Phosphorus-carbon heterocyclic chemistry : the rise of a new domain

839 Pages·2001·33.682 MB·English

by  François Mathey, Professor

Phosphorus-Carbon Heterocyclic
Chemistry
The Rise of a New Domain
Elsevier, 2001

Edited by: François Mathey
ISBN: 978-0-08-043952-5

Preface, Page vii, F. Mathey

Journal Codes for References, Pages ix-x

1 - The Birth of a New Domain, Pages 1-15, F. Mathey

2.1 - Three-membered Rings: 1. Phosphiranes and Phosphirenes, Pages 17-55,
F. Mathey and M. Regitz
2.2 - Three-membered Rings: 2. Diphosphiranes and Diphosphirenes, Pages 57-86,
Guita Etemad-Moghadam and Max Koenig
3.1 - Phosphetanne—Early Studies, Pages 87-103, Angela Marinetti and Duncan
Carmichael
3.2 - Four-membered Rings with 1 Phosphorus Atom, Pages 105-165, Takayuki
Kawashima and Renji Okazaki
3.3 - Diphosphetanes, Dihydrodiphosphetes, and Diphosphetes, Pages 167-193,
Martin Hofmann and Ulrich Zenneck
4.1 - Phospholanes and Phospholenes, Pages 195-218, T.P. Kee

4.2.1 - Five-membered rings. Phospholes: Early Literature 1953–1994, Pages 219-305,
Louis D. Quin
4.2.2 - Five-membered rings. Phospholes: Recent literature 1994-mid-1999,
Pages 307-362, Louis D. Quin and Gyöngyi S. Quin
4.3 - Heterophospholes, Pages 363-461, Alfred Schmidpeter

5.1 - Six-membered rings: Phosphinanes, Dihydro- and Tetrahydro-phosphinines,
Pages 463-483, Michael J. Gallagher
5.2 - Six-membered Rings: Phosphinines, Pages 485-533, Pascal Le Floch
by kmno4
5.3 - Six-membered Rings with Two or More Heteroatoms with at least One
Phosphorus Atom, Pages 535-630, Gottfried Maerkl and Peter Kreitmeier
6.1 - Macro- and Spiro-heterocycles, Pages 631-669, Michael Pabel and S. Bruce Wild

6.2 - Bicyclic and Polycyclic Systems with a Ring Junction Phosphorus Atom,
Pages 671-705, John C. Tebby
6.3 - Compounds with Phosphorus at the Spiro Position, Pages 707-751, J.C. Tebby

7 - Applications of Phosphorus Heterocycles in Homogeneous Catalysis,
Pages 753-772, F. Mathey
Author Index, Pages 773-846
by kmno4
Preface
As soon as Elsevier published Comprehensive Heterocyclic Chemistry II in 1996, it became obvious
that phosphorus heterocycles had reached a state of development where they had joined Oxygen, Nitrogen,
and Sulfur heterocycles as the fourth major strand of heterocyclic chemistry. It seemed natural to collect
all the phosphorus chapters of CHEC II and to offer them, after updating, as a single volume to specialists
of coordination chemistry, catalysis, and other areas who are involved in this field. In practice however,
this idea proved to be more difficult to achieve than expected. For instance, CHEC II treated both saturated
and insaturated rings in the same chapters and often did not cover the literature prior to 1982. Thus, this
book finally became much more than the initially planned compilation. In fact, it includes seven new
chapters, which cover the history of the domain, catalytic applications, phospholanes and phospholenes,
phosphinanes and phosphinines, early work on phosphetanes, together with recent work on phospholes.
Nine other chapters are updated versions of their CHEC II counterparts. The chapter on heterophospholes
comprises an updated combined version of three chapters of CHEC II. Finally, phosphole chemistry is
treated in two separate sections. The first one reproduces the initial chapter of CHEC II, with new schemes
and corrections, whilst the second one is an updating which can be read independently. The final result is
a huge book which comprehensively covers the chemistry of phosphorus-carbon heterocycles up to 1999
and also treats the most significant P-heterocycles including other heteroatoms such as heterophospholes,
heterophosphinines, and spirocyclic compounds. We hope that it will serve as the reference work for all
those interested in the domain for the years to come.
