p-Mediated rearrangements and 1,2-H shifts of indanylcarbenes
Robert A. Moss,* Wei Ma and Ronald R. Sauers*
Department of Chemistry, Rutgers University, New Brunswick, New Jersey 08903, USA.
E-mail: moss@rutchem.rutgers.edu
Received (in Corvallis, OR, USA) 15th December 1998, Accepted 29th January 1999
Absolute rate constants determined for 1,2-C and 1,2-H
migrations of cyclobutyl-, cyclopentyl-, benzocyclobutenyl-
and benzocyclopentenyl-(chloro)- or -(acetoxy)-carbenes re-
veal that ‘phenyl’ carbon migrations are preferred to
alternative 1,2-C shifts due to p-electronic effects.
1,2-Ac migration4,9 in ratios of 1.17 : 1 (hn) or 1.23 : 1 (78 °C);
no 1,2-C shift product was observed. Authentic 6 was readily
obtained by reaction of cyclopentanecarbaldehyde with Ac2O,10
whilst 7 was prepared from the same starting material via
conversion to E/Z-1-cyclopentylpropene with Ph3PNCHMe,
followed by oxidation with KMnO4.11
The 1,2-C migration of ‘phenyl’ carbon a is strongly preferred
to that of ‘benzyl’ carbon b in benzocyclobutenylcarbenes 1-Cl,
1-F and 1-OAc, whilst the 1,2-H shift is virtually non-
Indan-1-yl(chloro)carbene (4-Cl) produced H-shift products
(E/Z)-8-Cl, as well as the isomeric 1,2-C shift products
X
CHX
COAc
X
X
X
X
H
:
X
H
a
a
:
b
H
8-X
9-X
10-X
11
H
b
1-chloroindene (9-Cl, via Ca migration) and 2-chloroindene
(10-Cl, via Cb migration). The GC product distribution
(8-Cl : 9-Cl : 10-Cl) was 8.2: 1.0 : 0.03 (hn) or 10.0: 1.0: 0.03
(78 °C). Authentic chloroindenes were prepared by reactions of
a-tetralone (for 9-Cl) or b-tetralone (for 10-Cl) with PCl5 in
benzene.12 Alkenes 8-Cl (E/Z ~ 5) were synthesized in 60%
yield by NaOH-induced elimination of HCl from 1-dichloro-
methylindane, itself prepared by reaction of photogenerated
4-Cl with HCl in pentane.
From acetoxyindanylcarbene, 4-OAc, we obtained H-shift
products (Z/E)-8-OAc, C-shift products 9-OAc (via Ca migra-
tion) and 10-OAc (via Cb migration), and the acetyl migration
product, dione 11. The 8-OAc : 9-OAc : 10-OAc: 11 product
1-X
2-X
3-X
4-X
X = Cl, OAc, or F (for 1)
competitive.1 The 1,2-C shift specificity is attributable to
mediation by the phenyl p-system.1 Given the somewhat
unusual nature of cyclobutylcarbenes,2 such as 2-Cl3 and
2-OAc,4 where relief of strain may drive the 1,2-C shift ring
expansion, we now extend our studies to cyclopentylcarbenes
3-Cl and 3-OAc, as well as their benzo derivatives, the
indanylcarbenes 4-Cl and 4-OAc. The results imply that,
despite dominant reassertion of the 1,2-H shift with carbenes
3-X and 4-X, p-assisted 1,2-Ca migrations remain favored over
unassisted Cb shifts.
distributions
were
9.0 : 1.0 : 0.29: 5.3
(hn)
and
10.3: 1.0 : 0.24: 5.2 (78 °C). Authentic samples of 8-OAc,
9-OAc and 10-OAc were synthesized by reactions of indane-
1-carbaldehyde, a-tetralone or b-tetralone, respectively, with
Ac2O;10 whilst dione 11 was identified by its GC-MS and
associated cracking pattern.
Absolute rate constants (reproducibility ~ ±15%) for the
carbenic rearrangements were determined at 25 °C by laser
flash photolysis (LFP)9 of pentane solutions of the appropriate
diazirines (Amax = 0.5–0.7 at lmax) at 351 nm, using the
pyridine ylide method.4,9,13 Growth of the carbene–pyridine
ylide was monitored at 390 nm. Details of this methodology, as
applied to the carbenic rearrangements, have been de-
scribed.4,9
The absolute rate constants determined for the aggregate
rearrangements of carbenes 3-Cl, 3-OAc, 4-Cl and 4-OAc were
(where appropriate) partitioned into kH, kC and kAc, in accord
with the product distributions given above. In the case of 4-Cl,
the product distribution is likely biased by rearrangement
directly from the excited diazirine,1,3a,14 so that we partitioned
the aggregate rate constant using the 78 °C thermolysis product
distribution. Excited state diazirine rearrangements are not
expected in the acetoxycarbene series.4,15 The partitioned rate
constants appear in Table 1, together with related data for
carbenes 1-Cl and 1-OAc,1 2-Cl3 and 2-OAc,4 Me2CHCCl16
and Me2CHCOAc.4 The 2-Cl rate constants are corrected for
excited diazirine contributions,3 whereas the 78 °C thermal
product distribution is given for 1-Cl.1
All carbenes were generated from appropriate diazirine
precursors. 3-Chloro-3-cyclopentyldiazirine (lmax 348, 364 nm,
pentane) was prepared in 35% yield by NaOCl oxidation5 of
cyclopentanecarboximidamide·HCl,6 whereas 3-acetoxy-3-cy-
clopentyldiazirine (13%, lmax 342, 354 nm, pentane) was
obtained from a ‘modified’ Graham oxidation (LiOAc,
NaOCl)4 of the same amidine.† For the indanyl precursors,
indan-1-one was converted (40%) to 1-cyanoindane7 with
tosylmethyl isocyanide and NaOEt in DME. The cyanoindane
gave 58% of indane-1-carboximidamide·HCl upon reaction
with MeClAlNH2 (toluene, 80 °C, 30 h),8 and the amidine was
oxidized to 3-chloro-3-(indan-1-yl)diazirine (30%, lmax 330,
346, 360 nm, pentane) with NaOCl,5 or to the analogous
acetoxydiazirine (9%, 338, 352 nm, pentane) with LiOAc/
NaOCl.4† All diazirines were purified by chromatography on
silica gel and characterized by NMR and UV spectroscopy.
Carbenes 3-X and 4-X were generated by photolysis (l >
320 nm, 25 °C) and by thermolysis (78 °C) of pentane solutions
of the diazirines (A = 0.5–0.7 at lmax). Products were identified
by GC-MS, and confirmed by comparisons with independently
synthesized samples, or were isolated and characterized by
NMR and GC-MS. All new products gave acceptable elemental
analyses or high resolution MS molecular ions. All major
products ( > 5%) were characterized.
From 3-Cl, we obtained only 1,2-H shift product 5, which
was synthesized from cyclopentanone and Ph3PNCHCl. Car-
bene 3-OAc gave enol acetate 6 by 1,2-H shift, and dione 7 by
The most immediate observation from Table 1 is the
dominance of the 1,2-H shift in the chemistry of the five-
membered ring carbenes. In the absence of strain relief to drive
the ring expansion, as in cyclobutylcarbenes 2-OAc and 2-Cl,
CHCl
CHOAc
COAc
5
6
7
Chem. Commun., 1999, 467–468
467