Photocycloaddition of Some Difluoro(aminoenonato)boron Complexes with Arylalkenes

Article abstract of DOI:10.1002/hlca.200490027

The photocycloaddition of some difluoro [(methylamino- κ N)alkenonato-κO]boron complexes 1 with arylalkenes 2 is discussed. The resulting [2 + 2] photoaddition gave the cyclobutane and azetidine derivatives (Schemes 1, 3, and 5). Rearrangements of the cyclobutane gave 1,5-diketones derivatives (Schemes 2, 4, and 5). The yields of the photoadducts were governed by the reduction and oxidation potentials. Furthermore, the configurations of the products established high regio- and stereoselectivity, suggesting the presence of a singlet exciplex. The reactivity and the stereochemistry were rationalized by means of FMO (frontier molecular orbital) calculations.

Full text of DOI:10.1002/hlca.200490027

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Helvetica Chimica Acta Vol. 87 (2004)
Photocycloaddition of Some Difluoro(aminoenonato)boron Complexes
with Arylalkenes
by Kuniaki Itoh*a), Kazuhiko Okazakia), and Yuan L. Chowb)
a) Department of Chemistry, Faculty of Science, Kobe University, Nada-ku, Kobe, 657-8501, Japan
(tel. and fax: ‡81(78)8035695; e-mail: kuitoh@kobe-u.ac.jp)
b) Department of Chemistry, Simon Fraser University, Burnaby, B.C., V5A 1S6, Canada
The photocycloaddition of some difluoro[(methylamino-kN)alkenonato-kO]boron complexes 1 with
arylalkenes 2 is discussed. The resulting [2 ‡ 2] photoaddition gave the cyclobutane and azetidine derivatives
(Schemes 1, 3, and 5). Rearrangements of the cyclobutane gave 1,5-diketones derivatives (Schemes 2, 4, and 5).
The yields of the photoadducts were governed by the reduction and oxidation potentials. Furthermore, the
configurations of the products established high regio- and stereoselectivity, suggesting the presence of a singlet
exciplex. The reactivity and the stereochemistry were rationalized by means of FMO (frontier molecular
orbital) calculations.
Introduction. The photochemical cycloaddition of 1,3-diketones (via their enols)
with various olefins has been generally known as the de Mayo reaction [1], which,
because of its versatile intermediates, can be useful in natural-product synthesis [2].
Many reports concerning the mechanism of the photoaddition have appeared [3].
However, except for cyclic imino ketone compounds [4], the photoreaction of a
corresponding carbonyl compound containing an a-enamine group moiety (amino-
enone) has not been investigated, although it is known that the expected photoadducts
may serve in various ways as potential intermediates leading to N-atom-containing
compounds. Notably, irradiation of such aminoenones with a considerable number of
olefins yields no photoadducts [5]. Thus, because of the greatly increased reactivity of
1,3-diketones after complex formation with boron trifluoride [6], we prepared some
(aminoenonato)difluoroboron complexes. In a previous paper [7], we reported that, in
the reaction of cyclic and acyclic olefins, only a (3-amino-1,3-diphenylprop-3-en-1-
onato)difluoroboron complex (ˆdifluoro[(1Z)-2-(methylimino-kN)-1,3-diphenyl-
prop-1-en-1-ol-kO]boron) generated a reasonable amount of certain products and
furnished interesting derivatives from the [2 ‡ 2] photoadduct. Here, we examine the
photoreactions of some difluoroboron complexes, i.e., of 1a c, with arylalkenes 2a d
and report the photoproducts, the reactivity, and the reaction scheme.
Results. Irradiation of the B-complexes 1a c and trans-stilbene (2a) with a high-
pressure Hg lamp for 24 h in a Pyrex test tube gave the cyclobutane derivatives 3a c
and the azetidine derivatives 4a c (Scheme 1) Table 1. Each of the photoadducts was
obtained in pure form after column chromatography followed by recrystallization. In
the reaction of 1a and 1b, the yields of the cyclobutane derivatives 3a, b and of the
azetidine derivatives 4a, b were about equal, but, in the reaction of 1c, the cyclobutane
derivative 3c was obtained as the major product besides a small amount of 4c. The

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293
overall yield of the products increased with the number of Ph groups in the complex.
The structures of the products were determined on the basis of the spectral data.
‡
Typically, the MS of 3b showed a molecular-ion peak (M at m/z 403) indicating the addition of stilbene
(2a) to the B-complex 1b. Its 1H-NMR spectra displayed the signals due to the three CH groups (d 4.07, 4.33, and
5.11) characteristic of cyclobutane formation. A 13C-NMR signal for the imino C-atom was observed at d 183.8.
The configuration of 3b was mainly determined by NOESY (nuclear-Overhauser-effect spectroscopy)
experiments. A cross-peak appeared for HÀC(2)/HÀC(3), but no NOE was observed for HÀC(3)/HÀC(4),
indicating cis-orientation for the former and trans-orientation for the latter.
Scheme 1a)
a) For R1 and R2, see Table 1.
Table 1. Photocycloaddition of B-Complexes 1a c with trans-Stilbene (2a)
R1
R2
Products
Recovered 1
3
4
1a
1b
1c
Me
Ph
Ph
Me
Me
Ph
12%
39%
74%
14%
32%
8%
55%
The determination of the structure of the azetidine derivatives 4a c was considered in more detail. In a
COLOC (correlation by long-rang coupling) spectrum of 4b, e.g., a cross-peak appeared for the MeN protons (d
2.52) and C(4) of the ring (d 66.5), but no cross-peak existed for the MeN protons and the olefinic C-atom
(d 151.2). These observations indicated that 4b should have an azetidine rather than an oxetane structure. The
NOESY plots of 4b showed that both HÀC(3)/HÀC(4) and HÀC(4)/MeN are trans-orientated.