During this work, I have been fortunate enough to have the enthusiastic collaboration of most of the
authors who had already written the phosphorus chapters of CHEC II, including several of the founding
fathers of the field. Six new authors have joined the team. I also owe many thanks to Dr. Duncan
Carmichael who revised my English and to Mme. Fran9oise Girard who, in addition to drawing the
schemes in Chapters 4.2.1 and 4.3, helped me to manage the flow of manuscripts. At Elsevier, Dr. Guido
Zosimo-Landolfo promoted the project , whilst Drs. Adrian Shell and Dinali de Silva took care of the
publishing. Without the help of all these dedicated people, the book would have not existed.
F. Mathey
Palaiseau, January 2001
Journal Codes for References
ABC Agric. Biol. Chem. CHEC Comp. Heterocycl. Chem., 1st edi
ACH Acta Chim. Acad. Sci. Hung. tion
ACR Ace. Chem. Res. CHEC-II Comp. Heterocycl. Chem., 2nd edi
AC(R) Ann. Chim. (Rome) tion
ACS Acta Chem. Scand. CI(L) Chem. Ind. (London)
ACS(A) Acta Chem. Scand., Ser.A CJC Can. J. Chem
ACS(B) Acta Chem. Scand., Ser. B CL Chem. Lett.
AF Arzneim.-Forsch. COS Comp. Org. Synth.
AG Angew. Chem. CPB Chem. Pharm. Bull.
AG(E) Angew. Chem., Int. Ed. Engl. CPH Chem. Phys.
AHC Adv. Heterocycl. Chem. CPL Chem. Phys. Lett.
AJC Aust. J. Chem. CR CR. Hebd. Seances Acad. Sci.
AK Ark. Kemi CR(B) CR. Hebd. Seances Acad. Sci.,
ANY Ann. N.Y. Acad. Sci. Ser. B.
AP Arch. Pharm. (Weinheim, Ger.) CR(C) CR. Hebd. Seances Acad. Sci.,
APO Adv. Phys. Org. Chem. Ser. C
AX Acta Crystallogr. CS Chem. Ser.
AX(B) Acta Crystallogr., Part B CSC Cryst. Struct. Commun.
AX(C) Acta Crystallogr., Part C CSR Chem. Soc. Rev.
AQ(C) An. Quim. CZ Chem.-Ztg.
B Biochemistry DIS Diss. Abstr.
BAP Bull. Acad. Pol. Sci., Ser. DIS(B) Diss. Abstr. Int. B
BAU Bull. Acad. Sci. USSR, Div. Chem. DOK Dokl. Akad. Nauk SSSR
Sci. E Experientia
BBA Biochim. Biophys. Acta EGP Ger. (East) Pat.
BBR Biochem. Biophys. Res. Conmiun. EJIC Eur. J. Inorg. Chem.
BCJ Bull. Chem. Soc, Jpn. EJOC Eur. J. Org. Chem.
BEP Belg. Pat. EUP Eur. Pat.
BJ Biochem. J. CRV Chem. Rev. FES Farmaco Ed. Sci.
BJP Br. J. Pharmacol. FOR Fortschr. Chem. Org. Naturst.
BKS Bull. Korean Chem.Soc. FRP Fr. Pat.
BMS Bioorg. Med. Chem. Lett. G Gazz. Chim. Ital.
BRP Br. Pat. GEP Ger. Pat.
BSB Bull. Soc. Chim. Belg. GEP(O) Ger. Pat. Offen.
BSF Bull. Soc. Chim. Fr. H Heterocycles
BSF(2) Bull. Soc. Chim. Fr., Part 2 HC Chem. Heterocycl. Compd. [Weiss-
C Chimia berger-Taylor series]
CA Chem. Abstr. HCA Helv. Chim. Acta
CAR Carbohydr. Res. HOU Methoden Org. Chem. (Houben-Weyl)
CB Chem. Ber. IC Inorg. Chem.
