Anti-Bredt compound

Yes, XIV contains a bridgehead olefin. However, anti-Bredt compounds are not anymore the indiscretions they used to be years ago. Although still strained, they are capable of independent existence at room temperature, so much so that several members of this odd family have been synthesized. Then,... 

Violations of the rule are possible, especially if one of the rings has eight atoms or more, and structures such as 31 (Figure 3.43) are known as "anti-Bredt compounds. Anti-Bredt compounds with smaller rings may also... 

Still another example is the easy formation of anti-Bredt bicycloalkenones (see p. 188). As indicated above, a,a -dihalophosphoranes can be used to prepare 1,1-dihaloalkenes. Another way to prepare such compounds is to treat the carbonyl compound with a mixture of CX4 (X Cl, Br, or I) and triphenylpho-sphine, either with or without the addition of zinc dust (which allows less Ph3P to be used). " ... 

Compounds with at least one stereogenic center and one stereogenic axis (this also includes anti-Bredt-olefins that exist in two stable atropisomeric forms). 

For a review on chiral anti-Bredt rule compounds see M. Nakazaki, Top. Cun. Chem. 125. 1 (1984). 

Among a variety of compounds whose molecular characteristics fall into this category, our current interests have further limited our discussion to the doublebond systems which may conveniently be classified as follows 1. chiral ( )-cyclo-alkenes, 2. chiral anti-Bredt rule compounds, 3. trans-doubly bridged ethylenes ( betweenanenes ), 4. bridged allenes, and 5. overcrowded olefins. 

Considering the impressive accumulation of review articles 42a f) on the Bredt rule and number of papers reporting exotic anti-Bredt rule compounds, it is rather sur-... 

The chiral C2-conformer 42 of a ( )-cycloalkene can transform into the enantiomeric Cf-conformer 44 through a planar Cs-conformer 43, and this rope jump racemization can be prevented by anchoring one end of the unsaturated center onto the ring by means of an extra-bridge. This bridging creates a bicyclic anti-Bredt rule compound 45, revealing that all anti-Bredt rule compounds (45) with one double bond are necessarily asymmetric (C, symmetry) and have one asymmetric carbon atom. 

Comparison of (—)-(R)-( )-cyclooctene (50) and the (—)-(S)-anti-Bredt rule compound 51 would suggest their close structural relationship, and this is reflected in their respective absolute rotation values 5,16b) [otJoabs —458° (neat) and —725° (CHC13) as well as in their respective (- )-Cotton curves [0] —1.4x 10s at 196 nm (cyclohexane) and —13.6 x 10s at 213 nm (isooctane). 

The ( )-cyclooctene moiety can also be seen in bicyclo [4.2.1]l(8)-nonene (53) and bicyclo[4.2.1]l(2)-nonene (54), the smallest isolable members among anti-Bredt rule compounds 48). 

Since a single anti-Bredt rule compound is neccessarily asymmetric, combination of two of them should afford double anti-Bredt rule compounds ( bridgehead dienes ) either of Cs (or Q) symmetry or of C2 symmetry depending upon the ways in which the enantiomers are combined to make up the molecules. 

While the achiral double anti-Bredt rule compound 56 50) of Cs symmetry was isolated in an impure state and was found to be very labile toward 02 and heat, the chiral 57 of C2 symmetry was assumed to exist very briefly in the pyrolysis of 3,6-dimethylidene-l,7-octadiene51). 

This result suggests that the anti-Bredt s rule olefin if may be an isolable compound. A number of bicyclic bridgehead olefins (e.g. 52) have recently been prepared, and their stabilities related to the corresponding frans-mono-cyclic olefins 11S>116). The stability of 51 should correlate, then, with the... 

Generally, bicyclic compounds (71) containing a bridgehead double bond and with S 5 7 may be regarded as an anti-Bredt s rule compound, which is quite unstable because of high ring strain energy (33). 

In 1967, two American groups first succeeded in the synthesis of bicyclo[3.3.1]non-l-ene, an anti-Bredt s rule olefin (72) (34). Since then several isolable anti-Bredt s rule olefins with S = 7 have been prepared (35). The heterocyclic anti-Bredt s rule compound was first produced... 

Should one ask for direct enthalpy of formation investigations on anti-Bredt enamines, such studies are currently limited to l,6-imino[10]annulene (14) with its gas phase enthalpy of formation of 367.2 7.0 kJ mol-1. How do we affirm the decrease in resonance energy accompanying its geometric constraints There are no enthalpy of hydrogenation measurements for this compound, nor for any other 11-azabicyclo[4.4.1]-undecane (30) derivative71. However, we do know the gas phase enthalpy of formation... 

A successful example of insertion reactions in the synthesis of N-bridged compounds is the Rh2(OAc)4-catalyzed decomposition of /3-lactams 705, which was employed as a key step in the synthesis of several anti-Bredt /3-lactams with nitrogen in the bridgehead (86JA6431 89JA1073). 

Twist as well as pyramidalization are the typical deformations of bridgehead double bonds in bicyclic compounds of type 44 (Table 4). Structures of the type 44 comprise Bredt-olefms (formerly called anti-Bredt-olefins) and bridgehead olefins. In Bredt-olefins, the double bond is located at the bridgehead of a bicyclo[m.n.o]alkane skeleton with m,n,o 1, whereas... 

Bridgehead alkenes, often referred to as anti-Bredt molecules, are indeed unstable. The problem with these compounds can be understood by inspection of orbital overlap in such systems. The p-orbital at the bridgehead is far from the necessary coplanarity with the other p-orbital. In order to achieve n-bonding, the system must be strongly twisted. In the extreme cases, where the p-orbitals of the double bond are close to orthogonality, such molecules can be considered as electronic analogues of excited states of alkenes (Figure 2.26). 

Figure 6.103 Top Planar and twisted amide bonds. Bottom "Anti-Bredt s" amides - nitrogen atoms at the bridgehead of bicyclic compounds cannot participate in negative conjugation characteristic of amides.

---