In addition, the structure of 3b was confirmed by alkaline hydrolysis to give 1,2,3-triphenylhexane-1,5-
dione (5b) in 46% yield by a retro-aldol ring cleavage, and 3,4,5-triphenylcyclohex-2-en-1-one 6b in 42% yield)
by subsequent aldol-cyclization/dehydration (Scheme 2).
Irradiation of 1,1-diphenylethene (2b) and the B-complexes 1a c under conditions
similar to those used for trans-stilbene (2a) gave two isomers of the cyclobutane
derivatives, i.e. 7b,c and 8b,c by [2 ‡ 2] photoaddition (Scheme 3 and Table 2). Product
yields were remarkably lower than in the case of trans-stilbene, in particular, substrate
1a did not react. The ratios of the amounts of 7 and 8 were approximately equal. These
cyclobutane photoadducts 7 and 8 are regioisomers at the position of addition to 1,1-
diphenylethene (7: head-to-head; 8: head-to-tail). This position was determined from
1
the coupling of the protons of the cyclobutane ring in the H-NMR spectra (7b: J ˆ
8.8 Hz (d 3.64); 7c: J ˆ 9.5Hz ( d 4.01); 8b: s at d 4.55; 8c: s at d 4.83).

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Helvetica Chimica Acta Vol. 87 (2004)
Scheme 2
Scheme 3a)
a) For R1 and R2, see Table 2.
Table 2. Photocycloaddition of B-Complexes 1a c with 1,1-Diphenylethene (2b)
R1
R2
Products
Recovered 1
7
8
1a
1b
1c
Me
Ph
Ph
Me
Me
Ph
97%
74%
58%
10%
19%
7%
18%
Irradiation of styrene (2c) with the B-complexes 1a c under conditions similar to
those used for trans-stilbene (2a) gave only the 1,5-diketone derivative 9c (Scheme 4,
Table 3), no products being isolated from 1a and 1b, i.e., the product yield decreased
even more than in case of 2b with respect to 2a. Diketone 9c is derived by a retro-aldol
ring cleavage from the head-to-head (not the head-to-tail) cyclobutane adduct, as it is
the case for 1,3-diketones in the well-known de Mayo reaction.
Irradiation of the a-methylstyrene (ˆ2-methylprop-1-ene; 2d) and the B-com-
plexes 1a c under conditions comparable to those used for trans-stilbene (2a) gave the
cyclobutane derivatives (10b,c, 11b,c, and 12b, the azetidine derivative 13b, and the 1,5-
diketone 14c (Scheme 5, Table 4)). The product yields are much higher than in the case
of 1,1-diphenylethene (2b). The head-to-head adducts 10 and 11 are present in greater
amounts than the head-to-tail adduct 12, and compounds 10 with cis-positioned Ph
groups are much more abundant than the corresponding trans-isomers 11. The

Helvetica Chimica Acta Vol. 87 (2004)
Scheme 4a)
295
a) For R1 and R2, see Table 3.
Table 3. Photocycloaddition of B-Complexes 1a c with Styrene (2c)
R1
R2
Product 9
Recovered 1
1a
1b
1c
Me
Ph
Ph
Me
Me
Ph
ca. 100%
65%
27%
53%
Scheme 5a)
a) For R1 and R2, see Table 4.
Table 4. Photocycloaddition of B-Complexes 1a c with 1-Methylstyrene (2d)
R1
R2
Products
Recovered 1
10
11
12
13
14
1a
1b
1c
Me
Ph
Ph
Me
Me
Ph
ca. 100%
52%
14%
28%
8%
5%
17%
2%
26%
27%
configuration of these products was determined from the coupling of the methine
proton of the cyclobutane ring in the 1H-NMR spectra. (10b: J ˆ 6.3, 11.0 Hz (d 3.43);
11b: J ˆ 9.7 Hz (d 3.83); 10c: J ˆ 6.6, 11.6 Hz (d 3.86); 11c: J ˆ 9.3 Hz (d ˆ 3.10)). The
cis/trans forms were distinguished by the presence or absence of a cross-peak between

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the Me protons and the methine proton in the NOESY plots (11b: d 1.11 3.83; 11c:
d 1.10 3.10; 12b: d 1.92 3.72). In the case of 1c, a considerable amount of 1,5-
diketone 14c derived from the head-to-head cyclobutane adduct was obtained. As in
the case of trans-stilbene (2a), a small amount of the azetidine derivative 13b was
obtained.
Discussion.