CBR Chem. Br. ICA Inorg. Chem. Acta
CC J. Chem. Soc, Chem. Commun. IJC Indian J. Chem.
CCA Croat. Chem. Acta IJC(B) Indian J Chem., Sect. B
CCC Collect. Czech. Chem. Commun. US Int. J. Sulfur Chem.
CCR Coord. Chem. Rev. IJS(B) Int. J. Sulfur Chem., Part B
CEJ Chem. Eur. J. IZV Izv. Akad. Nauk SSSR, Ser. Khim.
CHE Chem. Heterocycl. Compd. (Engl. JA J. Am. Chem. Soc.
Transl.) JAP Jpn. Pat.
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JAP(K) Jpn. Kokai OS Org. Synth.
JBC J. Biol. Chem. OSC Org. Synth., Coll. Vol.
JC J. Chromatogr. P Phytochemistry
JCC J. Coord. Chem. PAC Pure Appl. Chem.
JCI I. Chem. Inf. Comput. Sci. PC Personal Communication
JCO J. Comput. Chem. PH 'Photochemistry of Heterocyclic Com
JCP J. Chem. Phys. pounds', ed. O. Buchardt, Wiley,
JCR(M) J. Chem. Res. (M) New York, 1976
JCR(S) J. Chem. Res. (S) PHA Pharmazie
JCS J. Chem. Soc. PHC Prog. Heterocycl. Chem.
JCS(C) J. Chem. Soc. (C) PIA Proc Indian Acad. Sci.
JCS(D) J. Chem. Soc, Dalton Trans. PIA(A) Proc Indian Acad. Sci., Sect. A
JCS(Fl) J. Chem. Soc, Faraday Trans. 1 PJC Pol. J. Chem.
JCS(Pl) J. Chem. Soc, Perkin Trans. 1 PMH Phys. Methods Heterocycl. Chem.
JCS(P2) J. Chem. Soc, Perkin Trans.2 PNA Proc. Nad. Acad. Sci. USA
JGU J. Gen. Chem. USSR (Engl. Transl.) PS Phosphorus Sulfur
JHC J. Heterocycl. Chem. QR Q. Rev., Chem. Soc.
JIC J. Indian Chem. Soc. RCR Russ. Chem. Rev. (Engl. Transl.)
JMC J. Med. Chem. RRC Rev. Roum. Chim.
JMR J. Magn. Reson. RTC Reel. Trav. Chim. Pays-Bas
JOC J. Org. Chem. S Synthesis
JOM J. Organomet. Chem. SA Spectrochim. Acta
JOU J. Org. Chem. USSR (Engl. Transl.) SA(A) Spectrochim. Acta, Part A
JPC J. Phys. Chem. SAP S. Afr. Pat.
JPO J. Phys. Org. Chem. SC Synth. Commun.
JPR J. Prakt. Chem. SH W.L.F. Armarego, 'Stereochemistry
JPS J. Pharm. Sci. of Heterocyclic Compounds', Wiley,
JSP J. Mol. Spectrosc. New York, 1977, parts 1 and 2
JST J. Mol. Struct. SL Synlett
K Kristallografiya SR Sulfur Reports
KFZ Khim.-Farm. Zh. SST Org. Compd. Sulphur, Selenium, Tel
KGS Khim. Geterotsikl. Soedin. lurium [R. Soc. Chem. series]
LA Liebigs Ann. Chem. SUL Sulfur Letters
M Monatsh. Chem. T Tetrahedron
MC Mendeleev Communications TA Tetrahedron: Asymmetry
MI Miscellaneous [book or journal] TAL Talanta
MIP Miscellaneous Pat. TCA Theor. Chim. Acta
MRC Magn. Reson. Chem. TH Thesis IJC(B)
MS Q.N. Porter and J. Baldas, 'Mass TL Tetrahedron Lett.
