A definite example of keeping the same optical structure occurs when the corresponding deuterosilane, [o:] D -4-32°, is reduced with lithium alumi num hydride. Resulting product is D-cK-naphthylphenylmethylsilane. Other optically active organosilicons, including R H Si°Br, R ; i Si°OH, and R 3 Si°OCH ; i , have also been made by Sommer's group.
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