The presented photocycloadditions revealed some interesting
features: i) The yields of the photoproducts are governed by both the reduction and
the oxidation potentials of the substrates. ii) The stereochemistry of the reactions shows
high regio- and stereoselectivity. The structures of the products have sterically
unfavorable cis- or endo-configuration. iii) The formation of the azetidine derivatives is
encountered less frequently than that of the cyclobutane derivatives. iv) In some [2 ‡ 2]
photoadducts, a retro-aldol reaction occurs, producing the 1,5-diketones. Some of these
phenomena (i) iii)) may be readily interpreted by assuming the presence of an adduct
(intermediate) such as the B-complex 1/arylalkene 2 complex, i.e., as an exciplex [7][8]
produced via a photoinitiated electron-transfer reaction. The difluoroboron complex is
known to possess a lower reduction potential than that of the parent didentate ligand
[9], and those of 1a c are lower by 0.4 0.5V. The oxidation and reduction potentials
of the B-complexes 1a c, the parent didentate ligands 1a' c', and the arylalkenes 2a
d are summarized in Tables 5 and 6. With an increasing number of phenyl groups, the
potentials of 1 decrease. Although the parent didentate ligands 1a' c' are not
photoreactive, their complexes 1a c are reactive. Thus, the difluoroboron complexes
are used as they are the stronger electron acceptors. The ease of adduct formation
suggests a high reactivity, in the order 1a < 1b < 1c. On other hand, the oxidation
potential of trans-stilbene (2a) is lower than that of the other arylalkenes 2b d by ca.
0.3 V, and its high reactivity is consistent with the adduct yields. The interaction occurs
between the excited difluoroboron complex as the electron acceptor and the
arylalkenes as the electron donor. That is, as described in Weller×s equation [10]
Table 5. Reduction Potentials of the B-Complexes 1a c and of Their Parent Didentate Ligands 1a' c' by Cyclic
Voltammetrya)
B-Complex
1st Wave
2nd Wave
Parent ligand
1st Wave
2nd Wave
1a
1b
1c
À 2.42
À 2.00
À 1.81
1a'
1b'
1c'
À 2.80
À 2.55
À 2.25
À 2.62
À 2.36
À 2.81
À 2.44
a) Conditions: 1 mm 1, MeCN, (Et4N)ClO4 (0.05m), glassy carbon cathode, reference electrode Ag/Ag ,
25 Æ 0.18.
‡
Table 6. Oxidation Potentials of the Arylalkenes 2a d by Cyclic Voltammetrya)
trans-Stilbene (2a) 1,1-Diphenylethene (2b) Styrene (2c) a-Methylstyrene (2d)
Peak potential [V] 1.21
1.48
1.57
1.45
a) Conditions: 1 mm 2, MeCN, (Et4N)ClO4 (0.05m), glassy carbon cathode, reference electrode Ag/Ag ,
25 Æ 0.18.
‡

Helvetica Chimica Acta Vol. 87 (2004)
297
(Eqn. 1), a one-electron transfer from a donor to an excited acceptor quantitatively
accounts for the stabilization. Weller×s classic empirical analysis of exciplex association
in terms of the excitation energies and redox properties of the partners provides a good
guide to the tendency of any photochemical system to undergo exciplex formation; for
this process in hydrocarbon solvent, Eqn. 1 holds.
ox
red
DH ˆ DE0,0 À [E (D) À E (A) ‡ 0.13 V]
(1)
1=2
1=2
The cyclobutane derivatives 3b,c and 10b,c, which are the major products (Tables 1
and 4), have a sterically unfavorable configuration between PhÀC(1) and PhÀC(4).
The product structures suggest that they arise from stable exciplexes possessing a
sandwich-type structure with strong p-orbital overlap between the aromatic moieties of
both compounds, as illustrated in the Figure [11]1). In other words, the rule of
conservation of orbital symmetry, commonly known as the WoodwardÀHoffmann rule
[12], plays an important role in determining the pathway of cycloaddition. Because of
the strong p-orbital overlap, the reaction of the B-complexes 1b,c with trans-stilbene
(2a) gives the cyclobutane derivatives 3b,c as the major products (Scheme 1). In
contrast, each of the reactions of 1,1-diphenylethene (2b) and a-methylstyrene (2d)
yields the regioisomeric cylcobutane derivatives 8b,c and 12b (Schemes 3 and 5, resp.).
These reactions suggest that the existence of a maximum secondary orbital overlap
influences the stereochemistry of the additions [13]. The Figure demonstrates the
overlap between PhÀC(3) of the trans-stilbene (2a) and PhÀC(5) of the B-complex 1c.
The degree of stereoselectivity seems to depend on the extent of p-orbital overlap in
the exciplex. For example, the photocycloaddition of 2-naphthonitrile with alkyl vinyl
ethers gives both endo- and exo-cycloadducts [14]. The photoaddition of furan to
1-naphthonitrile also gives a [4 ‡ 4] cycloadduct with an anti-configuration, but that of
2-naphthonitrile occurs in the syn-direction to produce a cage-like compound [15].
These data suggest that the flexibility of the structure of the exciplexes may be due to
the role that dipoleÀdipole interactions, the polarity, and the crowding of the exciplex
play in the orientation of cycloaddition [16].
Figure. Maximum-orbital-overlap −sandwich× model for the 1c 2a Exciplex
1
)
Exciplexes with olefins are assumed to be formed and act as the discrete intermediates in the
photocycloaddition although, unfortunately, the corresponding exciplex spectral bands have not been
detected.

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In the described reaction, a number of different stereoisomers were obtained.