Spectrometry of Heterocyclic Com UKZ Ukr. Khim. Zh. (Russ. Ed.)
pounds', Wiley, New York, 1971 UP Unpublished Results
N Naturwissenschaften URP USSR Pat.
NAT Nature USP U.S. Pat.
NEP Neth. Pat. YZ Yakugaku Zasshi
NJC Nouv. J. Chim. ZAAC Z. Anorg. Allg. Chem.
NKK Nippon Kagaku Kaishi ZC Z. Chem.
NMR T.J. Batterham, 'NMR Spectra of ZN Z. Naturforsch.
Simple Heterocycles', Wiley, New ZN(A) Z. Naturforsch., Teil A
York, 1973 ZN(B) Z. Naturforsch., Teil B
OM Organometallics ZOB Zh. Obshch. Khim.
OMR Org. Magn. Reson. ZOR Zh. Org. Khim.
QMS Org. Mass Spectrom. ZPC Hoppe-Seyler's Z. Physiol. Chem.
OPP Org. Prep. Proced. Int. ZPH Zh. Phys. Khim.
OR Org. React. ZPK Zh. Prikl. Khim.
1
The Birth of a New Domain
R MATHEY
Ecole Polytechnique, Palaiseau Cedex, France
1.1 INTRODUCTION 1
1.2 THE FOUNDATIONS 2
1.3 RECENT TRENDS 9
1.4 REFERENCES 13
1.1 INTRODUCTION
Phosphorus is certainly one of the most important non-metals of the Periodic Table. It plays a
central role in biochemistry, organic synthesis, coordination chemistry, homogeneous catalysis, solid state
chemistry and material sciences. With such a scope, one might wonder why its heterocyclic chemistry has
been underdeveloped for such a long time. It is startling that, whilst the first known P-C heterocycle, i.e.
the 1-phenylphosphinane, was prepared relatively early (in 1915 <15CB1473» (Equation (1)), the domain
really only began in 1953 with the discovery of the McCormack reaction between conjugated dienes and
halophosphines <53USP2663736-9> (Equation (2)).
PhPCl2 + BrMg(CH2)5MgBr | | (1)
P^
I
Ph
RPX2 ^ r\ ^^— ( ^ ). - (2)
X
X = Cl, Br R X
Why this should be is not obvious. It is clear that organophosphorus chemistry as a whole did not
progress significantly between the two World Wars, probably because the inherently complex nature
of this chemistry was not matched by the appropriate analytical tools. Without doubt, the discovery
of the ^^P NMR spectroscopy in the early fifties was a major turning point for phosphorus chemistry.
More subtly, organophosphorus chemistry in general, and its heterocyclic version in particular, does
not mimic organonitrogen chemistry. Whilst stimulating, the absence of a reliable model is obviously a
major handicap when trying to develop a new field. As an illustration, the McCormack reaction has no
counterpart in nitrogen chemistry. Thus, the development of phosphorus heterocyclic chemistry had to
rely on the imagination of a few dedicated chemists, and early developments were, inevitably, slow. The
1950 edition of Mann's book describing the heterocyclic derivatives of P, As, Sb and Bi comprehensively
surveyed all phosphorus heterocycles in 15 pages and 18 references <B-50MI(1)9>. Twenty years later, the
2 The Birth of a New Domain
second edition needed 354 pages and 503 references to cover the same topic <B-70MI(3)1>. By way of
comparison, the 1984 edition of Comprehensive Heterocyclic Chemistry <B-84MI5> which deals almost
exclusively with nitrogen, oxygen and sulphur heterocycles, needed ca. 8000 pages and 30000 references
to cover the fundamental aspects of its subject. The discrepancy reflects the fact that these more classical
heterocycles were essentially discovered during the 19th century, and are present in a number of natural
substances and in drugs, dyes and numerous other materials. Phosphorus heterocyclic chemistry cannot yet
compete at such a level of complexity but, nevertheless, is undergoing very rapid development. In 1981,
Quin's book dealing with P-C heterocycles <B-81MI4> already needed 430 pages and ca. 1100 references
to treat the topic. The size of the present work confirms the continuing expansion in the field. Furthermore,
the first applications in homogeneous catalysis do not lie far ahead (see Chapter 7). The future is thus
quite exciting. Few specialized fields of molecular chemistry present such a potential for growth and
applications.
