Chapter 5 Five-Coordinate and Quasi-Five-Coordinate Phosphorus Downloaded by STANFORD UNIV GREEN LIBR on September 25, 2012 | http://pubs.acs.org Publication Date: April 7, 1992 | doi: 10.1021/bk-1992-0486.ch005
JohnG.Verkade Department of Chemistry, Iowa State University, Ames, IA 50011
Some time ago we described the synthesis of two novel related classes of polycyclic phosphorus compounds known as prophosphatranes (I) and phosphatranes (II) (1-8). In more recent years we have extended these classes to include the analogous proazaphosphatranes (III) and azaphosphatranes (IV) (9-14).
Prophosphatranes I o,+
Phosphatranes II Of interest in systems of type I-IV is the relationship of the substituent Ζ or Z+ to the stabilization of structure I relative to II and that ofIIIrelative to IV. Although the former relationship is reasonably well characterized (4), the latter one is currently under investigation in our laboratories. By utilizing the proper combination of R and Z, it is possible to imagine the possibility of inducing long-range bridgehead-bridgehead 0097-6156/92/0486-0064$06.00/0 © 1992 American Chemical Society In Phosphorus Chemistry; Walsh, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
5. VERKADE
Five-Coordinate and Quasi-Five-Coordinate Phosphorus
65
interactions of varying strength of the sort depicted in V. Similarly, certain Ζ groups may giveriseto partial transannular interaction in quasiphosphatranes as shown in VI. o,+
0,+ ι
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R
Λ
Μ
OS Azaphosphatranes IV
Pro-azaphosphatranes III ζ R
|0,+
I
x
o,+
I
R
Quasi-azaphosphatranes V
Quasiphosphatranes VI
Synthesis of Azaphosphatranes Azaphosphatranes were accidentally synthesized for the first time as a result of a frustrated attempt to synthesize pro-azaphosphatranes 1 and 2 in the direct approach depicted in reaction 1. In the case of R = H, an insoluble polymer is slowly formed (75). When R = Me, we observed by P N M R spectroscopy the slow formation of 2 31
Ν P(NMe2) + (RNHCH CH ) N 3
2
2
3
N
^
-
R
(1)
p —
1 R=H 2 R = Me 3 R = CH Ph 2
oyer a period of weeks in an equilibrium mixture also containing mono and disubstituted product Upon work-up, the reaction mixture gave a mediocre yield (2050%) of 2 (70). This contrasts with the rapid and excellent conversion of (MeNHCH )3CMe in the presence of P(NMe )3 to P(MeNΠ)3CMe (76). 2
2
2
In Phosphorus Chemistry; Walsh, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
PHOSPHORUS CHEMISTRY
66
Rather than attempt to make the more sterically hindered 3 via reaction 1, we sought to speed up the formation of pro-azaphosphatranes by employing the more reactive phosphorus reagent indicated in reaction 2. Indeed the reaction was over within an hour at room temperature, but to our surprise, the azaphosphatranes 4-6 formed in virtually quantitative yield (14, 15). Thus in spite of the presence of the
H ClP(NMe2) + ( R N H O ^ C H ^ N Et N
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3
R α
> - < 5
2
(2)
R=H R = Me 6(C1) R = CH Ph 4(C1) 5(C1)
2
strong base Et3N for the neutralization of the HC1 formed, the latter protonated the proazaphosphatranes which (as is discussed later) are considerably more basic than any amine known (70,14, 75, 77). Reaction 2 also occurs quantitatively in CH2CI2 at 0 °C in the absence of Et3N. The rapid formation of 4(C1) in reaction 2, in contrast to the polymer formed in reaction 1, suggests that a protonated intermediate such as 4(C1) may facilitate rapid further nucleophilic attack on phosphorus of the pendant primary H
I
H.
Ν—PiNMe,), CI \^N(CH CH NH )
ClPiNMe^ + (H NCH CH ) N • 2
2
2
3
2
2
2
(3)
2
7(C1)
amine branches. Once 4 (CI) is formed, polymer formation (presumably by intermolecular transamination via the secondary amines) is inhibited. As might be expected then, 4(OTf) can be synthesized within an hour in quantitative yield via reaction 4 in which a proton source is present from the start in equimolar concentration. 4
PÎNMe ) +HOTf + ( H N Œ Œ ) N - ^ - ^ 4(OTf) () υ"ΐ ν-Λ The molecular structures of 4(C1) (74) and 5(BF4) (70) have been confirmed by X-ray means. Formally, these structures can be considered to contain a P(III) atom which is stabilized through proton-induced chelation by the tertiary nitrogen, rendering the Ρ(ΠΙ) atom five-cooniinate. Although a more sophisticated view of the bonding in these cations involves three-center four-electron MOs along the three-fold axis, the oxidation state of the phosphorus can be viewed as trivalent since electron withdrawal 2
3
2
2
2
3
2
2
In Phosphorus Chemistry; Walsh, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
5. VERKADE
Five-Coordinate and Quasi-Five-Coordinate Phosphorus
67
by the proton is compensated by electron donation from the axial nitrogen. The presence of these pentacoordinate structures is signalled by upfield *P chemical shifts in the -10 to -42 ppm region, with ^ P H coupling valuesfrom453 to 506 Hz (75). 3
It might be predicted that cationic compounds of type IV wherein Ζ is an alkyl carbocation would be stable. As will be discussed later, a C H 3 species does not induce transannulation, but the more electronegative CI^Br* cation does (9,18). Similarly, CI2 provides a C l which induces transannulation in 2 to give azaphosphatrane 9(C1) (9,18). +
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+
2
8 (Br) Z = CH Br,X = Br 9 (CI) Ζ = Cl, Χ = Cl 2
The examples of azaphosphatranes cited so far possess five-coordinate Ρ(ΙΠ) atoms which formally began as three-coordinate Ρ(ΙΠ) in a pro-azaphosphatrane. It is also possible to inducefive-coordinationin the P(V) pro-azaphosphatrane 10 with BF3 (72). The Ψ chemical shift of this compound (-2.2 ppm) is upfield as expected, and 3
10
11
no P=0 stretching frequency is detected in its IR spectrum. We observed similar results for the reaction of prophosphatrane 12 with BF3 (6) as well as with the other Lewis acids shown in reactions 7-13 (6, 7, 79). Although no structural metrics could be determined for 13-19, reaction 14 gave a crystalline product 21 which was structured by X-ray means (7, 79) whereas reactions 15-17 (7) did not provide single crystals. The robust character of the chelated structure of these P(V) azaphosphatranes and phosphatranes is undoubtedly a strong driving force for the diminution of the very strong P-chalcogen multiple bond to a formally single bond. A factor in the dibasic character of the axial chalcogen in some of the above reactions is undoubtedly the electron induction provided by the axial nitrogen. Evidence has also been adduced for a transannulated H3PO4 adduct of 12 in which two acidic hydrogens are hydrogen bonded to the P=0 oxygen while the third protonates this atom (6). That chelation plays a key role in the stabilization of azaphosphatranes and phosphatranes is consistent with our failed attempts to find P NMR evidence for pentacoordinate phosphorus species in mixtures of protonic acids, P(OR)3 or P(NMe2)3, and NR3. The observation 3 1
In Phosphorus Chemistry; Walsh, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
PHOSPHORUS CHEMISTRY
68
Δ A
V
A'
~|n 13 B F
o-J
A* lp
0
14 H
lp
1+ (8)
15 Η
Η
2+
(9)
16 Et Si lp
1+
(10)
17 Et Si Et Si 2+
(11)
3
3
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3
R
+
Et
+
H
+
A
V
A'
0-f