However, in most singlet-state reactions, the configuration of the olefins is retained in
the cycloadduct [8]. In other words, the major products (ca. 90%) derived from the cis-
and trans-olefins are cyclobutanes with the aryl substituents endo to the aromatic
moieties. Furthermore, Farid and co-worker [17] reported that a reaction of singlet-
excited phenanthrene with fumarate ester gave trans-cyclobutane derivatives together
with oxetane derivatives, but triplet-exciplexes gave both trans- and cis-cyclobutane
derivatives. Therefore, the present reaction may take place via a singlet-exciplex state
as does a similar photoaddition of the difluoro(1,3-dionato)boron complex to olefins [9]:
1
‡
B-Complex ‡ trans-stilbene (2a) ! (B-ComplexÀ TS )* ! azetidine 4‡ cyclobutane 3
It is well known that 3-amino-alk-2-en-1-ones exist in an aminoenone > iminoenal
tautomer equilibrium [18]. X-Ray analysis of the B-complex (1a c) has established
that the 6-membered chelate ring shows a shift to an enol form such as (O)ÀCˆCÀ is
1.367(7) ä, (N)ÀCˆCÀ is 1.404(8) ä, and (C)ÀCˆNÀ is 1.301(6) ä for 1c [19]. The
structures of the photoaddition products suggest that the excited B-complexes 1a c
react in the enolato form. Therefore, in the photoaddition, not only the cyclobutane
derivative but also the azetidine derivative should be obtained. The formation of the
azetidine derivatives 4a c and 13b is interesting, because it is known that the
photoaddition of the singlet state of rigid-framed cyclohex-2-en-1-one yields the
oxetane [17]. Also, there are a few example of the formation of azetidine by a
photoreaction [4].
We then examined and rationalized the reactivity and the stereochemistry by the
FMO2). The energy gap between the frontier orbitals indicates that the most favorable
interaction occurs between the LSOMO of the B-complexes 1a c in the singlet state
and the HOMO of the arylalkenes 2a d (Table 7). Furthermore, the atomic
Table 7. Energy Gap between LSOMO of 1 and HOMO of 2, and HSOMO of 1 and LUMO of 2 in the Singlet
State
B-Complex
Arylalkene
Energy gap [eV]
LSOMOÀHOMO
HSOMOÀLUMO
1a
2a
2b
2c
2d
2a
2b
2c
2d
2a
2b
2c
2d
2.156
2.659
2.769
2.578
1.991
2.494
2.604
2.413
1.671
2.174
2.284
2.093
3.206
3.743
3.811
3.843
3.336
3.873
3.941
3.973
4.158
4.693
4.763
4.795
1b
1c
2
)
Frontier MO (FMO) calculations were achieved by the PM3 method contained in the Win MOPAC
program (Ver. 3.0).

Helvetica Chimica Acta Vol. 87 (2004)
299
Table 8. Frontier Molecular Orbitals (LSOMO) of the B-Complexes
Coefficients of pp-orbital
B-Complex
C(1)
C(2)
C(3)
N
1a
1b
1c
singlet
triplet
singlet
triplet
singlet
triplet
À 0.189
À 0.615
À 0.164
À 0.156
À 0.002
0.190
À 0.495
À 0.109
À 0.572
À 0.552
À 0.106
0.533
À 0.080
À 0.012
À 0.112
À 0.077
0.006
0.604
À 0.237
À 0.551
À 0.484
À 0.185
À 0.058
0.108
coefficients of the NˆC bond and the CˆC bond are similar for 1a and 1b (Table 8).
Therefore, we concluded that the observed regioselectivity and reactivity could be
nicely explained. Furthermore, as in the previous discussion, the process via the singlet
exciplex is consistent with the atomic coefficients, especially in the case of the singlet
state of 1c.
The interesting characteristic of the present photocycloaddition is that it gives the
cyclobutane derivative via the [2 ‡ 2] photoadduct. This is in sharp contrast to the case
of 1,3-diketones (known as de Mayo reaction) [1]; moreover, the difluoroboron
complex yields no cyclobutane but, instead, produces 1,5-diketones 6 [6][9] from the
cyclobutane by a retro-aldol cleavage. This result seems to stem from a difference in the
coordination strength of the B-atom. The B-atom possesses a vacant p-orbital, which is
stabilized by ppÀpp conjugation to the neighboring lone pair of the hetero atoms. The
basicity of the N-atom is stronger than that of the O-atom; furthermore, the N-atom
receives a contribution from the Me group. Therefore, the coordination bond between
the N-atom and the B-atom is stronger than that between the N-atom and the O-atom,
and may overcome the strain energy possessed by the cyclobutane ring.
Experimental Part
1. General. The aminoenones and their B-complexes 1 were prepared and described previously [7][19][20].
IR Spectra: KBr tablets; Hitachi I-2000; in cmÀ1. 1H- and 13C-NMR Spectra: Jeol FX-90Q and Bruker AC-250
spectrometers, CDCl3 solns.; d in ppm rel. to SiMe4 as the internal reference, J in Hz. MS: ESCO EMD-05A; in
m/z. M.p.: Yanaco micro-hot-plate apparatus; uncorrected.
2. Photocycloaddition: General Procedure. A soln. of B-complex 1 (0.5mmol) in MeCN (25ml) and the
aryl alkene 2 (5mmol) in a Pyrex tube were degassed with N2 and irradiated at 08 for 10 h. Then the mixture was
evaporated and the residue chromatographed (silica gel, benzene Et2O 1:0 ! 4 :1).