1.2 THE FOUNDATIONS
After the initial discovery of 1-phenylphosphinane by Griittner and Wiemik <15CB1473> (Equation
(1)), Griittner and Krause prepared 1-phenylphospholane using the same procedure <16CB437>. In 1920,
Conant and Cook showed that phosphorus trichloride readily reacts with a,^-unsaturated ketones to give
1,2-oxaphospholenes <20JA830> (Equation (3)).
Ph
RT
PCI3. P h - ^0
Ac.O
Later, phosphorus trichloride was replaced by phenyldichlorophosphine <21JA1665>. With hindsight, this
reaction is the forerunner of the McCormack reaction discussed above (Equation (2)). Several years
elapsed before the next important step was made by Wittig and Geissler in 1953 <53MI2(580)44>, when they
isolated 5-phenyldibenzophosphole as a side-product of their classic work on pentaphenylphosphorane
(Equation (4)).
PhsP (/ ") (\ /) (4)
The same product was also obtained through the more conventional reaction of 2,2'-dilithiobiphenyl with
phenyldichlorophosphine. The phosphorus atom in 5-phenyl-dibenzophosphole is pyramidal and has a
fairly normal inversion barrier of 26 kcal mol~^ <71JA6205>. Thus no electronic delocalization of the
lone pair into the 5-membered ring occurs. Since it is readily available, 5-phenyldibenzophosphole is
used by coordination chemists as a non-flexible equivalent of triphenylphosphine. More significantly, the
dibenzophosphole unit served as a building block for Hellwinkel's synthesis of the first well-characterized
hexacoordinate phosphates with six P-C bonds <65CB576> (Equation (5)).
But, of course, the landmark of 1953 was the Dupont work describing the McCormack reaction for the
first time <53USP2663736-9> (Equation (2)). Suffice it to say that these patents formed the basis for almost
all progress in the field of 5-membered P-C rings for the next two decades. It is also interesting to note that
the phospholene oxides resulting from the hydrolysis of the McCormack cycloadducts immediately found
application in the catalytic decarboxylation of isocyanates into carbodiimides <62JA3673>. This reaction
has also been applied to generate heat-resistant polycarbodiimides from diisocyanates <62JA1493>.
The next event was the synthesis of pentaphenylphosphole by two groups working independently in
1959 <59JA3163, 59CI(L)1250>. The easiest preparation, from diphenylacetylene, is depicted in Equation (6).
Of course, the interest engendered by phospholes, concerned the degree of aromaticity in the ring. With
its heavy substitution, pentaphenylphosphole was obviously not the ideal species for such a study. Some
time later, Quin and his group prepared 1-methyl and 1-benzyl-phospholes and obtained an X-ray crystal
structure of the 1-benzyl derivative <67JA5984, 70JA5779> (Equation (7)).
The Birth of a New Domain
[pc^j^^ppccyy -+ 5 / ^
Li Li
(5)
Li PhPCl2
2 PhC=CPh LiC(Ph)=C(Ph)-C(Ph)=C(Ph)Li
THF
Br Br Br Br
Br2
(7)
P
O^ R 1
R
R = Me, CHoPh
The non-planarity of phosphorus - whose P-benzyl bond makes an angle of 67° with the mean plane of
the ring - casts serious doubt upon the aromaticity of the system. Almost at the same time, Mislow and
his group demonstrated that phospholes display a very low pyramidal inversion barrier, of ca. 16 kcal
mol~^ <71JA6205>. The most logical way to reconcile these data was to admit that the pyramidal ground
state of phospholes was weakly aromatic, whereas full delocalization took place in the planar transition
state as a result of the extensive overlap between the lone pair orbital and the n orbitals of the diene.
