Absolute Stereochemical Determination of Organic Molecules through Induction of Helicity in Host-Guest Complexes.

Stereochemistry is a fundamental molecular property with important ramifications for structure, function, and activity of organic molecules. The basic building blocks of living organisms (amino acids and sugars) exhibit a precisely selected set of molecular handedness that has evolved over millions of years. The absolute stereochemistry of these building blocks is manifested in the structure and function of the cell machinery (e.g., enzymes, proteins, etc.), which are essential components of life. In the many chemical subdisciplines, molecular stereochemistry is exceedingly important and is often a strong determinant of structure and function. Besides its biological implications, the centrally important role of stereochemistry in many disciplines of chemistry and related fields has led to tremendous effort and activity, highlighted by the success in stereoselective syntheses of a host of functionalities. In the present climate, it is often the difficulty of assigning absolute stereochemistry as opposed to synthesis, which has become a nontrivial challenge, requiring the attention of the community. There will not be a general solution to this problem, as each system will have its own unique requirements and challenges; however, the need for rapid, routine, and microscale analysis is apparent. This is especially true with parallel and high-throughput arrays for screening conditions and catalysts, generating a large number of samples that require analysis.In this Account, we summarize our contribution to this field through the development of molecular receptors for sensing molecular asymmetry. These methodologies strive to unambiguously assign the absolute configuration of asymmetric center(s). To accomplish this task, our laboratory has designed a variety of host molecules, bearing various binding elements, to form stable complexes with chiral molecules (guests). During this complexation event, the stereochemistry of a target molecule induces a supramolecular chirality (i.e., helicity) within the host system. The design of the host system is such that the helicity of the host/guest complex can be observed and assigned via Exciton Coupled Circular Dichroism (ECCD), a nonempirical technique for identifying handedness, which is correlated back to the absolute stereochemistry of the bound chiral molecule. Taking advantage of the high sensitivity of chiroptical techniques (in terms of the required amount of sample for analysis) and fast response time, these methodologies offer a microscale, rapid, and nonempirical solution for assignment of absolute stereochemistry.The first part of this Account describes application of porphyrin tweezers as reporters of chirality for the absolute stereochemical determination of various classes of organic molecules. This methodology is suitable to report the absolute configuration of organic molecules that contain binding elements (nitrogen or oxygen based functionalities). In the second part, host systems that do not require two sites of attachment to form ECCD active complexes will be described. This enables the absolute stereochemical assignment of challenging chiral molecules with functional groups lacking routine techniques for analysis.