3. Photoaddition of 1a with trans-Stilbene (2a). Difluoro{1-methyl-c-2-[1-(methylimino-kN)ethyl]-c-3,t-4-
diphenylcyclobutan-r-1-olato-kO}boron (3a): M.p. 166.0 167.08. IR: 1668, 1168, 1068, 1492, 1440, 1376, 896,
1
702. H-NMR (CDCl3): 1.18 (s, 3 H); 1.44 (s, 3 H); 3.23 (s, 3 H); 3.41 (d, J ˆ 11.0, 1 H); 4.02 (t, J ˆ 10.2, 1 H);
4.81 (d, J ˆ 11.7, 1 H); 7.26 (s, 5H); 7.33 ( s, 5 H). 13C-NMR (CDCl3): 22.4; 24.1; 35.3; 41.0; 51.5; 55.8; 71.7; 126.5;
‡
‡
‡
127.1; 128.1; 128.3; 129.3; 137.1; 138.2; 184.3. MS: 341 (M ), 322 ([M À F] ), 275([ M À BF2OH] ), 260, 205.
Anal. calc. for C20H22BF2NO: C 70.40, H 6.50, N 4.11; found: C 70.29, H 6.36, N 4.11.
Difluoro[(1Z)-1-(t-1,2-dimethyl-c-3,t-4-diphenylazetidine-r-2-yl-kN1)prop-1-en-2-olato-kO]boron (4a):
M.p. 156.5 157.58. IR: 1680, 1384, 1226, 1194, 1126, 918, 900, 758, 700. 1H-NMR (CDCl3): 1.55 (s, 3 H); 1.86
(s, 3 H); 2.46 (s, 3 H); 3.83 (s, 1 H); 4.49 (d, J ˆ 12, 1 H); 5.82 (d, J ˆ 12.0, 1 H); 7.11 7.57 (m, 10 H). 13C-NMR
(CDCl3): 21.7; 22.8; 32.4; 52.5; 66.3; 72.3; 98.7; 128.0; 128.7; 128.9; 129.6; 129.7; 129.9; 131.1; 134.5; 151.2. MS:
‡
‡
341 (M ), 275([ M À BF2OH] ), 260, 221, 206. Anal. calc. for C20H22BF2NO: C 70.40, H 6.50, N 4.11; found:
C 70.62, H 6.63, N 4.12.

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4. Photoaddition of 1b with trans-Stilbene (2a). Difluoro{c-2-[1-(methylimino-kN)ethyl]-1,c-3,t-4-triphen-
ylcyclobutan-r-olato-kO}boron (3b): M.p. 198.5 200.5 8. IR: 1670, 1498, 1450, 1374, 1166, 1062, 898, 758.
1H-NMR (CDCl3): 1.50 (s, 3 H); 3.24 (s, 3 H); 4.07 (d, J ˆ 10.5, 1 H); 4.33 (t, J ˆ 10.3, 1 H); 5.11 (d, J ˆ 11.2,
1 H); 6.95 7.49 ( m, 15 H). 13C-NMR (CDCl3): 22.4; 35.1; 42.4; 54.1; 54.7; 75.8; 125.7; 126.1; 126.9; 127.3; 127.7;
‡
128.1; 128.2; 129.2; 137.2; 137.9; 141.5; 183.8. MS: 403 ( M ), 383, 355, 260, 222. Anal. calc. for C25H24BF2NO:
C 74.46, H 6.00, N 3.47; found: C 74.42, H 6.07, N 3.52.
Difluoro[(1Z)-2-(t-1,2-dimethyl-c-3,t-4-diphenylazetidin-r-2-yl-kN1)1-phenylethen-1-olato-kO]boron (4b):
M.p. 152.5 153.58. IR: 1652, 1450, 1242, 1140, 1104, 922, 754, 696. 1H-NMR (CDCl3): 1.66 (s, 3 H); 2.52 (s, 3 H);
4.59 (q, J ˆ 11.5, 1 H); 4.59 (s, 1 H); 5.94 (d, J ˆ 11.5, 1 H); 7.23 7.57 (m, 15 H). 13C-NMR (CDCl3): 22.6; 32.3;
‡
52.9; 66.5; 72.5; 98.3; 125.5; 128.1; 128.5; 128.7; 128.8; 129.6; 129.8; 130.9; 134.1; 136.2; 151.2. MS: 403 (M ), 390,
337, 336, 261, 260. Anal. calc. for C25H24BF2NO: C 74.46, H 6.00, N 3.47; found: C 74.56, H 6.00, N 3.54.
5. Photoaddition of 1c with trans-Stilbene 2a. Difluoro{c-2-[(methylimino-kN)(phenyl)methyl]-1,c-3,t-4-
triphenylcyclobutan-r-1-olato-kO}boron (3c): M.p. 165.0 166.08. IR: 1656, 1498, 1448, 1152, 1068, 896, 770, 702.
1H-NMR (CDCl3): 3.25( s, 3 H); 4.23 (t, J ˆ 10.8, 1 H); 4.58 (d, J ˆ 10.8, 1 H); 5.07 (d, J ˆ 10.8, 1 H); 6.48 6.51
(m, 2 H); 6.91 7.55 (m, 18 H). 13C-NMR (CDCl3): 37.2; 43.7; 52.3; 56.5; 77.6; 125.9; 126.0; 126.3; 126.8; 127.5;
‡
127.6; 127.8; 128.1; 128.6; 128.8; 129.1; 130.8; 131.8; 138.1; 138.4; 142.1; 181.7. MS: 399 ([M À BF2OH] ), 382,
322, 173, 105. Anal. calc. for C30H26BF2NO: C 77.23, H 5.63, N 3.01; found: C 77.23, H 5.40, N 3.13.
