Page 1024 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1008 enthalpies follow the same trend. For imines, there are two potential hydrogen
abstraction sites, the CH or NH. Similarly for nitrones derived from aldehydes, there are
CHAPTER 11 two possible sites for abstraction. In both cases, the H abstraction from N is preferred.
Free Radical Reactions Either of these sites can be eliminated from competition by substitution. Another study
of this type included thiocarbonyl groups. 104 For methyl radicals, the barrier to addition
at sulfur (∼2 kcal/mol) is substantially lower than at carbon (∼8 kcal/mol), although
both are lower than for the carbonyl group. The relative reactivity of alkenes, carbonyl,
thiocarbonyl, imine, and nitrone groups is relevant in intramolecular additions, which
are discussed in Section 11.2.3.2.
The energetics of addition reactions of alkyl radicals with aldehyde groups have
been studied as a function of radical structure. 105 According to G3(MP2) computa-
tions, alkyl radicals show a lower barrier to carbonyl addition as they become more
substituted. As shown in Figure 11.8, these reactions interrelate acyl, alkoxyalkyl,
and alkoxyl radicals. For the simplest system, the addition at O is also energetically
favorable, but the activation barrier is higher. Experimental studies indicate that the
barrier for H abstraction from formaldehyde by primary radicals is around 7.5 kcal/mol,
which is significantly lower than is calculated. Hydrogen abstraction from the -
position is calculated to have a significantly higher barrier and is not competitive.

11.2.3.3. Radical Cyclizations Section C of Table 11.3 shows some reactions
involving cyclization of unsaturated radicals. This type of reaction is an important
application of free radical chemistry in synthesis, and is discussed more thoroughly
in Section 10.3.3 of Part B. Rates of cyclization reactions have also proven useful
in mechanistic studies, where they can serve as reference points for comparison with
other reaction rates.
Entry 30 is the case of ring opening of the cyclopropylmethyl radical, which was
discussed on p. 973. Note that the activation energy is somewhat higher than a normal
single bond rotation but less than that for cyclohexane inversion. Entry 32 shows that

H H
+ C X C X or X or + HC X
CH 3 CH 3 CH 2 CH 3 CH 4
H H

# These reactions are not
RCHCH O energetically competitive.
α-H abstraction
# R'
addition
R R' ΔH ΔH ‡ O
at carbon acyl H abstraction
–12.7 + 6.7 R C R' RCH O RC O + R'H
H CH 3
H addition at R'
H CH 3 CH 2 –11.7 + 3.8 oxygen R R' ΔH ΔH ‡
H (CH 3 ) 2 CH –11.6 ~ 0 H –10.3 +15.8
R C OR' CH 3
H (CH 3 ) 3 C –12.0 – 3.3
H
Fig. 11.8. G3(MP2) computational H and H ‡ in kcal/mol for reactions of alkyl radicals with
aldehydes.

104 D. J. Henry, M. L. Coote, R. Gomez-Balderas, and L. Radom, J. Am. Chem. Soc., 126, 1732 (2004).
105
H. Hippler and B. Viskolcz, Phys. Chem. Chem. Phys., 4, 4663 (2002).