Recent high-level calculations fully confirm this analysis <95AG(E)337> and show that the aromaticity of
the pyramidal parent phosphole is ca. 7 kcal mol~^ Phospholes are not planar because the aromaticity
of the planar state is insufficient to overcome the intrinsically high inversion barrier of phosphorus. Here
again, we see a sharp contrast with the corresponding nitrogen heterocycles. In the meantime, both the
synthesis and the chemistry of phospholes had been thoroughly investigated. Mathey's discovery that it is
possible to dehydrohalogenate the McCormack cycloadducts directly to phospholes at room temperature
using tertiary amines provided a useful practical advance <69CR(C)1066> (Equation (8)).
O
R'3N (- 2 HX)
+ (8)
CH2CI2 + hydrocarbon P
R X
R
This method, which has been refined several times <81S983>, remains the simplest and most general access
to phospholes. Its mechanism, which was fully understood only much later, involves a reversible [1,5] shift
of hydrogen from carbon to phosphorus. These shifts are the key feature of phosphole chemistry, but only
in 1981 was the first unambiguous demonstration of a thermal equilibrium between \H- and 2^-phospholes
brought to light <81JA4595> (Equation (9)). Trapping of the reactive 2//-phospholes by diphenylacetylene
gave the first known 1-phosphanorbomadienes. Very soon, the same type of transformation was shown to
occur with P-unsubstituted 1^-phospholes at low temperature <83JA687l>, thus explaining why it had been
impossible to characterize such species previously. In the absence of trapping reagents, these 2//-species
give P-P bonded [4 + 2] Diels-Alder dimers <82CC1272> (Equation (10)).
The Birth of a New Domain
le Me Me Me
H \M\
\ /
> 150°C PhC=CPh Me^
(9)
>^ P h ^^
Ph
Me Me
Me^
[4+2] ^ P.
(10)
<0°C
endo junction
Eventually, the unstable parent phosphole was fully characterized by ^H, ^^C and ^^P NMR spectroscopy in
1987 <87TL5025>. It must be stressed here that these [1,5] sigmatropic shifts are similar to those observed
for cyclopentadienes. From a theoretical standpoint, they reflect a,7t* overlap between the dienic system
and the exocyclic P-R bond existing as a result of the pyramidal geometry of phospholes. Surprisingly,
high-level computations later indicated that the parent 2//-phosphole appears to be slightly more stable
than its l//-isomer <93JOC5414>.
The sixties saw an acceleration in the development of phosphorus heterocyclic chemistry. 1962 saw
the first authenticated synthesis of phosphetanes from methyl-substituted olefins and halophosphines
<62JOC606> (Equation (11)).
1)PC13,A1C13
(11)
2)H20
The ionic mechanism implies a [1,2] methyl shift (Equation (12)).
Me
MQ Me
[l'2]Me
Me-Tf Me Me—r^ Me (12)
Me 1^ PCI2 Me PCI2
Me
Having an internal angle in the range of 80-90°, which favours the formation of pentacoordinate TBP
intermediates, phosphetanes have been ideal models for mechanistic studies of reactions involving P(III)
and P(IV) centres. Among these are nucleophilic substitutions at their chloro derivatives in the P(III) and
P(IV) states <72JCS(P1)713, 74JCS(D)633>.
Wagner's work describing a very simple synthesis of phosphiranes appeared just one year later, in 1963
<63USP3086053, 67JA1102> (Equation (13)).
