CH3 (δ 2.22, d, J ) 1.6 Hz) and for a single alkenyl proton
(δ 6.22, m). The faster eluting species displayed spectral data
that were strikingly similar to those reported for the charac-
terized reference compound des methyl 11a,7 allowing ready
assignment of 11a as a pyrrole. X-ray crystallographic analy-
sis of 11a later confirmed the structural assignment (see Sup-
porting Information). Examination of the crude reaction pro-
duct by 1H NMR prior to purification indicated the presence
of only 7 and 10; pyrrole 11 was not formed until exposure
of the crude thermolysate to SiO2. The ratio of isolated 7 to
isolated 11 occasionally was at variance with the 7/10 ratio
as it appeared in the 1H NMR spectrum of the crude thermo-
lysate, presumably as a consequence of differential chromato-
graphic stability, and so the latter value is reported as well.
In addition, 1H NMR-based examination of the crude thermo-
lysate immediately after reaction allowed identification of
signals consistent with the indole species 10, but these signals
decayed over the course of a few hours as the spectroscopic
signature for the pyrrole 11 grew in. In only a few cases
(6g, 6i) was this N-C(2) cyclopentannelated indole isolable,
but even in those instances, isomerization into the pyrrole
followed after a few more hours at room temperature. The next
three examples (6b-6d) demonstrated that both silyl ethers
(6b,c) and steric bulk (6d) at the R2 position are tolerated in
the transformation with little impact on yield. Interestingly,
comparing entries a-d reveals that the ratio of C(3)/N cycli-
zation is responsive to the size of the R2 substituent, with the
bulkiest entry (6d, R2 ) t-Bu) leading to the greatest selec-
tivity for the desired C(2)-C(3) cyclization regioisomer 7.
12g whose structure was secured by single-crystal X-ray
analysis (see Supporting Information). The final two entries,
6h and 6i, probed the capability of this cascade cyclization
sequence to deliver tetracyclic material. The cyclohexenyl
case 6h proceeded uneventfully to deliver a slightly biased
mixture of the pyrrole 11h and the indole product 7h in
excellent overall yield. The cyclopentyl lower homologue
6i, in contrast, yielded only a moderate amount of the
N-C(2) cyclized indole product 10i as the only tetracyclic
material to survive chromatography. The corresponding C(3)-
cyclized material 7i was observed in the crude thermolysate’s
1H NMR spectrum, at the ratio reported in Table 2, but it
decomposed upon attempted purification.
Thermolyses of the aryl-substituted 2-(allenyl)phenyl
azides 6j-6m did not provide tetracyclic material (Scheme
3). In each instance, the methylidene-containing products
Scheme 3. Thermolysis of Phenyl-Substituted
2-(Allenyl)phenyl Azides
The next two entries (6e,f) test the effect of a substituent
at the internal (R) position of the alkene. For both substrates,
the reaction proceeds similarly to the simpler R ) H cases
to afford nearly equal mixtures of the indole 7e/7f and pyrrole
11c/11f products. The R substituent resides at a position that
apparently exerts little steric or electronic influence on the
course of the reaction, as both the R ) CH3 and R ) Ph
cases proceed to product(s) in very similar yields/selectivities.
The Ph-bearing substrate 6g introduces at the alkene terminus
a group that might confer both electronically favorable (i.e.,
radical stabilizing; cf. 4) and sterically unfavorable charac-
teristics, and the tradeoff between these possibly opposing
effects appears to favor the latter. For the first time, the
C(2)-C(3) cyclized indole product did not survive attempted
chromatographic purification, and the regioisomeric N-C(2)
indole 10g was the only identifiable species isolated.
However, the indole 7g was detected in the crude thermoly-
sate admixed with 10g (1:1.1 ratio, Table 2). Hydrogenation
of 10g (Scheme 2) furnished the cis-disposed indole product
14j-14m were isolated in moderate yields. Presumably, a
diyl intermediate represented by 13 underwent H-atom trans-
fer, possibly via an intramolecular pathway as shown, to yield
the product alkene. These results stand as another point of
departure from the saturated tether series 1, where pendant
aryl rings (RkR1 ) aryl ring) did participate in diyl cycliza-
tions to afford benzannelated products 5 (RkR1 ) aryl ring
via alkene isomerization). The failure of the analogous species
13 to cyclize remains a mystery, but a rationale might be tied
to the different nature of the diyl intermediate in the unsatur-
ated series vis a´ vis the saturated series 1, as discussed below.
Given the similarities and the differences between the
reactions of the diyls derived from 1 and 6, it is appropriate
to consider the conceivable roles that the different candidate
diradicals might play in product formation. Figure 1 details
the plausible diyl (and related) options. Singlet σ/π orthogo-
nal diyl 15a is the likely first-formed species immediately
following N2 extrusion. This presumably short-lived diyl
could cyclize via initial C-C bond rotation and then C-N
closure to give products of the type 10, or it could undergo
singlet-triplet interconversion to give a new diyl that could
not cyclize directly and therefore might have a long enough
lifetime to realize other chemistry. As an alternative to direct
cyclization, singlet or triplet 15a could undergo an electronic
reorganization that is tantamount to placing one electron from
the nitrogen’s π orbital into its half-occupied sp2 orbital,
giving the orthogonal π/π diyl 15b (singlet or triplet).
Scheme 2. Formation of a Derivative Characterizable by
X-ray Crystallography
(7) Kashulin, I. A.; Nifant’ev, I. E. J. Org. Chem. 2004, 69, 5476-5479.
Org. Lett., Vol. 8, No. 14, 2006
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Feldman, Ken S.
