Intramolecular Homolytic Translocation Chemistry:  An ab Initio Study of 1,n-Halogen Atom Transfer Reactions in Some ω-Haloalkyl Radicals

Abstract

Ab initio calculations using all-electron (3-21G(()()), 6-311G) and pseudopotential (DZP) basis sets, with (MP2, QCISD) and without (UHF) the inclusion of electron correlation, predict that 1,n-halogen transfer reactions in the 5-halo-1-pentyl (6), 6-halo-1-hexyl (7), and 7-halo-1-heptyl radicals (8) proceed via C(s)- and/or C(2)-symmetric transition states (9-11), except for the 5-bromo-1-pentyl (6, X = Br) radical for which a C(s)-symmetric transition state (9) was located only at the UHF/3-21G(()()) level of theory and the 5-iodo-1-pentyl radical (6, X = I) for which no transition state (9) could be located at any level of theory used in this study. Energy barriers for these translocation reactions of between 120.0 (1,7-iodine transfer) and 191.0 kJ mol(-)(1) (1,5-chlorine transfer) are predicted at the MP2/DZP level of theory; QCISD/DZP (single-point) calculations predict similar energy barriers. These high energy barriers are a consequence of unfavorable factors associated with ring size and long carbon-halogen separations in transition states (9-11) which lead to significant deviations from the collinear arrangement of attacking and leaving radicals preferred in transition states involved in homolytic substitution reactions at halogen. The dependence of transition state energy on attack angle at halogen has been explored for the attack of methyl radical at chloromethane. At the MP2/DZP level of theory, attack angles of between 80 and 90 degrees are calculated to lead to increases in energy barrier of about 100 kJ mol(-)(1) when compared with the collinear (180 degrees ) arrangement of attacking and leaving groups. The mechanistic implications of these predictions are discussed.

Homolytic 1,5-transfer of chiral organosilicon groups from an enoxy oxygen to an alkoxy oxygen—implications for mechanism

“…¶ Most importantly, 7 obtained in this manner displayed an optical rotation, [a] D 20 , of 23.3° (cyclohexane), in excellent agreement with that measured for the compound obtained from the translocation reaction (Scheme 3). In the analogous enantiomeric series of reactions, (R Si )-(+)-6 provided a translocated silane (7) with an optical rotation of +3.3° when reacted with tributyltin hydride in the manner described above, strongly suggesting that the translocated product is (R Si )-(+)-7. We conclude from these observations that, in agreement with our computational data, 9 the translocated products (7) are in all likelihood formed from 6 in mechanisms involving frontside homolytic substitution at the silicon atom (transition state 2) resulting in retention of configuration rather than through a backside mechanism or a pathway involving a hypervalent intermediate such as 3.…”

“…When (S Si )-( 2)-6 was reacted with tributyltin hydride as described in our previous publication, 2 the translocated silane, 2-[(methyl(1-naphthyl)phenylsilyloxy)methyl]cycloheptanone (7) was isolated as a mixture of diastereoisomers in 78% yield after workup and chromatography (Scheme 3). When prepared in this manner, 7 proved to be a viscous oil with a measured optical rotation, [a] D 20 , of 23.3° (cyclohexane) that failed to provide crystals for X-ray analysis.…”

Intramolecular Homolytic Substitutions in Synthesis

“…7 Wild also determined the angular dependence of the MP2/DZP calculated energy for the transition state (1) involved in the degenerate reaction of methyl radical with chloromethane. As the angle (θ ) is reduced from 180…”

“…γ -Thiolactones were prepared in 86 and 75% yields from 7 and 8 respectively, while secondary iodides 9 and 10 gave the corresponding γ -thiolactones in moderate yields ( Figure 1). 16 A typical example of the application of this chemistry is in the preparation of 4 d from 1-(tert-butylthio)-2-(iodomethyl)hexane (7). In this case, 7 (316 mg, 1.00 mmol), n-tributyltin hydride (351 mg, 1.20 mmol), and AIBN (33 mg, 0.2 mmol) were dissolved in benzene (100 ml) in a 200-ml stainless steel autoclave equipped with an inserted glass liner.…”

“…Computational chemistry as well as carefully constructed experiments have shed light on the underlying reasons for the trends observed for these group transfer reactions. [7][8][9][10][11][12] Other than the substitution at hydrogen, which would appear to "go around corners" because of the orbitals involved during homolytic substitution, for the remaining main group elements, the backside mechanism requires a collinear (or nearly so) arrangement of attacking and leaving radicals with deviations from this geometry resulting in unfavorable processes with high-energy transition states. For example, Wild showed that 1,4-, 1,5-, and 1,6-homolytic translocations of chlorine, bromine, and iodine have prohibitively high-energy barriers as determined by MP2/DZP calculations (Scheme 2) consistent with substantially "bent" transition state geometries at the halogen (120-145…”