Difluoro[(1Z)-2-(t-1-methyl-2,c-3,t-4-triphenylazetidin-r-2-yl-kN1)-1-phenylethen-1-olato-kO]boron (4c):
M.p. 130.0 132.08. IR: 1650, 1498, 1450, 1358, 1154, 1102, 926, 754, 694. 1H-NMR (CDCl3): 2.02 (s, 3 H);
4.81 (s, 1 H); 5 .5 2 d(, J ˆ 11.3, 1 H); 5.98 (d, J ˆ 11.3, 1 H); 7.23 7.97 (m, 20 H). 13C-NMR (CDCl3): 34.3; 47.1;
66.3; 77.4; 99.1; 125.6; 128.1; 128.2; 128.6; 128.8; 128.9; 129.2; 129.6; 129.9; 130.1; 130.2; 131.0; 134.6; 136.2;
‡
150.9; 151.2. MS: 399 ([M À BF2OH] ), 382, 370, 322, 307. Anal. calc. for C30H26BF2NO: C 77.43, H 5.63, N 3.01;
found: C 77.56, H 6.00, N 3.47.
6. Hydrolysis of the Cyclobutane Derivative 3b. To 3b (1 mg, 0.13 mmol) in MeOH (30 ml), 10% aq. NaOH
soln. (0.05ml) was added. The mixture was refluxed for 2 h and then quenched with 5% aq. HCl soln. The
mixture was evaporated and the residue chromatographed (silica gel, hexane/acetone 6 :1): 5b (46%) and 6b
(42%).
Data of 1,2,3-Triphenylhexane-1,5-dione (5b): M.p. 83.5 84.0 8. IR: 1710, 1680, 1450, 1358, 1266, 864, 698.
1H-NMR (CDCl3): 2.00 (s, 3 H); 2.49 (dd, J ˆ 4.3, 16.0, 1 H); 2.66 (dd, J ˆ 9.8, 16.0, 1 H); 4.20 (ddd, J ˆ 4.3, 9.8,
10.8, 1 H); 5.11 (d, J ˆ 10.8, 1 H); 7.02 7.48 (m, 13 H); 7.92 8.06 (m, 2 H). 13C-NMR (CDCl3): 30.1; 45.2; 48.2;
‡
59.0; 126.4; 126.9; 128.0; 128.3; 128.5; 128.8; 133.0; 136.8; 137.0; 141.1; 199.2; 206.8. MS: 342 (M ), 341, 284, 283,
236. Anal. calc. for C24H22O: C 84.18, H 6.72%; found: C 84.16, H 6.74.
Data of 3,4,5-Triphenylcyclohex-2-en-1-one (6b): M.p. 107.4 108.28. IR: 1674, 1448, 1268, 768, 698.
1H-NMR (CDCl3): 3.45( d, J ˆ 8.0, 1 H); 3.47 (d, J ˆ 5.9, 1 H); 4.10 4.40 (m, 1 H); 5.00 (d, J ˆ 10.5, 1 H); 7.02
(s, 5H); 7.06 ( s, 5H); 7.23 7.50 ( m, 6 H); 7.83 8.07 (m, 4 H). 13C-NMR (CDCl3): 43.5; 45.8; 50.4; 126.4; 127.0;
128.0; 128.1; 128.2; 128.3; 128.5; 128.6; 128.7; 129.0; 129.2; 129.3; 132.8; 133.1; 137.0; 137.2; 137.3; 141.1; 198.5;
199.5. MS: 404, 299, 298, 284, 283. Anal. calc. for C29H24O2: C 86.11, H 5.98; found: C 86.19, H 5.93.
7. Photoaddition of 1b with 1,1-Diphenylethene (2b). Difluoro{c-4-[1-(methylimino-kN)ethyl]-1,2,2-
triphenylcyclobutan-r-1-olato-kO}boron (7b): M.p. 155.3 156.08. IR: 1682, 1494, 1440, 1142, 888, 760, 700.
1H-NMR (CDCl3): 2.06 (s, 3 H); 2.92 (s, 3 H); 2.97 (t, J ˆ 8.8, 1 H); 3.55 (t, J ˆ 8.8, 1 H); 3.64 (t, J ˆ 8.8, 1 H);
6.85 7.31 ( m, 15 H). 13C-NMR (CDCl3): 20.2; 34.6; 34.8; 41.0; 59.7; 82.1; 125.6; 126.0; 126.9; 127.7; 127.8; 127.9;
‡
128.0; 128.1; 141.6; 141.7; 146.1; 182.3. MS: 363 (M ), 347, 223. Anal. calc. for C25H24BF2NO: C 74.46, H 6.00,
N 3.41; found: C 74.16, H 6.24, N 3.45.
Difluoro{c-2-[1-(methylimino-kN)ethyl]-1,3,3-triphenylcyclobutan-r-1-olato-kO}boron (8b): M.p. 166.0
167.08. IR: 1682, 1498, 1446, 1160, 1132, 1078, 1056, 948, 892, 758, 704. 1H-NMR (CDCl3): 1.66 (s, 3 H); 2.98
(s, 3 H); 3.03 (d, J ˆ 13.2, 1 H); 3.74 (d, J ˆ 13.2, 1 H); 4.55 (s, 1 H); 7.13 7.46 (m, 15 H). 13C-NMR (CDCl3):
20.9; 34.7; 51.2; 55.6; 57.0; 71.9; 125.1; 126.9; 127.0; 127.1; 127.3; 128.0; 128.2; 129.0; 129.2; 144.0; 146.8; 147.1;
‡
180.9. MS: 403 (M ), 402, 347, 322, 283, 282, 260. Anal. calc. for C25H24BF2NO: C 74.46, H 6.00, N 3.47; found:
C 74.50, H 6.12, N 3.52.
