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- The Flippin–Lodge angle is one of two angles used by organic and biological chemists studying the relationship between a molecule's chemical structure and ways that it reacts, for reactions involving "attack" of an electron-rich reacting species, the nucleophile, on an electron-poor reacting species, the electrophile. Specifically, the angles—the Bürgi–Dunitz, , and the Flippin–Lodge, —describe the "trajectory" or "angle of attack" of the nucleophile as it approaches the electrophile, in particular when the latter is planar in shape. This is called a nucleophilic addition reaction and it plays a central role in the biological chemistry taking place in many biosyntheses in nature, and is a central "tool" in the reaction toolkit of modern organic chemistry, e.g., to construct new molecules such as pharmaceuticals. Theory and use of these angles falls into the areas of synthetic and physical organic chemistry, which deals with chemical structure and reaction mechanism, and within a sub-specialty called structure correlation. Because chemical reactions take place in three dimensions, their quantitative description is, in part, a geometry problem. Two angles, first the Bürgi–Dunitz angle, , and later the Flippin–Lodge angle, , were developed to describe the approach of the reactive atom of a nucleophile (a point off of a plane) to the reactive atom of an electrophile (a point on a plane). The is an angle that estimates the displacement of the nucleophile, at its elevation, toward or away from the particular R and R' substituents attached to the electrophilic atom (see image). The is the angle between the approach vector connecting these two atoms and the plane containing the electrophile (see the Bürgi–Dunitz article). Reactions addressed using these angle concepts use nucleophiles ranging from single atoms (e.g., chloride anion, Cl–) and polar organic functional groups (e.g., primary amines, R"-NH2), to complex chiral catalyst reaction systems and enzyme active sites. These nucleophiles can be paired with an array of planar electrophiles: aldehydes and ketones, carboxylic acid-derivatives, and the carbon-carbon double bonds of alkenes. Studies of and can be theoretical, based on calculations, or experimental (either quantitative, based on X-ray crystallography, or inferred and semiquantitative, rationalizing results of particular chemical reactions), or a combination of these. The most prominent application and impact of the Flippin–Lodge angle has been in the area of chemistry where it was originally defined: in practical synthetic studies of the outcome of carbon-carbon bond-forming reactions in solution. An important example is the aldol reaction, e.g., addition of ketone-derived nucleophiles (enols, enolates), to electrophilic aldehydes that have attached groups varying in size and polarity. Of particular interest, given the three-dimensional nature of the concept, is understanding how the combined features on the nucleophile and electrophile impact the stereochemistry of reaction outcomes (i.e., the "handedness" of new chiral centers created by a reaction). Studies invoking Flippin–Lodge angles in synthetic chemistry have improved the ability of chemists to predict outcomes of known reactions, and to design better reactions to produce particular stereoisomers (enantiomers and diastereomers) needed in the construction of complex natural products and drugs. (en)
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- Flippin–Lodge angle, , for a nucleophilic "attack" at a carbonyl-containing electrophile, in Newman projection, with electrophile plane I viewed edge-on. (en)
- Flippin–Lodge and Bürgi–Dunitz nucleophile approach trajectory angles, and , analogized to azimuth and altitude parameters in the celestial (horizontal) coordinate system. (en)
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- Both in this celestial coordinate system, and in the system to describe a nucleophile approaching a planar electrophile, the problem is to uniquely describe the location of a point off of a plane, relative to specific point on the plane . Hence, in both cases, the problem can be addressed using two angles, an altitude-type angle and an azimuth-type angle.
The nucelophile is represented here as :Nu. The substituents, R and R', attached to the carbon atom of the carbonyl group may be an atom such as hydrogen , alkyl groups such as methyl, ethyl, etc. , or other functional groups such as the O- and N-containing groups of esters and amides. In both panels, a second plane, plane II , is defined that is orthogonal to the first, containing the carbon and oxygen atoms of the carbonyl, C=O, and bisecting the R—•—R' angle . A third plane, mutually orthogonal to the first two, plane III , contains only the carbon atom of the carbonyl; it corresponds to the plane of the page in the Newman projection at right. In that projection, the vector pointing from :Nu to the carbon of the carbonyl is mirrored across plane I to make clear that the nucleophile can approach from either above or below this plane . The dotted line from :Nu to plane I indicates the computational process of mathematically projecting the geometric point representing :Nu onto plane I, which is sometimes a necessary maths operation .
Note, for sake of clarity of presentation in the image at left, the nucleophile and azimuthal angle, , are shown as displaced toward the R substituent, while in the Newman projection, the opposite is the case. In this article, the R substituent is arbitrarily being assigned to the more sterically bulky group, and so the displacement shown at right is most representative of the phenomena described throughout. Note also, while elevation in celestial applications is most easily measured as the specific altitude shown, the elevation of the nucleophile, , is most easily measured as the supplementary angle, Nu-C-O, hence its values are most often >90° . (en)
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- FlippinLodgeHeatchcockNewmanCorrectPlanesBoxed.jpg (en)
- BurgiDunitzFlippinLodgeCelestCoordAnalogyBoxedCorr.jpg (en)
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- The Flippin–Lodge angle is one of two angles used by organic and biological chemists studying the relationship between a molecule's chemical structure and ways that it reacts, for reactions involving "attack" of an electron-rich reacting species, the nucleophile, on an electron-poor reacting species, the electrophile. Specifically, the angles—the Bürgi–Dunitz, , and the Flippin–Lodge, —describe the "trajectory" or "angle of attack" of the nucleophile as it approaches the electrophile, in particular when the latter is planar in shape. This is called a nucleophilic addition reaction and it plays a central role in the biological chemistry taking place in many biosyntheses in nature, and is a central "tool" in the reaction toolkit of modern organic chemistry, e.g., to construct new molecules s (en)
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