R'
liq. NH3
C1CHR'-CH2C1 + 2 RPHNa (13)
P
I
R
The ^^P resonance of the parent phosphirane appears at -341 ppm <830MR494> and, of diamagnetic
phosphorus derivatives, only P4 itself resonates at higher field. The ring, having an internal angle of 47°,
is highly strained <69JA17>, but less than aziridine (C2H4X: X = PH, AE -22 kcal mol'^ X = NH, AE
-29.5 kcal mol"^) according to recent theoretical studies <89JPC3025>. Another feature brought to light
recently concerns the inversion of phosphorus in halophosphiranes, which apparently occurs via a turnstile
rotation <97CC1033>. Clearly, these rings will provide rich pickings for theoreticians! The cyclic strain
The Birth of a New Domain 5
induces a number of unusual ring-opening and ring-expansion reactions. The phosphorus lone pair displays
a very low nucleophilicity, but it has been possible to prepare the first well-characterized phosphiranium
salts by reaction with methyl triflate. These salts appear to be convenient precursors for non-stabilized
phosphenium ions which can be trapped by alkynes <95CC257>.
A year later, in 1964, Mahler described the first l,2-dihydro-l,2-diphosphete <64JA2306> (Equation
(14)).
F3C, CF3
CF. /CF
(CF3P)n ^ II +,,r^P P^r^ ^ ^
170°C,70h CF ^^ CF ^ ^P^ ^
(55%) ^ I (31%)
CF3
Later, this very simple synthesis was shown to be more general, but extensive studies of these rather
interesting molecules had to await the discovery of a more convenient route involving the ring-expansion
of phosphirenes <87AG(E)548> (Equation (15)).
Bu,P
y + RTClj
I
R
However, the major event of 1964 unquestionably took place during a lUPAC meeting on organophos-
phorus chemistry in Heidelberg. On that occasion, Braye described the synthesis of phospholide ions
by cleavage of the exocyclic P-R bond of phospholes. His results were published much later <71T5523>
(Equation (16)).
r\ ™F r\
\ y +M \ y> M++[R°] (16)
P P -
R
M = Li, Na, K R = Ph, Me, PhCH
It was subsequently demonstrated <71T5705> that the reaction proceeds through an unstable ion-radical
resulting from the addition of one electron to the ring. The cleavage takes place exclusively at the exocyclic
P-R bond. For some time, the phospholide ions were used merely as synthetic tools for the preparation of
new phospholes by reaction with various electrophiles (RX, H"^), and not characterized as such. Then in
1979, Quin and Orton reported the first ^^P data on these ions <79CC40l>. The ^^P resonances appear at low
field, i.e. +77 ppm for the parent (more classical phosphides resonate around 0 ppm). This deshielding is
associated with the presence of an in-plane lone pair at phosphorus which, through being weakly coupled
to the ring, induces a large paramagnetic shift <94JA9638>. Full ^H, ^^C and ^^P NMR characterization
came in 1987 <87TL5025>. Subsequently, the aromatic nature of these ions was demonstrated by the X-ray
crystal structure analysis of lithium tetramethylphospholide <89AG(E)1367>. The ring is planar, the P-C
bonds are short at 1.75 A, and the C-C bonds are equal at ca. 1.40-1.42 A. It is interesting to note that
the lithium is r^^-bonded. Recent theoretical studies calculate an aromatic stabilization energy for the free
parent ion at 29 kcal mol~^ <960M1755>, which is close to that of cyclopentadienide. Thus, phospholide
ions appear to be genuine aromatic species, even though all of their chemistry to date takes place at
phosphorus. Nonetheless, development of chemistry at the ring a-carbons is possible through a very
simple functionalization method that has been discovered recently <97AG(E)98, 980M2996> (Equation (17)).
The key step is the [1,5] sigmatropic shift of the functional group from phosphorus to the a-carbon. The
resulting 2//-phosphole is then deprotonated by the base as soon as it is formed. Obviously, the functional
group must be compatible with the base at temperatures where the migration takes place.
Perhaps the most important discovery of all appeared in 1966 when Markl described the first
phosphinine and paved the way for the development of the chemistry of dicoordinate phosphorus.
2,4,6-Triphenylphosphinine was obtained through a very simple O"^ ^- P exchange process in suitable
pyrylium salts <66AG(E)846> (Equation (18)). Several sources of PH3 such as P(CH20H)3, P(SiMe3)3 and
PH4I have been used. This initial discovery was later complemented by the preparation of the parent
compound by Ashe <71JA3293> (Equation (19)).