8. Photoaddition of 1c with 1,1-Diphenylethene (2b). Difluoro{c-4-[(methylimino-kN)(phenyl)methyl]-
1,2,2-triphenylcyclobutan-r-1-olato-kO}boron (7c): M.p. 166.0 168.08. IR: 1672, 1496, 1446, 1144, 1108, 892,
728, 698. 1H-NMR (CDCl3): 3.08 (s, 3 H); 3.19 (t, J ˆ 10.5, 1 H); 3.46 (dd, J ˆ 9.5, 10.5, 1 H); 4.01 (t, J ˆ 9.5,
1 H); 6.85 7.62 ( m, 20 H). 13C-NMR (CDCl3): 35.4; 37.2; 41.3; 59.9; 83.0; 125.6; 125.9; 126.2; 126.9; 127.1; 127.2;
127.8; 128.0; 129.7; 131.6; 132.8; 141.6; 146.0; 181.6. MS: 446, 398, 322. Anal. calc. for C30H26BF2NO: C 77.43,
H 5.63, N 3.01; found: C 77.50, H 5.56, N 3.03.

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Difluoro{c-2-[(methylimino-kN)(phenyl)methyl]-1,3,3-triphenylcyclobutan-r-1-olato-kN}boron (8c): M.p.
144.5 146.0 8. IR: 1662, 1496, 1446, 1354, 1084, 942, 882, 758, 702. 1H-NMR (CDCl3): 3.02 (s, 3 H); 3.18 (d, J ˆ
13.4, 1 H); 3.72 (d, J ˆ 13.4, 1 H); 4.83 (s, 1 H); 6.32 6.41 (m, 2 H); 6.87 6.90 (m, 2 H); 7.21 7.36 (m, 13 H);
7.42 7.53 (m, 3 H). 13C-NMR (CDCl3): 37.3; 52.1; 56.5; 57.8; 72.8; 125.2; 126.5; 126.7; 127.1; 127.2; 127.5; 128.0;
128.2; 128.6; 128.9; 130.0; 131.5; 132.1; 144.4; 146.9; 147.4; 180.7. MS: 400, 399, 322, 321, 307. Anal. calc. for
C30H26BF2NO: C 77.43, H 5.63, N 3.01; found: C 77.29, H 5.70, N 3.17.
9. Photoaddition of 1c with Styrene (2c). 1,2,5-Triphenylpentane-1,5-dione (9c): M.p. 93.5 94.0 8. IR: 1686,
1
1676, 1450, 1246, 1224, 1180, 754, 698. H-NMR (CDCl3): 2.35 2.21 ( m, 1 H); 2.52 2.66 (m, 1 H); 2.85 3.05
(m, 2 H); 4.78 (t, J ˆ 7.3, 1 H); 7.17 7.55 (m, 11 H); 7.89 (d, J ˆ 7.2, 2 H); 7.97 (d, J ˆ 7.2, 2 H). 13C-NMR
(CDCl3): 28.3; 36.0; 52.5; 127.2; 128.0; 128.3; 128.5; 128.7; 129.0; 132.9; 133.0; 136.7; 136.9; 139.2; 199.6; 199.8.
‡
MS: 328 (M ), 327, 223, 222, 209, 195. Anal. calc. for C23H20O2: C 84.12, H 6.14; found: C 84.16, H 6.33.
10. Photoaddition of 1b with 2-Phenylprop-1-ene (2d). Difluoro{c-2-methyl-c-4-[1-(methylimino-kN)eth-
yl]-1,t-2-diphenylcyclobutan-r-1-olato-kN}boron (10b): M.p. 165.0 166.58. IR: 1676, 1452, 1174, 1134, 1068, 906,
734, 700. 1H-NMR (CDCl3): 1.50 (s, 3 H); 2.21 (s, 3 H); 2.28 (dd, J ˆ 6.3, 11.0, 1 H); 3.17 (t, J ˆ 11.0, 1 H); 3.43
(dd, J ˆ 6.3, 11.0, 1 H); 6.88 7.26 (m, 10 H). 13C-NMR (CDCl3): 19.3; 27.2; 34.2; 34.8; 44.2; 53.0; 79.5; 125.4;
‡
126.3; 126.5; 127.1; 127.5; 144.0; 144.8; 183.7. MS: 340 ( M ), 322, 276, 260, 221. Anal. calc. for C20H22BF2NO:
C 70.40, H 6.50, N 4.10; found: C 70.35, H 6.74, N 4.15.
Difluoro{t-2-methyl-c-4-[1-(methylimino-kN)ethyl]-1c-2-diphenylcyclobutan-r-1-olato-kO}boron (11b):
M.p. 172.0 173.08. IR: 1674, 1498, 1446, 1368, 1142, 1082, 948, 890, 768, 702. 1H-NMR (CDCl3): 1.11
(s, 3 H); 2.16 (s, 3 H); 2.37 (t, J ˆ 9.7, 1 H); 2.94 (t, J ˆ 9.7, 1 H); 3.09 (s, 3 H); 3.83 (t, J ˆ 9.4, 1 H); 7.19 7.49
(m, 10 H). 13C-NMR (CDCl3): 19.9; 26.8; 34.5; 35.7; 49.4; 80.6; 125.9; 126.7; 127.0; 127.2; 127.6; 127.8; 141.9;
‡
144.3; 182.6. MS: 340 (M ), 322, 260, 258, 221, 203. Anal. calc. for C20H22BF2NO: C 70.40, H 6.50, N 4.10; found:
C 70.18, H 6.46, N 4.14.
Difluoro{t-3-methyl-c-2-[1-(methylimino-kN)ethyl]-1,c-3-diphenylcyclobutan-r-1-olato-kO}boron (12b):
M.p. 155.0 156.08. IR: 1684, 1448, 1110, 1076, 904, 758, 702. 1H-NMR (CDCl3): 1.47 (s, 3 H); 1.92 (s, 3 H);
2.37 (d, J ˆ 13.4, 1 H); 2.90 (s, 3 H); 3.25( d, J ˆ 13,4, 1 H); 3.72 (s, 1 H); 7.25 7.82 ( m, 10 H). 13C-NMR
‡
(CDCl3): 20.7; 29.2; 34.1; 42.2; 52.2; 71.4; 125.3; 127.1; 127.5; 128.2; 128.4; 142.9; 146.8; 181.0. MS: 341 (M ), 275,
260, 223, 203, 145. Anal. calc. for C20H22BF2NO: C 70.40, H 6.50, N 4.10; found: C 70.29, H 6.46, N 4.09.
Difluoro[(1Z)-1-(t-1,2,t-3-trimethyl-c-3-phenylazetidin-r-2-yl-kN1)-1-phenylethen-1-olato-kO]boron (13b):
M.p. 152.5 153.58. IR: 1658, 1446, 1358, 1226, 1178, 1110, 1060, 954, 920, 768, 702. 1H-NMR (CDCl3): 1.70
(s, 3 H); 1.83 (s, 3 H); 3.07 (s, 3 H); 3.28 (d, J ˆ 10.95, 1 H); 4.63 (s, 1 H); 4.91 (d, J ˆ 10.9, 1 H); 6.98 7.31
(m, 10 H). 13C-NMR (CDCl3): 19.2; 27.9; 39.8; 50.0; 61.3; 75.3; 102.9; 125.5; 125.6; 126.7; 127.8; 128.4; 128.7;
136.5; 144.3; 149.7; 149.8; 181.2. MS: 341, 298, 275, 260, 223, 222, 118. Anal. calc. for C 25H24BF2NO: C 74.46,
H 6.00, N 3.47; found: C 74.56, H 6.00, N 3.54.
11. Photoaddition of 1c with 2-Phenylprop-1-ene (2d). Difluoro{c-2-methyl-c-4-[(methylimino-kN)(phen-
yl)methyl]-1,t-2-diphenylcyclobutan-r-1-olato-kO}boron (10c): M.p. 164.5 165.2 8. IR: 1668, 1448, 1346, 1144,
1022, 922, 768, 700. 1H-NMR (CDCl3): 1.58 (s, 3 H); 2.15( dd, J ˆ 6.6, 11.6, 1 H); 2.99 (d, J ˆ 11.6, 1 H); 3.31
(s, 3 H); 3.86 (dd, J ˆ 6.6, 11.6, 1 H); 6.78 7.63 (m, 15 H). 13C-NMR (CDCl3): 27.4; 34.5; 37.7; 44.1; 53.2; 80.1;
125.5; 125.6; 126.6; 127.4; 127.6; 129.7; 131.7; 132.1; 143.9; 144.8; 183.4. MS: 337, 322, 285, 258. Anal. calc. for
C25H24BF2NO: C 74.46, H 6.00, N 3.47; found: C 74.41, H 6.14, N 3.55.
Difluoro{t-2-methyl-c-4-[(methylimino-kN)(phenyl)methyl]-1,c-2-diphenylcyclobutan-r-1-olato-kO}boron
(11c): M.p. 164.5 165.2 8. IR: 1668, 1448, 1346, 1144, 1022, 922, 768, 700. 1H-NMR (CDCl3): 1.10 (s, 3 H); 2.23
(dd, J ˆ 9.3, 9.7, 1 H); 3.11 (dd, J ˆ 9.2, 9.7, 1 H); 3.15( s, 3 H); 4.13 (dd, J ˆ 9.2, 9.7, 1 H); 7.20 7.40 (m, 4 H);
7.55 7.62 (m, 11 H). 13C-NMR (CDCl3): 22.3; 26.7; 36.2; 37.0; 39.6; 49.3; 80.9; 126.1; 126.6; 126.9; 127.4; 127.9;
129.6; 131.6; 132.9; 141.7; 144.0; 183.1. MS: 337, 322, 285, 258. Anal. calc. for C25H24BF2NO: C 74.46, H 6.00,
N 3.47; found: C 74.41; H 6.14; N 3.55.
2-Methyl-1,2,5-triphenylpentane-1,5-dione (14c): M.p. 99.5 100.0 8. IR: 1682, 1448, 1240, 980, 744, 712.
1H-NMR (CDCl3): 1.66 (s, 1 H); 2.44 2.55 (m, 3 H); 2.79 2.88 (m, 2 H); 7.19 7.42 (m, 11 H); 7.48 7.52
(m, 2 H); 7.79 7.83 (m, 2 H). 13C-NMR (CDCl3): 24.4; 34.3; 35.0; 54.4; 126.4; 128.2; 128.6; 129.2; 129.6; 127.2;
‡
131.9; 132.9; 136.7; 137.0; 143.8; 199.9; 203.2. MS: 237 ([M À PhCO] ), 223, 219, 204. Anal. calc. for C24H22O2:
C 84.18, H 6.48%; found. C 83.99, H 6.30.

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Received July 9, 2003